Study On Control And Protection Technology For Cascade Multilevel Converter And Its Application

Cascade Multilevel Converter (CMC) has got increasingly attentions all over the world due to its modular structure and redundant design. However, when the frequency scope of its output current is wider, especially in the case of the low frequency, the fluctutuion of the capacitor voltage will be so large that there is no reference of the choice of capacitor value and control strategy. Moreover, the operating conditions and the fault charateristics of CMC might be complicated under the conditions of AC system fault or CMC system fault. Therefore, the traditional control and protecion strategy of CMC will be difficult to meet the requirements satisfactorily. The main work of this dissertation is summarized as follows:The CMC control strategy with wider frequency scope of its output current is proposed. Under the condition of wider frequency scope, especailly in the case of the low frequency, the large magnitude fluctution of capacitor voltage might result in the problems such as the higher harmonic component, the unbalance of cascade unit voltages with small current and so on. As a result, this paper reveals the rule of he fluction of capacitor voltage. Based on this, the stable and balance control of capacitor voltage and current tracing control strategy of CMC which is suitable for wider frequency scope of output current is proposed. To prove the feasibility of the above method, a High-voltage High-power Low-frequency Current Generator(HHLCG) based on CMC system has been designed and applied to damp the subsynchronous resonance in the power plant. The experiment results demonstrate that the referenced low-frequency current can be traced with neglected error and DC capacitor voltage can be keep stable and balance using the proposed method. This research might has great value of practical engineering application.The control method under the abnormal condition of grid and CMC system is proposed. Aims at the requirement of low-voltage riding through of converter in power plants, a novel Converter Low-voltage riding-through Power (CLP) based on triple parallel interleaved Boost hardware topology and improved control strategy has been proposed and designed, as well as it has been applied to solve the problem of low-voltage riding through of converters in some power plants. Besides, the key factors of tolerant-control method design are emphasized based on the analysis of faults of CMC power units. Furthermore, a novel fault-tolerant control method under the condition of measuring units fault is proposed. This method can reduce the influence of the voltage fault by the three phase interlaced generating and weighted averaging of them. Finally, the simulation and experiment results demonstrate that the proposed tolerant is feasible and effective.Based on the analysis of typical fault characteristics, the protection scheme of multi-terminal direct current flexible system based on CMC is proposed. Firstly, the protection zone scheme and the typical fault characteristics in substations AC bus zone and converter zone are investigated. Secondly, the path of injecting short current under the faults of AC systems which contains the converter AC terminal bus is analyzed. At the same time, the feasible of a new protecion algorithm for turn-to-turn faults of reactor based on THD is researched. Based on the above results and protecion system design principles, the main protection and back-up protection scheme which has been applied in the practical engineering is proposed. Finally, the experiment results demonstrate that the proposed protecion scheme is reasonable.Aiming at the problem that the control and protection systems design of CMC is not universal in different applicaion fields, this dissertation proposes and designs a CMC universal integrated control and protection platform. Based on the analysis of the main function demands and design principles, a novel control scheme of the universal integrated platform which contains the layered framework and three level distributed controller with different control and protecion fuction has been illustrated. In addition, the universal integrated control and protection platform has made great contribution to the realization of above algorithm and solution. As a result, the platform proposed in this paper will play an increasingly imortant role in the research of control and protecion method in both steady state and dynamic state, at the same time, it provides a basic platform for the research and design of new products based on CMC.