Analysis of the Harmonic Characteristics of Multi-modular APF Shunt System Based on Symmetric Regular Sampling

Based on the mathematic model of the multi-modular APF parallel system comprising of N modules, the harmonic characteristics of the single module and the multi-modular shunt system on the basis of the regular sampling method are analyzed in this paper. The objective of the study above is to provide theory basis for the parameter design, circulating current analysis, coordinated control, and protection of the shunt system. Via the theory analysis and the experiment validations, some conclusions are obtained as follows. On the one hand, the low frequency modulation component of the output current from the shunt device compared with the single module is synchronous in the phase, and its amplitude is improved by N times. On the other hand, on condition that the carrier wave frequency which is defined as ωc is much larger than the modulation wave frequency, the high frequency switching components of the output current distribute near the the integral multiple of Nω_c, and three different harmonic characteristics exist in the frequency points of (2m-1)×(2N-1)ω_c, m×2Nω_c, and m×(2N-1)ω_c. Simultaneously, compared with the single module, their harmonic frequency are improved by N times, however, the harmonic amplitudes are reduced by N times.