High Voltage in Ships
We all know about the voltages used on board a ship. It is usually a 3phase, 60Hz, 440 Volts supply being generated and distributed on board.
Every day the owners and designers aim for bigger ships for more profitability. As the ship size increases, there is a need to install more powerful engines and other machineries.
This increase in size of machineries and other equipment demands more electrical power and thus it is required to use higher voltages on board a ship.
Any Voltage used on board a ship if less than 1kV
(1000 V) then it is called as LV (Low Voltage) system and any voltage above 1kV is termed as High Voltage.
Typical Marine HV systems operate usually at 3.3kV or 6.6kV. Passenger Liners like QE2 operate at 10kV.
Defination of HV:
The numerical definition of high voltage depends on context. Two factors considered in classifying a voltage as “high voltage” are the possibility of causing a spark in air, and the danger of electric shock by contact or proximity. The definitions may refer to the voltage between two conductors of a system, or between any conductor and ground.
In electric power transmission engineering, HIGH VOLTAGE is usually considered any voltage over approximately 33,000 volts. This classification is based on the design of apparatus and insulation.
The International Electro technical Commission and its national counterparts (IET, IEEE, VDE, etc.) define high voltage as above 1000 V for alternating current, and at least 1500 V for direct current—and distinguish it from low voltage (50–1000 V AC or 120–1500 V DC) and extra-low voltage (<50 V AC or <120 V DC) circuits. This is in the context of building wiring and the safety of electrical apparatus.
– In the United States 2005 National Electrical Code (NEC), high voltage is any voltage over 600 V (article 490.2).
– British Standard BS 7671:2008 defines high voltage as any voltage difference between conductors that is higher than 1000 V AC or 1500 V ripple-free DC, or any voltage difference between a conductor and Earth that is higher than 600 V AC or 900 V ripple-free DC.
WHAT IS CLASSED AS HIGH VOLTAGE?
In marine practice,
– voltages below 1,000Vac (1kV) are considered low voltage, and
– high voltage is any voltage above 1kV. Typical marine high voltage system voltages are 3.3kV, 6.6kV and 11kV.
THE MAJOR DIFFERENCES BETWEEN HIGH VOLTAGE SUPPLY AND LOW VOLTAGE SUPPLY ON BOARD SHIPS ARE:
1. High voltage systems are more extensive with complex networks and connections,
2. Isolated equipment MUST BE earthed down
3. Access to high voltage areas should be strictly limited and controlled
4. Isolation procedures are more involved
5. Switching strategies should be formulated and recorded
6. Specific high voltage test probes and instruments must be used
7. Diagnostic insulation resistance testing is necessary
8. High voltage systems are usually earthed neutral and use current limiting resistors
9. Special high voltage circuit breakers have to be installed
Why High Voltage in Ships?
– Higher power requirements on board vessels is the foremost reason for the evolution of HV in ships.
– Higher power requirements have been necessitated by development of larger vessels required for container transport particularly reefer containers.
– Gas carriers needing extensive cargo cooling Electrical Propulsion.
– For ships with a large electrical power demand it is necessary to utilise the benefits of a high voltage (HV) installation.
– The design benefits relate to the simple ohms law relationship that current (for a given power) is reduced as the voltage is increased. Working at high voltage significantly reduces the relative overall size and weight of electrical power equipment.
AS PER OHMS LAW
POWER = VOLTAGE x CURRENT
For a given Power,
Higher the Voltage, Lesser is the Current
440 KW = 440,000 Watts
= 440 Volts x 1000 Amps
=1100 Volts x 400 Amps
=11000 Volts x 40 Amps
– When large loads are connected to the LV system the magnitude of current flow becomes too large resulting in overheating due to high iron and copper losses.
P = VI CosФ
Copper loss =I² R [kW]
HV levels of 3.3 kV, 6.6 kV and 11 kV are regularly employed ashore for regional power distribution and industrial motor drives.
For example, a motor (let us assume a bow thruster), may be of a smaller size if it designed to operate on 6600 Volts.
For the same power, the motor would be of a smaller size if it is designed for 6600Volts when compared to 440Volts.
Thus these are the major reasons why recent ships have shifted towards high voltage systems.
The main disadvantage perceived by the user /maintainer, when working in an HV installation, is the very necessary adherence to stringent safety procedures.
Advantages/Disadvantages of using HV Advantages:
Advantages:
For a given power, Higher voltage means Lower current, resulting in:
– Reduction in size of generators, motors, cables etc.
– Saving of Space and weight
– Ease of Installation
– Reduction in cost of Installation
– Lower losses – more efficient utilization of generated power
– Reduction in short circuit levels in the system which decides the design and application of the electrical equipment used in the power system.
Disadvantages:
1. Higher Insulation Requirements for cables and equipment used in the system.
2. Higher risk factor and the necessity for strict adherence to stringent safety procedures.
Marine Electrical System
– Some installations may feed the ships sub stations directly with HV and step- down to 440 V locally.
– The PEM drives in this example are synchronous motors which require a controlled low voltage excitation supply current to magnetise the rotor poles.
– This supply is obtained from the HV switchboard via a step-down transformer but an alternative arrangement would be to obtain the excitation supply from the 440 V ER sub switchboard.
- Hazardous Electrical Voltage Training ChecklistThe training requirements below apply to all employees who face a risk of electrical shock that is not reduced to a safe level by electrical installation requirements and
who must work on or near energized components.
All Qualified High Voltage Electrical Workers who work on high voltage equipment (> 600 volts) are required
to be trained on safety-related work practices that pertain to their jobs and in the following topics below:
The skills and techniques necessary to distinguish exposed live parts from other parts of electrical equipment.
• The skills and techniques necessary to determine the nominal voltage of exposed live parts.
- The clearance distances and the corresponding voltage to which the Qualified Person will be exposed.
• Safely de-energizing of parts and subsequent electrical lockout and tagging procedures as required by the electrical standard.
• Proper precautionary work techniques.
• Proper use of PPE to include non-conductive gloves, aprons, head protection, safety glasses, and face shields.
- Proper selection and use of rated test instruments and equipment, including the capability to visually inspect all parts of the test equipment for defects.
• Use of insulating and shielding materials for employee protection to include auxiliary shields, guards, mats, or other specific equipment.
• Proper use of insulated tools or other non-conductive devices such as fuse pullers, fish tapes, hot sticks, ropes, or handlines.
• The importance of illumination and to work only in properly illuminated areas.
- Proper work techniques for work in enclosed or confined work spaces.
• Removal or special handing of any conductive materials and equipment.
• Proper and safe use of portable ladders around electrical equipment.
• Removal of any conductive jewelry or apparel.
• Proper alerting techniques such as using safety signs and tags, barricades,attendants, and work practices.
• Any other safety related work practice not listed above but necessary for them to safely do their job
Electric Shock:
Voltages greater than 50 v applied across dry unbroken human skin can cause heart fibrillation if they produce electric currents in body tissues that happen to pass through the chest area.
Accidental contact with high voltage supplying sufficient energy may result in severe injury or death. This can occur as a person’s body provides a path for current flow, causing tissue damage and heart failure. Other injuries can include burns from the arc generated by the accidental contact. These burns can be especially dangerous if the victim’s airways are affected.
Hazards of High Voltage
Arcing:
An unintentional electric arc occurs during opening of a breaker, contactor or switch, when the circuit tries to maintain itself in the form of an arc.
During an insulation failure, when current flows to ground or any other short circuit path in the form of accidental tool slipping between conducting surfaces, causing a short circuit.
Results of an electric arc:
Temperatures at the arc terminals can reach or exceed 35,000° f or 20,000˚c or four times the temperature of sun’s surface. The heat and intense light at the point of arc is called the arc flash.
Air surrounding the arc is instantly heated and the conductors are vaporised causing a pressure wave termed as ARC BLAST.
Hazards of an Arc Flash:
– During an arc flash, sudden release of large amounts of heat and light energy takes place at the point of arc.
– Exposure frequently results in a variety of serious injuries and may even be fatal, even when the worker is ten feet or more from the arc center.
– Equipments can suffer permanent damage.
– Nearby inflammable materials may be ignited resulting in secondary fires.
Hazards of Arc Blasts & ejected materials:
– An arc flash may be accompanied by an arc blast
– The arc blast causes equipment to literally explode ejecting parts with life threatening force. – Heated and vaporised conducting materials surrounding the arc expand rapidly causing effects comparable to an explosive charge.
– They may project molten particles causing eye injuries. The sound that ensues can harm the hearing.
- Potential injuries:
– At some distance from the arc, temperatures are often high enough to instantly destroy skin and tissue. Skin temperatures above 100˚C ( about 210˚F) for 0.1sec result in irreversible tissue damage, defined as an incurable burn.
– Heated air and molten materials from arc faults cause ordinary clothing to burst into flames even if not directly in contact with the arc. Synthetic fibers may melt and adhere to the skin resulting in secondary burns.
– Even when safety goggles are worn, arc flash may cause severe damage to vision and or blindness. Intense UV light created by arc flash can damage the retina. Pressure created from arc blasts can also compress the eye, severely damaging vision.
– Hearing can also be affected by the loud noise and extreme pressure changes created by arc blasts. Sound blasts with arc blasts exceed 140dB which is equal to an airplane taking off. Sudden pressure changes exceeding 720lbs/sq.ft for 400ms can also rupture eardrums. Even at lesser pressure, serious or permanent damage to hearing may occur.
Short Circuit
A short circuit ( or a fault ) is said to have taken place when the current is not confined to its normal path of flow but diverted through alternate path(s).
– During short circuit, the current rises much above the normal value.
– Short circuit level is the maximum possible current that flows at the point of fault during a short circuit.
Effects of short circuit:
High currents during Short circuits can cause damage to electrical installation by giving rise to excessive
Thermal Stresses, Mechanical Stresses , Arcing.
Methods adopted to prevent effects of short circuit in a system:
– A well-designed Protective Relay system trips out a breaker(s) and isolates the faulty circuit from the power source within a short time to prevent/minimise effects of high short circuit current, as and when it occurs.
– The equipment in the system, the cables, the switchgear, the busbar, the generators are designed to withstand the effects of short circuit during that short period.
Calculation of the short circuit levels in the system is therefore required to help in:
a. Designing an appropriate Protective Relay System
b. Choosing the right switchgear with suitable short circuit withstand capacity to be used in the system.
