Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
1999-03-18
2001-06-19
Trieu, Van T. (Department: 2736)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S651000, C361S042000, C361S044000
Reexamination Certificate
active
06249230
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a ground fault detector system for detecting a ground fault in a multi-phase ungrounded or high impedance grounded electrical power distribution network, and a ground fault detector used therewith. More particularly, the present invention relates to such a ground fault location system where the detector detects an artificial injected signal.
BACKGROUND OF THE INVENTION
In a power distribution network, one or more distributed phases can become faulted to ground. Reasons for such a ground fault are numerous and known, and include damage to electrical insulation and the connection of a defective device to the power network, among others.
In high-impedance grounded networks and ungrounded networks (i.e., any network where there is no significant current path to ground), phase-to-ground faults produce relatively insignificant values of fault current. Typically, in small isolated-neutral industrial installations, the ground-fault current for a phase fault may be well under an amp. Correspondingly, in a large plant containing miles of cable, such current may be no more than 20 amps. Such currents usually are not of sufficient magnitude for the operation of zero-sequence over-current relays, ground fault relays, fault-sensing circuit breakers, fuses, and similar protective devices to locate and remove such faults. Such protective devices do not have the sensitivity necessary due to the complexity of the current flow pattern between the distributed cable capacitance and the fault.
The primary indication of a ground fault in such a network is a shift in ground potential with respect to phase voltage. Although such indication can be sensed and employed as an alerting device, such indication does not assist in determining the location of the ground fault, which may be anywhere within the network.
An accurate ground fault location system is an important tool that is useful in minimizing the economic and maintenance impact of faults on ungrounded and high impedance grounded networks. The benefits of accurate fault location include reduced switching operations required to isolate a faulted feeder section or load; reduced search efforts to locate a fault, thereby facilitating faster repair time; and greater likelihood that a detected fault will be addressed before a second fault occurs in the network. Accordingly, a need exists for an accurate ground fault location system and a detector therefor.
SUMMARY OF THE INVENTION
The present invention satisfies the aforementioned need by providing a ground fault location system and a detector therefor. The ground fault location system is used in a multi-phase ungrounded or high-impedance grounded electrical power distribution network. The system comprises a signal generator and the ground fault detector. The signal generator is coupled to the network at a first particular network location and generates for each phase of the network an individual non-DC ground fault detection voltage signal between such phase and ground.
The ground fault detector is coupled to the network at a second particular network location separate from the first location and comprises a summing device and an annunciator. The summing device is coupled to all of the phases of the network at such second location, and sums any current passing therethrough and produces at an output thereof a sum signal representative of such summed current. The annunciator is coupled to the output of the summing device to receive the sum signal, and provides an indication when the sum signal is non-zero.
Each phase of the network at the second location has a distribution current passing therethrough and the sum of the distribution current from each phase is normally substantially zero. The zero-sum distribution current at the second location results in a substantially zero sum signal and the annunciator does not provide the indication based on such distribution currents.
When the second location is on a path between the first location and a ground fault, the individual voltage signal on at least one of the phases generates a fault current on such phase through such path. The generated fault current results in a non-zero sum signal and the annunciator provides the indication.
When the second location is not on a path between the first location and a ground fault, none of the individual voltage signals on any of the phases generates a fault current on any phase through such path. The lack of a generated fault current results in a substantially zero sum signal and the annunciator does not provide the indication.
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Baldwin Thomas L.
Imece Ali F.
Novosel Damir
ABB Power T&D Company Inc.
Trieu Van T.
Woodcock Washburn Kurtz Mackiewicz & Norris
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