A harmonic compensation approach for interlinking voltage source converters in hybrid AC-DC microgrids with low switching frequency

In recent years, the hybrid AC-DC microgrid has been well accepted as it combines the advantages of both AC and DC systems. As the microgrid contains both DC sub-grids and AC sub-grids, interlinking DC-AC converters are essential. Meanwhile, considering the nonlinear AC loads may deteriorate the voltage quality of the AC bus, embedding an ancillary harmonic compensation function to the interlinking converters is promising. However, the conventional harmonic control methods used for active power filters (APFs) may not be suitable for the interlinking converters due to the main purpose of it is to exchange real and reactive power between the DC and AC sub-grids. The switching frequency is preferred to be lower than the APFs when the capacity of the microgrid is large. At low switching frequency, harmonic compensation performance or even the system stability may be affected. In this paper, a harmonic compensation approach suitable for hybrid AC-DC interlinking converters at low switching frequency is proposed. Through feeding the PWM reference signal with the harmonic compensation component directly to avoid the multi-loop control path of the fundamental component, the proposed method can achieve the effective harmonics compensation without being limited by the closed-loop control bandwidth. The proposed method, modeling approaches, stability analysis, as well as detailed virtual impedance design are presented. Experimental verification is also provided.