High Voltage Safety and Precautions
Making personal contact with any electric voltage is potentially dangerous. At high voltage (>1000 V) levels the electric shock potential is lethal. Body resistance decreases with increased voltage level which enhances the current flow. Remember that an electric shock current as low as 15 mA can be fatal. So,the risk to people working in HV areas is greatly minimised by the diligent application of sensible general and company safety regulations and procedures.
Personnel who are required to routinely test and maintain HV equipment should be trained in the necessary practical safety procedures and certified as qualified for this duty.
Approved safety clothing, footwear, eye protection and hard hat should be used where danger may arise from arcs, hot surfaces and high voltage etc.
Safety equipment should be used by electrical workers includes insulated rubber gloves and mats. These protect the user from electric shock.
Safety equipment is tested regularly to ensure it is still protecting the user. Testing companies can test at up 300,000 volts and offer services from glove testing to Elevated Working Platform or EWP Truck testing.
A insulated material or rubber mat can be used as a dead front of all electrical installations and equipments.
The access to HV switchboards and equipment must be strictly controlled by using a permit-to-work scheme and isolation procedures together with live-line tests and earthing-down before any work is started. The electrical permit requirements and procedures are similar to permits used to control access in any hot-work situation, e.g. welding, cutting, burning etc. in a potentially hazardous area.
HIGH VOLTAGE SAFETY RULES AND PROCEDURES
All safety rules presented in this document are intended to ensure safe working conditions while working with potentially dangerous voltages. It is assumed that all personnel working with potentially dangerous voltages have been trained in basic electrical safety procedures.
1. This guidance does not apply where equipment has been isolated, discharged, disconnected and removed from the system or installation.
2. Equipment that is considered by an Authorised Person (HV) to be in a dangerous condition should be isolated elsewhere and action taken to prevent it from being reconnected to the electricity supply.
3. All working on, or testing of, high voltage equipment connected to a system should be authorised by a permit-to-work or a sanction-for- test following the procedures as described in Practical Exercises no. 4
4. No hand or tool (unless the tool has been designed for the purpose) must make contact with any high voltage conductor unless that conductor has been confirmed dead by an Authorised Person (HV) in the presence of the Competent Person (HV).
5. Where any work or test requires an Accompanying Safety Person (HV) to be present, he/she should be appointed before that work or testing can begin.
6. Voltage test indicators should be tested immediately before and after use against a test supply designed for the purpose.
7. Where the procedures involve the application of circuit main earths, the unauthorised removal of such earths should be prevented, wherever practicable, by the application of safety locks.
8. Where the procedures involve the removal of circuit main earths, that is, testing under a sanction-for-test, the earths will be secured with working locks. The keys to these locks will be
retained by the Duty Authorised Person (HV), who will remove and replace the earths as requested.
Precaution prior to live voltage and phasing checks:
1. Where live phasing is to be undertaken, the area containing exposed live conductors should be regarded as a high voltage test enclosure.
2. Approved equipment used for live voltage and phasing checking at high voltage should be tested immediately before and after use against a high voltage test supply.
3. Live voltage and phase checking on high voltage equipment may only be undertaken by a Authorised Person (HV), with assistance if necessary from a Competent Person (HV)
acting on verbal instructions from the Authorised Person (HV). Neither a permit-to-work nor a sanction-for-test is required, but the Authorised Person (HV) and any assistant should
be accompanied by an Accompanying Safety Person
(HV).
Testing at high voltage:
1. Where high voltage tests are to be undertaken, a sanction-for-test should be issued to the Competent Person (HV) who is to be present throughout the duration of the tests.
2. The areas containing exposed live conductors, test equipment and any high voltage test connection should be regarded as high voltage enclosures.
High voltage test enclosures:
1. Unauthorised access to a high voltage test enclosure should be prevented by, as a minimum, red and white striped tape not less than 25 mm wide, suspended on posts, and by the display of high voltage danger signs. An Accompanying Safety Person (HV) or the Duty Authorised Person (HV) should be present throughout the duration of the tests, and the area should be continually watched while testing is in progress.
Work on busbar spouts of multi-panel switchboards
When work is to be carried out on busbar spouts, the following operations should be carried out in strict sequence:
a. the Authorised Person (HV) should record
the details of necessary safety precautions and switching operations on a safety programme and produce an isolation and earthing diagram;
b. the section of the busbar spouts on which work is to be carried out must be isolated from all points of supply from which it can be made live;
c. the isolating arrangements should be locked so that they cannot be operated, and shutters of live spouts locked shut. Caution signs should be fixed to the isolating points;
d. where applicable, danger signs should be attached on or adjacent to the live electrical equipment at the limits of the zone in which work is to be carried out;
- busbars should be checked by means of an approved voltage indicator to verify that they are dead, the indicator itself being tested immediately before and after use. The checking with the voltage indicator should be done on the panel to which the circuit main earths are to be applied. This test should also be made on the panel on which the work is carried out;
f. circuit main earths should be applied at a panel on the isolated section of the busbar other than that at which work is to be done using the method recommended by the switchgear manufacturers. The insertion of hands or any tool into the contact spouts for this purpose is not an acceptable practice;
g. an earth connection should also be applied to all phases at the point-of-work;
h. the permit-to-work should be issued to cover the work to be done. During the course of the work, where applicable, the earth connection(s) at the point-of-work may be removed one phase at a time. Each phase earth connection must be replaced before a second-phase earth connection is removed;
j. on completion of the work, the permit-to-work should be cancelled.
Definition of safety terms:
Definitions presented here are those deemed necessary and suitable for electrical laboratory applications present in the Electronics and Electrical Engineering Laboratory. They should not be assumed to be directly related to definitions presented in other electrical standards or codes.
High Voltage: Any voltage exceeding 1000 V rms or 1000 V dc with current capability exceeding 2 mA ac or 3 mA dc, or for an impulse voltage generator having a stored energy in excess of 10 mJ. These current and energy levels are slightly below the startle response threshold.
Moderate Voltage: Any voltage exceeding 120 V rms (nominal power line voltage) or 120 V dc, but not exceeding 1000 V (rms or dc), with a current capability exceeding 2 mA ac or 3 mA dc.
Temporary Setups: Systems set up for measurements over a time period not exceeding three months.
Test Area: Area in which moderate voltages are accessible, and which has been clearly delineated by fences, ropes, and barriers.
Troubleshooting: Procedure during which energized bare connectors at moderate or high voltages might be temporarily exposed for the purpose of repair or problem diagnosis.
Inter lock: A safety circuit designed to prevent energizing high- or moderate-voltage power supplies until all access doors are closed, and to immediately de-energize such power supplies if the door is opened. Note that this function does not necessarily ensure full discharge of stored energy.
Bare Conductor: A conductor having no covering or electrical insulation whatsoever.
Covered Conductor : A conductor enclosed within a material of composition or thickness not defined as electrical insulation .
Insulated Conductor: A conductor encased within material of composition and thickness defined as electrical insulation.
Exposed Conductor: Capable of being inadvertently touched or approached nearer than a safe distance by a person. It applies to parts that are not suitably guarded, isolated, or insulated.
Unattended Operation: The operation of a permanent setup for electrical measurements for a time period longer than can be reasonably attended by staff.
Enclosed: Surrounded by a case, housing, fence or wall(s) that prevents persons from accidentally contacting energized parts.
Temporary Setups
When troubleshooting a setup with exposed or bare conductors at high or moderate voltages, it may be necessary to temporarily bypass safety interlocks. Such procedures may only be performed under two-person operating conditions.
In instances where troubleshooting a system or particular equipment becomes frequent (at least once every six months) Group Leader approval is required. In all cases two staff members must be present when high voltage is energized and the interlock(s) bypassed. When troubleshooting a single piece of equipment in such a way that personnel may have access to high or moderate voltage (for example, repairing an instrument), two persons should be present.
The “keep one hand in the pocket” rule is strongly encouraged.
Signs and Warning Lights
DANGER HIGH VOLTAGE signs must be on display on all entrances to all test areas where bare conductors are present at both moderate and high voltages. These signs should be in the vicinity of the test area and on the outside of the door leading to the laboratory area.
A warning light, preferably flashing, must be on when high or moderate voltages are present, and ideally should be activated by the energizing of the apparatus. The warning light must be clearly visible from the area surrounding the test area. In special cases where such a light interferes with an experiment, it can be omitted with special permission from the Group Leader and Division Chief.
In all cases where there is direct access from the outside hallway to the area where high or moderate voltages are present, a warning light, DANGER HIGH VOLTAGE sign, a safety interlock (for high voltages) and a locked door are required.
For unattended setups with bare conductors at high or moderate voltage, a warning sign with the names of two contact persons and the dates of unattended operation must be posted on the door leading to the high-voltage area. In addition, written notice of unattended testing of high or moderate voltage with bare conductors must be sent to the NIST Fire Department (in Gaithersburg) or to the Engineering, Safety, and Support Division (in Boulder) clearly stating the anticipated dates of operation. A warning light on or near the door to the laboratory must be illuminated when high or moderate voltages with bare conductors are present.
Grounding Stick
Before touching a high-voltage circuit or before leaving it unattended and exposed, it must be de-energized and grounded with a grounding stick. The grounding stick must be left on the high-voltage terminal until the circuit is about to be re-energized. Grounding sticks must be available near entrances to high-voltage areas. Automatic grounding arrangements or systems that employ audible warning tones to remind personnel to ground the high-voltage equipment are strongly encouraged for two-person operation, and are mandatory for one-person or unattended operation.
For systems with bare conductors at moderate voltages, the use of a grounding stick is strongly recommended, particularly if the setup contains energy-storage devices.
Modes of Operation
Two-person: Two-person operation is the normal mode of operation where high or moderated voltages are present. Allowed exceptions are:
When all potentially dangerous voltages are confined inside a grounded or insulated box, or where the voltages are constrained in a shielded cable, or where the is no access to bare conductors
When one-person or unattended operation setups have been designed and approved according to the rules set out in this document and with appropriate approval.
It is presumed that both individuals participating in two-person operation will follow basic high-voltage safety procedures and will monitor each other’s actions to ensure safe behavior.
One-person: One-person operation of systems using high and moderate voltages with bare or exposed conductors, may be approved, after appropriate review and authorization, in order to provide for the efficient use of staff for long-term applications where it is judged that safety would not be compromised.
