Two Cases Of Feeder Protection
On transformer feeders, the transformer and line or cable are connected in series and form a unit. In this manner, one CB can be saved. On feeders with tee-offs, transformers along the feeder are directly connected.
In this case, a substation is saved. In both cases, the protection is faced with particular difficulties. The Siemens SIPROTEC product range will be used for this purpose.
The differential protection for this application must include the special features required in transformer differential protection: ratio and vector group adaptation, as well as in-rush blocking and stabilizing against over-fluxing.
In addition, the protected object must be allowed to extend over larger distances, which means that the measured value transferral must have the properties of a feeder differential protection. The normal feeder differential protection was however also applied. For this purpose, the CTs at one end was connected in the delta to provide the vector group adaptation. The inrush blocking was provided at both ends by means of supplementary relays.
With numerical technology, a relay is now available which includes the properties of a transformer and feeder differential protection in a single unit (7SD84).
Optionally, a differential relay with integrated distance zones (7SL86) could be chosen. In this case, under-reaching zones would be graded into the transformer from both sides. This would provide fast protection even when the signal transmission fails, and also allow a rough discrimination between primary and secondary winding faults.