Adaptive digital distance protection for series compensated transmission lines
Series capacitors offer considerable technical and economical advantages in long distance a.c. transmission. In particular, their excellent reliability and minimal maintenance requirements make series compensation the most cost effective method of enhancing the power transfer capability of an existing or proposed interconnection. E.H.V. lines employing series capacitors however, pose difficult problems for the line protection relays, not encountered with plain feeders. One important cause of these problems is the resonance between the series capacitor and the line series inductance, which in turn imposes a sub-synchronous oscillation on the system signals. Also, the rapid changes in the circuit parameters, resulting from the operation of the capacitor protection equipment, namely the spark gaps, introduce some difficulties for the line protection schemes, especially an impedance measuring relay. These spark gaps are installed in parallel with the series capacitors to prevent the development of very high voltages across the capacitor which could cause excessive damage to the equipment. The work presented herein describes a new digital distance relay suitable for series compensated line applications. The errors in the impedance measurement for a phase to earth fault when the spark gaps do not flash over, are discussed and new methods are proposed to compensate for these errors. The new concept of a complex residual compensation factor, as opposed to a real one, is also discussed. A new adaptive filtering is incorporated in the relay in order to minimise the detrimental effect of the subsynchronous oscillation on the relay decision logic. Finally, the relay is thoroughly tested for many different system configurations, to fully evaluate the relay response.