Unattended: It is recognized that in order to run efficient calibration services and maintain appropriate delivery schedules, unattended operation of systems using high and moderate voltages may be necessary. In such cases, unattended operation is permitted.
with appropriate review and authorization, for systems having no bare or exposed conductors, and where required warning signs, lights, and barriers are present.
Unattended operation of setups with bare or exposed conductors at high and moderate voltages may be necessary under special circumstances, such as for unusually long data- acquisition periods. This is meant to be a rare occurrence. Should this mode of operation be frequently employed, then the apparatus should be modified to enclose all potentially dangerous voltages.
Circuit Breakers & Disconnects
Circuit breakers, disconnects or contactors used to energize a high-voltage source must be left in an open position when the supply is not in use. Laboratories should always be left in a configuration that at least two switches must be used to energize high-voltage circuits. Whenever possible a “return-to-zero-before energizing” interlock should be incorporated into the high-voltage supply.
Proper Circuit Design Recommendations
– Draw the circuit and study it before wiring it for operation at high voltage.
– Make sure all devices that require grounding are securely grounded.
– Allow adequate clearances between high-voltage terminals and ground.
– Solicit a second opinion before operation for the first time.
Transformers and Variacs:
– Make certain that one terminal of each transformer winding used to provide a separately derived system (this excludes the winding connected to the power supply) as well as the transformer or Variac case are properly grounded.
– The common terminal of a Variac should be connected to the supply neutral.
– Cascade transformers and, in some cases, isolation transformers are exceptions.
- General Information PERMIT-T0-WORK:– Issued by an authorised person to a responsible person who will perform the task of repair/maintenance.
– Generally valid only for 24-Hrs. Permit to be re-validated by the permit-holder if work extends beyond 24 Hrs. after issue Formats will vary and be customized for a particular vessel/marine installation.
Permit To Work- BROAD GUIDELINES:
Prepared in duplicate copy and has at least five sections:
– 1st section states the nature of work to be carried out.
– 2nd section declares where electrical isolation and earthing have been applied and where Danger /Caution notices have been displayed.
– 3rd section is signed by the Person receiving the Permit acknowledging that he is satisfied with the safety precautions taken and the Isolation/ Earthing measures adopted.
– 4th section is signed by the Permit-holder that the work has been completed/suspended.
– 5th Section is signed by the Issuing authority cancelling the Permit.
High Voltage Safety and Precautions
For the purposes of safety, HV equipment includes the LV field system for a propulsion motor as it is an integrated part of the overall HV equipment. From the HV generators, the network supplies HV motors (for propulsion, side thrusters and air conditioning compressors) and the main transformer feeders to the 440 V switchboard. Further distribution links are made to interconnect with the emergency switchboard.
HV Circuit breakers and contactors
Probably the main difference between a HV and an LV system occurs at the HV main switchboard. For HV, the circuit breaker types may be air-break, oil-break, gas-break using SF6 (sulphur hexafluoride) or vacuum-break. Of these types, the most popular and reliable are the vacuum interrupters, which may also be used as contactors in HV motor starters.
Each phase of a vacuum circuit breaker or contactor consists of a fixed and moving contact within a sealed, evacuated envelope of borosilicate glass. The moving contact is operated via flexible metal bellows by a charging motor/spring or solenoid operating mechanism. The high electric strength of a vacuum allows a very short contact separation, and a rapid restrike-free interruption of the arc is achieved.
When an alternating current is interrupted by the separating contacts, an arc is formed by a metal vapour from the material on the contact surfaces and this continues to flow until a current zero is approached in the a.c. wave form. At this instant the arc is replaced by a region of high dielectric strength which is capable of withstanding a high recovery voltage. Most of the metal vapour condenses back on to the contacts and is available for subsequent arcing. A small amount is deposited on the shield placed around the contacts which protects the insulation of the enclosure. As the arcing period is very short (typically about 15 ms), the arc energy is very much lower than that in air-break circuit-breakers so vacuum contacts suffer considerably less wear.
Because of its very short contact travel a vacuum interrupter has the following advantages:
– compact quiet unit
– minimum maintenance
– non-flammable and non-toxic
– The life of the unit is governed by contact erosion but could be up to 20 years.
In the gas-type circuit breaker, the contacts are separated in an SF6 (sulphur hexafluoride) gas which is typically at a sealed pressure chamber at 500 kPa or 5 bar (when tested at 20° C).
HV Insulation Requirements
The HV winding arrangements for generators, transformers and motors are similar to those at LV except for the need for better insulating materials such as Micalastic or similar.
The HV windings for transformers are generally insulated with an epoxy resin/powdered quartz compound. This is a non-hazardous material which is maintenance free, humidity resistant and tropicalised.
Conductor insulation for an HV cable requires a more complicated design than is necessary for an LV type. However, less copper area is required for HV conductors which allows a significant saving in space and weight for an easier cable installation. Where the insulation is air (e.g. between bare-metal live parts and earth within switchboards and in terminal boxes) greater clearance and creepage distances are necessary in HV equipment.
INSULATION RESISTANCE TESTS OF HV EQUIPMENT:
– A 5000 Vdc Megger, Hand-cranking or Electronic can be used for equipments upto 6.6KV.
– For routine testing of IR, 5000 Vdc must be applied for 1 minute either by cranking at constant speed with a Hand-cranking megger or by maintaining a 5000 Vdc continuously by a PB in an Electronic Megger.
IR values taken at different temperatures are unreliable, particularly if the temperature differences are more than 10°C.
SAFETIES OF IR TEST TO HV EQUIPMENTS
1. Before applying an IR test to HV equipment its power supply must be switched off, isolated, confirmed dead by an approved live-line tester and then earthed for complete safety.
2. The correct procedure is to connect the IR tester to the circuit under test with the safety earth connection ON. The safety earth may be applied through a switch connection at the supply circuit breaker or by a temporary earth connection local to the test point. This is to ensure that the operator never touches a unearthed conductor.
3. With the IR tester now connected, the safety earth is disconnected (using an insulated extension tool for the temporary earth). Now the IR test is applied and recorded. The safety earth is now reconnected before the IR tester is disconnected.
This safety routine must be applied for each separate IR test.
At prescribed intervals and particularly after a major repair work on an equipment or switchgear, a Polarisation Index(PI) may be taken to assess the condition of insulation of the equipment. PI readings are less sensitive to temperature changes.
POLARISATION INDEX ( PI ):
When the routine IR value tests (taken at different temperatures) are doubtful or during annual refit or after major repairs are undertaken, a PI test is conducted.
– PI value is the ratio between the IR value recorded after application of the test voltage continuously for 10 minutes to the value recorded after 1 minute of application.
– PI value= 2.0 or more is considered satisfactory.
A motor-driven megger is essential for carrying out a PI test.
High Voltage Equipment Testing
The high voltage (e.g. 6.6 kV) installation covers the generation, main supply cables, switchgear, transformers, electric propulsion (if fitted) and a few large motors e.g. for side-thrusters and air conditioning compressors. For all electrical equipment the key indicator to its safety and general condition is its insulation resistance (IR) and this is particularly so for HV apparatus. The IR must be tested periodically between phases and between phases and earth. HV equipment that is well designed and maintained, operated within its power and temperature ratings should have a useful insulation life of 20 years.
Large currents flowing through machine windings, cables, bus-bars and main circuit breaker contacts will cause a temperature rise due to I2R resistive heating. Where overheating is suspected, e.g. at a bolted bus-bar joint in the main switchboard, the local continuity resistance may be measured and checked against the manufacturers recommendations or compared with similar equipment that is known to be satisfactory.
A normal ohmmeter is not suitable as it will only drive a few mA through the test circuit. A special low resistance tester or micro-ohmmeter (traditionally called a ducter) must be used which drives a calibrated current (usually I = 10 A) through the circuit while measuring the volt-drop (V) across the circuit. The meter calculates R from V/I and displays the test result. For a healthy bus-bar joint a continuity of a few mΩ would be expected.
Normally the safe testing of HV equipment requires that it is disconnected from its power supply. Unfortunately, it is very difficult, impossible and unsafe to closely observe the on-load operation of internal components within HV enclosures. This is partly resolved by temperature measurement with an recording infra-red camera from a safe distance. The camera is used to scan an area and the recorded infra-red image is then processed by a computer program to display hot-spots and a thermal profile across the equipment.
Safety testing of HV equipment:
Normally the safe testing of HV equipment requires that it is disconnected from its power supply. Unfortunately, it is very difficult, impossible and unsafe to closely observe the on-load operation of internal components within HV enclosures. This is partly resolved by temperature measurement with an recording infra-red camera from a safe distance. The camera is used to scan an area and the recorded infra-red image is then processed by a computer program to display hot-spots and a thermal profile across the equipment.
SANCTION-FOR-TEST SYSTEM
– following work on a high voltage system, it is often necessary to perform various tests. testing should only be carried out after the circuit main earth (CME) has been removed.
– a sanction-for-test declaration should be issued in an identical manner to a permit to work provided and it should not be issued on any apparatus where a permit to work or where another sanction-fortest is in force.
Note That:
A sanction-for-test is not a permit to work.
An example of a sanction-for-test declaration is shown in the code of safe working practices (COSWP) 2010 edition annex 16.2.1.
Additional Procedures Needed for HV systems
Limitation of access form
When carrying out high voltage maintenance, it may be dangerous to allow anyone to work adjacent to high voltage equipment, as workers may not be familiar with the risks involved when working on or nearby high voltage equipment. The limitation of access form states the type of work that is allowed near high voltage equipment and safety precautions. the form is issued and signed by the chief engineer AND electrical officer, and countersigned by the persons carrying out
the work.
Additional Procedures Needed for HV systems
Earthing Down
Earthing down is a very important concept to understand when
working with high voltage systems. It is important to ensure that any stored electrical energy in equipment insulation after isolation is safely discharged to earth. the higher levels of insulation resistance required on high voltage cabling leads to higher values of insulation capacitance (c) and greater stored energy (w). this is demonstrated by the electrical formula:
energy stored 
joules = (capacitance x voltage²)/2
Earthing down ensures that isolated equipment remains safe. Additional Procedures Needed for HV systems
There are two types of earthing down a high voltage switchboard:
1. CIRCUIT EARTHING
– an incoming or outgoing feeder cable is connected by a heavy earth connection from earth to all three
conductors after the circuit breaker has been racked out. This is done at the circuit breaker using a special key. This key is then locked in the key safe. The circuit breaker cannot be racked in until the circuit earth has been removed.
