Earth Fault Relay Settings

The earth/ground fault relay setting has nothing to do with the phase setting. You may have a phase setting of 600, 800 or 1200 amps. This setting is dependent on the transformer capacity,load, phase conductor ampacity and other criteria. The earth/ground relay is set at a much lower setting such as 250 amps. The actual earth fault is much lower that a 3 phase fault and the relay must be able to detect this low ampacity.

I agree. I think your feeder over current relay is set to control the over load problem.

The O/C setting depends on the Transformer rating & also the full load of the system. E/F relay setting has nothing to do with the O/C relay setting. its always advisable to set the E/F relay setting at the minimum as possible to avoid system instability because of heavy inrush current due to any earth fault.

oc relay settings depends on the different loads on different feeders .An overcurent relay is essentially a sensor that is connected between the power supply and the load and upon the detection of a overcurrent condition due to overloads or other electrical faults, disconnects the load from the power supply. An oc relay is usually designed to operate over a wide range of values and the user must set the trip current based upon the specifications of the loads in the feeder circuit where as the neutral wire of all feeders are connected to one common earth. so earth fault settings r similar for all feeders. let me explain you in detail abt earth fault.

During balanced conditions, current through the neutral is 0. If there is an unbalanced fault (one phase or two phase to ground fault) the current through neutral is no longer zero . Presence of current in the neutral indicates that there is a fault present.So the ground relays can be set to low value to detect this current.

idmt relays are generally non unit type protection- ie it may operate for faults out side the intended zones. it is an advantage and disadvantage. advantage-it can be set for back up.disadvantage-it has to be co ordinated hence may have to be time dalayed. since the positive sequence impedance is continuous and omnipresent phase fault relays usually have to be set higher to grade with starting currents of motors for example..zero sequence circuits get a break whenever a delta star trafo.is encounterd.hence grading can start a fresh at each discontinuity and e/f relays as such can be set usually set more sensitive.they have to grade only with fewer sections and spill currents. it is also incidentally more advantageous since earth faults give rise to step and touch potentials communication interference ....with numerical relays one can easily build up interlocking hence the time settings can be small

Dear Guest, Different feeders can have different Phase current setting based on the size of cable and maximum permissible load current in that feeder.For example , Four feeders in a station may have over-current settings of 400A for three feeders and 300A for one feeder.The Over current relay will operate whenever the Fault current or load current exceeds this setting .

Suppose in a feeder with setting 400A setting. There is a load current of 200A and ther accurs an earth fault of 100A ,the phase fault relay will not operate as the total current is lessthan the the set value for phase fault relay and therefore an E/F relay is required which is sensittive to clear such faults

This is the purpose for which the E/F relay is included in the 11KV feeder protection scheme .

The E/F relay will have current flow, only when there is an earth fault .That is why it possible to set E/F relay at minimum possible setting.

That is the reason Why when phase fault protection settings are different for various feeder, the earth Fault relay is set at a minimum possible setting commonly in all the feeders

The minimum setting is normally decided considering following factors 1)The burden of relay at set plug setting 2) The CT ratio adopted 3) The 11 KV fault level of Station 4) The Knee point Voltage of the CTS connected 5) The normal service leakage current, Etc

Dont hesitste ask, if the above explanation not cleared your doubts .

sir,

I want to know the time Vs current retting setting.

Say load current750 A, what willbe the earth fault current setting & its time to tripping. Also over current relay setting(current and time).

How time setting is calculated

The over current relay 's time and current settins are designed to follow international standards, For example , the IEC curves are designed as follows

t = TMS x 0.14 / (Ir power 0.02 - 1 )

This is standard inverse Charecteristic is for 10/ 3 Sec curve

Suppose You Have 1000/1 Amp CT and you want this relay to be set at 750 A, then you have to select 75% current setting or 0.75 A settings

The time delay can be set in two different ways. You can have an external timer to trigger tripping as per the time delay intended.

If the relay have designed operating characteristic like the one shown above,then you can set the time of opration by setting suitable TMS. For this you must Know the fault current

Then Ir will be = Fault Current / (CT ratio x current setting )

For example if you want a tripping ting time of 200ms, and the fault current expected is 3500A

The equation to be solved is 0.2 = TMS x 0.14 / ( 3500 / (1000x 0.75) to the power of 0.02 - 1 )

The above is Over current setting.

For earth fault setting you can adopt 20 % as current setting and 0.05 as time lever setting

Dear sir

where are u working. You can go for lower setting.

raju