Restricted Earth Fault Protection-CT SELECTION-PART 1 :RATIO SELECTION

SELECTION OF PROPER CURRENT TRANSFORMER IS ONE OF THE IMPORTANT PART OF A PERFECT PROTECTIVE SYSTEM. IF THE PROTECTION IS DONE BY DIFFERENTIAL THEN IT HAS THE MOST IMPORTANCE.....

Any protective system is basically a "Detection system" which detects any abnormality. CTs are the eyes of a relay. A relay monitors the system condition through the CT.

As mentioned in the article "Restricted Earth Fault Protection Philosophy"  REF  is basically a differential protection.

Needless to say that differential protection requires most sophisticated and accurate CTs, as very marginal error can cause mal-operation of the relay.

For metering the CT classes are 0.2, 0.5, 1 etc.

For non differential protection the CT classes are 5P10, 5P20, 10P10, 10P20 etc.

But for differential protection the CT class shall be of special type called "PS " class CT also noted as "PX" class CT.

A CT is specified by mainly-

1. Ratio,

2. CT class & its related parameter

For REF protection CT class shall be PS. which has the following characteristics-

1. Knee point Voltage

2. Magnetizing current

3. CT resistance.

So, specification of a CT for restricted earth fault protection (or differential protection) shall be like below

1. CT RATIO
2. CT CLASS- PS or PX
     2.1 KNEE POINT VOLTAGE= Vk (Say)
     2.2 MAGNETIZING CURRENT(Imag)=30 mA MAX AT Vk/2 (IT IS A SPECIFIED VALUE)
     2.3 CT RESISTANCE(Rct)= <=5 Ohm (IT IS ALSO A SPECIFIED VALUE.)

PART 1- CT RATIO SELECTION FOR REF

In this article we shall summarize the methods for CT ratio selection.

As mentioned in the article "Restricted Earth Fault Protection Philosophy" for high impedance REF ratio of LCT & NCT shall be same.

This requirement does not create any problem if the neutral point is solidly earthed.

SOLIDLY EARTHED LT TRAFO/https://electricaltechnologyrishi.blogspot.com

As per the picture beside say, a 11/0.433 kV 2.5 MVA transformer needs REF protection, having a solidly earthed Neutral.

Iflc(secondary)=

2500/(1.732*0.433)=3333.53 A.

With 20% margin=4000A

So line CT ratio shall be 4000/1A or 4000/5A.

Since the neutral is solidly earthed earth fault current will be very high(fault impedance not considered), nearly to phase fault current. Hence selection of 4000/1 or 4000/5A NCT & LCT shall serve the purpose.

But there shall be problem if neutral of the transformer is earthed through any impedance to have a restricted earth fault current.

Suppose a 132/11.5 kV, 30 MVA transformer need REF protection, where 11.5 kV side is earthed through an NGR restricting the earth fault current say to 400 A

RESISTANCE EARTHED HT TRAFO/
https://electricaltechnologyrishi.blogspot.com

For this reference Transformer secondary full load current is=

30,000/(1.732*11.5)=1506.17 A

With 20% margin= 1807.41A

Line CT may be selected as 1800/1 A or 1800/5 A.

But what about the Neutral CT.

Say  we have selected 1800/1 A CT, then it will see (400*1/1800)=0.22 A earth fault current. So, the relay must have pick up ability at minimum 0.22 A, If for such selection we use a relay that has the minimum pick up setting of 0.3 A, the system will not work.

Also it is to be remembered that 400 A earth fault current will flow if the fault is is at 100% of the winding, and there is no other fault resistance. But in practical case there shall be fault resistance (say core of the transformer/ transformer tank etc) and the fault may have happened inside the  winding then it will offer very low earth fault current which may be difficult to detect. Let us see an example.

(N.B-Also as the E/F is restricted to 400 A then the transformer core must be capable of with standing 400A current, this to be specified to the transformer manufacturer)

Say,

for the shown diagram an internal earth fault has occurred at 40 % of the transformer winding. So, if we neglect the fault impedance then actual earth fault current shall be 40% of 400A i.e. 160A only, & CT secondary current or relay current shall be=160*1/1800=0.088 A. Addition of fault impedance will further reduce fault current.

So, CT with higher ratio will reflect low amount of current to the Relay. 

DETERMINATION OF % OF UNPROTECTED WINDING

It a very common issue every protection engineer shall encounter. What shall be the % of winding that will remain unprotected.!!!!!!!!

Let's assume that an REF relay has a minimum sensitivity of 0.01 A, i.e. at least  0.01 A current to relay coil is required for the relay to pick up.

So, if a fault occurrs at 4.5% of the winding then primary or actual earth fault current shall be 4.5%*400=18 A (We have neglected all fault impedance)

So CT secondary current shall be 18*1/1800=0.01 A.
So we can see that as per the CT & relay selection upto 4.5 % of the transformer winding shall remain unprotected, we haven't take fault impedance in account, in actual the percentage shall be higher.
So, what shall be the solution!!!!!!

There are few options.

1. We can select 1800/1 A CT as line & neutral CT, and select another CT with lower ratio say 200/1 A with a stand by earth fault relay for the remaining 4.5% winding protection.

2. Select 1800/1 A CT as line CT & 200/1 or 400/1 A CT as neutral CT with biased low impedance REF scheme(beyond scope of this article)

3. Third option is the most interesting one, select both NCT & LCT as 400/1 A. 

But is is possible???? Lets see.

The line CT shall experience 1500*1/400=3.75 A current during normal operation also. So the CT secondary shall have the capability to withstand 3.75 A current.

Such CT shall be specified as 400/1 A, Ith=1800 A

(Ith is the thermal withstand current rating, i.e. the CT shall be thermally capable of withstanding 1800/1 A ratio)

Since all the LCTs are residually connected it will only respond when there is earth fault, you can refer to the article  "CT connection for sensing earth faults" for details on this connection.

So, these are techniques used for CT ratio selection for Restricted earth fault protection.

In the next part we shall discuss about the other characteristics like selection of knee point voltage & purpose & design of Stabilizing resistors which are also equally important for a successful REF installation.

Please see article Restricted Earth Fault Protection-CT SELECTION-PART 2 :KNEE POINT VOLTAGE AND STABILIZING RESISTOR SELECTION for the next part.

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