DC Voltage Unbalance Analysis And Control For Cascaded H-bridge Statcom

In recent years,as the voltage level of the power distribution network in the power system has become higher and higher,the number of various loads connected to the grid has continued to increase,and the subsequent three-phase imbalance and power factor drop problems have become increasingly serious.Therefore,the cascaded H-bridge STATCOM began to be widely used in power distribution systems.Studies have shown that when the three-phase asymmetry occurs in the system,not only the negative sequence current of the system will increase,but also the unbalance of active power between the three phases of the supporting STATCOM,which will cause the unbalance of the voltage between the STATCOM phases;at the same time,Problems such as differences in the switching devices of the H-bridge sub-modules or inherent limitations of the modulation method used can also lead to unbalanced DC voltages of the sub-modules in the phase,thereby affecting the operating performance of the equipment.The active power model in the traditional abc coordinate system contains a large number of trigonometric functions and power calculations,which are complex and require too much calculation.In order to eliminate the trigonometric function and power calculation in the traditional interphase DC side voltage equalization control based on the traditional abc coordinate system,the active power model is established on the dq coordinate system,and the calculation is different from the abc coordinate on the convenient and simple active power.Power model.In order to improve the traditional phase-to-phase voltage equalization control strategy based on negative sequence current injection,the negative sequence current command is simplified according to the active power model based on the dq coordinate system,and a voltage equalization control strategy based on the traditional negative sequence current injection is proposed.Improved negative sequence current feedforward closed-loop control voltage equalization control strategy with stronger adjustability and faster response.The RT-LAB real-time simulation platform based on OP5600 verifies the improved negative sequence current feedforward closed-loop control voltage equalization control strategy.The simulation results show that the voltage equalization control strategy based on the negative sequence current feedforward closed-loop control proposed in this paper can not only be implemented quickly The phase-to-phase DC voltage is balanced,and the controller shows strong robustness and fast response speed when the voltage drops.In order to further stabilize the DC side voltage of the H-bridge sub-module,quantify the negative impact of the single-stage frequency multiplication CPS-SPWM modulation method on the phase voltage,and use a detailed mathematical analysis to prove that due to the fact that each H-bridge sub-module has a different throughput per unit time Equal to cause voltage imbalance in the phase.The improvement of the active voltage vector superposition control strategy solves the problem that the traditional active voltage vector control strategy is prone to oscillation due to the introduction of current,and reduces the influence of the double frequency of the DC side voltage on the system.It is proved by simulation that the improved intra-phase voltage equalization control strategy based on active voltage vector superposition not only has strong anti-interference ability,but also has fast control speed,which can quickly suppress the DC voltage imbalance of the sub-modules in the phase.