An Improved Current-limiting Control Strategy for Shunt Active Power Filter

Some types of short-circuit or open-circuit fault in the distribution power system usually bring about a strong impulse current, which will result in the over-current fault of the shunt active power filters (SAPF) connected in the distribution power system to suppress the harmonic currents. Therefore, a novel current-limiting control strategy for SAPF is proposed in this paper. With this method, when one or more phase currents of the compensation instruction currents exceed the instantaneous current limit, the exceeded part of the phase current having the maximum instantaneous absolute value is truncated, while the other phase currents are limited proportionally. Theoretical analysis, simulation and experimental results show that, on the one hand, compared with the traditional proportional current-limiting control method, the new method has much faster response speed to overcome the problem of over-current protection delay; on the other hand, compared with the traditional truncated current-limiting control method, the sum of the instruction currents equals to zero, so no extra harmonic currents, especially 3k-th harmonic currents, where k = 1, 3, 5,..., are included in the APF output currents other than the compensation harmonic currents, and furthermore, in the three-phase four-wire system, no extra zero-sequence harmonic currents exist in the neutral line. As a result, APF has the best compensation effect so that the least harmonic currents flow into the power system.