| Static Synchronous Compensator(STATCOM), a new power quality control device, adopts full-controlled power electronics components as the core and plays a significant role in the improvement of grid power factor and power quality. Compared with the conventional reactive power compensation device, it has better response speed, dynamic property, compensation precision, as well as advantages of filtering harmonics and compensating the unbalance. By connecting H-bridge power cells in series, the STATCOM with H-bridge cascaded structure can upgrade the working voltage of the device and increase the compensation capacity, making this kind of STATCOM device the future direction in high voltage and high power field. This paper mainly conducted in-depth study of reference current detection, current tracking control and DC capacitor voltage balancing control of H-bridge cascaded STATCOM as well as other key technologies. Detailed research contents are as follows:The premise for H-bridge cascaded STATCOM to achieve accurate compensation to the reactive power of the grid is to get the accurate reference directive current. Based on conventional instantaneous reactive power theory, the ip-iq method has wide practical engineering applications due to its simplicity in implementation and accuracy in testing. However, the phase locked loop(PLL), abc/dq transformation of coordinates, low pass filter(LPF) and other links will have impact on the detection speed. In view of this problem, this paper proposed a novel detection method for fundamental positive sequence active current based on fan-in vector transformation. By means of fan-in vector transformation the voltage and current in three-phase system are normalized into the voltage and current in single-phase system which simplifies the detection process of fundamental positive sequence active current in three-phase system. And the transformed voltage and fundamental current contain only positive-sequence component. Through the method based on constructing cascaded delayed signal cancellation theory, the fundamental positive sequence voltage and current are extracted rapi dly; then, positive sequence active current can be obtained by simple active calculation. Compared with ip-iq method, the proposed method greatly reduced calculated quantity of the system, which boosted the detecting speed.Controlling the output current of H-bridge cascaded STATCOM to rapidly and accurately track the variation of the reactive current of the system is the key to realize the effective compensation to the system. But the system itself is a nonlinear, multivariable, strong coupling system,and the traditional linear control method has been unable to accommodate new industrial needs. Concerning this issue, this paper proposed the nonlinear control method of H-bridge cascaded STATCOM based on passivity theory which can improve the dynamic compensation performance and the stability of the device. After analyzing the Mathematical model of H-bridge cascaded STATCOM, an Euler-Lagrange(EL) system model is built to guarantees the dynamic stability of Lyapunov function. By analyzing the principle of the dc capacitor voltage balance, the method which reduces the expected stable equilibrium points of the system is proposed and, hence, the computational efficiency of the system is improved. To improve the convergence speed of the system, the damping which can accelerate the system energy dissipation is injected into the system and the amount of the injected damping is thoroughly analyzed. Finally, a discrete-time state observer is designed to predict the system reference directive current, which not only improves the real-time performance of control algorithm and dynamic performance of systems, but also reduces the harmonic content in output current of the device. At the same time, a discrete-time sliding mode observer is built to observe the output voltage, and as a result, the error derived from DC capacitor voltage fluctuation during formula computing.To secure the voltage balance of the independent dc suspended capacitor of H-bridge cascaded STATCOM is key to maintaining the stability of the device. Regarding to this problem, in this paper, on the basic of 3 levels balance control strategy for dc capacitor voltage, the new overall voltage control, clustered balancing control and individual balancing control are proposed. Also, the proportional resonant(PR) controller is proposed for overall voltage control. Compared with the PI controller, it has better performance in terms of response time and transient process. Moreover, the active/disturbances rejection controller(ADRC) which has been widely used in other control fields is first proposed for clustered balancing control. And it can realize the dynamic compensation for outside disturbance. Compared with the traditional PI controller, ADRC is insensitive to parameters, making the parameter selection much easier. Finally,the method is developed by shifting the modulation wave vertically for individual balancing control. By judging the voltage and the charge or discharge state of the capacitor in one cell, the duty cycle of the drive pulse is adjusted in real time to change the charge or discharge time of the capacitor, and hence, the individual balancing control can be realized. This method is easy to implement through FPGA. Using this method and carrier phase shifting PWM coop eratively, a good individual balance can be achieved.On the basis of the system implementation technology of the 10 k V ±2Mvar H-bridge cascaded STATCOM, this paper details the design philosophy of the main circuit parameter, the design procedure of hardwar e circuit in power cells, control system structure of the device and the way to realize the control. Finally, experiments of passivity-based control and dc capacitor voltage balancing control are conducted on device respectively, and then, the validity of hardware as well as software design approach and control methods are verified. |
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