Search

In this paper, the control strategy of modular multilevel matrix converter (M3C) is studied in the scenario of fractional frequency offshore wind power. The M3C system is compared with the traditional rotating synchronous generator system in terms of structural parameters and motion equations, and the feasibility of the M3C inverter to simulate the external characteristics of the synchronous generator is analyzed. With the goal of converter participating in system frequency regulation and stabilizing system voltage, based on the control theory of virtual synchronous generator (VSG), the control scheme design of virtual synchronous generator of M3C converter is completed, and a control strategy suitable for fractional frequency offshore wind power M3C converter is established. Finally, using PSCAD/EMTDC simulation platform, the correctness of this control strategy is verified. With the same system structure and parameters, the control strategy is compared with the decoupling control strategy of double dq coordinate transformation. The results show that the VSG control strategy can make the frequency converter have inertia and damping characteristics. The VSG control can participate in the system frequency regulation, and has good control response characteristics.

Modular multilevel matrix converter (M3C) is a new topology of AC-AC power converter, which has very broad application in low frequency transmission system, high-power asynchronous motor speed regulation and low offshore wind power transmission. Due to the power coupling effect of the two frequencies, the capacitive voltage of M3C bridge arms is prone to instability when the grid voltage is asymmetric. Firstly, the arm power of M3C under unbalanced input conditions is calculated, and the power distribution law among the arms of two different power balance method is summarized. On this basis, the influence of low-frequency circulating currents on the arm power is analyzed. Under the premise of ensuring the balance of input and output power of the system, a capacitor voltage balance control strategy based on the construction of circulating currents is proposed, which avoids the introduction of negative sequence current on the grid side. Under unbalanced conditions, the capacitor voltage can be quickly balanced by the capacitor voltage closed-loop control and direct power compensation strategy. The constructed low-frequency circulating currents only flow inside the converter, and will not affect the decoupling operation of M3C input and output sides. The effectiveness of the proposed control strategy is verified by a 220 kV, 400 MW M3C system implemented in MATLAB.

Low-frequency transmission system (LFTS) based on modular multilevel matrix converter (M3C) has great application potential in offshore wind power transmission,urban power grid and long-distance power transmission. In order to study the startup process and the control strategies of LFTS and connect the converter station to the low-frequency transmission line without voltage or current impulse of M3C,a startup method of undisturbed grid connection is proposed. Firstly,the structure and operating principle of the LFTS are analyzed. Then,the selection principle of sub modules and the charging process of M3C are analyzed with two-terminal LFTS as the target. Taking the advantage that the blocked M3C does not exchange power with the low-frequency power grid when the capacitor is at rated voltage,the problems of over-voltage and over-current that tend to occur during LFTS startup can be solved by flexible control of M3C and reasonable design of startup strategy,without additional control devices and detecting the synchronization of the breaker. Finally,a real time digital simulation system (RTDS) is built to verify the proposed startup method. The simulation results show that converter stations connect to the low-frequency transmission line without voltage or current impulse. Also,the proposed strategy is correct and has a great prospect of the engineering application.