||Due to the limited capacity of a single inverter, parallel inverters are often used for the interconnection of renewable energy generation systems. The equal-current sharing is the commonly-used control method for parallel inverters; however, the output efficiency based on the equal-current sharing is low at light loads. Besides, more inverters in parallel will cause the lower output efficiency at the light loads and higher failure rate. In order to improve the output efficiency of parallel inverters at light loads, a novel equal-current-sharing-based control strategy for parallel inverters is proposed in this thesis. The proposed control strategy uses the conversion efficiency curve of a single inverter to calculate the conversion efficiency of different number of parallel inverters under different load conditions. The number of parallel inverters achieving optimal output efficiency can then be calculated; therefore, the connection and disconnection of different number of parallel inverters under different load conditions can be determined. A fully digital inverter realized by Texas Instruments (TI) TMS320F28335 that rated output voltage 220V, output power 2kW and frequency 60Hz is designed and implemented in this thesis. Three implemented inverters are used to validate the performance of the proposed control strategy for parallel inverters at light loads. Experimental results show that the proposed method can effectively improve the conversion efficiency of parallel inverters at light loads.|
In addition, when the number and capacity of parallel inverters are not the same, the failure rate of parallel inverters should be different. Therefore, the different number of parallel inverters and their reliability costs is discussed in this thesis. The optimal number and capacity of parallel inverters for cost minimum considering the proposed light-load control strategy, inverter costs and reliability costs through the net present value (NPV) of economic analysis in the lifetime is then investigated. For a photovoltaic generation system with installed capacity of 100kW, the optimal number and capacity of parallel inverters with rated capacities of 5kW, 10kW, 25kW, 50kW and 100kW is designed. Simulated results show that comparing with the installation of a single 100kW inverter in 20-year lifetime; extra profit of 23 thousands USD can be obtained through the proposed light-load control strategy under the same per kW inverter cost.