Apparatus for protection of power-electronics in series compensating systems

摘要

An active power line conditioner circuit having a series compensation system arrangement utilizes a transformer-coupled three-phase voltage-source inverter. A conventional metal oxide varistor (MOV) and bypass breaker combination are provided on the line side of the coupling transformer. A solid-state thyristor shorting ("crowbar") switch is provided on the secondary side of the coupling transformer across the outputs of the three-phase voltage-source inverter (VSI). The crowbar switch circuitry utilizes a diode bridge with a thyristor on the dc side of the bridge resulting in the simplification that only one active device need be gated. Crowbar switching logic activates the crowbar switch based on the exceeding of a threshold voltage measured at the dc power source link to the voltage-source inverter. A direct measure of dc source link voltage is employed to determine when to trigger the conduction of the crowbar thyristor switch. The three-phase voltage-source inverter is provided with overcurrent protection circuitry that immediately blocks the active portion of each phase-leg whenever the current exceeds a predetermined level. In an alternative embodiment of the present invention, a thyristor-switched MOV device is provided across the dc source link in parallel with the dc source output and the dc link capacitor for fast switching of slight overcurrents which are passed by the VSI. This allows operation with a dc source voltage close to maximum system ratings, thereby making better utilization of the VSI power electronics.