| With the gradual expansion of the power system and the increasing power system load in China,the problem of load imbalance has become more prominent in the power system.To effectively solve the problem of load imbalance,reduce harmonic pollution in the power system,improve system stability,and ensure that the power system has good power quality,flexible AC transmission system(FACTS)technology has emerged.Static synchronous compensator(STATCOM),as an important component of flexible AC transmission systems,has been widely used in reactive power compensation of unbalanced loads.This paper studies the control strategy of cascaded H-bridge STATCOM under unbalanced load conditions.The working principle,mathematical model,and topology of cascaded H-bridge STATCOM were analyzed,and a suitable main circuit topology was selected.The working state of cascaded H-bridge STATCOM was analyzed,and the principles of single-polarity double-frequency modulation and bipolar modulation in CPS-PWM were explored.It was concluded that single-polarity double-frequency modulation should have better output waveform and lower harmonic content when applied to STATCOM devices,and therefore single-polarity double-frequency modulation was chosen as the modulation method in this paper.The effectiveness of this method was then verified through simulation.In response to the phenomenon of three-phase current imbalance caused by unbalanced three-phase loads,which can lead to three-phase current imbalance in the power grid,this paper first analyzes the mathematical model of unbalanced loads cascaded with H-bridge STATCOM.Then,based on the mathematical model,a feedforward decoupling control strategy based on positive and negative sequence decomposition is designed.The direct current voltage fluctuation problem and voltage imbalance mechanism of the cascaded Hbridge STATCOM are analyzed,and a phase-averaged voltage control method is proposed to suppress voltage fluctuations and maintain system stability.A method of injecting zerosequence current is proposed to maintain phase-to-phase voltage balance.Finally,a model is built in simulation software to verify the effectiveness of the feedforward decoupling control strategy and the phase-averaged voltage control strategy.In some specific situations,the traditional PID control technology exhibits disadvantages such as slow dynamic response,large overshoot,and poor robustness.To enhance the dynamic response and anti-interference ability of the cascaded H-bridge STATCOM in the case of voltage drop and load mutation,this paper proposes replacing traditional PI control with the self-disturbance rejection control(SDRC)technology.Through simulation experiments,the effectiveness of the SDRC technology using the LADRC algorithm instead of traditional PI control is demonstrated,showing that the LADRC control technology has many advantages,such as fast dynamic response,strong tracking ability,and good robustness.This paper explores how to suppress direct current voltage fluctuations while reducing the capacitance value on the direct current side of the inverter.The principle of the direct current voltage double-frequency fluctuation and the influence of the direct current peak voltage on the capacitance value are analyzed.The specific implementation method of direct current voltage fluctuation suppression in the cascaded H-bridge STATCOM is studied.The active power decoupling(APD)technology,specifically the Buck-type decoupling circuit in the APD technology,is proposed to suppress and absorb the double-frequency fluctuation of the direct current voltage.A simulation experiment platform is built to verify the effectiveness of the APD technology.Based on the simulation platform,a master-slave control experimental platform based on DSP+FPGA is further built.Relevant programs are written,and the effectiveness of the APD technology in suppressing direct current voltage fluctuation is verified through experiments,showing good experimental results. |
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