US 5,514,964 AGranted

System for monitoring a dual voltage ungrounded system for leakage currents

AssigneeSquare D Company

InventorsStephen P. Benesh, David W. Hasenohrl

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Abstract

A system contained on a single isolation panel determines the hazard current of a dual voltage, ungrounded, polyphase system. The system uses a single isolation transformer having dual secondary windings for generating the dual voltages. A first line isolation monitor or LIM, is microcontroller based and continuously measures the fault impedance of one of the output voltages of the polyphase, ungrounded or isolated system. A second LIM continuously measures the fault impedance of the other output voltage of the 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 hazard current is displayed and if it exceeds a predetermined threshold, the LIM will provide audio and visual alarms. A serial communications channel allows the LIM to communicate with other intelligent devices. The LIM has means for self-calibration and self-testing while on-line and during a power-up sequence. Line frequency is also determined by the LIM. The two LIMs can be used to monitor the dual distribution system at the same time without interfering with each other.

A system for continuously monitoring a hazard current between line conductors of a dual voltage, ungrounded polyphase power distribution system and ground, said system comprising;

A. an isolation transformer, said transformer having a primary winding coupled to a grounded power system, a first secondary winding for generating a first output voltage on line conductors of said dual voltage distribution system, and a second secondary winding for generating a second output voltage on line conductors of said dual voltage distribution system;
B. a first line isolation monitor (LIM) coupled to said first secondary winding and to said ground;
C. a second LIM coupled to said second secondary winding and to said ground;
D. wherein said first LIM and said second LIM inject a test current into said ground, said test current having a frequency at a multiple of line frequency of said grounded power system, and wherein said frequency of said test current from said second LIM is different from said test current from said first LIM;
E. wherein said first LIM and said second LIM have a frequency selective voltmeter for measuring a test voltage and a line voltage between each of said line conductors and ground, said test voltage responsive to said test current;
F. wherein said frequency selective voltmeter in said first LIM includes a frequency selective filter having a center frequency selectively tuned to said line frequency when measuring said line voltages of said first output voltage to ground and tuned to said test current frequency of said first LIM when measuring said test voltages;
G. wherein said frequency selective voltmeter in said second LIM includes a frequency selective filter having a center frequency selectively tuned to said line frequency when measuring said line voltages of said second output voltage to ground and tuned to said test current frequency of said second LIM when measuring said test voltages;
and
H. wherein said first LIM and said second LIM include means for calculating a hazard impedance by dividing said measured test voltage by said test current, and further including means for calculating said hazard current from said hazard impedance by dividing said hazard impedance by a largest line voltage of said measured line voltages between each of said line conductors and ground.

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Abstract

Claims11