Experimental studies on fault current limiter by voltage source inverter with line voltage harmonics compensation

Recently, many distributed generators have been connected to a power system. These may cause a large current if some system fault occurs in the power system, therefore a short-circuit capacity of the power system should be carefully considered. However, the short-circuit capacity also depends on a line impedance. A large line impedance will suppress the increase of short-circuit capacity, but it will cause the other difficulties about a line voltage harmonics. Increase of nonlinear loads such as diode rectifier circuits results in the line voltage distortion on an electric power distribution line. In the circumstances, a useful apparatus which can both reduce the short-circuit capacity and suppress the line voltage distortions is expected to provide a solution against these difficulties. Authors have developed a converter-based FCL as an advanced power system protection apparatus. The FCL consists of a series inductor and a self-commutated voltage source inverter connected in series with a line by a coupling transformer. It utilizes a variable reactance which is realized by suitable operation of the series inverter. During a fault in a power system, the series inverter is activated as an additional inductor to reduce the fault current best. But in a standby mode, it behaves like a capacitor for compensating a voltage drop across the inserted series inductor. This paper proposes that the FCL compensates line voltage harmonics as well as the voltage drop across the series inductor in the standby mode. The operating characteristics and effectiveness of the FCL are confirmed by experimental studies with a laboratory scale model.