A power quality solution with low inverter ratings

The feasibility of a three-phase current-fed inverter for active filtering of distribution system harmonics was investigated since most of the published works in this field have employed voltage-fed inverters.; The study showed that the voltage-fed hybrid filter generally has less conduction losses than the current-fed one no matter what types of passive filters are employed. From the loss standpoint, the current-fed inverter is suitable for the role of active filter only when it is in series with a parallel-resonant passive filter tuned at the fundamental frequency, instead of the popular compound series-resonant filters tuned at 5th, 7th, and 11th harmonics. The function of the parallel-resonant filter, inserted in between the point of common coupling and the output capacitor of the current-fed inverter, is to prevent high fundamental current from circulating in the inverter circuitry. The power (VA) ratings of the semiconductor components and power losses of the inverter therefore are reduced.; The study also revealed that, in contrast to the voltage-fed inverter-based hybrid filter, which requires both the load voltage and the line current to be sensed and functions correctly only under closed-loop feedback control, the current-fed inverter-based hybrid filter can perform active filtering simply by scaling the fundamental removed load current to be the duty ratio for the inverter.; Simulation works confirmed the analysis. With the duty-ratio directly derived from the load current, the current-fed inverter-based active filtering on a prototype one-branch distribution system loaded with a 3kW full-bridge diode rectifier has been done successfully. The results of the experiment are in accordance with the analysis. Using 3kW as the power base and selecting the semiconductor devices with appropriate ratings for the inverters, the inverter losses are 0.82% p.u. for the current-fed/parallel-resonant filter, 0.08% p.u. for the voltage-fed/parallel-resonant filter, and 2.09% p.u. for the current-fed/compound series-resonant filter with the same capacitance (150{dollar}mu{dollar}F) in each passive filter. Increasing the capacitance increases the inverter losses in the conventional hybrid filter, but does not increase the inverter losses in the proposed configuration. However, the VA ratings of the parallel-resonant passive filter are twice the ratings of the series-resonant filter, and high inductances are required.; In summary, the proposed hybrid filter, consisting of a three-phase current-fed inverter and a parallel-resonant passive filter tuned at the fundamental frequency, is suitable for active filtering of distribution systems with low overall losses and low VA ratings of the inverter components. The control of this current-fed combination is simpler than the control of various voltage-fed combinations.