Research Of Active Damping For LCL Grid Connected Inverter Based On Reducing Delay And Sampling Compensation

With the rapid development of human society,our demand for energy is increasing day by day.Facing the limited energy and environmental pollution caused by the use of primary energy,new renewable energy has aroused a great of concern.As an interface between distributed generation and grid,Grid connected inverter is widely used in the field of renewable energy.Compared with the single L converter,the LCL converter has a unique advantage in reducing the quality and volume of the converter and suppressing the high frequency switching ripple.In this paper,the problem of active damping is studied by using the capacitive current feedback single-phase LCL converter.Firstly,taking the analogy control of capacitor current feedback active damping(AC-CCFAD)of LCL type single-phase grid connected converter as an example,through the equivalent transformation,it reveals that the AC-CCFAD is equal with the passive damping with filter capacitor paralleled a resistance.Then giving the digital control of the capacitor current feedback active damping(DC-CCFAD)of single-phase LCL type grid connected converter as an example,through equivalent transformation,it is equivalent to filter capacitor shunt impedance,which can be equivalent to the parallel of frequency dependent resistance and impedance.The equivalent parallel resistance provides damping,and it will become negative from positive at the 1/6 sampling frequency.The positive and negative dividing frequency of equivalent shunt impedance is 1/3 sampling frequency.When the resonant frequency is less than 1/3 sampling frequency,the positive value of shunt impedance is equivalent to the inductance which will weaken the filter so that the resonance frequency is becoming larger.On the contrary,the negative value of shunt impedance will enhance the filter so that the resonance frequency decreases.The boundary frequency of resistance and reactance is changing with the sampling and calculation delay.With the decrease of the delay,the frequency dividing point which decide the positive or negative of the resistance and reactance increases gradually.The equivalent parallel resistance and reactance are constant positive in the Nyquist frequency when the sampling and calculation delay is zero.Grid connected current sampling and calculation delay mainly affects the phase margin of the system.With the decrease of the outer loop delay,the phase margin increases gradually.However,as long as the equivalent shunt resistance has the boundary frequency point,the system can not be stable at the boundary frequency position.The stability of the system will be affected by the change of system parameters,moreover,the damping characteristics of the system are closely related to the frequency,and a small change of the frequency will cause the change of the damping effect.With the measure of half step delay of sampling and calculation not only in the capacitor current damping loop but also in the loop of the grid control,the dividing frequency point can be increased to 1/4 sampling frequency,compared with the one step delay the dividing frequency point increased as well as the phase margin.With the method of half step delay and capacitor current sampling loop cascaded compensation function the boundary frequency can be further improved to 1/2 sampling frequency position.As a result the system can be damped in the whole Nyquist frequency.In this way,the equivalent parallel resistance of the system becomes a quantity that does not vary with the frequency,so that the damping effect will not change greatly with the change of frequency.Finally,a model of DC-CCFAD of LCL type single-phase grid connected converter is built in MATLAB to verify the effectiveness of the proposed method.The experimental platform is also built,through the experiments to verify the new method.