| In the field of'smart grid', in intelligent electric transmission and distribution, custom power technologies, STATCOM (STATic synchronous COMpensator) is the advanced and complexed device in dynamic var compesation. STATCOM has nice performance in response speed, stabilized power voltage, reduced grid loss, increased transmitting capacity, limite of transient voltage improvement and reduced harmonic.STATCOM is the basic technology of the FACTS (Flexible AC Transmission System) by power electronics conversion, therefore based on the multiple pulse and multilevel converter's topology reseach, STATCOM of multilevel inverter for high power medium voltage applications is particularly discussed. The order of the disquisition is the cascade multilevel inverter, multiple pulse converter, neutral point clamped(NPC) converter, hybrid multilevel converter, reactive current detection and auxiliary protection system. The follo wings are the main work:High power cascade STATCOM is a typical nonlinear system, its control theory algorithm is complex and abstract. According to the dynamic math model of a active and reactive current for cascade STATCOM, a nonlinear dynamic control strategy for reactive power is obtained. Using this control method of two-input two-output linearization, the output variables decoupling control is achieved. Under the condition of occurring short circuit fault and high power load changing abruptly, the nonlinearity and two-output decoupling controller has nice dynamic performance.There are several types of selective harmonic elimination PWM (SHPWM) scheme. Based on the analysis of these SHPWM schemes, we proposed an improved modulation scheme that is carrier overlapping switch frequency optional-pulse width modulation (CO-SFO-PWM), this modulation scheme may be suitable for cascaded multilevel inverter of three-phase systems. The scheme combines the advantages of carrier overlapping and switch frequency optional, which enables it to increase the fundamental amplitude of the output line voltage and improve the utilization ratio of DC voltage. Besides, this scheme allows the output line voltage to have better harmonic characteristics, and in particular, significantly improve the distortion ratio of output line voltage at low modulation.The main feature of the multipulse rectifier lies in its ability to reduce the line current harmonic distortion. This is achieved by the phase shifting transformer, through which some of the low-order harmonic currents generated by the six-pulse rectifiers are canceled. In general, the higher the number of rectifier pulses, the lower the line current distortion is. A dual 18-pulse rectifier topology is proposed. The topology achieves harmonic performance of 36-pulse rectifiers by using relatively simple 18-pulse transformers. It can also neutralize the 5th and 7th harmonics caused by unbalanced secondary windings. In the case of non-ideal phase shifting transformers (with phase shifting errors or unbalanced leakage inductances), we can find that the effect of harmonic cancellation is still obvious under the condition of minimizing the errors.Neutral point clamped converter which dc input voltage of the inverter is normally split by two cascaded dc capacitors, providing a floating neutral point. With a finite value of the capacitor, it can be charged or discharged by neutral carrent, causing neutral-point voltage deviation. There are three neutral points in NPC/H-bridge inverter, whose potential may need a tight control to avoid any voltage deviation. However, for the electronic power system using a multipulse rectifier as a front end, the rectifier can be designed such that its midpoints can be directly connected to the neutral points of the inverter. In doing so, the inverter neutral voltages are fixed by the rectifier and thus do not vary with the inverter operating conditions.The factors which influence the total harmonic distortion(THD) of phase voltage and bring about the deviation of DC line voltages of five-level NPC inverter fed by dual 18-pulse rectifier under in-phase disposition modulation are also analyzed. Meanwhile, the relationship between the THD of phase voltage of five-level NPC/H bridge inverter and the deviation of DC line voltages and load condition, output frequency and the value of DC line capacitors is discussed.Hybrid multilevel converter is cascading multiple complex modules with different H bridge, various voltage levels and different DC supply sources. Considering the drawbacks of current flow backwards in traditional modulation method for composite cascade multilevel converter, an improved topology and a novel control strategy is proposed. The main converter operates at the fundamental frequency based on command voltage, the auxiliary converter adopts carrier phase shifted PWM technology(CPS-PWM) at a higher frequency to enhance the quality of output voltage waveforms. The amplitude of command voltage is greater than output voltage amplitude of the main converter at any time, to ensure that output voltage variation of both converters keeps in the same direction.The low field intensity high energy ZnO Varistor is the best one of nolinear absorption element for overvoltage protection, its I-V Characteristics, equivalent circuit as well as overvoltage protection are discussed when it is used in power electronics devices. The capability of restricting voltage, the energy capacity under different conditions, the requirement of bearing pulse-load, the application topologys in the medium voltage high power converter and the parameter selection are described in detail.A simple tracking algorithm of single phase reactive current detection scheme is proposed. Within just one caculation period, the product of the load current and sinusoidal waveform voltage value which were be the same of frequency and amplitude with the supply voltage but lagging 1/4 cycle is caculated by the phase locked loop circuit. The product were stored into a storage queue of sampling datas, the reactive current was fast trackled by comparing the sampled datas which in front and in post of the fundamental cycle. The effect of tracking the current variation and the anti-interference capability is obvious. The algorithm is simple and fast, only needs 2 multiplication machines in all.The cooling modus of medium voltage high power STATCOM is circulating cooling by highly purified water. The isoelectric connecting scheme is proposed. Each cooling branch that between the two terminal whose instantaneous electric potential difference were be zero or approximent zero, so the case of cooling terminal ionization erosion, water leakage even insulation leakage were be decreased greatly. By designed the heat emission balance for every one cooling branch, the electric-construction of STATCOM is modified.
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