On C-MMC Based HVDC Technology For Bulk Power Overhead Line Transmission

In contrast to the well-known two-level or three-level converters, modular multilevel converter (MMC) offers many features, such as excellent output voltage quality, high modularity, less losses, and low switching frequency, etc. Hence, the MMC has been identified as the most promising converter for high-voltage direct current (HVDC) flexible system. The original MMC employs a great number of series-connected half-bridge submodules (HBSM) in each arm; however, it does not have the ability to clear off the fault current arising from the dc-side short circuit just by the converter action. Therefore, it is very hard for the MMC with HBSMs extending to overhead line power transmission. A new breed of MMC adopting clamp double sub-modules (CDSM), namely, C-MMC, was highlighted most recently, which has dc fault current blocking capability. Due to this fault-suppressing feature, the C-MMC is extremely suitable for overhead line power transmission; consequently, applications of the HVDC flexible system can be greatly extended. Unfortunately, few literatures are reported on C-MMC nowadays; hence, it is meaningful to study it thoroughly and systematically. This thesis focuses on operation characteristic and bulk power overhead line transmission application of C-MMC. The main works are organized as follows.(1) Impedance-frequency characteristics and dc fault blocking mechanism of the C-MMC are studied. Continuous mathematical model and state space equations of the C-MMC under steady operation are derived, which proves that the C-MMC has asymptotic stability. Equivalent formulas of the ac/dc side impedance are deduced. An impedance calculation approach adopting Test Signal Method is proposed, and its detailed implementation process is presented. The results shows:a) the ac port displays variable impedance due to non-linear modulation coupling, and the dc side can be equivalent to a single-tuned filter; b) changing of operating points and control modes do little effects on the ac/dc side impedance; c) the dc side impedance can be affected by the circulating current. By establishing equivalent circuit during dc fault, the fault current blocking mechanism are investigated. To achieve better operating benefits and transient characteristic, a modified C-MMC topology is proposed, including:a) each arm adopts both HBSMs and CDSMs; b) damping resistors are embedded in series with the clamp diodes.(2) A three-level SM fault redundancy protection and control strategy is designed. The SM is divided into three protection zones, and each zone is protected by a dual-thyristor scheme. A SM fault-tolerate procedure is proposed, to cope with element failures at different zones. In converter-level, an improved nearest level control with energy balance strategy is proposed. In this method, the capacitor voltage references are dynamically adjusted; in addition, by introducing fault factor, the capacitor voltage measured values are renewed to avoid switching events of faulty elements. Concepts of dynamic spare elements and real-time safety margin are proposed. Several approaches are applied to expand the available number of the dynamic spare elements, including transformer tap changer control and power adjusting strategy, etc. To suppress the dc ripple current after removing the faulty SMs, a voltage compensation control strategy is proposed. Its core idea is to introduce an additional offset component in the arm voltage reference, therefore, the ac current can distribute symmetrically between the up arm and the down arm of the faulty phase.(3) Start-up and fault-restart control strategies of the C-MMC-HVDC system are designed. The self-start pre-charge procedure is analyzed, which can be divided into two basic stages, i.e., static charging stage and dynamic charging stage. According to the theorem of electrical circuit, the equivalent circuit at static charging stage is simplified; in addition, estimation formula of the maximum charging current is derived and selection principles of the current-limit resistor are presented. A simple grouping sequentially controlled charge method is proposed. This method makes the capacitors be charged group by group so that each capacitor can share sufficient stored energy. Furthermore, auto-restart control sequences under temporary and permanent dc faults are designed.(4) Capacity extending technology using series/parallel connected C-MMCs and switching off/on strategies of basic converter units (BCU) are studied. To achieve high voltage and power level, a bipolar HVDC flexible topology with combined C-MMC is presented. In this topology, each pole consists of BCUs in series or in parallel; between the positive pole and the negative pole is the neutral bus, which is connected to the ground electrode. BCUs’switching off/on procedures are classified as series-type and parallel-type, and each type’s switching control sequences are designed. A novel control strategy with bypassing valve group is proposed, to switch off/on series connected BCUs quickly.(5) Steady-state operation and low dc voltage ride-through control strategies of the improved LCC-C-MMC HVDC system are studied. The inverter side of this hybrid system is made up of the combined C-MMCs, to match the bulk power requirement of the LCC. To solve the low dc voltage ride-through problem, which is caused by the ac grid fault on the rectifier side, two improved control strategies with bypassing valve group are proposed, namely, backup constant current control and backup constant voltage control. These two approaches take the inverter side and rectifier side as the dominant station, respectively. The third harmonic injection modulation and a reative power dynamic adjusting strategy are used to expand the steady operating area and transient operating ability. When the fault is very serious, by appling the bypassing valve strategy, the BCU at high voltage terminal is switched off to make the system operate under the half-voltage mode, then, the healthy BCU is controlled to transmit power continuously.(6) A general loss calculation method is proposed, which is suitable for different cell concepts, i.e., HBSM, CDSM and full-bridge submodule (FBSM). The procedure of this approach is as follows:in step-one, the time-domain waveform of each element is derived, according to the operating parameters and the modulation control strategy; in step-two, the characteristic of semiconductor device is acquired based on the data provided by manufactures; in step-three, the losses distribution are calculated based on the current, the voltage and the switching instants of each device. This method can quickly calculate the power loss distribution of the MMC in various operation conditions, which also can take the additional capacitor voltage optimization control into consideration and is easy programmatic implementation.