Ground fault detection system

Details Ground fault detection system Ground fault detection system

2002-09-06

2004-01-13

Toatley, Jr., Gregory J. (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

Ground fault protection

C361S042000

Reexamination Certificate

active

06678132

ABSTRACT:

RELATED APPLICATIONS
None.
FIELD OF THE INVENTION
The present invention relates to ground fault protection, and more particularly to a DC ground fault sensor system for detecting ground fault conditions.
BACKGROUND OF THE INVENTION
Electrically powered automobiles are vehicles that do not depend on internal combustion engines for propulsive power, but rather on relatively large electric traction batteries. The traction battery of an electric automobile is engaged with an electric traction motor for propelling the automobile, and the traction battery is rechargeable to permit repeated use of the traction battery.
The skilled artisan will appreciate that a traction battery must have a relatively large capacity, and must deliver a relatively large amount of power, compared to a conventional 12 volt automobile storage battery. It is further understood that because power is directly proportional to battery voltage and system current, the high power delivery requirements which must be satisfied by traction batteries necessarily mean that higher electrical voltages will be present in electric automobiles than in automobiles powered by fossil fuels, which typically require only a comparatively low power, low voltage storage battery for energizing auxiliary loads when the internal combustion engine is not operating.
Hybrid electric vehicles (HEVs) combine the internal combustion engine of a conventional vehicle with the battery and electric motor of an electric vehicle. This results in an increase in fuel economy over conventional vehicles. This combination also offers extended range and rapid refueling that users expect from a conventional vehicle, with a significant portion of the energy and environmental benefits of an electric vehicle. The practical benefits of HEVs include improved fuel economy and lower emissions compared to conventional vehicles. The inherent flexibility of HEVs also permits their use in a wide range of applications, from personal transportation to commercial hauling.
Because electric or hybrid electric vehicles require little or no combustion of fossil fuels, such vehicles produce little or no environmentally harmful emissions, in contrast to fossil fuel powered vehicles. Such vehicles are become increasingly attractive alternatives to fossil fuel powered cars. However, because of the high voltage requirements of its traction battery an electric or hybrid electric vehicle raises significant electrical safety concerns.
For example, unwanted electric current flow outside of the intended electric circuit flow (i.e. ground fault conditions) may cause significant damage to electronic components within a system (such as an HEV propulsion system), thereby disabling or even destroying the electronic equipment. In addition, such ground fault conditions may result in an electric shock, which can have graver consequences when the shock is caused by contact with a high voltage traction battery system, as compared to a conventional, relatively low voltage automotive storage battery system. To reduce the likelihood of such shock, many traction battery systems are not grounded to the automobile chassis, in contrast to conventional automotive storage battery systems. Instead, many traction battery systems have a closed loop return path, so that the negative power conductor of the system (i.e., the electrical current return path) is isolated from the chassis of the electric or hybrid electric vehicle.
FIG. 1
shows a detection circuit for detecting a DC ground fault. A high impedance network
700
is coupled between first positive power conductor
200
and second negative power conductor
300
and operates to balance the high voltage battery string equally between the positive and negative voltage values with respect to the ground reference potential (i.e. chassis) voltage
540
. The high impedance network
700
comprises resistors Ra, Rb, . . . , Rn connected in series and having a first terminal
710
coupled to first power conductor
200
, a second terminal
720
coupled to second power conductor
300
and a third terminal
730
coupled to ground reference potential
540
via Resistor R
168
. Preferably, each of the resistors Ra, . . . , Rn is of equal resistance and arranged such that the magnitude of the voltage is centered equally above and below chassis. Sensing resistor R
168
operates to detect a shift in this centering due to an undesired impedance fault by sensing the induced current through R
168
.
While such isolated systems may minimize the likelihood of a significant electric shock to a person in the event of a short circuit or low impedance connection between the positive or negative power conductors and chassis, such a circuit provides only for detection of an unwanted electrical path between the chassis (reference potential) and either positive or negative power conductors. It fails to provide any detection of an unwanted electrical path between chassis and the center of the energy storage string and also fails to provide any compensation for re-equalizing the voltage potential between the two power conductors and the chassis. A ground fault detection system for sensing a DC ground fault condition within the high voltage energy storage and actively compensating to correct such condition is desired.
SUMMARY OF THE INVENTION
A fault detection system for detecting an unwanted electrical path between a reference potential and at least one of: a) a floating power source, b) a first power conductor coupled to a first terminal of the power source, and c) a second power conductor coupled to a second terminal of the power source. The system comprises an impedance network electrically coupled to the first and second power conductors and having an output terminal for providing a first voltage signal with respect to the reference potential, and an amplifier circuit responsive to the first voltage signal and to a reference voltage for generating an amplified signal indicative of the existence of the unwanted electrical path when the difference between the first voltage signal and the reference voltage exceeds a predetermined value. In the case of an unwanted electrical path between the chassis reference and a node within the floating power supply, the system is operative to detect a signal indicative of an impedance imbalance from the center point and to provide a compensating impedance for equalizing the voltage between the first and second power conductors with respect to chassis.
A fault detection system for detecting and compensating for an unwanted electrical path between a reference potential and at least one of a floating power source, a first power conductor coupled to a first terminal of the power source, and a second power conductor coupled to a second terminal of the power source, the system comprising an impedance network electrically coupled to the first and second power conductors and having an output terminal for providing a first voltage signal with respect to the reference potential, a high gain amplifier arrangement responsive to the first voltage signal and to a reference voltage for generating an amplified signal indicative of the existence of the unwanted electrical path when the difference between the first voltage signal and the reference voltage exceeds a predetermined value; and a compensating circuit having an input electrically coupled to the output of the high gain amplifier, and first and second outputs electrically coupled via conduction paths to the first and second power conductors, respectively, for providing a compensating signal to at least one of the first and second power conductors, the compensating signal in accordance with the magnitude of the amplified signal for reducing the difference between the first voltage signal relative to the reference voltage.
A fault monitoring apparatus comprises a floating power source having a first terminal coupled to a first power conductor and a second terminal coupled to a second power conductor; an impedance network having a first terminal electrically coupled to the first power conductor, a seco

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