CT connection to sense earth fault/Earth Faults Detection Philosophy

CT CONNECTION FOR EARTH FAULT DETECTION

Earth fault relays like 51N/50N detects earth faults because of the CT connection. There are three major types of connections for 51N/50N.

i.Residually connected CTs.

ii.CBCT(core balance CT)/ZCT(Zero sequence CT) connection.

iii.CT connected in transformer neutral.

Residual CT connection_CBCT connection/electricaltechnologyrishi.blogspot.com

RESIDUAL CONNECTION:

Current flowing through AN= I _a =I_a1+I_a2+I_a0

Where I _a is the current through the secondary of the CT and

I_a1= Positive sequence current

I_a2=Negative sequence current

I_a0=Zero sequence current

Current flowing through BN= I _b =I_b1+I_b2+I_b0

and

Current flowing through CN= I _c =I_c1+I_c2+I_c0

Remember that we can divide current into these three components under any circumstances. In case of a healthy and absolutely balanced system  value of Negative sequence component and Zero sequence component shall be Zero.

Current flowing through NO and hence current flowing through the 50N or 51N relay coil is-

= I _a + I _b + I _c

= (I_a1+I_a2+I_a0) + (I_b1+I_b2+I_b0) + (I_c1+I_c2+I_c0)

= (I_a1+ I_b1+ I_c1) + (I_a2+ I_b2+ I_c2) + (I_a0+ I_b0+ I_c0)

Now vector sum of both positive sequence current and negative sequence currents are zero, but vector sum of zero sequence current is not zero.

i.e.

I_a1+ I_b1+ I_c1 = 0

I_a2+ I_b2+ I_c2 = 0

But,

I_a0+ I_b0+ I_c0 ≠ 0

During normal operation for a balanced system there is only positive sequence component. Even if there is unbalancing there shall be positive sequence component and negative sequence component. But no zero sequence components will be there unless there is an earth fault.

Zero sequence current flows only when an earth fault occurs. 

So for non-earth fault condition earth fault relay shall remain non responsive.

Only during earth fault there is current through the coil of 51N, for which it shall be energised

CBCT CONNECTION:

CBCT also senses I _A + I _B + I _C as similar to the residual connection. Only difference is here, the vector addition happens in the PRIMARY circuit. Then it transforms to the secondary circuit according to the CT ratio and thus the relay coil.

So the explanation given above is also applicable here.

For the same reason stated earlier CBCT can’t sense positive or negative sequence current. It can only senses zero sequence current, which only flows due to earth fault.

But it shall be remembered that earthed armour of a cable should not pass through the CBCT.

If so done earth fault current shall flow through the faulted phase and the Armour, in opposite phase direction resulting net current of 0. This will not allow the relay to energize.

E/F CASE WHEN THE EARTHED ARMOUR PASSES
THROUGH THE CBCT/electricaltechnologyrishi.blogspot.com

So, as shown in the picture, it is clear that if Armour is passed through the CBCT, earth fault current through the faulty phase and Armour shall oppose each other resulting net current through the CBCT as 0. So it is mandatory that cable Armour shall be cut before entering CBCT.

CT CONNECTED TO TRANSFORMER NEUTRAL:

E/F relay is connected to transformer neutral through a protection class CT as shown. The entire earth fault current shall return through the earthed neutral. So this CT can sense E/F current without any difficulty. In this configuration also Sum of I _A + I _B + I _C  flows through the CT primary, and sum of the currents are transformed as per CT ratio as input of the relay.

 

CONNECTION OF STAND BY EARTH FAULT RELAY (51 NS) & RESTRICTED EARTH FAULT RELAY (64)
 AT AT TRANSFORMER  NEUTRAL

So, these are the CT connections used for Earth fault detection and sensing in industries. Connection and operation of 64 (restricted earth fault relay) is not mentioned here, which shall be given under a different Article.
(Please refer the link 
https://electricaltechnologyrishi.blogspot.com/2018/08/restricted-earth-fault-protection.html)

Note: 

1. All the current addition shown here is vector addition.
2. For positive and Negative sequence component one phase shall be 120 degree apart from the other,  however for zero sequence component phase displacement between the phases is 0 degree.

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