US 5,446,682 AGranted

System for calibrating a line isolation monitor

AssigneeSquare D Company

InventorsDonald R. Janke, James A. Rodrian

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Abstract

A line isolation monitor (LIM) indicates the maximum hazard current of an ungrounded polyphase power distribution system. The LIM is microcontroller based and continuously monitors a fault impedance for each phase of the distribution system. The fault impedance is determined by the LIM by injecting a continuous sine wave measurement current into a ground terminal to generate a measurement voltage across the fault impedance. Using the measurement voltage and current, the LIM calculates the fault impedance and, using this impedance, calculates the hazard current based on the maximum line to ground voltage of the ungrounded system. The LIM has means for self-calibration and self-testing while on-line and during a power-up sequence. These tests compare measured and theoretical values of circuit parameters of voltage and current sensing circuits. During this calibration check, the monitoring circuits of the LIM are removed from the isolated system and known voltage and current signals are injected internally to the monitoring circuits to verify the component parameters. If the calibration parameters are outside tolerances by a first, low level, the LIM annunciates a warning, but will continue to monitor the hazard current. If the deviation is greater than a second, higher level, the LIM will indicate a fault and remove itself from service.

Claims13

Dependent claims

A system for self-calibrating a line isolation monitor that continuously displays a hazard current between line conductors of an ungrounded polyphase power distribution system and ground, said monitor having a current sensing circuit for measuring a test current injected into said power distribution system, a voltage sensing circuit for each of said line conductors, said voltage sensing circuits for measuring voltages between said line conductors and ground, said voltages consisting of a line voltage and a test voltage generated across a hazard impedance between said line conductors and ground by said test current, said self-calibrating system comprising;

A. a sine wave generator for generating a calibration test voltage, said test voltage having a predetermined frequency at a multiple of a determined line frequency of said power distribution system;
B. a calibration control for selecting inputs to said voltage sensing circuits, said inputs coupled to said line conductors when said monitor is in a normal mode of operation, and said inputs coupled to said sine wave generator when said monitor is in a calibration mode, said voltage sensing circuits to measure said voltages between said line conductors and ground when in said normal mode and to measure said test voltage when in said calibration mode;
C. a first comparator for comparing outputs of said voltage sensing circuits with predetermined parameters when in said calibration mode, said predetermined parameters calculated as theoretical values based on component values and tolerances within said voltage sensing circuits, said first comparator for outputting a first signal representative of a deviation from said predetermined parameters;
D. a second comparator for comparing said test current with a theoretical value based on component values and tolerances within said current sensing circuit, said second comparator for outputting a second signal representative of a deviation from said theoretical value;
E. an output circuit for producing a visual and an audible indication of a warning condition if either of said first or second signals representing a deviation from said predetermined parameter and theoretical value, respectively, exceed a first, predetermined amount, and for producing a visual and an audible indication of a fault condition if either of said first or second signals exceed a second, greater, predetermined amount when in said calibration mode;
and
F. wherein said line isolation monitor periodically self-calibrates at predetermined times without interfering with said continuous display of said hazard current.

4 more elements

A method of self-calibrating a microcontroller-based line isolation monitor for continuously displaying a hazard current between line conductors of an ungrounded polyphase power distribution system and ground, said monitor including a current sensing circuit for measuring a test current injected into said power distribution system and a voltage sensing circuit for each of said line conductors, said voltage sensing circuits for measuring voltages between said line conductors and ground, said voltages consisting of a line voltage and a test voltage generated across a hazard impedance between said line conductor and ground by said test current, said method of self-calibration comprising;

A. generating a calibration test voltage:
B. selecting inputs to said voltage sensing circuits, said inputs coupled to said line conductors when said monitor is in a normal mode of operation, and said inputs coupled to said calibration test voltage when said monitor is in a calibration mode, said voltage sensing circuits to measure said voltages between said line conductors and ground when in said normal mode and to measure said test voltage when in said calibration mode;
C. comparing outputs of said voltage sensing circuits with predetermined voltage parameters when in said calibration mode, said predetermined parameters calculated as theoretical values based on component values and tolerances within said voltage sensing circuits and outputting a signal representative of a deviation from said predetermined voltage parameters;
D. providing a visual and an audible indication of a warning if said output signal representing a deviation exceeds a first, predetermined amount, and providing a visual and an audible indication of a fault condition if said output signal exceeds a second, greater, predetermined amount when in said calibration mode;
E. comparing said test current with a theoretical value based on component values and tolerances within said current sensing circuit and outputting a signal representative of a deviation from said theoretical value;
F. providing a visual and an audible indication of a warning if said output signal representing a deviation from said theoretical value exceeds a first, predetermined amount, and providing a visual and an audible indication of a fault condition if said output signal exceeds a second, greater, predetermined amount when in said calibration mode;
and
G. wherein said line isolation monitor periodically self-calibrates at predetermined times without interfering with said continuous display of said hazard current.

5 more elements

Dependencies are inferred from the claim text.