2. BUSBAR EARTHING
– when it is necessary to work on a section of the
busbars, they must be completely isolated from all possible electrical sources. This will include generator incoming cables, section or bus-tie breakers, and transformers on that busbar section. The busbars are connected together and earthed down using portable leads, which give visible confirmation of the earthing arrangement.
High voltage safety checklists for the following can be found in onboard “Company Safety Manual” and sample can be found in the “Code of Safe working Practices for Merchant Seaman (COSWP)” 2010 edition:
• working on high voltage equipment/installations
• switchgear operation
• withdrawn apparatus not being used
• locking off
• insulation testing
• supply failure
• entry to high voltage enclosures
• earthing
• working on high voltage cables
• working on transformers
• safety signs
• correct personal protective equipment
Personnel should not work on High Voltage equipment unless it is dead, isolated and earthed at all high voltage disconnection points. The area should be secured, permits to work or sanction for test notices issued, access should be limited and only competent personnel should witness the testing to prove isolation.
Work Procedures in High Voltage
Working procedures are divided in to three distinct groups.
1. Dead working
2. Live working
3. Working in the vicinity of live parts
Dead Working:
Work activity on electrical installations which are neither live nor charged, carried out after taking all measures to prevent electrical danger.
Precautions before starting work
– Obtain PTW/Sanction- to-Test Permit before commencing work
– Test and prove that the equipment is DEAD before earthing. (with a HV line tester)
– Earth the equipment
Working in the vicinity of live parts:
– All work activity in which the worker enters the vicinity of live zone with his body or with tools and equipment without encroaching in to live zone.
– Using the correct personal protective equipment (PPE) and following safe work practices will minimize risk of electrical shock hazards
HIGH VOLTAGE EQUIPMENT
A typical high voltage installation will incorporate only high voltage rated equipment on the following:
1. Generating sets
2. High voltage switchboards with associated switchgear, protection devices and instrumentation
high voltage cables
3. high voltage/low voltage step-down transformers to service low voltage consumers
4. high voltage/high voltage (typically 6.6kV/2.9kV) step-down transformers supplying propulsion converters and motors
5. high voltage motors for propulsion, thrusters, air conditioning and compressors
A high voltage electrical shock is a significant danger to any person carrying out electrical work. Any simultaneous contact with a part of the body and a live conductor will probably result in a fatal electric shock. There is also a risk of severe burn injuries from arcing if conductors are accidentally short-circuited.
A high voltage electric shock will almost certainly lead to severe injury or a fatality.
Factors that could increase the risk of receiving an electric shock:
1. High voltage work may be carried out close to a person that is not familiar with high voltage hazards. therefore, the area must be secured from the surrounding non-electrical work and danger notices posted.
2. Areas of earthed metal that can be easily touched increase the possibility of electric shock from a high voltage conductor.
Dangers Working With High Voltage Equipments
- High voltage insulation testing (flash testing) can be particularly hazardous when several parts of the equipment are energised for a period of time.
4. Equipment using water as part of the high voltage plant can lead to an increased risk of injury.
5. Using test instruments when taking high voltage measurements can increase the risk of injury if the protective earth conductor is not connected. This can result in the enclosure of the instrument becoming live at dangerous voltages.
6. High voltage equipment will store energy after disconnection. for example, on a 6.6kv switchboard, a fatal residual capacitive charge may still be present hours or even days later.
7. if, during maintenance, a high voltage circuit main earth is removed from the system, it must not be worked on as the high voltage cabling can recharge itself to a high voltage (3–5kv).
Dangers Working With High Voltage Equipments
TRANSFORMER TESTING & MAINTENANCE
What is a transformer?
Transformer is a static device which transforms a.c. electrical power from one voltage to another voltage keeping the frequency same by electromagnetic induction.
Main features of transformer:
Outdoor,oil cooled, 3 phase,50hz
Primary is delta connected and secondary is star connected.
Naturaly cooled
Amongst all the types of transformers this is the most required and most used type.
Parts of transformer:
- MAIN TANK
- RADIATORS
- CONSERVATOR
- EXPLOSION VENT
- LIFTING LUGS
- AIR RELEASE PLUG
- OIL LEVEL INDICATOR
- TAP CHANGER
- WHEELS
- HV/LV BUSHINGS
- FILTER VALVES
- OIL FILLING PLUG
- DRAIN PLUG
- CABLE BOX
TESTING OF TRANSFORMER:
- Testing is carried out as per PMS or Company checklist.
- Routine , type tests & special tests
- Routine tests ( to be carried out on each job):
- Measurement of winding resistance
- Measurement of insulation resistance
- Seperate source voltage withstand test
(high voltage tests on HV & LV)
4.Induced over voltage withstand test (dvdf test)
5.Measurement of voltage ratio
6.Measurement of no load loss & current.
7.Measurement of load loss & impedence.(efficiency & regulation)
8.Vector group verification
9.Oil bdv test.
10.Tests on oltc (if attached)
MAINTENANCE OF TRANSFORMER
– Transformer is the heart of any power system. Hence preventive maintenance is always cost effective and time saving. Any failure to the transformer can extremely affect the whole functioning of the organization.
MAINTENANCE PROCEDURE
OIL :
- Oil level checking. Leakages to be attended.
- Oil BDV & acidity checking at regular intervals. If acidity is between 0.5 to 1mg KOH, oil should be kept under observation.
- BDV, Color and smell of oil are indicative.
- Sludge, dust, dirt ,moisture can be removed by filtration.
- Oil when topped up shall be of the same make. It may lead to sludge formation and acidic contents.
- Insulation resistance of the transformer should be checked once in 6 months.
- Megger values along with oil values indicate the condition of transformer.
- Periodic Dissolved Gas Analysis can be carried out.
MAINTENANCE BUSHINGS
- Bushings should be cleaned and inspected for any cracks.
- Dust & dirt deposition, Salt or chemical deposition, cement or acid fumes depositions should be carefully noted and rectified.
MAINTENANCE
- Periodic checking of any loose connections of the terminations of HV & LV side.
- Breather examination. Dehydration of Silica gel if necessary.
- Explosion vent diaphragm examination.
- Conservator to be cleaned from inside after every three years.
- Regular inspection of OIL & WINDING TEMPERATURE METER readings.
- Cleanliness in the Substation yard with all nets, vines, shrubs removed.
Work on distribution transformers
When work is to be carried out on the connections to, or the windings of, a distribution transformer:
a. the Authorised Person (HV) should record
the details of necessary safety precautions and switching operations on a safety programme, and produce an isolation and earthing diagram;
b. the switchgear or fuse gear controlling the high voltage windings should be switched off, and a safety lock and caution sign fitted;
c. the low voltage windings of the transformer switch or isolator should be switched off, and a safety lock and caution sign fitted, or other physical means should be used to prevent the switch being energised during the course of work;
d. where applicable, danger signs should be attached on or adjacent to the live electrical equipment at the limits of the zone in which work is to be carried out;
e. the transformer should be proved dead at the points-of-isolation if practicable;
f. an earth should then be applied to the high voltage winding via the switchgear and a safety lock fitted. If the proprietary earthing gear is available for the low voltage switchgear, it should be fitted and safety locks applied (it is advisable to retest for dead before fitting this earthing gear);
g. before a permit-to-work is issued – the Authorised Person (HV) should, at the point- of-work in the presence of the Competent
Person (HV), identify and mark the transformer to be worked on. The permit-to-work and the key to the key safe should then be issued to the Competent Person (HV);
PROTECTION OF TRANSFORMERS
- The best way of protecting a transformer is to have good preventive maintenance schedule.
- Oil Temperature Indicators.
- Winding Temperature indicators.
- Buchholz Relay.
- Magnetic Oil level Gauge.
- Explosion Vent.
- HT fuse & D.O. fuse.
- LT circuit breaker.
- HT Circuit breaker with Over load, Earth Fault relay tripping.
- Oil Surge Relay for OLTC.
- PRV for OLTC.
- HORN GAPS & Lightening Arrestor.
- Breather.
FAILURES & CAUSES
- Insufficient Oil level.
- Seepage of water in oil.
- Prolonged Over loading.
- Single Phase loading.
- Unbalanced loading.
- Faulty Termination (Improper sized lugs etc)
- Power Theft.
- Prolonged Short Circuit.
- Faulty operation of tap changer switch.
- Lack of installation checks.
- Faulty design
- Poor Workmanship
-Improper formation of core.
– Improper core bolt insulation.
– Burr to the lamination blades
– Improper brazing of joints.
– Burr /sharp edges to the winding conductor.
– Incomplete drying.
– Bad insulation covering.
– Insufficient cooling ducts in the winding.
- Bad Quality of raw material.
- Transit damaged transformers.
- After failure , transformer is removed and replaced with new/repaired one without removing the cause of failure which results in immediate or short time failure.
HIGH VOLTAGE EQUIPMENT MAINTENANCE
- MAINTENANCE OF SWITCHGEAR ENCLOSURES
- Strictly adhere to required procedures for system switching operations. Switching, de-energizing and energizing shall be performed by authorized personnel only.
- Install temporary grounding leads for safety.
- Remove necessary access and coverplates.
- Fill out inspection test form. Record data in reference to equipment.
- Completely isolate switchgear enclosure to be tested and inspected from sources of power.
- Mechanical Inspection:
I. Check mechanical operation of devices.
II. Check physical appearance of doors, devices, equipment and lubricate in accordance with manufacturer’s instructions.
III. Check condition of contacts.
IV. Check disconnects, starters, and circuit breakers in accordance with inspection and test reports and procedures.
V. Check condition of bussing for signs of overheating, moisture or other contamination, for proper torque, and for clearance to ground.
VI. Inspect insulators and insulating surfaces for cleanliness, cracks, chips, tracking.
VII. Report discovered unsafe conditions.
VIII. Remove drawout breakers and check drawout equipment.
IX. Check cable and wiring condition, appearance, and terminations. Perform electrical tests as required.
X. Inspect for proper grounding of equipment.
XI. Perform breaker and switch inspection and tests
- Cleaning:
i. Check for accumulations of dirt especially on insulating surfaces and clean interiors of compartments thoroughly using a vacuum or blower.
ii. Remove filings caused by burnishing of contacts.
iii. Do not file contacts. Minor pitting or discoloration is acceptable.
iv. Report evidence of severe arcing or burning of contacts.
v. Degrease contacts with suitable cleaners
- Electrical Testing:
i. Check electrical operation of pilot devices, switches, meters, relays, auxiliary contacts, annunciator devices, flags, interlocks, cell switches, cubicle lighting. Visually inspect arrestors, C/T’s and P/T’s for signs of damage. Record data on test report form.
ii. Megger test insulators to ground.
iii. Megger test bussing phase to ground, and phase to phase, using a 1000 volt megger.
iv. DC hipot phases to others and to ground using step voltage method as specified for cables with withstand levels held for not less than one minute. Record decay curve, current versus time to completion of test, and indicate withstand level.
