Research On Several Control Technologies For Modular Multilevel Converter Based HVDC

Modular multilevel converter(MMC) is considered the most promising topology for high voltage direct current(HVDC) application, due to its modularity, scalability, low switching frequency and excellent output voltage waveforms. Thus, MMC-HVDC has become the development trend of voltage source converter based HVDC(VSC-HVDC). The control strategy of MMC-HVDC is its key technology and directly determines its operation performance. The research achievements on control strategy for MMC-HVDC under balanced voltage conditions are rich and mature. However, the research achievements on control strategy for MMC-HVDC under unbalanced voltage conditions are relatively few, which has become a difficult and hot research. Although some scholars have carried out related research and have achieved a certain progress, there are still some problems to be solved. In the normal operation, three-phase grid voltages are almost balanced. In general, three-phase grid voltages are seriously unbalanced when power system fault occurs. Therefore, research on control strategy for MMC-HVDC under unbalanced voltage conditions is of great significance to optimize its control performance and improve its ability of fault ride through. MMC-HVDC plays a more and more important role in the field of supplying power to weak ac system and passive network. If passive network contains asymmetric load and nonlinear load, it is difficult for the existing control strategies to make the inverter station to provide high quality voltage for passive network. Therefore, it is necessary to develop the high performance control strategy for the inverter station. The main work of this paper is to study the control strategy for MMC-HVDC. This paper studies the following aspects in-depth:(1) AC-side control strategy for MMC-HVDC under unbalanced voltage conditionsAC-side generalized power model of MMC-HVDC is established and then deadbeat direct power control strategy is presented in this paper. This control strategy can regulate active power and reactive power directly and omits current inner loop. Moreover, it has a simple control structure and fast dynamic response. In order to make this strategy applied to unbalanced power grid, ac-side power is analyzed in-depth and power compensation strategy is proposed. In addition, the computation formulas of power compensation components for three control objectives are given in this paper.A novel unbalanced control strategy is presented so as to improve operation performance of MMC-HVDC under unbalanced grid voltages. On the basis of the current PI closed-loop control method in the positive synchronous reference frame, the novel control strategy develops an auxiliary controller aiming to suppress negative currents, active power fluctuation or reactive power fluctuation, which avoids the decomposition of the positive and negative sequences of currents and voltages and complicated calculation of the reference currents. Second order generalized integrator(SOGI), reduced order generalized integrator(ROGI) and reduced order vector proportional integrator(ROVPI) are regarded as alternatives for the auxiliary controller and they are compared and researched. Research results show that SOGI-based auxiliary controller will produce third positive sequence harmonic, resulting in the current distortion, and ROVPI has better control precision and phase margin than ROGI. Therefore, ROVPI is selected as the auxiliary controller.The correctness and effectiveness of the above two unbalanced control strategy are validated by simulation results on PSCAD/EMTDC.(2) Circulating current suppression strategy for MMC-HVDC under unbalanced voltage conditionsPhase-unit instantaneous energy expression is derived under unbalanced voltage conditions in this paper. Then, the mechanism of circulating current and the composition of circulating current in each case are analyzed. A novel circulating current suppression strategy in three-phase stationary frame is proposed which is based on PI regulator and improved resonant(IR) regulator in parallel. The circulating current suppression strategy is executed in abc stationary reference frame so that phase-locked loop and rotating coordinate transformation are omitted, leading to a simple control structure. What’s more, positive-, negative-and zero-sequence component of second harmonic circulating current can be eliminated simultaneously and this method can achieve good control performance under balanced and unbalanced voltage conditions. Simulation results show that the proposed circulating current suppression strategy can eliminate the circulating current under balanced and unbalanced voltage conditions.(3) Arm current control strategy for MMC-HVDC under unbalanced voltage conditionsArm current consists of dc current, ac-side current and circulating current. Ac-side current control and circulating current suppression can be achieved simultaneously by means of regulating arm current. Two arm current control strategies are presented in this paper:1) The arm current control strategy for MMC-HVDC based on decoupler and proportional integral(PI) regulator plus vector proportional integral(VPI) regulator,2) Arm current control strategy based on model predictive control with simplified two-step-ahead prediction for MMC-HVDC.For the first method, the coupling relationship between the input and the output are uncovered by calculating and analyzing relative gain array(RGA). The decoupler is proposed in order to simplify the design of the controller. PI-VPI regulators are employed in three stationary frame to control dc current and ac-side current and eliminate second harmonic circulating current. In order to avoid the subjectivity and blindness of VPI controller design, a detail design procedure is proposed in this paper.For the second method, the discrete mathematic model regarding arm current as state variable is established. Based on the discrete mathematic model, model predictive control(MPC) with one-step-ahead prediction, named ls-MPC, is employed to control arm current. To improve the control performance, MPC strategy with two-step-ahead prediction, named 2s-MPCI, is employed. However, the number of calculations increases significantly. To reduce the calculation burden, a two-step-ahead prediction considering the same control action during two control cycles, named 2s-MPCII, is employed. The calculation of 2s-MPCII is still heavy for an MMC with a large number of SMs. Therefore, to further reduce the calculation burden, a simplified MPC strategy, named 2s-MPCIII, is proposed. The detailed implementation procedure and flow charts of the four MPC strategies are described. Furthermore, four MPC strategies are compared in control performance, average switching frequency and calculation burden.The two arm current control strategies can both work well under balanced and unbalanced voltage conditions, as well as asymmetry of the upper and lower arm. Compared to the first method, the second method omits decoupler and modulator, has a simpler control structure and better dynamic performance. Simulation results validate the effectiveness of two arm current control strategies.(4) High performance control strategy for the inverter station of MMC-HVDC connected to passive networkThe traditional control method with outer voltage loop and inner current loop of inverter station supplying for passive networks exists some shortcomings, such as a complicated control structure, four PI parameters needed to be adjusted, a slow response for load disturbance and poor voltage quality when supplying unbalanced or nonlinear load. Therefore, this paper establishes a continuous mathematical model of MMC-HVDC inverter side in stationary reference frame, and then the discrete mathematical model, i.e., predictive equation, is derived, and then model predictive direct voltage control(MPDVC) for inverter station is proposed. To reduce the computational burden of MPDVC, deadbeat model predictive direct voltage control(DBMPDVC) is presented, which combines deadbeat control with DBMPC. The detailed implement procedure of MPDVC and DBMPDVC are presented. Furthermore, the equivalence between MPDVC and DBMPDVC are analyzed qualitatively. For DBMPDVC, PI regulators and current inner loop are omitted, the control structure is simple, and voltage can be controller directly. Simulation results on PSCAD/EMTDC reveal that DBMPDVC has a good steady-state and dynamic performance and can provide high voltage quality power for a passive network under various operation conditions.