Considerations in the Application of Modern Protection Schemes for Short Transmission and Distribution Lines with Tapped Transformers

Abstract

In Alberta, Industrial System Designates (ISD) own and operate transmission, sub transmission and distribution power lines that connect their generation facilities to loads within their designated areas. Such lines feed well pads, mines and loads outside the Central Processing Facilities. Differing from traditional utility lines, some of these lines may have as many as four or five taps and transformers directly connected. Communication facilities may not be available for each tap. An added challenge for protection is the supply transformer neutral resistance grounding to limit line ground fault current. This presentation discusses the development, application, checking and testing of relevant protection schemes. With diversity of ISD systems, this presentation is intended as a case study for reference purposes and not as a cookie-cutter tool for ISD line protection. Firstly the presentation introduces primary and backup protection schemes for a short multiple taps 72 kV line. These schemes utilize two multifunction numerical relays and a sensitive ground fault relay. Phase and residual ground differentials are supervised by distance, and other protection functions. Such supervision would have not been traditionally used if the line's protection would have sufficient terminal communication and switching facilities. Multi-tap, limited communication, high source to line impedance ratios and consideration for fault resistance, forced the selection of specific scheme to maximize sensitivity. With the availability of smart relays at affordable costs, sensitive schemes can be utilized, however careful supervision becomes necessary to avoid relay mal-operation. Finally the presentation addresses testing concerns. Smart relay's tests are primarily complicated. The presentation provides insights on requirements to facilitate relay testing and satisfy the ultimate purposes of ensuring the relay secure and reliable operations.