.
- Electrical Testing:v. Maximum DC hipot test levels shall be as follows:
a) 25kV class 50kV DC
b) 15kV class 28.5kV DC
c) 5kV class 9kV DC
vi. Test contact resistance across bolted sections of buss bars. Record results and compare test values to previous acceptance and maintenance results and comment on trends observed.
9. At completion of inspection and test, remove temporary grounds, restore equipment to serviceable condition and recommission equipment.
10. Compare test results to previous maintenance test results
- MAINTENANCE OF HIGH VOLTAGE AIR/OIL CIRCUIT BREAKERS:1. Strictly adhere to required procedures for system switching operations. Switching, de-energizing and energizing shall be performed by authorized personnel only.
2. Completely isolate circuit breakers to be worked on from power sources.
3. Install temporary grounds.
4. Remove circuit breaker from cubicle unless bolted type.
5. Record manufacturer, serial number, type and function of breaker, reading of operations counter, date of inspection, and signature of person responsible for inspection on report sheet.
- Mechanical Inspection:
Inspect for:
I. accumulations of dirt, especially on insulating surfaces.
II. condition of primary contact clusters.
III. condition of control wiring plug-in contacts.
IV. condition of moving and fixed main contacts, excessive heating or arcing.
V. condition of arcing contacts.
VI. cracks or indications of tracking on insulators.
VII. tracking or mechanical damage to interphase barriers.
VIII.flaking or chipping of arc chutes.
IX. broken, damaged or missing springs on operating mechanism.
X. damage to or excessive wear on operating linkage, ensure all clevis pins are securely retained in position.
Inspect for:
XI. correct alignment of operating mechanism and contacts.
XII. evidence of corrosion and rusting of metals, and deterioration of painted surfaces.
XIII. Oil breakers only:
a) Refer to manufacturer’s maintenance manual for special tools that may be required to check oil breaker contacts.
b) Check oil holding tanks in accordance with manufacturer’s instructions.
c) Check for proper oil level and condition of level gauge.
- Cleaning:
i. Remove accumulations of dirt from insides of cubicles with vacuum cleaner and/or blower.
Ii. Clean insulating surfaces using brush or wiping with lint free cloth.8. Check fixing bolts of hardware and breaker components for tightness.
9. ‘Dress’ pitting on contact surfaces, using a burnishing tool. ‘Dress’ major arcing on contacts to smooth condition. Remove filings before switchgear is re-energized. Report unsafe conditions resulting from severe arcing or burning of contacts.
- On completion of foregoing tasks, lightly lubricate bearing points in operating linkage with manufacturer’s specified lubricant. Operate breaker several times to ensure smoothness of mechanical operation.11. Check potential and current transformer cable connections for tightness.
12. Replace inspection lamp where fitted.
13. On first inspection, record data to auxiliary equipment, i.e. primary fuses, potential transformer, potential fuses, and current transformers. Record serial numbers, catalogue numbers, sizes, ratios.
14. On completion of inspection and test, remove temporary grounds. Restore equipment to serviceable condition.
- Electrical Maintenance Tests:
a) General:
i. Test contact resistance across closed line-load contacts, and line and load circuit breaker plug-in clusters. Record results. Clean contacts using appropriate tools to get lowest contact resistance reading possible.
ii. Test insulation resistance for all phases to others and to ground.
iii. Test electrical function in accordance with breaker manufacturer’s instructions and drawings.b) Air Breakers:
i. Prior to hipot test being carried out, ensure surrounding primary connections to main equipment are properly grounded and isolated.
ii. DC hipot test at test levels indicated for switchgear enclosure.
c) Oil Breakers:
i. Do not perform DC hipot tests on oil circuit breakers.
ii. Dielectric (hipot) test on insulating oil per ASTM D877. Compare dielectric strength test results to previous test data where applicable, and comment on changes.
FUSED OR UNFUSED LB AND NLB DISCONNECT SWITCHGEAR
.1 Strictly adhere to required procedures for system switching operations. Switching, de-energizing and energizing shall be performed by authorized personnel only.
.2 Completely isolate switchgear to be worked on from power sources.
.3 Remove access covers and plates.
.4 Test and discharge equipment to be worked on.
.5 Install temporary safety grounds.
.6 Report manufacturer, serial number, type, function of switchgear assembly, date of inspection, and signature of person responsible for inspection.
- Mechanical Inspection: inspect for:
I. accumulations of dirt, especially on insulating surfaces.
II. condition of moving and fixed contact, excessive heating or arcing.
III. cracks, or tracking on insulators.
IV. tracking or mechanical damage to interphase barriers.
V. chipping or flaking of arc chutes or arc shields.
VI. fixing bolts being fully tightened where bolted-on shields are fitted.
VII. overheating or arcing on fuses and fuse holders.
VIII. correct fuse clip tension.
IX. broken, missing or damaged springs on operating mechanism.
X. damage to or excessive wear on operating linkage. Check that all clevis pins are securely retained in position.
XI. correct alignment of contact blades and operating linkage.
XII. corrosion & rusting of metals, deterioration of painted surfaces.
XIII. proper operation of key interlock or other mechanical interlock (if applicable).
XIV. evidence of corona deterioration.
- Cleaning:I. Remove accumulations of dirt from insides of switchgear cubicles using vacuum cleaner and/or blower.
II. Clean insulating surfaces using brush or wiping with lint free cloth.
III. Do not file contacts. Minor pitting or discoloration is acceptable.
IV. Report evidence of severe arcing or burning of contacts.
V. Degrease contacts with suitable cleaners.
9. Check that connections, including current limiting fuses, are secure. Torque to manufacturer’s requirement.
- Electrical Maintenance Tests:
I. Test insulation resistance for all phases to others and to ground.
II. Test contact resistance across switch blade contact surfaces.
III. Test electrical charging mechanism of switch if applicable.
IV. Test electrical interlocks for proper function.
V. DC hipot test phases to the others and to ground using step method to levels specified for switchgear.
VI. Operate blown fuse trip devices if applicable.11. After testing is completed, remove temporary grounds and restore equipment to serviceable condition.
- MAINTENANCE OF PROTECTION RELAYS1. Strictly adhere to required procedures for system switching operations. Switching, de-energizing and energizing shall be performed by authorized personnel only.
2. Completely isolate protective relays to be tested and inspected from sources of power.
3. Set and test protective relays to “as found” settings or to new settings provided by Minister prior to maintenance commissioning.
4. Use manufacturer’s instructions for information concerning connections, adjustments, repairs, timing, and data for specific relay.
- Mechanical Inspection of Induction Disc Relays:
I. Carefully remove cover from relay case. Inspect cover gasket. Check glass for tightness and cracks.
II. Short-circuit current transformer secondary by careful removal of relay test plug or operation of appropriate current blocks.
III. Ensure disc has proper clearance and freedom of movement between magnet poles.
IV. Check connections and taps for tightness.
V. Manually operate disc to check for freedom of movement. Allow spring to return disc to check proper operation.
VI. Check mechanical operation of targets.
VII. Check relay coils for signs of overheating and brittle insulation
- Cleaning:
I. Clean glass inside and out.
II. Clean relay compartment as required. Clean relay plug in contacts, if applicable, using proper tools.
III. Remove dust and foreign materials from interior of relay using small brush or low pressure (7 lbs.) blower of nitrogen.
IV. Remove rust or metal particles from disc or magnet poles with magnet cleaner or brush.
V. Inspect for signs of carbon, moisture and corrosion.
VI. Clean pitted or burned relay contacts with burnishing tool or non-residue contact cleaner.
- Electrical Testing: Tests for typical overcurrent relays include:
I. Zero check.
II. Induction disc pickup.
III. Time-current characteristics.
IV. Target and seal-in operation.
V. Instantaneous pickup.
VI. Check C/T & P/T ratios and compare to coordination data.
VII. Proof test each relay in its control circuit by simulated trip tests to ensure total and proper operation of breaker and relay trip circuit by injection of the relay circuit to test the trip operation.
- Solid State Relays:
I. Inspect and test in accordance with manufacturer’s most recent installation and maintenance brochure.
II. Perform tests using manufacturer’s relay test unit as applicable, with corresponding test instructions.
III. If the manufacturer’s tester is not available, use a relay tester unit approved by relay manufacturer, with proper test data and test accessories.
IV. Proof test each relay in its control circuit by simulated trip tests to ensure total and proper operation of breaker and relay trip circuit by injection of relay circuit to test trip operation.
V. Check C/T and P/T ratios and compare to coordination date.9. At completion of inspection and test, restore equipment to serviceable condition and recommission equipment. Compare test results to previous maintenance test results.
- MAINTENANCE OF OVERHEAD RADIAL POWER LINES:1.
Strictly adhere to required procedures for system switching operations. Switching, de-energizing and energizing shall be performed by authorized personnel only.
2. Completely isolate overhead radial power lines to be tested and inspected from sources of power.
3. Install temporary grounding leads for safety.
4. Inspect insulators and insulating surfaces for cleanliness, cracks, chips, tracking, and clean insulators thoroughly.
- Check cable connections to insulators and check cable sag between poles. Report discovered unsafe conditions.
6. Visually check wooden poles and sound test with 18 oz. wooden mallet.
7. Visually inspect metal line structures for rust, deterioration, metal fatigue, and report discovered unsafe conditions.
8. Inspect crossarms, bolts, rack assemblies, guys, guy wires, and dead ends. Report discovered unsafe conditions.
9. Visually inspect grounding connections.
10. On completion of inspection, remove temporary grounding, restore equipment to serviceable condition
- SURGE ARRESTORS:
1. Strictly adhere to required procedures for system switching operations. Switching, de-energizing and energizing shall be performed by authorized personnel only.
