Low current ground fault relay

Details Low current ground fault relay Low current ground fault relay

2000-02-04

2001-12-04

Sherry, Michael J. (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

Motor protective condition responsive circuits

C361S029000, C361S042000, C361S047000, C361S078000

Reexamination Certificate

active

06327124

ABSTRACT:

BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates generally to the field of ground fault relays (GFRs) and, more particularly, to GFR devices in which a sensitive relay detects insulation degradation in large electric motors and cables supplying large electric motors. In current practice, the GFR typically reads the current leakage to ground when the motor is turned on and during running of the motor. If the leaking current is above some preset value, the GFR senses this situation and its output, which is normally connected to the motor controller, then causes the motor to be switched off. The present invention operates in two (2) modes. The first mode is a “look ahead” mode prior to the motor being turned on, and the second mode is a “real time” mode after the motor is turned on. In both modes, the GFR evaluates the insulation resistance to ground.
II. Description of the Related Art
Motor control circuits for protecting an electric motor against ground faults are well known in the prior art, when used in both single phase motors and in three phase motors. Typically, three phase motors draw large currents when running and this is especially true in mining operations. However, to be effective, a GFR, in such an environment as a mining, must sense very small amounts of leakage current.
For instance, in U.S. Pat. No. 5,386,183 issued to Cronvich et al., a ground fault relay apparatus for sensing a ground fault in a motor control system is shown wherein the system generates an unrectified signal indicating the current level in two (2) or more conductors of a motor, and then generates from the first and second unrectified signals an unrectified sum signal indicative of the sum of the current levels and then has means for comparing the rectified sum signal to a designated limit to compare that signal with a preset signal to turn off a motor in which the ground fault currents are excessively large.
In U.S. Pat. No. 5,715,125 issued to Neiger et al., an intelligent ground fault circuit relay is shown wherein the relay automatically tests its internal circuitry on a periodic basis to provide a high probability of proper operation in the event of a ground fault in the motor circuit. In addition, this invention discloses a method of monitoring the ground leakage current and to adjust the trip threshold of the GFR to prevent nuisance tripping. Further, this particular patent discloses systems for monitoring the wiring system for detecting a miswired condition and to set various alarms to indicate the various monitored results to a user.
SUMMARY OF THE INVENTION
In accordance with the present invention and the contemplated problems which have existed and continue to exist in this field, the present invention provides a GFR for protecting large electrical motors which draw very large currents when running, but, to be effective, the GFR must sense very small leakage currents in order to protect the motors and operating personnel. The electronic circuitry and hardware of the present invention will provide the sensing operation for effecting desired results. The present invention operates in two (2) modes identified as the “look ahead” mode and the “real time” mode. The “look ahead” mode utilizes a high-voltage coupling device to connect the GFR to the circuit being protected which usually consists of an electrical power cable and an electrical motor. In this mode the GFR is determining the resistance value of the leakage path to ground before the motor is turned on. The “real time” mode utilizes a current transformer to sense the leakage current to ground in the same circuit as previously described. In this mode the GFR is determining the leakage current to ground after the motor is turned on. This invention uses a method of monitoring the ground leakage current which synchronizes the GFR's operation during the motor start-up and shut-down process so as to prevent nuisance tripping when operating in both the current and resistance modes.
In the “look ahead” mode a signal is supplied by the GFR to the motor circuit before the motor is turned on. In the “look ahead” mode the GFR is measuring the resistance that exists between the motor circuit and ground. The relay then compares the measured resistance value with the trip value previously set in the GFR. The GFR only allows the motor to be turned on if the measured value of resistance to ground is greater than the trip value previously set in the GFR. Obviously, the motor is not allowed to turn on if the measured resistance value is less than the preset trip value.
In the “real time” mode, the motor is turned on and the current transformer is measuring the leakage current which may exist between the motor circuit and ground. The GFR compares the leakage current value with a current trip value. If the measured current value is less than the trip preset current value the motor circuit remains turned on. If the measured current value is greater than the preset current trip value the motor circuit is turned off. In addition, before the motor circuit is turned off, the time duration of the measured current value must exceed a time delay preset in the GFR.
In the “real time” mode the current transformer secondary winding is providing a signal to the GFR. Within the GFR the signal is passed through a noise filter to eliminate the extraneous signals which are not desired, thereby improving the electrical noise immunity which results in greater leakage current and line-to-ground resistance measuring accuracy, and less nuisance tripping at lower leakage current or higher resistance value threshold settings. Then the signal is fed to a converter which changes this AC signal to a DC RMS signal and feeds that signal to a programmable gain amplifier. The amplifier is programmable in order to set various signal gain levels for the signal received for the current transformer. This increases the accuracy of measurement when the range is wide, even by several orders of magnitude. The filtered DC RMS converted and now amplified signal is then fed from the amplifier to an analog to digital converter. The digitized value is then fed to a processor circuit which will digitally compare the received signal to the set value in the processor circuit. If the measured value originating from the current transformer in the motor circuit exceeds the selected preset trip value for a time period greater than the preset time delay, then the relay's processor calls for the GFR relay contacts to cause the deactivation of the main contactor which in turn de-energizes the motor circuit.
Other objects, advantages and capabilities of the invention will become apparent from the following description taken in conjunction with the accompanying drawings showing the preferred embodiment of the invention.


REFERENCES:
patent: 3656136 (1972-04-01), Blair
patent: 4716487 (1987-12-01), Horvath et al.
patent: 5386183 (1995-01-01), Cronvich et al.
patent: 5481194 (1996-01-01), Schantz et al.
patent: 5675465 (1997-10-01), Tanaka et al.
patent: 5715125 (1998-02-01), Neiger et al.
patent: 5751132 (1998-05-01), Horvath et al.
patent: 5847908 (1998-12-01), Herbstritt
patent: 5945802 (1999-08-01), Konrad et al.

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