Restricted Earth Fault Protection

Dear friend,

This is well replied in J & P switchgear book.

No. of CTs in LV switchgear depends upon location of neutral CT. I will explain in details.

The Neutral terminal of transformer secondary is connected to two outgoing connections(one to earthing & second to neutral core or busbar)as you have mentioned. The branching point is the key. If neutral CT is located before branching(i.e between transformer neutral terminal & the branching point),only 3 CTs(one in each phase)is required. But if neutral CT is located after branching point(as per your case), one more CT is needed in neutral bus bar of LV switchgear.

You can refer circuit diagram in J & P switchgear book

D.D.Desai

Many thanks for good explanation.

The total connected load to LV switchgear is balanced three phase (30A current flowing in neutral bus is measured when phase currents are about 1700A).

The Min. setting value for R.E.F relay is 200A (10% of 2000A). So why it is necessary to have another CT on neutral bus?

The total connected load to LV switchgear is balanced three phase (30A current flowing in neutral bus is measured when phase currents are about 1700A).

The Min. setting value for R.E.F relay is 200A (10% of 2000A). So why it is necessary to have another CT on neutral bus?

Insuch case 4CTs are adequate. 3CTs in phases and one CT in neutral earthing strip

to ensure a good level of protection a earthing transformer is required on the delta side and CT to be provided on the neutral obtained thus.rest becomes similar to star side.

How many CT's is required for Restricted Earth Fault Protection (64/87N) of a 11KV/0.4KV, Dyn11 power transformer. I have considered one CT on the neutral point in transformer box and three CT's on three phases in downstream LV switchgear. Is it necessary to have another CT on the neutral bus of downstream LV switchgear. the transformer has two neutral bushings. one for connection to system earhing well and another for connection of neutral cables.

REF can be provided with 4CTs and 5CTs. If 4CT method is adopted 3CTs in live phases and one CT shall be placed in neutral before earth point. In5CT method 3CTs in phases ,one CT in functional neutral and another in neutral earthing strip