When determining relay settings, Protection Engineers use Isc data from short-circuit computer programs to obtain the current flowing through the system for various faults. The program can be directed to show the contribution of current by any element in the system and for any fault in the system. Mutually coupling data must be entered into the program for those lines installed close to each other to get the result of any coupling voltages and currents during fault conditions.
This presentation will explain the miss-operation of a mutually coupled 69 kV line to a 230kV line that occurred on December 23rd, 2013 in the San Diego Gas & Electric 230/ 69 kV system.
The actual misoperation was due to a wrong directional decision made by a microprocessor relay for a duration of 4ms before dropping out. By this time a trip and a transfer trip was given to the line, isolating it from the 69 kV systems.
This line was protected with Phase Distance and Ground Directional over-current primary relay and Phase and Ground Directional over-current back up relaying, on one terminal and with Phase and Ground Distance protection for the Primary relaying and Directional Phase and Ground over-current backup protection over Audio Tone communication channel. Settings for this line were done using our traditional 80-85 percent PUTT line reach with the traditional higher setting for the Instantaneous ground elements due to Mutual coupling.
The purpose of this presentation is to share with other Protection Engineers the knowledge and experience gained by the study of a utility relay misoperation and the solution used to fix this problem by introducing Negative Sequence Elements logic to increase security and sensitivity in the relay setting and also explains how to avoid future relay misoperations.