| As a novel static var compensator,STATCOM(static synchronous compensator)has the advantages of continuous adjustment,rapid response,and nearly sinewave output current.Using the series-connected IGBTs technology to improve the voltage level directly,STATCOM could work on higher voltage and larger capacity electric power system.When series-connected IGBTs STATCOM(SCI-STATCOM)operates under unbalanced conditions,the voltage fluctuation on dc capacitor caused by negative-sequence component would influence the safe and stable operation of SCI-STATCOM in extreme cases.In this paper,the internal relationship between the relevant operating parameters and the voltage fluctuation on dc capacitor of SCI-STATCOM was studied.The range,which can withstand the voltage fluctuation on dc capacitor and operate safely under unbalanced conditions,of SCI-STATCOM was analyzed.A method to determine whether the device can operate stably under specific conditions was proposed.The specific research contents are as follows:First of all,for the three topologies of series-connected IGBTs,MMC and cascaded H-bridge,three topologies’ characteristics were analyzed.Otherwise,the number of key components,the number of IGBTs,harmonic characteristics and operation reliabilities of the three topologies were compared.It is considered that the topology of series-connected IGBTs should be applied in power system at least to the 35 kV and above,and more economical when a single converter’s rated capacity within a single IGBT’s.Secondly,according to the power conservation theory,the voltage fluctuation model of SCI-STATCOM dc capacitor under unbalanced conditions was established and simplified,and the mathematical relationship between the dc voltage ripples rate and eight main parameters was obtained,which include voltage unbalance,current unbalance,capacitance and voltage of dc capacitor,et al.Based on this,the sensitivity of eight key parameters for the voltage fluctuation on dc capacitor was analyzed.The calculation results show that the modulation ratio,dc capacitance,series inductance,positive-sequence voltage phase and negative-sequence voltage phase have great influence on the dc voltage fluctuation of the SCI-STATCOM.When SCI-STATCOM under extreme unbalanced conditions,the absolute sensitivities of modulation ratio and dc capacitance to dc voltage ripples rate are 21.299 and 17.039 respectively,which are much larger than other parameters.Since the series inductance is limited by the operating loss and stability of the device,considering the operability of parameter adjustment,it is effective to suppress the dc voltage fluctuation of SCI-STATCOM by optimizing the modulation ratio and the capacitance of dc capacitor.Finally,the extreme value of voltage unbalance and loads current unbalance in different typical application scenario of high-voltage SCI-STATCOM was discussed.Combined with the simplified model of dc voltage fluctuation under above two extreme unbalanced conditions,the judgement method of the stable operation range of SCI-STATCOM was obtained,which bases on the limit of voltage fluctuation on dc capacitor.The simulation results on PS CAD shows that the simplified model of voltage fluctuation on SCI-STATCOM dc capacitor proposed in this paper was accurate,and the judgment method about voltage stability on SCI-STATCOM dc capacitor under above two conditions was correct and effective. |
