ENGINEERING ARTICLES

It is assumed here that the transmission has multiple grounding points at wye connected transformer neutrals, located throughout the system. When this condition is satisfied, any arcing fault between a phase conductor and the ground will be supplied by zero-sequence currents originating in the neutral connection of the high-voltage transformer banks. We often refer to these neutral connections as the "sources" of ground current, since very little current would flow to the ground fault if there were no grounded neutrals to provide a complete circuit for the fault current. When there are multiple ground sources, the current flowing to the ground may be very large. Any current flowing to the ground contains zero-sequence components and, under grounded conditions, a zero-sequence voltage will be measured at any nearby relay installation. Negative-sequence currents and voltages will also be observed, and these are sometimes used by the protective system. However, most ground ...

An important aspect of transmission line protection is related to the fast detection and clearing of ground faults on transmission systems that have grounded neutrals. In the protection of transmission lines, ground faults are given special treatment. Ground faults are detected using different relays than those used for phase faults, although it is possible that phase relays may detect and properly clear ground faults. Ground relays, however, take advantage of unique features of the power system that make it possible to detect grounded conditions very quickly. IMPORTANCE OF GROUND FAULT PROTECTION: Most high-voltage and extra-high voltage transmission lines are grounded neutral transmission systems, either solidly grounded or grounded through a resistance or a reactance. It has been estimated that, on these high-voltage systems, over 90% of all transmission line faults are ground faults. It has been observed by one protection engineer that, on 500 kV transmission lines, one-line...

There are three basic types of relays that are used for ground relaying; overcurrent relays, distance relays, and pilot relays. Ground relays are almost always completely independent of phase relays, even though any fault current, including ground fault current, will flow through one or more of the phase relays. The ground relays, however, can be provided with much greater sensitivity to the zero-sequence currents by using higher tap settings. This means that the ground relays will pick up much faster than phase relays for a fault involving the ground. 1) OVERCURRENT RELAYS: Directional or non-directional overcurrent relays are widely used at most voltage levels because of their low cost and reliable service record. Many relay engineers prefer an overcurrent relay with an inverse or very inverse time-current characteristic. This means that the pickup will be very fast for close-in faults and delayed for faults at the end of the transmission line. This delay makes coordinatio...

There are several ways in which the system and the generator may interact with subsynchronous effects. A few of these interactions are basic in concept and have been given special names which are discussed here. •Induction generator effect • Torsional interaction • Transient torque 1) INDUCTION GENERATOR EFFECT: Induction generator effect (IGE) is caused by self-excitation of the electrical network. The resistance of the generator to subsynchronous current, viewed looking into the generator at the armature terminals, is a negative resistance over much of the subsynchronous frequency range. This is typical of any voltage source in any electric network. The network also presents-a resistance to these same currents that is a positive resistance. However, if the negative resistance of the generator is greater in magnitude than the positive resistance of the network at one of the network natural frequencies, growing subsynchronous currents can be expected. This is the conditi...