Improvement Of Detection Method Of Harmonics And Instantaneous Reactive Current In Low Voltage Distribution Network With Three-phase Four Wire System Supply Mode

With the rapid development of the power industry and the semiconductor industry,the converter equipment has been widely used.At the same time,a large number of nonlinear loads increase,the voltage distortion harmonic pollution is serious,and the serious reactive power consumption affects the quality of power supply and the safe and stable operation of power grid heavily.The hannonic suppression and reactive power compensation have become the most urgent problems to be solved.The three-phase four wire power supply mode is the main power supply mode of the low voltage distribution network in our country,but now the most popular reactive power compensation device SVG is not particularly extensive in the three-phase four wire system.This paper introduces the working principle of SVG,the characteristics of the work,the current control mode,analyzes the main circuit topology of SVG in three-phase four wire power grid,and through making a comparative analysis of them,the paper selects the most suitable topology to design.One of the most important aspects of controlling harmonic pollution is to put unity power factor converter into distribution network.While the application of this principle is that combining the multiplex technology and PWM technology in the main circuit of converter in SVG reduce the harmonic pollution.With the above control technique,the converter device in SVG can easily filter the high frequency harmonics and its low frequency harmonics can be eliminated easily.Whether the reactive current detection is accuracy directly restricts the compensation performance of reactive power compensation device.This paper mainly analyzed two kinds of reactive current and hannonic detection method which is the most efficient detection method in SVG and introduced the theoretical basis of them.Through the formula derivation and simulation analysis,it is pointed out that the dq detection method can be directly applied to the reactive current detection in three-phase four wire system.In this respect,ip-iq detection method and pq detection method failed to exceed the dq detection method,the zero sequence current separation should be applied in the three-phase four wire system.This paper also analyses the above three harmonic and reactive current detection method performance in the unbalanced operation,the simulation analysis reach a conclusion that under unbalanced grid voltage conditions,the PLL failed to accurately extract the voltage,seriously affect the detection accuracy of reactive current.Aiming at the above problems,this paper proposes an improved reactive current detection method.Aiming at the shortcoming that the ip-iq method can not be applied directly in the three-phase four wire system,this paper put forward the method of separating the zero sequence current;according to the error brought by the phase locked loop(PLL),the amplitude integrator is designed to replace it,each of which are:positive sequence voltage gain link and amplitude integrator screening AC link.The Simulink simulation of the improved method is based of ip-iq detection method,through the contrast with the traditional methods,the paper verified the improved method has the following advantages of accuracy and real-time.