Simulation And Research Of HVDC System Based On Modular Multilevel Converter

High voltage direct current based on voltage source converter(VSC-HVDC) transmission technology has becoming one of the most important methods to transmit clean energy and suppling power offshore because of its some special technical features, such as the capability for four quadrant operation, adjustability for active or reactive power, smooth power flowing, no the lowest limiting power, lower low-order harmonic, unnecessary for power filter and SATCOM. Currently, the hot-point for VSC-HVDC flexible transmission technology, which is based on VSC and pulse-width modulation (PWM) technology, is focus on the converter topology, mathematical model, modulation method of system, protective strategy of controlling and so on. This manuscript is aimed one kind of VSC-modular multilevel converter (MMC), and the performance is compared when we use different sub-modular(SM). We analyze the short-coming of these sub-modules, and it is solved by using new mixed-MMC topology and novel MMC controlling strategy.Firstly, we introduce the current development status of the VSC-HVDC flexible and its advantage when it is applied in clean energy transmission. The mathematical model of MMC is analyzed, and the principle is also introduced for these three sub-modules - half sub-modular, full sub-modular, and clamp the twin module. According to the corresponding analyzing results, the MMC based on half sub-modular need less electron device and lower energy wastage. Comparatively, the MMC based on the full sub-modular have the capability of clean DC faults with more electron-devices and higher energy wastage, and the MMC based on clamp the twin module synthesize these two sub-modules. Above all, we take a new mixed MMC topology, and it is consisted by half sub-module and clamp the twin module. Therefore, this mixed MMC employ the capability of clean DC faults with less electron device and lower energy wastage.Secondly,we investigate system controlling of VSC-HVDC Flexible transmission technology.In this manuscript, the carrier phase shifted PWM(CPS-PWM) is introduced, and its function in MMC system and the output result for different pulse shape are also analyzed. Besides that, we also introduce the controlling strategy for system-level controlling system, converter-level controlling system and converter valve-level controlling system, and the controlling strategy of inner and outer loop for converter-level controlling system are described in detail. Above all, a new controlling method for MMC system is proposed, and the model predictive control(MPC) is also developed. Applying this new method, we can define a cost function based on the discrete mathematical model of MMC, and the corresponding best solution of the cost function can be obtained. The switch status of each MMC sub-modular can be get according to the best solution, and the balance of capacitor-voltage between MMC sub-modules can be achieved. Therefore, the circulating current is restrained.Finally, we construct MMC-HVDC system model based on PSCAD/EMTDC software, and the fault analysis for MMC with mixed sub-modules. The analysis results show that the new mixed-MMC is able to pass through block, and the corresponding electron device can be reduced. Meanwhile, we validate the controlling strategy of the new MPC. The results show that the new MPC can balance the capacitor-voltage between MMC sub-modules and restrain the circulating current.