Research On Statcom Under Three-phase Load Unbalance In Low-voltage Distribution Network

The improvement of power quality is an important prerequisite for the rapid development of the national economy,especially in the low-voltage distribution network such as electric locomotives,electric arc furnace with volatility and impact load access,resulting in voltage instability,three-phase imbalance.The stabilized grid voltage can be realized by controlling the reactive power compensation.At present,the device which has significant advantages in compensating reactive power,three-phase imbalance and harmonic suppression is the distributed synchronous compensator(DSTATCOM).In order to suppress the harmonics better,the LCL filter is used at the DSTATCOM output to filter the electric current.Based on the analysis of the working principle and basic structure of DSTATCOM,its main circuit structure with output LCL filter is determined.According to the basic principle of the circuit,the mathematical model of synchronous rotating coordinate system and two-phase stationary coordinate system under three-phase balanced load and unbalanced load are deduced respectively,and the main circuit parameters and LCL filter parameters are designed.When the three-phase load imbalance,using the DSC(delayed signal cancellation method)to separate its positive and negative sequence current,the separation of the positive and negative sequence components are controlled independently.In this paper,dq-PI dual-closed-loop control and ?-?-coordinate PR control are chosen as the control strategies.As the former control strategy requires multiple Park transformation,according to the characteristics of mathematical models also need to decoupling control,virtually increase the complexity of the control system.And use the latter PR control current loop,we can directly track the amount of sine adjustment.Therefore,the control system can not only save a lot of Park transformation,but also avoid the decoupling control,especially in the three-phase load imbalance,the system is more simplified,real-time better and more accurate.Finally,the simulation results show that the analysis of four control results under the control of the dq coordinate system and the ?? coordinate system in the two coordinate systems when the balanced load and the unbalanced load are analyzed.Grid voltage and current to achieve the same phase,so that the power factor are almost 1,but also can be seen in the PR control in a shorter period of time to achieve the same compensation with the PI control effect.