| The modular multilevel converter (MMC) is well scalable to high-voltage levels of power transmission based on cascaded connection of multiple sub-modules (SMs) per arm, which also means a high number of output voltage levels. Due to the high-quality output voltage, the low switching losses as well as the use of standard power electronic devices, the MMC topology is considered suitable for high-voltage direct current (HVDC) transmission applications. The MMC based HVDC transmission system has bright prospects in renewable power integration, grid interconnection, power supply to offshore loads, city centre infeed and etc.. The control strategy, which is one of the key technologies in MMC-HVDC transmission system, is studied in this dissertation. The main contents are listed as follows:(1). The nearest level modulation (NLM) strategy is introduced to the firing control of the MMC with a high number of voltage levels. The Fourier series representation of the output voltage waveform is derived, by which the fundamental-wave and harmonic components are calculated.(2). The capacitor voltage balancing problem of the MMC is investigated. The requirement to reduce the switching frequency of power electronic device is not satisfied with the straightforward capacitor voltage balancing strategy, which results in high switching losses. To solve this problem, an improved capacitor voltage balancing strategy for the MMC is proposed. The improved capacitor voltage balancing strategy focuses on the sub-modules whose capacitor voltage exceeds the limits, while the switching states of the other sub-modules are maintained to some degrees by employing the maintaining factor. Therefore, the switching frequency and the switching loss of power electronic device are reduced.(3). The sub-module faults in MMC are analysed, which will cause oscillations in DC voltage and DC current, as well as the outage of the converter. Different redundancy control and protection strategies to deal with the sub-module faults in cascaded H-bridge converter are compared, and then a redundancy control and protection strategy for the sub-module faults in MMC is proposed. In this strategy, one or several redundant sub-modules are arranged in the hot stand-by state, which can be put into operation very quickly, while the others are arranged in the cold stand-by blocking state to protect the IGBTs in them.(4). According to the generalized MMC model, the dynamics of the positive-, negative-, and zero-sequence components are derived. Then, a dual current control scheme with positive-and negative-sequence current controllers is applied to MMC. The power controller to eliminate the negative-sequence current components and the other one to eliminate the DC voltage ripples are compared. A zero-sequence current controller is also proposed in addition to the positive-and negative-sequence current controllers. Moreover, the dynamic power limiting control is proposed to regulate the power limitations and protect the power semiconductors under different grid fault conditions. The distribution of DC current to three phase units is equal only in balanced grid conditions, but is not equal in unbalanced grid conditions.(5). According to the MMC mathematical model, the direct voltage control and the direct current control are proposed for the control of the MMC-HVDC system connected to small passive networks.In the direct voltage control, the amplitude of the AC-bus voltage is regulated by feed-through of the command reference along with a PI feedback compensator. Nominal system frequency of the inverter AC-side is ensured by setting the angular frequency of the modulation signal at the nominal value.In the direct current control, a double closed-loop controller containing an inner fast current loop and an outer voltage loop is proposed for the control of the inverter station. The angular frequency of synchronous phase angle provided for the dq transformation of the inverter controllers is set at the nominal value, and thus a constant system frequency of the inverter AC side is ensured. |
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