2. Completely isolate surge arrestors to be tested and inspected from sources of power.
3. Install temporary grounding leads for safety.
4. Inspect surge arrestors for cleanliness, cracks, chips, tracking and clean thoroughly.
5. Perform insulation power factor test. Record results.
6. Perform grounding continuity test to ground grid system, record results.
7. On completion of inspection and testing, remove temporary grounds, restore equipment to serviceable condition.
DISCONNECTION PROCEDURE:
Safety of Disconnection Switch:
1. When a disconnect switch is installed in this manner, the frame of the disconnect switch, the upper and lower steel operating rod and the switch handle are all bonded together and connected to the common neutral and the pole’s ground rod, effectively eliminating any insulating value of the insulated insert. The electrical worker operating the switch has no protection and could have as much as full system voltage from the worker’s hands on the switch handle to the worker’s feet.
2. The use of rated rubber gloves can eliminate touch potential if the switch were to fail and go to ground. But there is also the hazard of step potential for the worker operating the switch, and rated rubber gloves does nothing to eliminate step potential. Also, the maximum ASTM rating for rubber gloves is limited to 36 kV, eliminating worker protection from higher voltages.
3. Properly installed ground mats provide the best protection for workers operating disconnect switches while standing on the ground.
If the disconnect switch were to fail and go to ground, the switch handle could be energized at potentially full system voltage, say 7,200 volts, energizing the switch handle at 7,200 volts, less the voltage drop in the grounding conductor from the switch handle to the ground mat (typically 20 to 25 volts).
– But if the worker were wearing rated rubber gloves and standing on a ground mat attached to the switch handle, would they be safe? Yes!
– If they were not wearing rated rubber gloves but still standing on a ground mat attached to the switch handle, would they be safe? Yes!
– When the worker wears rated rubber gloves while standing on a ground mat attached to the switch handle, the gloves are insulating the worker from the 20 to 25 volts developed across the ground mat and switch handle; well below any hazardous voltage. They are safe with or without rated rubber gloves if they are standing on a ground mat properly connected to the switch handle.
PPE to WORK in HV
HV Disconnection Procedure:
Almost every major line or equipment in a substation has associated with it a means of completely isolating it from other energized elements as a prudent means of insuring safety by preventing accidental energization. These simple switches, called disconnects, or disconnecting switches. They are usually installed on both sides of the equipment or line upon which work is to be done.
How to operate these switches:
1. They should not be operated while the circuit in which they are connected is energized, but only after the circuit is deenergized.
2. They may be opened by means of an insulated stick that helps the operator keep a distance from the switch.
3. Locking devices are sometimes provided to keep the disconnects from being opened accidentally or from being blown open during periods of heavy fault currents passing through them.
ISOLATION OF ANY HIGH VOLTAGE EQUIPMENT:
What is isolation:
Isolation is a means of physically and electrically separating two parts of a measurement device, and can be categorized into electrical and safety isolation. Electrical isolation pertains to eliminating ground paths between two electrical systems. By providing electrical isolation, you can break ground loops, increase the common-mode range of the data acquisition system, and level shift the signal ground reference to a single system ground. Safety isolation references standards have specific requirements for isolating humans from contact with hazardous voltages. It also characterizes the ability of an electrical system to prevent high voltages and transient voltages from transmitting across its boundary to other electrical systems with which you can come in contact.
- Isolation of individual circuits protected by circuit breakersWhere circuit breakers are used the relevant device should be locked-off using an appropriate locking-off clip with a padlock which can only be opened by a unique key or combination. The key or combination should be retained by the person carrying out the work.
Note
Some DBs are manufactured with ‘Slider Switches’ to disconnect the circuit from the live side of the circuit breaker. These devices should not be relied upon as the only means of isolation for circuits as the wrong switch could easily be operated on completion of the work.
- Isolation of individual circuits protected by fuses
Where fuses are used, the simple removal of the fuse is an acceptable means of disconnection. Where removal of the fuse exposes live terminals that can be touched, the incoming supply to the fuse will need to be isolated. To prevent the fuse being replaced by others, the fuse should be retained by the person carrying out the work, and a lockable fuse insert with a padlock should be fitted as above. A caution notice should also be used to deter inadvertent replacement of a spare fuse. In addition, it is recommended that the enclosure is locked to prevent access as stated above under ‘Isolation using a main switch or distribution board (DB) switch-disconnector’.Note
In TT systems, the incoming neutral conductor cannot reliably be regarded as being at earth potential. This means that for TT supplies, a multi-pole switching device which disconnects the phase and neutral conductors must be used as the means of isolation. For similar reasons, in IT systems all poles of the supply must be disconnected. Single pole isolation in these circumstances is not acceptable.
High voltage insulation testing (flash testing) can be particularly hazardous when several parts of the equipment are energized for a period of time.
Isolation Procedure:
1. Isolate from all sources of supply.
2. Prevent unauthorised connection by fixing safety locks and caution signs at points-of- isolation.
2. Fix danger signs on live equipment adjacent to the point-of-work.
PROVING THE SYSTEM IS DEAD:
How to prove:
Before starting work it should be proved that the parts to be worked on and those nearby are dead. It should never be assumed that equipment is dead because a particular isolation device has been placed in the off position.
1. The procedure for proving dead should be by use of a proprietary test lamp or two pole voltage detectors.
2. Non-contact voltage indicators (voltage sticks) and multi-meters should not be used.
3. The test instrument should be proved to be working on a known live source or proprietary proving unit before and after use.
4. All phases of the supply and the neutral should be tested and proved dead.
Proving dead unused or unidentified cables
• Where there is uncertainty regarding isolation when removing unidentified cables or proving dead an ‘unused’ cable, particularly where insufficient conductor is exposed to enable the use of test probes, those conductors should be assumed to be live until positively proven to be dead and any work carried out on them should employ live working practices until the conductors are proved dead.
• Clamp meters can be used as a means of identifying cables by testing for current flow in the conductors.
• Non-contact voltage indicators (voltage sticks) can also be useful in these situations to test for voltage where cables without a metallic sheath are to be identified. However, once insulation is pared using live working practices to reveal the underlying conductors, contact voltage detectors should be used as the means of proving dead.
Prove Dead:
(i) Prove dead with a high voltage potential indicator at all accessible points-of-isolation.
(ii) Where appropriate, prove dead on the low voltage side of a transformer, that is LV feed pillars, LV distribution boards etc.
EARTHING AND DISCHARGING OF HV:
Earthing down is a very important concept to understand when working with high voltage systems.
It is important to ensure that any stored electrical energy in equipment insulation after isolation is safely discharged to earth.
The application of earthing on high voltage conductors is controlled in accordance with the provisions of the Power System Safety Rules.
The following general requirements and principles are applicable for portable earthing.
Safety:
1. Always carry earthing equipment below shoulder level;
2. Ensure that clamps and leads are kept a safe distance from any high voltage conductor;
3. Apply clamps to stirrup (if provided) or a horizontal conductor where possible;
4. Avoid clamp application to bushing caps and to braids; and
5. Position clamp so that tension on the earth lead is minimised.
6. Assemble and inspect earthing equipment on the ground;
7. Extend or otherwise prepare any earthing equipment such as shot gun sticks on the ground.
8. Proving High Voltage Conductors De-Energised
9. Do not allow any part of the earthing system to encroach on safe approach distances. Where practicable, keep the earthing leads away from the body;
10. Earthing equipment is to be removed carefully from high voltage conductors to prevent the equipment encroaching on or coming into contact with adjacent live high voltage conductors.
11. Check that the rating of the earthing equipment is appropriate for the fault level at the location at which it is to be applied.
12. Ensure that the earthing equipment is in a serviceable condition. Any portable earthing equipment found to be defective shall be removed from service for repair or disposal.
There are two types of earthing down a high voltage switchboard:
1. CIRCUIT EARTHING
– an incoming or outgoing feeder cable is connected by a heavy earth connection from earth to all three
conductors after the circuit breaker has been racked out. This is done at the circuit breaker using a special key. This key is then locked in the key safe. The circuit breaker cannot be racked in until the circuit earth has been removed.
2. BUSBAR EARTHING
– when it is necessary to work on a section of the
busbars, they must be completely isolated from all possible electrical sources. This will include generator incoming cables, section or bus-tie breakers, and transformers on that busbar section. The busbars are connected together and earthed down using portable leads, which give visible confirmation of the earthing arrangement.
Earthing Procedure:
(i) Earth conductors at all points-of-isolation and fix safety locks to earths.
(ii) Identify with certainty or spike underground cables at the point/s of work if the conductors are to be cut or exposed.
(iii) Earth overhead lines near the working places.
ISSUING OF A PERMIT-TO-WORK or
SANCTION- FOR-TEST:
1. Before a permit-to-work or a sanction-for-test is issued, the Authorised Person or Electrical Engineer should identify the equipment on which the work or test is to be undertaken.
2. If the work involves, or may involve, obtaining access to items of equipment over which confusion could occur, the Authorised Person (HV)/ Electrical Engineer should identify such items to the Competent Person (HV) and apply temporary marking to them.
3. Before issuing a permit-to-work or sanction-for- test, the Authorised Person (HV)/ Electrical Engineer should show the Competent Person (HV) the isolation and earthing diagram and indicate the safety arrangements at the points-of-isolation and at the point-of-work or test.
- The Authorised Person (HV)/ Electrical Engineer should ensure that the Competent Person (HV) understands all the relevant safety procedures and precautions.
5. If the Competent Person (HV) thereafter accepts the permit or sanction, that person becomes responsible for the defined work or test until the permit or sanction is cancelled.
6. Mark the point-of-work.
7. Issue the permit-to-work, isolation and earthing diagram, and the key to the safety key box to the Competent Person (HV).
8. Authorised Persons (HV)/ Electrical Engineer undertaking tasks requiring permits-to-work or sanctions-for-test should issue the documents to themselves.
9. Adjust mimic diagram and complete the site logbook.
10. All such documents must be countersigned by a site- certified Authorised Person (HV)/ Electrical Engineer before the work or test starts.
Undertake the work:
The Competent Person (HV) is to undertake or directly supervise the work and on completion, or when the work is stopped and made safe, is to return the original of the permit-to-work, the isolation and earthing diagram and the Competent Person’s (HV) key to the safety key box to the Duty Authorised Person (HV)/ Electrical Engineer, and complete part 3 of the permit retained in the pad.
