| With the rapid development of industry and the increasing concern about environment and fossil fuels,lots of nonlinear,unbalanced loads and renewable energy sources are connected to distribution networks,producing large amounts of power quality problems,such as: reactive power,harmonics,negative-sequence currents and voltage sags,which increases power loss and threaten the sage operation of power grid.The static synchronous compensator(STATCOM)using voltage source converter has been widely utilized to correct the power factor,to compensate the unbalanced loads and enhance the voltage stability in distribution and transmission grid.With the development of power electronics technology,various topologies of multilevel converters have been studied.Among these,the cascaded H-bridge multilevel converter has been an attractive topology for STATCOM in middle and high voltage grid due to its advantages such as: transformer-less,modularization structure,fast dynamic response,desirable output performance and less resonance.With the support of the National High Technology Research and Development of China(‘863' Program)(2011AA05A301),“Key technology research of grid-connected large scale photovoltaic power plant”,this paper concentrate on the key theory and technology of cascaded multilevel STATCOM,and mainly focus on the structure of output filter,the current control method,the stability of voltage control loop,the compensation strategy of unbalanced load and the reactive power support strategy under asymmetrical voltage condition for cascaded STATCOM.The emphasis and achievement of the paper are mainly reflected on the following aspects:(1)A modified finite control set based model predictive current control method is proposed for cascaded multilevel converter.In the conventional finite control set model predictive current control method,only one optimal output voltage determined by the cost function is applied during the entire control period,which brings the drawbacks of large calculation amounts,current error and high current ripple,etc.Based on the states model of cascaded converter,the relationship between output voltage level and the current slope within one control period is analyzed firstly in this paper,and then,a modified current predictive control method is presented.In the proposed control method,a linear combination of two different current slops is utilized to regulate current within one period.With this method,the current error could be eliminated at the end of each control period;as a result,the steady control performance is improved.Since this method is relied on the finite control set model predictive control,the dynamic response is also perfect.Meanwhile,a voltage balancing control method based on sorting method is also improved,switch constrains are employed in this method to simplify the complex procedure of switch action selection and to enhance the efficiency of sorting algorithm.Furthermore,the stability of the capacitor voltage control loop is researched for cascade multilevel STATCOM.According to operation principle of multilevel STATCOM,the mathematic model of capacitor voltage control loop is built,and then its stability is analyzed considering the filter in the feedback loop.Two filters included second order low pass filter and second order band stop filter are analyzed,and the results show that the filter plays important roles in the design of control parameters.In addition,the design rules of control parameters are also presented based on the stability analysis for STATCOM.(2)A comprehensive compensation method of delta-connected STATCOM under asymmetrical voltage conditions is proposed.The asymmetric active powers of three-phase clusters produced by line voltages and phase currents were analyzed firstly.According to the phasor analysis,constrains of voltage phasors and current phasors under reactive current compensation mode,negative-sequence current compensation mode and comprehensive compensation mode are deduced.On the basis of this,the analytical expressions of circulating current under the three modes are derived.Furthermore,a comprehensive compensation method for delta-connected STATCOM is proposed.In the proposed method,phase current references are calculated directly with the instantaneous voltage and current signals,which could enhance the dynamic behavior and cluster-balancing control performance especially under asymmetrical voltage conditions.(3)Focusing on the reactive power control of delta-connected cascaded STATCOM under asymmetrical voltage condition,a flexible reactive power support strategy is proposed.The delta-connected STATCOM is employed to improve the ac network operation during asymmetrical voltage condition.A reactive power support strategy is presented for delta-connected STATCOM.With this strategy,STATCOM could inject positive-sequence reactive power to raise positive-sequence voltage and inject negative-sequence reactive power to suppress negative-sequence voltage flexibly.Furthermore,a safe operation is guaranteed all the time with this method as the phase current reference is reduced when the maximum value of phase current was larger than the rated current,which is important for STATCOM especially under asymmetrical grid fault condition.Meanwhile,three current reference calculation methods include: active power ripple elimination,reactive power ripple elimination and balanced positive sequence curren under asymmetrical voltage condition are also compared and analyzed,which is beneficial for the control of STATCOM under abnormal voltage condition.(4)A LC coulped cascaded STATCOM and its control method are researched.The output filter of LC coulped cascaded STATCOM is formed by L seriesed with C.Compared to the conventional L-filter STATCOM,LC coulped cascaded STATCOM have advantages such as: high capacitive reactive power rating,high efficiency.The working principle and operating performance of LC coulped STATCOM are analyzed,and the discrete state model is builted.On the basis of this,a state feedback control method is proposed for STATCOM to damp the resonance of the LC-filter and improve its fundamental-frequency gain.Additionaly,a capacitor voltage observer is also built to estimate the voltage of output capacitor in the state feedback control method instead of voltage sensor.Finally,the feasibility and effectiveness of the LC coulped STATCOM and state feedback control method is verified with simution results.(5)Taking a 10 k V/±6MVar cascaded STATCOM for example,this paper also introduces the hardware and software design.The design procedures of topology,dc-link capacitors,inductance,IGBTs and its drivers and the diagram of control system are illustrated in detail.Taking the equivalent series resistance in account,the impedance and current support capacity of dc-link capacitors is discussed,and the design solution for capacitor considering rated voltage,rated current and power loss is also presented.Finally,the feasibility of the design is verified with low voltage and high voltage experiments. |
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