Three-phase Voltage Source PWM Rectifier With LCL Filter

PWM rectifier is widely used in power generation of solar, wind and other renewableenergy sources. The rated power of PWM rectifiers used in these generation systems isusually very high. In order to reduce switching losses and electromagnetic interference, lowswitching frequency is generally used. To achieve the grid harmonic requirements, usingconventional L filter will require larger inductance. Thus it will increase costs, reduce thecurrent loop response speed and increase the inductive voltage drop, the DC voltage willneed to be increased. However, the LCL filter which has third-order filter characteristics,needs smaller inductance to achieve the same harmonic requirements. So the LCL filter caneffectively reduce the system cost. But the LCL filter will resonate at certain frequencies,and it will lead to system instability.This paper firstly describes the working principle of PWM rectifier, then derives themathematical models of PWM rectifier in the three-phase stationary coordinatesystem(ABC) and the two-phase rotating coordinate system (dq) based on L and LCL filterrespectively.The frequency characteristics of the LCL filter is analyzed. The result shows that theLCL filter’s frequency response is the same as the L filter’s below the resonance frequency,and LCL filter’s frequency response attenuates faster than L filter’s. Then the method ofdesign LCL filter parameters is introduced. The design steps, constraints, and efficient wayto verify the design are given.Based on control strategy of PWM rectifier with single L filter, the voltage loop andcurrent loop controller are designed. Detailed analysis of the passive damping method andthree kinds of active damping methods is presented. The analysis of the impact of thedigital control process on control systems is also presented. Using Matlab/Simulink verifiesthe effect of the designed control strategy and damping strategy.Finally, the experimental result using active damping method is presented.It showsthat the LCL filter resonance has been effectively suppressed, the grid current has lowharmonic distortion and fast dynamic response, the output voltage is stable and has stronganti-disturbance capacity.