Check the equipment:
If the work has been completed, check to ensure it is safe to energize. If the work has been stopped, check the equipment has been made safe.
Cancel the permit-to-work:
(i) Cancel the permit-to-work by signing the “completion of work” part and by cancel the permit in the presence of the Competent Person (HV).
(ii) File the isolation and earthing diagram in the operational procedure manual and permit-to-work in respective file.
(iii) Return key to key safe.
Issue the sanction-for-test :
(i) The Competent Person (HV) is to be shown the safety arrangements at all the point/s of isolation and at the locations of the test, and is to initial the isolation and earthing diagram.
(ii) Issue the sanction-for-test, isolation and earthing diagram, and the key to the safety key box to the Competent Person (HV).
(iii) Retain working lock keys, and remove and replace earths as requested.
Cancel the sanction-for-test:
(i) Cancel the sanction-for-test by signing part 4 and by destroying the sanction in the presence of the Competent Person (HV).
(ii) File the isolation and earthing diagram in the operational procedure manual.
(iii) Return key to key safe.
RE-ENERGIZING EQUIPMENT:
i. Conduct tests and visual inspections to ensure all tools, electrical jumpers, shorts, grounds, and other such devices have been removed.
ii. Warn others to stay clear of circuits and equipment.
iii. Each lock and tag must be removed by the person who applied it.
iv. Visually check that all employees are clear of the circuits and equipment.
PROTECTIONS OPERATING PRINCIPLES
SF6 Circuit Breaker:
A circuit breaker in which the current carrying contacts operate in sulphur hexafluoride or SF6 gas is known as an SF6 circuit breaker.
SF6 has excellent insulating property. SF6 has high electro-negativity. That means it has high affinity of absorbing free electron. Whenever a free electron collides with the SF6 gas molecule, it is absorbed by that gas molecule and forms a negative ion.
The attachment of electron with SF6gas molecules may occur in two different ways,
These negative ions obviously much heavier than a free electron and therefore over all mobility of the charged particle in the SF6 gas is much less as compared other common gases. We know that mobility of charged particle is majorly responsible for conducting current through a gas.
Working of SF6 Circuit Breaker:
The working of SF6 CB of first generation was quite simple it is some extent similar to air blast circuit breaker. Here SF6 gas was compressed and stored in a high pressure reservoir. During operation of SF6 circuit breaker this highly compressed gas is released through the arc in breaker and collected to relatively low pressure reservoir and then it pumped back to the high pressure reservoir for re utilize.
The working of SF6 circuit breaker is little bit different in modern time. Innovation of puffer type design makes operation of SF6 CB much easier. In buffer type design, the arc energy is utilized to develop pressure in the arcing chamber for arc quenching.
During opening of the breaker the cylinder moves downwards against position of the fixed piston hence the volume inside the cylinder is reduced which produces compressed SF6 gas inside the cylinder. The cylinder has numbers of side vents which were blocked by upper fixed contact body during closed position. As the cylinder move further downwards, these vent openings cross the upper fixed contact, and become unblocked and then compressed SF6 gas inside the cylinder will come out through this vents in high speed towards the arc and passes through the axial hole of the both fixed contacts. The arc is quenched during this flow of SF6 gas.
During closing of the circuit breaker, the sliding cylinder moves upwards and as the position of piston remains at fixed height, the volume of the cylinder increases which introduces low pressure inside the cylinder compared to the surrounding. Due to this pressure difference SF6 gas from surrounding will try to enter in the cylinder. The higher pressure gas will come through the axial hole of both fixed contact and enters into cylinder via vent and during this flow; the gas will quench the arc.
PROTECTIONS OPERATING PRINCIPLES
Principles of Distance Relays
Since the impedance of a transmission line is proportional to its length, for distance measurement it is appropriate to use a relay capable of measuring the impedance of a line up to a predetermined point (the reach point).
Such a relay is described as a distance relay and is designed to operate only for faults occurring between the relay location and the selected reach point, thus giving discrimination for faults that may occur in different line sections.
The basic principle of distance protection involves the division of the voltage at the relaying point by the measured current. The apparent impedance so calculated is compared with the reach point impedance. If the measured impedance is less than the reach point impedance, it is assumed that a fault exists on the line between the relay and the reach point.
The reach point of a relay is the point along the line impedance locus that is intersected by the boundary characteristic of the relay.
Since this is dependent on the ratio of voltage and current and the phase angle between them, it may be plotted on an R/X diagram. The loci of power system impedances as seen by the relay during faults, power swings and load variations may be plotted on the same diagram and in this manner the performance of the relay in the presence of system faults and disturbances may be studied.
Solid State Switching Principle
– High voltage testing does not usually require high power.
Thus special methods may be used which are not applicable.
– Then generating high voltage in high power applications.
– In the field of electrical eng. & applied physics, high voltages are required for several applications as:
1. a power supply (eg. hv dc) for the equipments such as electron microscope and x-ray machine.
2. required for testing power apparatus – insulation testing.
-High impulse voltages are required for testing purposes to simulate over voltages due to lightning and switching.
Solid State Switching Principle
Solid State Switching Principle
- Both full-wave as well as half-wave circuits can produce a maximum direct voltage corresponding to the peak value of the alternating voltage.
- When higher voltages are required voltage multiplier circuits are used. The common circuits are the voltage double circuit
- Used for higher voltages.
- Generate very high dc voltage from single supply transformer by extending the simple voltage doubler circuit.
Electric Propulsion and High Voltage Practice
Marine Electric Propulsion
Integrated electric propulsion (IEP) or full electric propulsion (FEP) or integrated full electric propulsion (IFEP) is an arrangement of marine propulsion systems such that gas turbines or diesel generators or both generate three phase electricity which is then used to power electric motors turning either propellers.
It is a modification of the combined diesel-electric and gas propulsion system for ships which eliminates the need for clutches and reduces or eliminates the need for gearboxes by using electrical transmission rather than mechanical transmission of energy.
Electric propulsion for many new ships is now re-established as the popular choice where the motor thrust is governed by electronic switching under computer control.
The high power required for electric propulsion usually demands a high voltage (HV) power plant with its associated safety and testing procedures.
Passenger ships have always been the largest commercial vessels with electric propulsion and, by their nature, the most glamorous. This should not, however, obscure the fact that a very wide variety of vessels have been, and are, built with electric propulsion.
Early large passenger vessels employed the turboelectric system which involves the use of variable speed, and therefore variable frequency, turbo-generator sets for the supply of electric power to the propulsion motors directly coupled to the propeller shafts. Hence, the generator/motor system was acting as a speed reducing transmission system. Electric power for auxiliary ship services required the use of separate constant frequency generator sets.
A system that has generating sets which can be used to provide power to both the propulsion system and ship services has obvious advantages, but this would have to be a fixed voltage and frequency system to satisfy the requirements of the ship service loads. The provision of high power variable speed drives from a fixed voltage and frequency supply has always presented problems. Also, when the required propulsion power was beyond the capacity of a single d.c. motor there was the complication of multiple motors per shaft.
Developments in high power static converter equipment have presented a very convenient means of providing variable speed a.c. and d.c. drives at the largest ratings likely to be required in a / marine propulsion system.
The electric propulsion of ships requires electric motors to drive the propellers and generator sets to supply the electric power. It may seem rather illogical to use electric generators, switchgear and motors between the prime-movers (e.g. diesel engines) and propeller when a gearbox or length of shaft could be all that is required.
There are obviously sound reasons why, for some installations, it is possible to justify the complication of electric propulsion:
- Flexibility of layout
- Load diversity between ship service load and propulsion
- Economical part-load running
- Ease of control
- Low noise and vibration characteristics
FLEXIBILITY OF LAYOUT
The advantage of an electric transmission is that the prime-movers, and their generators, are not constrained to have any particular relationship with the load as a cable run is a very versatile transmission medium. In a ship propulsion system it is possible to mount the diesel engines, gas turbines etc., in locations best suited for them and their associated services, so they can be remote from the propeller shaft. Diesel generator sets in containers located on the vessel main deck have been used to provide propulsion power and some other vessels have had a 10 MW generator for ship propulsion duty mounted in a block at the stern of the vessel above the ro-ro deck.
Another example of the flexibility provided by an electric propulsion system is in a semi-submersible, with the generators on the main deck and the propulsion motors in the pontoons at the bottom of the support legs.
LOAD DIVERSITY
Certain types of vessels have a requirement for substantial amounts of electric power for ship services when the demands of the propulsion system are low. Tankers are one instance of this situation and any vessel with a substantial cargo discharging load also qualifies. Passenger vessels have a substantial electrical load which, although relatively constant, does involve a significant size of generator plant. There are advantages in having a single central power generation facility which can service the propulsion and all other ship loads as required.
ECONOMICAL PART-LOAD RUNNING
Again this is a concept that is best achieved when there is a central power generation system feeding propulsion and ship services, with passenger vessels being a good example.
It is likely that a typical installation would have between 4-8 diesel generator sets and with parallel operation of all the sets it becomes very easy to match the available generating capacity to the load demand. In a four engine installation for example, increasing the number of sets in operation from two that are fully loaded to three partially loaded will result in the three sets operating at a 67% load factor which is not ideal but also not a serious operating condition, It is not necessary to operate generating sets at part-load to provide the spare capacity to be able to cater for the sudden loss of a set, because propulsion load reduction may be available instantaneously, and in most vessels a short time reduction in propulsion power does not constitute a hazard.
The propulsion regulator will continuously monitor the present generator capability and any generator overload will immediately result in controlled power limitation to the propulsion motors. During manoeuvring, propulsion power requirements are below system capacity and failure of one generator is not likely to present a hazardous situation.
EASE OF CONTROL
The widespread use of controllable pitch propellers (cpp) has meant that the control facilities that were so readily available with electric drives are no longer able to command the same premium. Electric drives are capable of the most exacting demands with regard to dynamic performance which, in general, exceed by a very wide margin anything that is required of a ship propulsion system.
LOW NOISE
An electric motor is able to provide a drive with very low vibration characteristics and this is of importance in warships, oceanographic survey vessels and cruise ships where,/-for different reasons, a low noise signature is required. With warships and survey vessels it is noise into the water which is the critical factor whilst with cruise ships it is structure borne noise and vibration to the passenger spaces that has to be minimised.
