DCS protection configuration PFSK151 3BSE018876R1

DCS protection configuration PFSK151 3BSE018876R1

DCS protection configuration PFSK151 3BSE018876R1

A current differential protection including power transformers must be compensated for transformer turns ratio and phase shift/vector group. In RED670 this compensation is made in the software algorithm, which eliminates the need for auxiliary interposing current transformers. Necessary parameters, such as transformer rated voltages and phase shift, must be entered via the Parameter Setting tool or the LHMI. Another concern with differential protection on power transformers is that a differential protection may operate unwanted due to external earth faults in cases where the zero-sequence current can flow only on one side of the power transformer but not on the other side, as in the case of Yd or Dy phase shift/vector groups.

This is the situation when the zero-sequence current cannot be transformed from one side to the other side of the transformer. To make the differential protection insensitive to external earth faults in these situations, the zero-sequence current must be eliminated from the terminal currents so that it does not appear as a differential current. This was previously achieved by means of intermediate current transformers. The elimination of zero-sequence current is done numerically and no auxiliary transformers are necessary. Instead it is necessary to eliminate the zero-sequence current by proper setting of the parameter ZerSeqCurSubtr.

If the power transformer is of the type Dyn, where yn-windings currents are measured, then the zero-sequence component will be subtracted when these currents are transformed to the HV-side. 6.3.2.2 Small power transformers in a tap If there is a line tap with a comparatively small power transformer (say 1-20MVA) , line differential protection can be applied without the need of current measurement from the tap. It works such that line differential protection function will be time delayed for small differential currents below a set limit, making coordination with downstream short circuit protection in the tap possible. For differential currents above that limit, the operation will be instantaneous in the normal way. Under the condition