Research On Static Var Generator Research Under Grid-voltage Unbalance

PWM SVG has the advantages of controlled DC side voltage, bidirectional power flow, low harmonic content, no pollution to the power grid. Mainly for the control strategy of SVG includes double closed loop control, nonlinear control and intelligent control. The above control methods are based on the assumption that the power grid is symmetrical. However, when the grid is asymmetry, negative sequence voltage and negative-sequence current appear in the grid, the above control methods lead to that even-order harmonics appear in the DC voltage. Fluctuating DC voltage can cause odd harmonics current. DC bus capacitors and power quality of life are affected, severe cases may cause equipment to be burned.When grid is asymmetric, different control targets are realized under different constraints. Commonly used control algorithms includes proportional resonant (PR) control, dual dq control algorithms, formulas sub-sequence method, nonlinear control and so on.Under unbalanced grid, when reactive compensation is implemented fully, the three-phase asymmetry current is obtained. With the variation of grid asymmetrical rate, SVG compensation power is also changed. In this paper, the theory also has been analyzed detailed.To achieve control of SVG, rapid separation of positive and negative sequence is needed. In many references, different solutions have been proposed, they can be summarized for four kinds:(1) Harmonic filtering method, second harmonic in the dq coordinate system is filter out by secondary notch. Notch filter parameters design and digital implementation are complex. Dynamic performance of the system is affected.(2) Double dq control algorithm, by the double dq transformation, the fundamental positive and negative sequence components can be separated. Howere, the structure of the method is complex, amount of calculation is large. In order to ensure the stability of the system, the bandwidth of the system is reduced.(3) Based on the second order generalized integrator method. The three-phase voltage signals shift 90 degree phase angle by using second-order generalized integrator However, the instantaneous symmetrical components calculation increases the complexity of computation and configuration of the system. (4) Nonlinear control method. The method has the disadvantage as follows:dependence on the parameters, large computation, long control cycle, poor real-time, control complexity.At the same time needs stateful measurable. Therefore, the application of this method is limited.The positive and negative separation based on instantaneous value voltage method is presented in this paper. AC forgotten iterative algorithm was used to filter separate signals. This algorithm has the characteristics of short delay and high precision.Firstly, the grid SVG mathematical models and power exchange model have been analyzed under grid unbalanced.Considering power tube, the ability of the SVG's compensation power is analyzed theoretical under grid unbalanced.Maximum compensation power as the goal, SVG's operation mode can be divided into two modes. In mode 1 control, reactive power is compensated fully. In mode 2 control, compensation power is up to the max.