Abstract
A virtual synchronous machine (VSM) is a converter which, compared to other types of converters, has more friendly interactions with the power grid because it is able to simulate the external characteristics of a synchronous machine, which can provide virtual inertia and damping. When the grid voltage is unbalanced, there will be negative sequence current and power oscillations. There will also be double-frequency ripples on the DC bus, which affect the normal operation of the DC power source or load. In order to solve these problems, a comprehensive control strategy is proposed in this paper. The principle of a VSM operated as a current source converter, also called VISMA, is used in the design. A complex coefficient filter is applied to separate the positive and negative sequence components of the grid voltage. By analyzing the reasons of power oscillations under unbalanced voltage, the electrical simulation part of the VSM is improved to achieve several objectives: to suppress negative sequence current and DC voltage ripples. Additionally, the rated voltage in the reactive control part is adaptively adjusted to stabilize the system. The validity of the proposed control strategy is verified by simulation and experiment.
Highlights
With the development of the power grid, power electronics is widely applied in the system [1,2].when the proportion of generators or loads interfaced by switched power converters reaches a high level, their impact on the stability and robustness of the power grid cannot be ignored [3].The power electronic converters, as a common interface, do not have damping and inertia
With the difference between virtual synchronous machine (VSM) and traditional converter control strategy, the unbalanced control strategy cannot be directly used in VSMs
A three‐phase converter rectifier based on the VSM control strategy was designed to verify the validity of the theoretical analysis and research. dSPACE‐1103 was used in the control system for the rectifier. dSPACE is a real‐time development and test platform based on MATLAB developed by German dSPACE company
Summary
With the development of the power grid, power electronics is widely applied in the system [1,2]. VSM can be applied in a rectifier and an inverter, which emulates the steady state and transient behavior of the synchronous motor and generator [8] In this way, the total inertia of the grid is increased. With the difference between VSM and traditional converter control strategy, the unbalanced control strategy cannot be directly used in VSMs. There is already relevant research on the three-phase unbalanced control of VSMs. A multiobjective self-synchronized VSM strategy in unbalanced power grid was proposed in [14], with a second-order generalized integrator applied to deal with the double-frequency ripple. In [16], an improved control strategy based on a double decoupled synchronous reference frame was proposed to compensate the negative current sequence. Simulation and experimental results prove the correctness and validity of the proposed control strategy
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