To improve open ring security, one or more sectionalizer can be used between the primary feeder and the normal open point (NOP). Such schemes have not proven popular due to two limitations:
- lack of remote indication of the sectionalizing breaker status which means that in case of fault, customers could be off supply for a long time if nobody alerts the utility.
- long fault clearance times at the primary feeder breaker cause stresses on the MV cable sheaths and extend the duration of the voltage dip.
Motorized sectionalizing CBs equipped with remote terminal units (RTUs) and telecommunications equipment have been introduced to overcome the first limitation. Supply restoration can take place within minutes, gaining a significant improvement in security and availability.
To avoid the penalty of long tripping times, overcurrent with
logic selectivity could be used. In this scheme, the IDMT relays at the primary feeder and at the sectionalizers need to have a start output contact and a block input signal.
Three sectionalizing CB with IDMT( inverse definite minimum time)protection and logic discrimination, effectively creating four zones are shown in Figure.