Electricity: electrical systems and devices – Safety and protection of systems and devices – Ground fault protection
Reexamination Certificate
2000-05-24
2001-11-20
Huynh, Kim (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Ground fault protection
C361S030000, C388S903000
Reexamination Certificate
active
06320731
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to drive arrangements for motors and generators which are subject to internal wiring faults.
Motors and generators, referred to herein as “electric machines”, are typically arranged with phase windings which are connected internally in a wye or delta configuration. These connections provide a low impedance path for fault current to flow in the event of an inter-phase fault. Similarly, other winding arrangements that do not provide any isolation between phases result in a low impedance path for inter-phase fault currents to flow. For example, when a phase-to-phase short occurs because the insulation between two phase conductors deteriorates or when two or more phases are connected externally during or as a result of maintenance work high fault currents are produced through the low impedance paths between phases.
For a bolted phase-to-phase fault, fault current contributes to the fault from the source since a continuous circuit through the fault is provided by the interconnections of the phases at the source. Fault current is contributed to faulted electric machine stator windings because of the continued rotation of a permanent magnet field within the faulted stator winding. Such fault current flows through the fault because a continuous circuit is provided by the interconnected phases of the windings.
Electric machines with controllable fields such as wound field synchronous machines have the ability to limit the duration of fault current contribution by de-energizing the magnetic field. Other machines, such as the permanent magnet type cannot limit duration of fault current contributed by the machine during a coast down subsequent to power circuit tripping as a result of the fault. A three-phase fault is similar to the above described phase-to-phase fault except that it involves all three phases instead of just two phases. Continued operation with a faulted three phase machine, if possible at all, would require increasing the machine leakage reactance which would require a trade-off with respect to normal performance characteristics.
A phase-to-ground fault occurs when motor insulation fails between a conductor and a ground such as a grounded motor structure. Phase-to-ground fault current magnitudes have previously been mitigated by utilizing a high resistance ground impedance between a multiple phase system neutral and ground. However, with typical delta or wye connected systems, this resistance to ground consists of a single resistor connected to the neutral of a wye system or the derived neutral of a delta system and the value of that resistance must be such that the impedance is less than the impedance resulting from the total system charging capacitance which, in a delta or wye system, consists of all three phases in parallel. This results in relatively larger amounts of available ground fault current for a single phase line-to-ground fault than for a single phase circuit. Moreover, grounding arrangements in such systems have no ability to detect phase faults.
Interphase fault detection arrangements have been disclosed in the prior art. For example, the patent in Hasegawa U.S. Pat. No. 4,363,065 discloses an interphase fault detection arrangement in which each phase has independent detection windings and includes a corresponding ground resistor in which fault detection is dependent upon voltage sensing across each ground resistor. The Patent to Gale et al. U.S. Pat. No. 5,587,864 discloses protection circuitry for short circuits and ground faults utilizing phase current sensors and differential sensors. The Baker et al. U.S. Pat. No. 5,521,787 discloses differential current fault protection utilizing both single and multiphase current sensing and the patents to Traub U.S. Pat. No. 3,584,259 and Lardennois U.S. Pat. No. 3,999,104 are directed generally to phase fault sensing utilizing independent resistors connected to ground.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a fault tolerant circuit arrangement for multiple phase electric machine which overcomes disadvantages of the prior art.
Another object to the invention is to provide a fault tolerant circuit arrangement for multiple phase electric machines having independent phase connections and a monitoring system which overcomes disadvantages of the prior art.
These and other objects of the invention are obtained by providing a fault tolerant circuit arrangement for multiple phase electrical machines having independently powered phase circuits each having a high impedance connection to ground so that the only path to ground resulting from a single insulation failure is through a high impedance connection and a monitor for at least one phase circuit to detect a phase circuit-to-phase circuit or phase circuit-to-ground insulation failure and an indicator for indicating a detected insulation failure. The circuit arrangement may be applied to multi-phase motor drive systems to enhance survivability of the systems by mitigating available fault current during the most common modes of electrical failure. While the circuit arrangement may be used with synchronous, induction or permanent magnet motors, it is especially beneficial when applied to permanent magnet motors in which the magnetic field cannot be de-energized at the onset of a fault.
As used herein the term “inter-phase fault” refers to faults involving multiple phases and the term “intra-phase fault” refers to a fault within a single phase, that is, from one winding turn to another winding turn.
With the circuit arrangement of the invention, fault current is mitigated for multi-phase, phase-to-phase and line-to-ground faults by utilizing independent motor phase windings in conjunction with an electric drive configuration which provides isolation between phases from the source of the fault and a high resistance system ground on each isolated phase. The same fault tolerant circuit arrangement is applicable to generators.
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Eaves Stephen S.
Kasson Matthew L.
BakerBotts LLP
Electric Boat Corporation
Huynh Kim
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