Earth Fault Relay Settings

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Anonymous

Poster Earth Fault Relay Settings

03/31/2010 12:19 AM

Hi All,

I have newly joined a 33/11 KV sub-station as a maintenence engineer(as a

trainee),one thing i noticed there is the overcurrent relay settings(the TMS

and plug settings) for the phase over current unit is different for the different

LVs of different feeders,but the earth fault unit settings is same for all the

feeder LVs i.e the time multiplier is set at 0.03 and plug setting is 25% of the

rated tab.

Can anyone explain me the reason.

Thanks in advance

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#1 "Re: Earth Fault Relay Settings" by wareagle on 03/31/2010 9:12 PM (score 1)

wareagleGuru

Join Date: Jun 2008Location: AlabamaPosts: 1561Good Answers: 59

Re: Earth Fault Relay Settings03/31/2010 9:12 PM

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.

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1 of 5 6/20/2013 1:42 PM

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#3

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Anonymous

Poster Re: Earth Fault Relay Settings

03/31/2010 11:51 PM

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

problem.

SABNAVEESActive Contributor

Join Date: Nov 2008Posts: 23

Re: Earth Fault Relay Settings04/01/2010 3:06 AM

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.

SHAMITHGAMBHIR1Power-User

Join Date: Feb 2010Location: dubai,mangalore,bangalore.....Posts: 149Good Answers: 8


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.

__________________

shamith gambhir, electrical engineer

JansCommentator

Join Date: Feb 2009Location: Tamil NaduPosts: 60


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


2 of 5 6/20/2013 1:42 PM

#6

#8In reply to #6

#9In reply to #8

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

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Better not count the cent if the risk is many many dollars.

Panneer

SelvamCommentator

Join Date: Jul 2009Location: chennai, IndiaPosts: 55Good Answers: 1


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 .

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Anonymous

Poster Re: Earth Fault Relay Settings

07/05/2010 8:06 AM

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

Panneer Re: Earth Fault Relay Settings08/02/2010 6:27 AM


3 of 5 6/20/2013 1:42 PM

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SelvamCommentator

Join Date: Jul 2009Location: chennai, IndiaPosts: 55Good Answers: 1

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

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rajukailasActive Contributor

Join Date: Dec 2009Posts: 14


Dear sir

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

raju

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#1 "Re: Earth Fault Relay Settings" by wareagle on 03/31/2010 9:12 PM (score 1)

Users who posted comments:

Anonymous Poster (3); Jans (1); Panneer Selvam (2); rajukailas (1); SABNAVEES (1);

SHAMITHGAMBHIR1 (1); wareagle (1)

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