For very high power, the most favoured option is to use a pair of high efficiency, high voltage a.c. synchronous motors with fixed pitch propellers (FPP) driven at variable speed by frequency control from electronic converters. A few installations have the combination of controllable pitch propellers (CPP) and a variable speed motor. Low/medium power propulsion (1-5 MW) may be delivered by a.c. induction motors with variable frequency converters or by d.c. motors with variable voltage converters.
The prime-movers are conventionally constant speed diesel engines driving a.c. generators to give a fixed output frequency. Gas turbine driven prime- movers for the generators are likely to challenge the diesel option in the future.
Conventionally, the propeller drive shaft is directly driven from the propulsion electric motor (PEM) from inside the ship. From experience obtained from smaller external drives, notably from ice-breakers, some very large propulsion motors are being fitted within rotating pods mounted outside of the ship’s hull. These are generally referred to as azipods , as the whole pod unit can be rotated through 360° to apply the thrust in any horizontal direction, i.e. in azimuth. This means that a conventional steering plate and stern side-thrusters are not required.
Ship manoeuvrability is significantly enhanced by using azipods and the external propulsion unit releases some internal space for more cargo/passengers while further reducing hull vibration.
Gradual progress in the science and application of superconductivity suggests that future generators and motors could be super-cooled to extremely low temperatures to cause electrical resistance to become zero.
Marine Electric Propulsion
– Podded drives offer greater propulsion efficiency and increased space within the hull by moving the propulsion motor outside the ships hull and placing it in a pod suspended underneath the hull.
– Podded drives are also capable of azimuth improving ship maneuverability. Indeed, podded drives have been widely adopted by the cruise ship community for these reasons.
– The motors being manufactured now are as large as 19.5 MW, and could provide the total propulsion power.
– In an AC drive, a frequency converter is used to control the speed and torque of electric motor. The speed of the AC electric motor can be controlled by varying the voltage and frequency of its supply. A frequency converter works by changing the constant frequency main electrical supply into a variable frequency output.
– The ideal simplicity of the induction motor, its perfect reversibility and other unique qualities render it eminently suitable for ship Propulsion.
Electric propulsion
– Diesel-Generator sets to produce electricity to common grid for propulsion and ship use.
– Variable speed drives to rotate fixed pitch propellers.
– Commonly used in Cruise vessels, LNG tankers, Off-shore vessels and Ice breaking vessels due to reduced fuel oil consumption, lower emissions and increased pay-load
- Large Diesel Engine for Main Engine
- Configuration of Electric Propulsion
- System for Ships
- Configuration of Electric Propulsion
- System for Ships
- Configuration of Electric Propulsion
- System for Ships
- Configuration of Electric Propulsion
- Configuration of Electric Propulsion
- System for Ships
- Configuration of Electric Propulsion
- System for Ships
- Configuration of Electric Propulsion
- System for Ships
- Comparison with Conventional and
- Electric Propulsion system
SHORE SUPPLY FACILITIES
Shore power supply facilities have adopted high voltage rather than low voltage by necessity in order to keep the physical size of related electrical equipment such as shore connection cables manageable.
Inevitably high voltage would otherwise introduce new risks to ship’s crew and the shipboard installations if necessary safety features were not built into the HVSC system or safe operating procedures were not put in place.
Those onboard systems that are designed to accept high voltage shore power, typically involving the following things:
– incoming power receptacles,
– shore connection switchgear,
– step-down transformer or isolation transformer,
– fixed power cables,
– incoming switchgear at the main switchboard and
– associated instrumentation. HVSC is often referred to as cold ironing.
The system nominal voltage is considered to be in the range from 1 kv ac to 15 kv ac.
Infrastructure Considerations
Electrical System Grounding Philosophy:
The manner in which electrical system is grounded (e.g., ungrounded system, solid neutral grounding system, low impedance neutral grounding system, or high impedance neutral grounding system), including ground potential transformer method. Circuit.
protection strategy is built around the selected method of system grounding in terms of over voltage prevention, over current prevention or continued operability under single phase grounded condition.
SYSTEM GROUNDING COMPATIBILITY
Arrangements are to be provided so that when the shore connection is established, the resulting system grounding onboard is to be compatible with the vessel’s original electrical system grounding philosophy (for instance, the shipboard ungrounded power distribution system is to remain ungrounded, or the shipboard high impedance grounding system is to remain high impedance grounded within the design grounding impedance values). Ground fault detection and protection is to remain available after the shore connection has been established.
Cable Management System:
The cable management system is the ship’s interface point with the shore power system. The cable management system is typically composed of flexible hv cables with the plug that extends to the shore power receptacle, cable reel, automatic tension control system with associated control gears, and instrumentation. shore power is fed to the shore connection switchboard via the cable management system.
SHORE CONNECTION SWITCHBOARD
where no cable management system is provided onboard, the shore connection switchboard is normally the ship’s interface point with the shore power system. hv shore power is connected to this shore connection switchboard by means of an hv plug and socket arrangement. the shore connection switchboard is provided with a shore power connecting circuit breaker with circuit protection devices.
ONBOARD RECEIVING SWITCHBOARD
The receiving switchboard is normally a part of the ship’s main switchboard to which the shore power is fed from the shore connection switchboard.
EQUIPOTENTIAL BONDING
Equipotential bonding between the ship and the shore is to be provided. An interlock is provided such that the HV shore connection cannot be established until the equipotential bonding has been established. The bonding cable may be integrated into the HV shore power cable. If the equipotential bonding cable is intended to carry the shipboard ground fault current, the cable size is to be sufficient to carry the design maximum ground fault current.
LOAD TRANSFER
Temporary Parallel Running:
Where the shipboard generator is intended to run in parallel with the shore power for a short period of time for the purpose of connecting to the shore power or back to ship power without going through a blackout period, the following requirements are to be complied with:
i) Means are to be provided to verify that the incoming voltage is within the range for which the shipboard generator can be adjusted with its automatic voltage regulator (AVR)
ii) Means are to be provided for automatic synchronization
iii) Load transfer is to be automatic
iv) The duration of the temporary parallel running is to be as short a period as practicable allowing for the safe transfer of the load. In determining the rate of the gradual load transfer, due regard is to be paid to the governor characteristics of shipboard generator in order not to cause excessive voltage drop and frequency dip.
Load Transfer via Blackout
Where load transfer is executed via blackout (i.e., without temporary generator parallel running), safety interlock arrangements are to be provided so that the circuit breaker for the shore power at the shore connection switchboard cannot be closed while the HV switchboard is live with running shipboard generator(s).
Safety Interlocks
An interlock, which prevents plugging and unplugging of the HV plug and socket outlet arrangements while they are energized, is to be provided.
Marine Shore Connection Concept
Shore Connection System Solutions
(Example)
On board Installation
Handling of HV Plug
While the HV shore connection circuit breakers are in the open position, the conductors of the HV supply cables are to be automatically kept earthed by means of an earthing switch. A set of pilot contactors embedded in the HV plug and socket-outlet may be used for this purpose. The earthing switch control is to be designed based on a fail-to-safe concept such that the failure of the control system will not result in the closure of the earthing switch onto the live HV lines.
HV Shore Connection Circuit Breakers
Arrangements are to be provided to prevent the closing of the shore connection circuit breaker when any of the following conditions exist:
i) Equipotential bonding is not established
ii) The pilot contact circuit is not established
iii) Emergency shutdown facilities are activated
iv) An error within the HV connection system that could pose an unacceptable risk to the safe supply of shoreside power to the vessel. These errors may occur within the alarm system, whether on board the ship or at the shoreside control position, or within any relevant
safety systems including those which monitor system performance.
v) The HV supply is not present
HVSC Circuit Breaker Control:
HV shore connection circuit breakers are to be remotely operated away from the HVSC equipment.
HV shore connection circuit breakers are to be made only when it has been established that personnel are evacuated from the HV shore connection equipment compartments. The operation manual is to describe these established procedures.
HVSC Emergency Shutdown:
In the event of an emergency, the HV system shall be provided with shutdown facilities that immediately open the shore connection circuit breaker. These emergency shutdown systems are to be automatically activated.
Any of the following conditions are to cause emergency shutdown of the shore power supply:
i) Loss of equipotential bonding
ii) High tension level of HV flexible shore connection cable, or low remaining cable length of cable management system
iii) Shore connection safety circuits fail
iv) The emergency stop button is used
v) Any attempts to disengage the HV plug while live (this may be achieved by the pilot
contactors embedded in the plug and socket such that the pilot contactors disengage before the phase contactors can disengage)
Tests of HV Switchboards
Type Test
HV switchboards are to be subjected to an AC withstand voltage test in accordance with Table-2
or other relevant national or international standards. A test is to be carried out at the manufacturer’s test facility in the presence of the Surveyor.
Onboard Test
After installation onboard, the HV switchboard is to be subjected to an insulation resistance test in accordance with Table-2 in the presence of the Surveyor.
EQUIPMENT DESIGN:
Air Clearance
Phase-to-phase air clearances and phase-to-earth air clearances between non-insulated parts are to be not less than the minimum, as specified in Table
Creepage Distance
Creepage distances between live parts and between live parts and earthed metal parts are to be adequate for the nominal voltage of the system, due regard being paid to the comparative tracking index of insulating materials under moist conditions according to the IEC Publication 60112 and to the transient overvoltage developed by switching and fault conditions.
Shore Connection Switchboard:
Construction
The HV shore connection switchboard is to be designed, manufactured and tested in accordance with a recognized standard code of practice as given by IEC.
Circuit Breaker
i) Shore connection HV circuit breaker is to be equipped with low voltage protection (LVP)
ii) The rated short-circuit making capacity of the circuit breaker is not to be less than the prospective peak value of the short-circuit current
iii) The rated short-circuit breaking capacity of the circuit breaker is not to be less than the maximum prospective symmetrical short-circuit current
iv) HV shore connection circuit breaker is to be remotely operated
- HV Circuit Breakers may beAir-Break (scarcely used)
2. Oil-Break (not used in ships)
3. Gas-Break (SF – 6 – Sulphur Hexafluoride)
4. Vacuum-Break (Most Popular)
Reference Articles, Books and websites:
1. Electric Propulsion Systems for Ships by Dr. Hiroyasu Kifune
2. Standard Safety of High Voltage by Chris Spencer
3. www.marineinsight.com
4. www.abb.com
5. Practical Marine Electrical Knowledge by D.T. Hall
6. Low and high voltage supply by Henning E. Larsen
7. www.imtech.com
8. www.skysail.info
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