Hv Grounding

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Earth Connections • Distribution systems of these high voltage usually have their neutral points earthed to the ship’s hull through a resistor • The resistor in neutral line limits earth faults currents and protects equipment

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Types of Neutral Connections • Insulated Neutral System • Earthed Neutral System • Resistance earth Neutral System

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Insulated Neutral to Earth • This system is totally insulated from the ship’s hull • This system maintains continuity of power supply to the equipment even in the event of single phasing fault. • This ensure power supply to critical equipment • The power supply to the equipment can disrupt only if two single phase faults occur simultaneously in two lines which is then equivalent to short circuiting faults • But such fault occur very rare OLADOKUN

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Insulated Earth Neutral

Gen

Load

Earth

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Insulated Earth 3-Phase

Gen

Earth OLADOKUN

Load

Single Earth Fault no effect D1MC Semester 1 / Energy Conservation Management ( ECM06) / May 2007 /

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Insulated Earth 3-Phase

Gen

Earth OLADOKUN

Load

Double Earth short Circuit D1MC Semester 1 / Energy Conservation Management ( ECM06) / May 2007 /

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Neutral Earthed System • Earthed Neutral is done in 3-phase system for supply voltage of 3.3-6.6 kV and above • If a solidly earthed system is used for such high voltage then magnitude of earthed current will be extremely high which can damage the equipment • The earth current is limited by connecting a resistor in series between earth and neutral point • The earth current due to single phasing is limited not to exceed rated current at maximum load

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Neutral Earth 1-Phase

Gen

Load

Earth

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Neutral Earth 3-Phase

Gen

Earth OLADOKUN

Load

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• • • • • • • • • • OLADOKUN

8.LL. 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 High Voltage Equipment D1MC Semester 1 / Energy Conservation Management ( ECM06) / May 2007 /

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• voltage level which enhances the current • flow. Remember that an electric shock • current as low as 15 mA can be fatal

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• • • • • • • • • • • • • • • • • • • • • • • • • • • • •

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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. The access to HV switchboards and equipment must be strictly controlled by using a permit-to-work scheme and isolation procedures together wlth liae-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-utork situation, e.g. welding, cutting, burning etc. in a potentially hazardous area. All work to be carried out on HV equipment is subject to an Electrical Permit to Work (EPTW). I .+III

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• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • OLADOKUN •

EPTW The format of a permit will vary for different companies and organisations. The broad guidelines for the necessary declarations and procedures are outlined below: Before work is commenced on HV equipment an EPTW must be issued. This permit is usually the last stage of a planned maintenance task which has been discussed, prepared and approved by the authorising officer to be carried out by the responsiblep erson. The carbon-copied permit, signed by the responsible person, usually has at least five sections with the first stating the work to be carried out. The next section is a risk assessmendt eclaring wherb electrical isolation and earthing has been applied and where danger/caution notices have been displayed then the permit is signed as authorised by the Chief Electrotechnical Officer (CETO) or Chief Engineer. In the third section, the person responsible for the work (as named in section one) signs to declare that he/she is satisfied with the safety precautions and that the HV circuit has been isolated and earthed. Section four relates to the suspension or completion of the designated work. Finally, the last section cancels the permit with a signature from the authorising officer. A Permit-to-Work is usually valid only for 24 hours. Some marine and offshore companies will also require an associated Electrical lsolation Certiftcatet o declare and record exactlv where the circuit isolation and earthing has been applied before the EPTW can be authorised. A SanctiontoTest safety certificate may also be

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• HV live-line testing components.

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• • • • • • • • • • • • • • • • • • • • • • • • • •

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it must be tested and proaed dead after disconnection and isolation. This can only be carried out by using an approved live-line tester as shown in Fig. 8.29. The tester itself must be proaen before and after such a test. This is checked by connecting the tester to a known HV source (supplied either as a separate battery operated unit or included as an internal self-test facility). Two people should always be together when working on HV equipment. * Earthing-down Before work can be allowed to commence on HV equipment it must be earthed to the hull for operator safety. As an example, consider the earthing arrangements at an HV switchboard. Here, the earthing-down method is of two types: Y Circuit Earthing: After disconnection from the live supply, an incoming or outgoing feeder cable is connected by a manually operated switch to connect all three conductors to earth. This action then releases a

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Portable earthing connectors.

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• • • • • •

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Remember to alwavs connect the common wire to earth fiist before connecting the other wires to the three phase connections. When removing the earthing straps, always remove the earth connection last.

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• QUESTION ANSWER • Why is earthing down considered essential So that the worker can be assured that • during HV maintenance? the equipment (u^4 -himself) cannot experience any accidentally applied

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voltage because the earth connection bonds the circuit to earth (zero volts).

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• • • • • • • • • • • • • • • • • • • • • • • • • • •

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permissiae-ketyo allow the circuit breaker to be withdrawn to the TEST position. The circuit breaker cannot be re-inserted until the earth has been removed and the key restored to its normal position. V Bus-bar Earthing: When it is necessary to work on a section of the HV switthboard bus-bars, they must be isolated from all possible electrical sources. This will include generator incomers, section or bus-tie breakers and transformers (which could back-feedo)n that bus-bar section. Earthing down is carried out at a bus-section breaker compartment after satisfying the permissive key exchanges. In some installations the application of a bus-bar earth is by a special earthing circuit breaker which is temporarily inserted into the switchboard solely for the bus-bar earthing duty. For extra confidence and operator safety, additional earthing can be connected local to the work task with approved portable earthing straps and an insulated extension tool, e.g. at the terminals of an HV motor as shown in Fig. 8.30.

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