System for monitoring a dual voltage ungrounded system for leakage currents

摘要

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.