Research On The Control Strategy Of AC Side And Protection For The Three-phase Four-wire STATCOM Based On MMC

In distribution system,the large number of nonlinear loads such as power electronic equipment,electric railway and so on,lead to the serious distortion of current waveform.The power quality problem has been paid more and more attention.D-STATCOM is an effective device to improve the power quality in distribution system.Due to the limitation of compensation capacity and voltage level of D-STATCOM,a three-phase four-wire STATCOM based on modular multilevel converter(MMC)is proposed in this paper.Firstly,the topology structure and working principle of MMC are analyzed,and the mathematical model of single phase MMC is established.Its power unit can be realized in the four quadrant operation.MMC can be regarded as a controlled voltage source,and the voltage of the upper and lower bridge arm can be control by triggering pulse.And then realize the control of the output voltage of the AC side.As for pulse modulation technology,CPS-SPWM and CD-SPWM were compared in this paper.The results show that CPSSPWM technology can realize the equilibrium distribution of the active power at the bridge between each power unit,and effectively improve the equivalent switching frequency.The CPS-SPWM technology is used to modulate the pulse width in this paper.Secondly,the topology structure of the three-phase four-wire MMC-STATCOM is constructed.By analyzing its mathematical model and working principle,it can be known that the conversion of capacitive and inductive conditions can be realized by controlling the amplitude and phase of the output voltage of MMC-STATCOM.For the detection of the compensation instructions current,the ip-iq method based on instantaneous reactive power theory is adopted.And the sliding mean algorithm is adopted for the filter,which has faster calculation and smal er occupied memory.In the control strategy of AC side,the principle of direct current method and indirect current method are analyzed.In order to maintain the stability of the DC side voltage,sharing control and equalization control strategy are adopted in the capacitor voltage.When the power system is not in the normal state,in order to prevent the MMC-STATCOM from being damaged,it is needed to take some protective measures.As for the situation of the compensation instructions current exceeding the al owable range,the cutoff current limiting protection strategy and the same proportion current limiting protection strategy are proposed.Finally,the three-phase four-wire MMC-STATCOM model is built on the Matlab/Simulink.The simulation analysis is carried out on the comprehensive condition,including harmonic,reactive,and asymmetrical current.The results show that the compensation effect is fine,and the DC side voltage can be stabilized near a given value,which verifies the effectiveness of the control strategy.In addition,the change of the compensation current before and after the protection is analyzed,which proves the feasibility of the protection strategy.