This invention relates generally to ground faults in compensated electric power system distribution networks, and more specifically concerns a system for detecting such ground faults which has high sensitivity but which requires information only from the protected power line and which uses generally known power line constants.
Medium voltage power distribution systems (greater than 1 KV) typically are grounded in some manner. System grounding involves a number of operational issues and effects, including the minimizing of voltage and thermal stresses on equipment, compliance with personnel safety standards, avoidance of communication system interference and helping in the rapid detection and subsequent clearing (elimination) of ground faults, i.e. faults which occur between one phase of the three-phase power line and ground.
It has been estimated that 80 percent of all power system faults in a medium voltage distribution network are single phase-to-ground faults. Such ground faults have a number of potentially serious effects, depending upon the particular system used for grounding. These effects can include a significant hazard to human safety; thermal stress on the equipment due to the presence of the fault current from the faulted phase; voltage stress, which could be both transient and sustained; interference with telecommunications, and a disruption of power to customers in the area of the fault served by the faulted feeder.
Different grounding systems are used in different countries. The more common types of grounding include isolated neutral, where the neutral point has no intentional connection to ground, i.e. the system is connected to ground through distributed line-ground capacitance. Countries using the isolated neutral system include Italy, Japan and Russia. Solid grounding systems are characterized by the power system neutral points being connected to earth, with no intentional impedance between neutral and earth. Great Britain uses a unigrounding type of solid grounding, while the USA, Canada, Australia and Latin America use a multi-grounding solid grounding system. France and Spain utilize a low impedance grounding system, in which the system is grounded through a low impedance resistor or reactor. For high impedance grounding systems, grounding is obtained through a high impedance resistor or reactor.
Still other countries use a resonance grounding approach. Resonance grounding systems include a reactor which is generally tuned to the system phase-to-ground capacitance. This reactor is a coil, generally referred to as a Petersen coil. Those power systems using resonance grounding are also sometimes referred to as compensated power systems.
Resonance grounding systems restrict the magnitude of ground fault current and reduce many of the significant problems of ground faults, with the possible exception of voltage stress. Some of the other systems, like the solid grounding (multigrounding) system, without any impedance between the neutral and the earth, have some operational advantages, but large magnitudes of fault current occur with a ground fault. The occurrence of a ground fault in such systems requires immediate interruption to prevent/minimize the risk to human safety and to avoid other problems. Fast interruption of the faulted line, even for temporary faults, is important.
The major disadvantage to the power systems which restrict the magnitude of ground fault current, such as the compensated power systems, is the problem of ground faults being difficult to detect because of low magnitude fault current. Actual phase-to-ground faults can thus go undetected for a considerable time. The power system can continue to operate, however, with little risk to human life. It is still desirable, however, that a ground fault be detected, because a second fault located elsewhere on the system does permit large magnitude fault currents to flow. Ground faults are typically easier to detect in those systems which produce large magnitude fault current when a ground fault occurs, but such faults must be detected quickly; the fault current moreover must be interrupted as soon as possible, by interrupting service to the faulted portion of the line.
Accordingly, it is desirable to have a ground fault detection system which has the required sensitivity to detect high resistance (very low fault current) ground faults for use with resonant grounded power systems, but which is also practical in its implementation. Further, it is desirable for such ground fault detection system to use only information (current and voltage) from the protected line, since such a system can be used outside of a substation.
Accordingly, the present invention is a system and method for detecting ground faults in a power distribution network, including the determination of zero sequence voltage (V0) and zero sequence current (I0) quantities present on a protected power line and the calculation of a conductance value from the V0 and I0 quantities. The calculation is enabled by enabling circuits only under selected power line conditions to insure the accuracy of a ground fault determination. The conductance-related value is then compared against a first threshold value to determine a forward fault and a second threshold value to determine a reverse fault.