Research On Control Strategy And Application Of Cascaded STATCOM With LCL Filter

At present, with the wide application of non-linear, frequent fluctuation and unbalanced equipment in such areas as electronic electrical and industrial devices, the problems of three-phase asymmetric load, lower power factor, greater energy loss of power grid, harmonic pollution and voltage fluctuations are the importance factors influencing electrical power quality. Consequently, harmonic suppression and reactive power compensation are research focus in recent years. Compared with conventional reactive power compensation devices, the Static Synchronous Compensator(STATCOM a.k.a. SVG) has its unique excellent characteristics of regulating dynamic reactive power, compensating harmonic and asymmetric currents. Large numbers of STATCOM are used to solve voltage dynamic stability problems. So, the necessity of carrying out the key technologies of cascaded STATCOM with LCL filter becomes apparent in this paper.The neutral of three-phase Y configuration is generally indirectly grounded in 6k V and 10 k V system. Meanwhile, neutral ungrounded method is adopted in STATCOM. So, research of real-time voltage between the above neutrals is necessary for a more accurate model analysis of STATCOM. The paper proposes a new modeling method bases on the analysis of neutral voltage and switch function of HBridge for the first time. Because the current control strategy of STATCOM in AC side in d–q coordinates is a high order, nonlinear and strong coupling dynamic system, the high-precision linear state feedback method is proposed in the paper.In the paper, LCL filter is used as output filter to obtain high slew rate of low frequency compensation current bandwidth and sufficient attenuation for high frequency currents. Because of the resonance phenomenon in the application of LCL filter, its parameters influence the performance in such areas as resonant frequency, harmonic attenuation ratio and resistance loss characteristics. Meanwhile, with the detailed analysis of resistance loss characteristics caused by harmonics and attenuation of switching ripple, the heating problem of damping resistor is solved. And on this basis the compound active control strategy is proposed. Moreover, in order to improve stability performance and widen the bandwidth of harmonics control for LCL filters with passive damping control method, a new control loop composed of Proportion-Integral(PI), LCL filter, 2nd order low pass filter, and correction element is used to realize the stability of current control-loop and tracking accuracy.As to the problem of the great amplitude and phase detection error for harmonics and steady state error of close-loop control system in the case of non-ideal grid frequency and rapid time-varying harmonic currents, a new Phase-Locked Loop(PLL) method is integrated with detection approach of harmonics and frequency adaptive control strategy based on the d–q synchronous reference frame and Discrete Fourier Transform(DFT). By contrast, using the new detection method, the system harmonics detection speed is accelerated and its detection accuracy is also improved significantly. Furthermore, to achieve zero steady-state tracking errors with high compensation precision for rapid time-varying harmonic currents, two kinds of improved controller are presented in paper. On the one hand, the general Proportional-Resonant control(PR) has narrower stability compensation bandwidth and control system may be left in an unusable state, on account of open-loop unstable poles. Aiming at that, the new composite controller(Proportional-Integral Resonant, PIR) that improves stability compensation bandwidth and the polar distribution based on ensuring the stability of current control-loop and tracking accuracy of harmonics by using PI control part, stability control correction element and stability bandwidth correction element is presented. On the other hand, to improve the compensation precision and fundamental period time delay response characteristic of the repetitive control, the new control system is made of double closed loops: the inner correction loop and the outer improved repetitive control loop, and satisfies high compensation precision during the load fluctuations. Corresponding, PI control part, stability bandwidth correction element and 2nd order low pass filter are adopted in the inner current control loop which can track the reference current quickly. Tracking error of the inner control loop is further reduced with the outer improved repetitive control loop. Moreover, the new PLL method proposed in the paper is used to ensure frequency adaptive performance of the compound controllers.Aiming at the DC bus voltage balance problem which exists in phases of three-phase Y cascaded structure, active power injection model analyses of the three phases are compared by zero-sequence voltage and negative-sequence voltage injection. Through above analysis, a zero sequence voltage injection method based on the d–q synchronous reference frame and DFT is proposed in the paper. A new dynamic hierarchical control strategy for capacitor voltage, which is integrated the zero sequence voltage injection method, is adopted to guarantee the voltage balance of the DC bus voltage. The PI correction control based on 2nd harmonic compensation is adopted, guaranteeing the total DC bus voltage balance. Separate voltage balance control method with SPWM and switching control strategy of each H-Bridge with carrier disposition PWM are used to realize the voltage balance control of each H-Bridge. In addition, loss averaging control strategy based on pulse space ratio modulation is proposed to balance the action times of each IGBT switch device.Furthermore, based on the key research technologies of the ±6Mvar medium-voltage cascaded STATCOM, both simulation models and experiments devices are employed to verify the feasibility and effectiveness of the proposed control strategy. In this paper, the corresponding hardware frame and soft-ware system of the system are illuminated. Finally, several relevant experiments of harmonic compensation based on the research platform are presented.