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Earth fault protection
Earth fault protection from the output of sumation of three phase current transformers.
By Anonymous on 6 April, 2006 - 3:46 pm

In a 11KV switchgear incomer the earth fault protection has been provided by using the sumamtion of three out puts of the mounted current transformers in three phases.

Some times it is observed that during starting of a largest motor feeder in the switchgear the protection mal faucntions causing outage of the switchgear.( Earth fault relay opeartion may be due to saturation of Current transformers resulting unablance current outputs in CT secondaries).

The current setting is 8% & cannot be increased further as the 11KV system is resistance earthed.

The time setting also cannot be incresed for the incomer from fault level point of view.

Now my question is whether is there any method avilable to make the above protection reliable.

By Phil Corso, PE on 9 April, 2006 - 5:50 pm
1 out of 1 members thought this post was helpful...

Responding to Anonymous' Apr 6, 3:46pm post about false tripping of earth-fault relaying (EFR):

A) CT saturation is unlikely. If it is, then, your entire protective relaying reliability becomes suspect. That is, if starting current of
the largest motor causes CT saturation they are useless for 3-ph and ph-ph fault current levels. The most likely cause is residual-spillover due to starting-current asymmetry.

B) There are a number of fixes, the most common being time-grading. If the motor feeder is equipped with an instantaneous EFR, that is, a
window-type CT enclosing all phase conductors, then, protective relaying coordination requires that the swgr incoming feeder EFR provide backup
for an outgoing feeder! Hence, delaying it 0.1 or 0.2 seconds is tolerable, especially if the swgr bus is insulated!

C) There is another possibility for false operation of the incomer EFR. Sometimes an unbalanced CT burden will exacerbate the problem of residual current flow. How many phase-relays are installed? Two or three?

Phil Corso, PE {Boca Raton, FL, USA}
[] (

By Rajesh sharma on 25 April, 2006 - 12:34 am

We had solved several problem of similar type in LT systems. Best Method to connect a stablising resistor between star point of CT to ground. If you can send details of motor I can calculate the value of resistor & this will solve your problem.

Rajesh Sharma

By Anonymous on 14 May, 2006 - 5:17 pm

can you please explain how it works, why should the differential CT currents flow through the stabilizer resistors and get gnded

By mathew chandy on 27 June, 2006 - 11:36 pm

Motor details are as follows:

1120 KW, 3.3 KV, 3 ph, 2 pole, FLC-220A, Locked rotor curret 1320 A, Start time 4 sec. Motor is connected to 11/3.3KV transformer. Motor is designed to operate in VFD/as well as DOL mode & operates perfectly in the VFD mode. Starting of the motor on DOL trips 11 kV swgr on earth fault. Settings were raised to 0.4In & 400msec and the relay (Micom P123) still trips on E/F. Based on the above reply will you be able to recommend a suitable stabilising resistor? Please let me know if you need any further information.

best regards,
mathew chandy

1 out of 1 members thought this post was helpful...

Responding to Mathew Chandy's Jun 27, 11:34 pm query... how is earth-fault relay connected:

A) In parallel with 3 line-current CTs?

B) In the neutral residual circuit of phase-current CTs that are wye-connected?

C) Window or Core-type CT surrounding phase conductors?

D) Is restricted earth-fault technique being used? if so, what is maximum earth-fault current?

E) Does relay operate if motor is started in VFD mode?

F) Where are current measurements made? At 11kV? (hopefully not)? Or 3.3kV?

I apologize for misinterpreting the data you furnished:
G) What is the nominal current-rating of the CTs that are connected to the EF relay?

Caution! If the EF relay is already connected between the CT star-point and ground (earth), then, a stabilizing resistor would place it in parallel with the EF relay. Thus its proper operation could be further jeopardized!

Phil Corso, PE {Boca Raton, FL, USA}
[] (

By Phil Corso, PE on 2 July, 2006 - 7:02 pm
1 out of 1 members thought this post was helpful...

Two problems appear on this thread: one involving a 10-MW, 111kV motor (anonymous); the other an 1,120-kW, 3.3kV one (mathew chandy)!

In either case it is well worth knowing the nominal current rating (not their ratio) of the CTs connected to the offending EF relay!

Please clarify!

Phil Corso, PE {Boca Raton, FL, USA}
[] (

By Anonymous on 1 July, 2006 - 10:16 am

Thanks for your response. The Motor rating is 10MW, 11KV motor.