| With the diversification of electrical equipment,the non-linear loads connected to the power system are also increasing,which brings serious harmonic pollution to the power grid.At present,the widely used method is to eliminate the harmonic components in the power grid through the compensation function of the active power filter(APF).However,in the actual use process,real-time compensation without deviation in the true sense is difficult to achieve.Even because of the delay problem,harmonic pollution may be amplified,which seriously affects the compensation performance of APF for harmonics.In order to reduce the negative impact caused by the delay,eliminate the error between the compensation current output by the APF and the real harmonic current in the load current,and achieve accurate and real-time compensation effects,the paper mainly does the following research work:(1)The paper analyzes each link of the system that generates delay,focusing on the research and compensation of the delay of the harmonic detection link.And establish the mathematical model of the three-phase three-wire parallel APF under the a-b-c coordinates.According to the system configuration,the paper analyzes the links in the system where the delay occurs.APF delay includes A/D conversion delay,harmonic detection delay,current control delay,injection circuit delay and inverter switch action delay,etc.(2)Based on the harmonic detection method of instantaneous reactive power theory,an improved phase shift control harmonic detection method is proposed for the delay problem in the harmonic detection part.This method performs phase compensation on the rotating coordinate system during the inverse transformation of the fundamental DC component to compensate the fixed pha se delay time of the filter.(3)The research calculates the parameters of the three-phase APF main circuit,determine the DC side voltage,capacitance and inductance values,and build the APF simulation model based on the improved harmonic detection method,including the overall model diagram,the harmonic current detection module,and the DC side voltage control module and current control strategy module.(4)Through simulation,the effects of APF harmonic compensation under different load conditions are analyzed,and the steady-state indicators of the ip-iq detection method and the improved phase shift control detection method are horizontally compared: the voltage fluctuation of the DC side voltage is less than 1%,the harmonic current detection deviation of the improved detection method is reduced to 0.76%;in the four cases of phase-balanced resistive load,three-phase balanced resistance resistive load,three-phase unbalanced resistive load,and three-phase unbalanced resistance resistive load,compared with the ip-iq detection method,the improved method reduces the power grid current THD reduction percentage by more than 20%.(5)Through simulation,the dynamic performance of the system is analyzed.When the APF is connected to the main system,the waveform of the grid current is completely stable and presents a sine wave after one cycle.In the case of a sudden load change,or when an unbalanced load is connected,the grid current reaches a balanced state after one cycle. |
