Maloperation of Transformer Differential protection
Maloperation of Transformer Differential protection
(OP)
Differential protection of a 31.5MVA, 33/6.9kV, Dyn11 transformer has maloperated in a case. The relay is numerical type and the recorded currents are balanced (about 230A) on the 33kV side of the transformer and match with the b-phase current (about 1150A-after applying transformation ratio)on the 6.6kV. The a- and c- phase currents are exactly half of the b-phase current (about 575A). The differential current in a-phase has exceeded the set value and caused the trip.
The investigation revealed an open circuit in a-phase ct secondary wiring on 6.6kV of the transformer. Every thing has been checked and found in order.
How to correlate the recorded currents with the trip or is there some other reason for the trip.
Any help.
Thanks in anticipation.
The investigation revealed an open circuit in a-phase ct secondary wiring on 6.6kV of the transformer. Every thing has been checked and found in order.
How to correlate the recorded currents with the trip or is there some other reason for the trip.
Any help.
Thanks in anticipation.






RE: Maloperation of Transformer Differential protection
RE: Maloperation of Transformer Differential protection
Ia=Ic, Ib=2Ia=2Ic, and Ia=Ic=Ib/2
The CTs are assumed to be on individual CT cores, i.e. there is not one core for all three CTs.
RE: Maloperation of Transformer Differential protection
The relay operated correctly because there is unbalance in current between primary and secondary side.
It is not clear where the current was measured. Presumably inside the relay i.e.by the relay. It appears that two phases are acting as return path for the current in the third phase. The relay is receiving input from only one phase and two phase inputs are missing. Recommend to check secondary wiring.
RE: Maloperation of Transformer Differential protection
But a diff operation is consistent with a broken CT connection - it will lead to a difference between HV and LV as far as the relay is concerned.
Bung
Life is non-linear...