Consideration from Viewpoint of Output Impedance Concerning Control Method for Parallel Operation of CVCF Inverters

This paper describes the study results of the inverters running in parallel. It is a problem how to suppress the circulating current which flows among the inverters. In the consideration of the control algorithm, it is important to reveal the output impedance characteristics of the inverters. The impedance rules the circulating current. First, Simulations demonstrate that the output impedance depends on the control scheme of the output voltage and characteristics of the circulating current differ. The output impedance was calculated in cases on a single-phase CVCF inverter. The impedance is calculated by the disturbance transfer function when the output current is assumed as the disturbance. The validity of these calculation results was confirmed by using the simulations. Next, as examples, two control schemes and their experimental results are demonstrated. The two schemes are: “PQ droop method” based control and “common current distribution method” based control. In the former case, since the output impedance is resistive, the phase of the output voltage is adjusted by using the reactive component of the circulating current. Figure 1 shows the latter control system. The inverters have an instantaneous output voltage feedback system with minor current loop. The load current iL is detected by a common current sensor and distributed to the inverters. In the case of the inverter standalone operation, the inverter output current iout is fed back, and in case of the inverter parallel-operation, the load current iL/n is fed back instead of iout. Table 1 indicates the calculation results of the output impedance on the Fig. 1. When iout is not fed back (SW1 = 0), the output impedance becomes resistance. When iout is fed back (SW1 = 1), the value of the impedance becomes 1/10 of the value of the impedance when SW1 = 0. This characteristic can bring the following matters for the inverters working in parallel. Because the load current is fed back (SW1 = 0), in the inverters running in parallel, the output impedance to the load is same as that of the stand-alone inverter. Consequently, the inverters can offer the same output-voltage performance as that of the stand-alone inverter. When the inverter output current is not fed back (SW1 = 1), the output impedance between inverters is ten times the output impedance to the load, and then the circulating current hardly flows between inverters. Figure 2 indicates the output voltage waveforms when two 100 kVA inverters are connected in parallel. 1.4% of the total harmonic distortion of the waveforms is obtained at 100% capacitive rectifier load. In this case, each inverter shares the load within 5% of error. Fig. 1. Parallel operation controller for inverter with instantaneous output voltage control