| Flexible low-frequency AC transmission adopts voltage source-type AC-AC commutator,which not only has flexible control capabilities such as power flow regulation,reactive power compensation,voltage dynamic support,but also can use the existing AC circuit breaker and relay protection technology,easy to be networked and easy to realize multi-terminal interconnection.Modular multilevel matrix converter(M3C)is a typical converter used in flexible low-frequency AC transmission.It can achieve arbitrary amplitude,phase and frequency output and is suitable for high-voltage and large-capacity situations.It has a good application prospect.At present,existing literatures have studied the main circuit parameters of M3C,but there are still problems such as large capacitance and inductance,and large number of bridge arm submodules,which lead to large area and high cost of converter station.Starting with two valve control freedom,circulating current and common-mode voltage,this paper explores appropriate control strategies to optimize the main circuit parameters of M3C,realize the lightweight design of M3C,and improve the economy of flexible low-frequency AC transmission.Based on the generation mechanism of circulating current,the amplitude expression of circulating current for M3C is derived in this paper,which can assist the parameters design of the inductance in bridge arms for M3C.Then,a control strategy for circulating current based on the network topology of M3C is proposed.Circulating current of 9 bridge arms can be controlled only by stacking the control voltage components of circulating current in a set of large linearly independent branches of circulating current(4 bridge arms)in M3C,which can be combined with the voltage equalization control strategy of bridge arms.The proposed analysis and control method of circulating current can be extended to various modular multilevel cascaded converters.In this paper,the injection control of common-mode voltage is studied,and the analysis shows that the maximum modulation ratio of M3C can be increased to 1.1547 and the number of arm submodules can be reduced by 13.4%when the common-mode voltage is injected into the modulation voltage of bridge arms.Then,the injection influence analysis of common-mode voltage is carried out.The results show that the injection of common-mode voltage has little effect on the capacitor voltage fluctuation and the bridge arm current amplitude,that is,it has little effect on the selection of capacitor and inductance.After common-mode voltage is injected,the main frequency component of the circulating current remains unchanged,that is,the control strategy of circulating current proposed in this paper is still effective.At the same time,in order to prevent the common-mode voltage from affecting the AC power grid and transformer on both sides,the transformers on both sides of M3C should adopt angle connection in the valve side,star connection and neutral ground connection in the network side.The main circuit parameters of M3C are optimized and verified by simulation in this paper.The results show that the inductance design value of bridge arms can be reduced by 60%and the capacitance design value of modules can be reduced by 16.5%when considering the proposed control strategy of circulating current in the design of main circuit parameters for M3C.Considering the phase lock angle error and the control voltage component of circulating current which needs about 2%margin,the injection of common-mode voltage can reduce the number of arm submodules by 10.3%in actual.Finally,a simulation model with 220kV and 330MVA flexible low-frequency transmission engineering is built on the PSCAD simulation platform to verify the effectiveness of the proposed control strategy and the correctness of the optimization of main circuit parameters. |
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