| For the randomness of wind power, the access of wind farm to the grid will impact the regional voltage stability; at the same time, the stability of power system would affect the access of wind farm, which will further deteriorate the grid voltage stability. The voltage stability is particularly important in the grid with large wind power penetration; static synchronous compensator (STATCOM) can improve the transient voltage stability of power system. In this paper, it takes the±1.8Mvar cascade multilevel STATCOM as the background, from which the the modeling is analyzed and the parameters are designed for the main circuit of delta-connected cascade multilevel STATCOM, furthermore, the key technologies of DC capacitor voltage balance control, system voltage unbalance control, grid current waveform control and phase-locked loop are all detailedly studied.Main circuit analysis is the research foundation of cascade multilevel STATCOM. A mathematical model of cascade multilevel STATCOM is established that the delta-connected cascade multilevel STATCOM can be equivalent into three single-phase chains, and its steady-state expressions are deduced. On this basis, the parameters of main circuit for±1.8Mvar cascade multilevel STATCOM have been designed, while the effections of capacitor, inductor and switching frequency on device performance have been discussed and analyzed.The methods of phase-locked-loop (PLL) for cascade multilevel STATCOM are analyzed. A comparison is given among three types of commonly used phase-locked loop method, whose dynamic performance and steady-state precision are analyzed based on the effection to output characteristics of STATCOM, and several ways against of system phase mutation for cascade multilevel STATCOM are shown.DC capacitor voltage balance control is one of the key problems for cascade multilevel STATCOM. Considering that the imbalance of DC capacitor voltage is caused by the inconsistency of active power absorbed and consumed by chain, a balance control strategy based on active voltage vector superposition is proposed, in which an active voltage component is superposed to chain's output voltage to change its absorbed active power. A general analytical method based on vector analysis is also presented, by which the performance of balance control strategy can be analyzed, including stability and regulation capacity. To find out the most appropriate balance control strategy, a comparison still based on vector analysis among the proposed and other two commonly used methods is provided, from which it can be known that the proposed balance control strategy has the advantage of good stability and strong regulation capacity, and simulations are performed to prove it.System voltage imbalance control is another key issue of cascade multilevel STATCOM. The control method has been analyzed for cascade multilevel STATCOM under unbalanced system voltage in the paper, from which it can be known that STATCOM needs to output positive sequence, negative sequence and zero sequence components at the same time to output any controlled reactive current, based on which the individual pahse instantaneous current control strategy has been proposed, and two modified compensation modes are studied:reactive control mode and voltage control mode. The cascade multilevel STATCOM can maintain normal operation under unbalanced voltage by using the individual phase instantaneous current control strategy and the unbalanced system can be effectively compensated by the two modified compensation modes. Simulation results show that the operation capability of cascade multilevel STATCOM under unbalanced voltage can be effectively enhanced by the proposed control strategy and it can compensate system reactive power in accordance with the designed compensation mode.The problem of low-order harmonic current for delta-connected cascade multilevel STATCOM has been studied. Traditional PI controller can not track sinusoidal command current without steady-state error, and has insufficient harmonic suppression ability for its limited bandwidth. A novel low-order harmonic suppression method based on hybrid controller is proposed, which is composed by proportional controller and repetitive controller. The hybrid controller has been detailedly designed; compared to the proportional controller, it can track sinusoidal command current without steady-state error and has good harmonic suppression ability for its sufficient bandwidth. A comparison of simulation is given between hybrid controller and proportional controller, from which the effectiveness and feasibility of proposed method has been proved.A prototype of cascade multilevel STATCOM has been built. The proposed control strategies including DC capacitor voltage balance control, individual phase instantaneous current control and system voltage imbalance control are all implemented in the prototype, and many experimental researches have been made, from which the effectiveness and feasibility of proposed methods have been proved. In addition, all control strategies have been verified by the industrial products of Yanshan petrochemical 6kV/±1.8Mvar and Jilin wind farm 6kV/±8Mvar cascade multilevel STATCOM.
|
PDF Full Text Request