Protection Question No.35 - Transformer secondary side protection from primary
Can a 3500 KVA, 6.3/11 KV power transformer HV side be protected against all faults by installing protection relays only on its primary 6.3 KV side? Overhead line is directly connected at 11 KV side of transformer.Transformer is delta(6.3 KV) - wye ( 11 KV) type with neutral grounded solidly ... an idea is to install 51N on neutral of grounded Y at transformer secondary and take its signal to primary side protection relay?
How fault currrents on Y side of transformer reflected on delta side? Can overcurrent relays decide about fault on Y side by sensing current from delta side?

#1
Wed, May 13, 2009 - 10:23
As you know, the protection requirements of each side of mentioned transformer is different and designer shall consider  suitable protection system for each side. Regarding your other question about effect transferring of secondary earth fault to primary phase side, you can refer to Protection Riddle No.1.
Indeed, zero sequence current cannot transfer from secondary side to primary phase side in DYn connection, but other sequence element of earth fault currents such as negative and positive sequence can appear in primary side and some protection system such as differential protection can detect them.

#2
Thu, June 11th, 2009 - 11:17
The protection at transformer primary-delta side will protect the E/F at secondary-star side also.  Only thing, the current seen by the relay at primary(D) will exclude the zero seq current.  that is the current in primary-delt widing is 2/3 part and in the phase connection of primary side √3 * 2/3 = 1.15.  So I think  the relay at primary-delta widning  protect secondary for E/F at secondary-star also.

#3
Thu, June 11th, 2009 - 14:03
In Dyn connection with solidly neutral grounding system, the fault current is controlled mainly by the leakage reactance of the winding, which varies in a complex manner with the position of the fault.

The variable fault point voltage is also an important factor, as in the case of impedance earthing. For faults close to the neutral end of the winding, the reactance is very low, and results in the highest fault currents. The variation of current with fault position is shown in below Figure.

As you see for faults close to the neutral end of the winding primary the current protection systems cannot have suitable function against secondary fault which can be very sever. Therefore in some case primary side protection cannot cover all secondary fault situations.