Research On Three-phase Grid-Connected Inverters With LCL Filter

With the development of economic, we society need more and more energy. On theone hand fossil energy’s reserves decrease ceaselessly, on the other hand using of fossilenergy will result in significant amount of greenhouse gas and pollution gas, so it spurs usto find alternative energy sources. In the recent decade, the industry of new energy hasgained great development, such as photovoltaic industry and wind power industry.grid-connected converters which have a intermediate position between new energy and gridhave gained great development in the recent decade.Single L filters are used in low power range grid-connected converters and it hasadvantages of easy design and regulation. It has drawbacks of very high dc-link voltage andswitching frequencies. LCL filters are usually used in medium and high power rangegrid-connected converters. LCL filters have stronger harmonic suppression capability thansingle L filters. Because LCL filters are three-order systems, control strategy of LCL filtersis very complicated.First of all, we analysis that LCL filters have advantages than single L filters andmathematical models of LCL filters are deduced in abc-frame, stationary-frame andsynchronous-frame coordinate system. Input current ripple of the dc-link filters inthree-phase grid-connected converters is deduced in this paper. We have an interview ofthree-phase PLLs.The definition of PR controller is given, the relationship between PR controller instationary-frame and PI controller in synchronous-frame is deduced. Practical PR filterrealization and discrete-time implementation are introduced later.The control strategy of three-phase grid-connected converters which use grid-sidecurrent as control target is proposed here. The drawbacks of single current loop of grid-sidecurrent are discussed. Then control strategy which uses capacitance current as inner loopand grid-side current as outer loop is presented. We get Mathematical models ofthree-phase grid-connected converters in stationary-frame, use PR controller as outer controller and proportion controller as inner controller. Parameters of controllers areanalyzed and designed. Then experiments have been done to verify the validity.As Repetitive control strategy is very effective for harmonic suppression, we introduceRepetitive control strategy with dual current loop control strategy both in stationary-frameand synchronous-frame. Then experiments have been done to verify that this controlstrategy is effective for controlling both fundamental component and harmonicscomponents of current.