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Research On Key Techniques Of Medium Voltage Cascade Multilevel STATCOM

Posted on:2013-02-05Degree:DoctorType:Dissertation
Country:ChinaCandidate:C X MiaoFull Text:PDF
GTID:1112330362466276Subject:Power electronics and electric drive
Abstract/Summary:PDF Full Text Request
With the development of computer technology and power conversion technology,a considerable amount of high-voltage, medium-voltage and high-power nonlinearequipment has been widely used. Although the application of these devices bringsconvenience to people, it causes many problems on power quality. Among which,problems such as over low power factors, severe harmonic pollution, voltagefluctuation and flicker are comparatively more evident. Medium voltage StaticSynchronous Compensator (STATCOM) becomes a hot topic for the research onmodern reactive compensation technology as it can not only solve aforementionedproblems but also improve the power quality. As regards numerous topologicalstructures of STATCOM, there are no multiple transformers in cascade multi-leveltopologies. Besides, small low-voltage capacitance meter is applied to realizehigh-voltage and high-capacity. Thus, STATCOM has wide application prospectsunder the conditions of medium and high voltage. This paper deeply andsystematically studies key technologies under the background of the development ofmedium-voltage H-bridge cascade multi-level STATCOM (±2.8MVar).This paper puts forward a level synthesis PWM (LSPWM) that can be applied to2H-bridge cascade STATCOM. On the basis of volt-second balance of vertical levels,this method applies the association between adjacent level and expected level to getthe actuation duration of levels. This method is more effective with short operationtime and high control efficiency. As the utilization ratio of DC voltage is too lowwhen LSPWM method is adopted, level synthesis-switching frequency optimizedPWM (LS-SFOPWM) is put forward by injecting zero sequence components intomodulation wave. This method improves the utilization ratio of DC voltage (anincrease of1.15times) based on having similar harmonic characteristics to LSPWM.The circuit structure of medium-voltage cascade multi-level STATCOM based onLCL filter is brought forward. This structure can inhibit high-frequency switchingharmonics and further improve the performance of STATCOM. By switching functionmodeling, the STATCOM based on the structure of LCL filter is modeled and themain circuit parameters of cascade STATCOM (±2.8MVar) are designed. This modelis used to analyze the correlation among LCL filter parameters, based on which anapproach to design filter that can effectively inhibit the high-frequency components ofcurrent and restrict the total filter inductance from dropping is brought forth. Aftersimulation, the results show that the high-frequency filtering effect of LCL filter is better than that of L filter. Under the condition when there is no great change in theoverall costs of LCL structure, more excellent harmonic properties can be obtained.A method based on regulating synchronous rotary angles for detectingfundamental positive sequence voltage is proposed. By this method, positive andnegative sequence voltages can be precisely detected when the three-phase voltagesare unbalanced. Furthermore, p-q detection method for improving the detector offundamental positive sequence voltage and ip-iqdetection method for adjustingsynchronous rotary angles are proposed by introducing the aforementioned methodinto them. In this case, the reactive current can be accurately detected when thethree-phase grid voltages are unbalanced or harmonic. Through emulation verification,it is proved that this method has high detection accuracy and good dynamicperformances. Meanwhile, it can satisfy the requirements for detecting compensatedreactive current.An approach to control voltage by binary particle swarm optimization, fuzzy PIand overlapping positive and negative voltage is put forward on the basis of principlesof instantaneous power balance to compensate the imbalance of system voltage. Thismethod reduces the number of PI controller and complexity of parameter tuning. Bybinary particle swarm algorithm, several aspects are improved in fuzzy PI as follows.It becomes easy to effectively and automatically to select fuzzy control rules, whichrealizes the optimum extraction of fuzzy control rules. Moreover, the PI parameterscan be modulated online to meet the requirements of various working conditions. Therobustness and dynamic performance of the control system are improved as well.In this paper, the internal model control is applied to the control of STATCOM,which realizes the compensation current control more ideally. Three-level balancecontrol method is proposed to balance the capacitor voltages by controlling thebalance of H-bridge energy. The DC-bus capacitor voltage is controlled as a whole,between and within phases. Under this situation, the balance between phase-to-phasevoltage and internal phase voltage is relatively better realized. After simulating thismethod, it is testified that the approach to balance capacitor voltages is effective.The proposed key technology of STATCOM is applied to practical projects todesign and develop medium-voltage H-bridge cascade multi-level STATCOM(±2.8MVar) for6kV substation with coal mine ground of Zaozhuang Coal MiningGroup. This device was connected to the grid in2011, while the operation effectindicated that the STATCOM designed in this paper can better realize the dynamic reactive power compensation and harmonic suppression. As a whole, the intendedpurpose of design was achieved, which verified the effectiveness of relevant methodsproposed in this paper.
Keywords/Search Tags:medium voltage STATCOM, H-bridge cascade, LS-SPOPWM, LCL filter, voltage and current measure, internal model control, DC capacitor voltagebalance
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