A New Distribution Network Power Quality Compensator For Three-Phsae Four-Wire System And Its Control Strategy

Due to the rapid growth of emerging electricity load,the demand for power quality is also increasing.At the same time,the non-linear loads and unbalanced loads are increasing in the three-phase four-wire distribution system,resulting in power quality problems such as reactive power,negative sequence,and neutral currents,which has caused great influence to the power grid.Therefore,it is particularly important to study the power quality problems and compensation technology and take effective measures to carry out comprehensive management.The emergence of the static compensator(DSTATCOM)of the distribution network has brought a new revolution to modern static var compensation devices.It has become one of the important components of flexible AC transmission system with the advantages of fast response,high control accuracy,dynamic continuous compensation,and the effective suppression of voltage fluctuation and flicker.Firstly,a new type of capacitive coupling DSTATCOM topology for three-phase four-wire system is proposed to avoid the complex topology and difficult control of the traditional "split" three-phase four-wire DSTATCOM.It is not only avoids the problem of voltage equalization on the DC side of the split-capacitor three-bridge topology,but also reduces a pair of bridge legs compared to the three-phase four-arm topology,making the system more stable and lower cost.Then the principle of the system is analyzed,and the parameter design method of the series capacitor is deduced.The neutral current output capability is compared with that of the split-capacitor three-bridge topology.The simulation verifies the correctness and effectiveness of the proposed structure.Secondly,A three-phase four wire power distribution network power quality compensator based on series and parallel structure is proposed using a similar"bloking" concept to avoid the problems of large capacity and high cost of the split three-phase four wire DSTATCOM topology.The proposed system can solve the problems of large transformer capacity and unsatisfactory treatment effect in the traditional transformer parallel compensation mode when dealing with the neutral current.At the same time,it does not need to introduce the neutral current into its own loop,which can achieve full-line reduction of neutral current.The mathematical model of the new series compensator is analyzed and detailed analysis of unbalanced load voltage and compensation principle are presented.The comparison is made with traditional transformer parallel compensation from the aspects of capacity and effect.The correctness and the analysis of proposed strategy is verified by the simulation results.Then,a new positive and negative sequence double loop superposition control strategy is proposed to avoid large error caused by traditional direct current method based on instantaneous reactive power.And it can also avoid traditional compensation in positive and negative sequence synchronous rotating coordinate system without considering voltage unbalance.The proposed control strategy can achieve the compensation of reactive power and negative sequence under the condition of unbalanced system voltage.The mathematical model of the power distribution network DSTATCOM in the positive and negative sequence equivalent circuits is established.And the reactive power,the DC side capacitor voltage and the transfer function of the negative sequence control loop are deduced respectively.Modeling and simulation are carried out by Matlab/Simulink.The results show the correctness and effectiveness of the control strategy proposed in this chapter.Finally,the parameters of the main components of DSTATCOM were studied and designed.A prototype of three-phase four-wire low-power experiment was made and its control performance and power quality compensation effect were verified by experiments.