| 摘要 |
A static protective relay comprises a power directional unit for controlling a circuit breaker which couples a primary feeder or a source of supply of a 3-phase power distribution system to an alternating current secondary network through a network transformer. The power directional unit senses reverse power flow above a preselected threshold level from the network to the network transformer to energize a tripping coil of the circuit breaker. Included in the power directional unit is a voltage deriving circuit producing a voltage proportional to the system current through the circuit breaker, a modulator which modulates the voltage by a voltage proportional to the system voltage at the circuit breaker to obtain an output proportional to real power flow through the circuit breaker, and a detector which energizes the tripping coil upon the presence of a reverse power flow signal from the modulator. The power directional unit also includes a source of variable bias comprising circuitry for providing a bias signal which varies both in magnitude and in phase and is a function of the phase angle of the phase current. The variable bias is supplied as an input to the detector to provide a power directional unit exhibiting improved sensitivity to highly capacitive phase currents.
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