Abstract
For a VSC-HVDC transmission system based on a hybrid topology converter of full-bridge and half-bridge, a kind of virtual synchronous generator (VSG) control strategy which can be applied to modular multilevel converter (MMC) grid-connected structure was researched and proposed. First, based on the conventional VSG control strategy, the energy stored in the equivalent capacitor of MMC power module was used to imitate the rotor inertia of synchronous generator. The characteristics of generator can be simulated during transient frequency fluctuations and it can help relieve the power fluctuations. Secondly, with respect to the structural characteristics of the direct grid connection through the reactor on the AC side of the MMC, which is unlike the microgrid inverter, there are no additional filter capacitors. So, the existing commonly used VSG control strategy of the microgrid inverter and double-closed-loop structure composed of filter capacitor voltage and AC current cannot be directly applied. For this, a method where the given values of inner current loop are calculated by grid impedance matrix was used. So, a double-closed-loop control structure composed of a power outer loop based on VSG control and a current inner loop is obtained. It can effectively improve the current control capability during the transient process. Finally, a hybrid MMC simulation model was built based on PSCAD to verify the correctness and effectiveness of the proposed methods.
Highlights
With the development of microgrid technology and the continuous expansion of new energy grid-connected capacity, the proportion of large-capacity converters represented by VSC-HVDC (Voltage Source Converter Based on High Voltage Direct Current) in the power system is increasing fast
VSC-HVDC system based on virtual synchronous generator (VSG) control can emulate synchronous generator characteristics
Capacitor voltage average of arms and active power can be controlled by adjusting the angular frequency of voltage vector of converter side. e grid synchronization angle is generated by the VSG control loop
Summary
With the development of microgrid technology and the continuous expansion of new energy grid-connected capacity, the proportion of large-capacity converters represented by VSC-HVDC (Voltage Source Converter Based on High Voltage Direct Current) in the power system is increasing fast. In [8], the VSG control structure is designed for the module multilevel converter (MMC) based on half-bridge power unit and the stability range of the control parameters is analyzed for accessing weak AC system. The grid frequency feedback value is dependent on the phase-locked loop (PLL) in the control structure, where it is difficult to achieve smooth switching between islanding and grid-connected operation modes, and it is useful for VSC-HVDC converters. In order to achieve fast DC fault clearing and suppress DC short-circuit current rise, the hybrid topology usually sets a DC loop to generate the DC component of the upper and lower bridge modulation signals. Vj (j a, b, c) is an AC control signal of three phases. en it is added to the output signal Vdc_com of the DC regulator, and the outputs Vju, Vjn (j a, b, c) are the upper and lower arm control signals, respectively. e three-phase grid voltage Usj is connected with the PLL to obtain the grid synchronization angular velocity ω, which provides synchronous phase for the coordinate transformation (dq synchronous rotation frame and abc stationary frame)
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