Date Code 20080110 Protection Functions 3-33
SEL-387E Instruction Manual
Figure 3.14: REF Function, 32IOP Setting Guide
If you want to protect a single wye winding in, for example, a delta-wye transformer, set 32IOP at
1, 2, or 12, which is the number of the relay winding input associated with the line-end CTs of the
protected winding. The relay uses residual current from that single winding input as the Operate
quantity.
Figure 3.14 shows neutral CT input connected to the Winding 3 input of the relay, as it
must be for every case where REF protection is to be used.
If you want to protect an autotransformer, set 32IOP at 12 and connect the primary and secondary
side CTs to relay Winding Inputs 1 and 2. You could also use this setting for the single winding
mentioned above, if that winding had two breakers and two sets of CTs at the line end. You
would also have to connect these CTs to Winding Inputs 1 and 2. Such a connection would be
typical in ring-bus or breaker-and-a-half configurations. With 32IOP set at 12, the relay sums the
residual currents from the Winding 1 and Winding 2 inputs to create the Operate quantity.
Calculation of the residual current at each relay winding input is as follows:
IRW
n
= IAW
n
+ IBW
n
+ ICW
n
(
n
= 1, 2)
For the neutral CT connection, the relay uses only one of the three ABC inputs, e.g., IBW3, so the
residual current for input 3 would be IRW3 = 0 + IBW3 + 0 = IBW3.
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