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Protection riddle no.8 - Risk of operation in power plant

Each power unit includes one auxiliary medium voltage switchgear to supply internal consumptions. Also common switchgear is considered for power plant common consumers and a long interconnection cable (approximately 700 m) between each MV and common panel for emergency using. This interconnection path is used instead of incoming units when their incomings can not be energized via normal path.
One day, the head of electrical commissioning group called the office engineers to help. He said,” we can not run unit 2 switchgear via normal path, and we have decided to use interconnection of common switchgear. But when we close the related circuit breaker, its relevant sensitive earth fault trips it”.
The setting of mentioned relay is similar to the other same relays in output feeders and it is adjusted at 2 A ( in primary side ) for all sensitive earth fault; because that medium voltage system is resistance grounded with maximum 10 A earth fault currents. Also all those earth fault relays are connected to special core balance current transformers.

He said they have never had any problem with other output feeders. They didn’t have enough time, and wanted permission to increase the setting of earth fault relay in commissioning duty. They knew well this activity may be accompanying a great risk.

Can you take that risk of operation?
Author : Hamid
Mon, November 26, 2007 - 13:56

Consider standard 11 KV over head line feeder protection having one module consisting over current protection for two phases and an earth fault protection. The earth fault peotection is taken from the current flowing through the cable connecting the CT neutral and relay neutral which can be termed as a residual current. Considering slightly different actual ratios for the three numbers CTs, it is standard practice to set a minimum pick up for the earth fault relay.
The CTs used have a metering core, a normal protection core. Some CTs have an additional differential protection core which will have only lesser error. Metering CTs will saturate at double the full load current.

Restricted earth fault(REF) relay of any transformer is preferable to be taken from a differential protection core of a CT. The standard settings used in Kerala State Electricity Board(KSEB) are for EF relay or REF relay taken from normal protection core is 20% of full load setting. If REF is relay taken from standard differential core, if available is 5% only.

Around thirty years ago, on an experimental basis the normal earth fault relay settings were reduced to 10% of full load current for selected feeders and kept unaltered for one year. There were much more tripping compared to previous year for a particular season. Hence it was decided to revert to previous settings.

When making purchase of instrument transformers, INSIST a graph for maximum percentage error versus times of full load current. For metering cores this must be from zero load to full load. For normal protection CTs it must be from two times to twenty times full load because these CTs are supposed to saturate above twenty times full load. In case of differential protection CTs the curve must be available from double full load to eighty times full load.

Any differential relay must withstand eighty times full load for one second and any normal protecion relay must withstand twenty times full load for three seconds. These are not based on any BS but the values used for ENGLISH ELECTRIC (NOW GEC ALSTHOM) RELAYS manufactured in India.

In above posted case find the maximum possible error from graphs supplied by the CT manufactures for a zero impedence EF. As a thumb rule give double of such setting arrived.

Maximum fault current can easily be found in case of secondary of a substation transformer by assuming the higher voltage bus is an INFINETE BUS.Actual fault current will be less than this value, but definetely will not go below 80% 0f this value.

Once I had commissioned a 11 KV panel fed by a 110KV/11KV transformer. The panel had relays from another new Indian manufacturer. During lightning the differention relay of the transformer FAILED with good amount of smoke.Two times the relay was replaced by a relay of same manufacturer and again previous result. Then on my recommendation, the differential relay was replaced by an ENHLISH ELECTRIC differential relay and the problem never occured again.

Author : ernairnp
Tue, November 27, 2007 - 17:16

Sensitve earth fault relays are always followed with time delay function.At the time of closing the breaker instantaneous relays will have a tendency to operate either due to line capacitance charging or due to unbalance magentic inrush current.If the time delay is kept to avoid maltripping that is sufficient.

Author : smuthukrshna
Wed, November 28, 2007 - 15:56

The discussion was not the MALOPERATION due to INRUSH CURRENTS,but MALOPERATION due to errors in the OUTPUT OF CORE BALANCE CTS.

I have not discussed earth fault protection using current measured in neutral of either Genertor or Transformer. In that case the time delay for inrush currents is taken care of by a DEFINITE TIME DELAY relay.

In the case discussed by me an IDMT telay is used with the MINIMUM PICK UP SETTING is decided by the maximum possible error in CT ratios of the three individual CTs.

Author : ernairnp
Wed, November 28, 2007 - 15:57
I have fully understood Mr.Nair's advise of setting /C.T errors for SEF protection.
Time delay helps here also similar to other cases.

Author : smuthukrshna
Wed, November 28, 2007 - 16:00
Dear my friends,

Your discussion is very useful for me.
But according to commissioning report, even without any load current on that mentioned cable, they had measured more than 2A in secondary side of core balance CT.
Regarding to cable length, I think that problem involved to AC cable capacitance current of three phase medium voltage cables.
What is your opinion?
Author : Hamid
Thu, November 29, 2007 - 11:00

Relay must be set above normal current due line capacitance.
Third hormonics current may also flow in residual circuit.
SEF Relay must have tuned to operate only for fundamental frequency components.

Author : S.Muthukrshna
Thu, November 29, 2007 - 11:02

Cable earth leakage(fault) and or inproper c.t installation can not be ruledout

Author : S.Muthukrshna
Thu, November 29, 2007 - 11:03
From latest description of Mr. Hamid, it seems that PILOT WIRE PROTECTION is used. In that case my earlier messages are not relevant to this particular case.

PW protection is a diffeential protection and the setting recommended as per manual of the relay is to be used. This is because relevant currents are passed through an impedence and the voltage drop on this impedence is used to activate the relay.In any case if there is a tripping on no load, increase setting as done alone is the solution.

Author : N P NAIR
Thu, November 29, 2007 - 11:04

It is NOT pilot wire protection.It is same as residual e/f protection .Instead of adding three phase current from individual current transformers the three currents magnetic fluxes are added in a single core called corebalance c.t by inserting all three phase wires into the single core.The secondary turns which are wound on the core will develop voltage only when there is unbalance current in such a fashion when all the three are added is not resulting zero flux in the core.(Nothing but zero sequence current reflection).

Author : S.Muthukrishnan
Mon, December 10, 2007 - 10:21
According to general recommendation in network with impedance earthed neutral, if a relay supplied from a ring CT is used, the pick-up may be set at 1.5 times the zero sequence capacitive current of the feeder (of the order of 3 Amps/km for high voltage cables). 
Author : Hamid
Sun, January 13, 2008 - 19:48
The earth fault was due to charging currents which was drawn at switching by the cable dielectric so a 1cyc. time delay may solve the problem it wont be unsafe if the setting of E/F relay is slightly changed. 
Author : BALAJI
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