Research On AC-AC Type Soft Open Point(AC-SOP) Topology And Control Strategy

To cope with the greenhouse effect and achieve low-carbon energy production,nations around the world are increasing the development and utilization of renewable energy.The penetration of distributed generations(DGs)such as wind power and PV are gradually increasing among the distribution system.Through this,the proportion of traditional power generation based on thermal power has been reduced.The issues of bidirectional power flow and voltage profile caused by DGs in distribution networks are becoming more serious,which is a great challenge to the safety and reliability of distribution network.The flexible interconnection of the distribution network is an important means to deal with the above problems.The traditional radial network can be upgraded to the flexible meshed configuration by the interconnected equipment.The penetration of DGs,as well as the power supply quality can be increased by intelligent control the power flow and loads.First,this paper analyzes the impact on the distribution system by bidirectional power flow and other problems.Hereafter,the difference between present interconnection devices is compared and a novel direct type SOP(AC-AC type soft open point,AC-SOP)is proposed.AC-SOP cascades unipolar AC-AC converters(UAACs)and bipolar AC-AC converters(BAACs)through the multi-winding transformer.It is able to control the amplitude and phase angle of output voltage in four quadrants independently without the help of DC components or DC-link.The principle of AC-SOP topology and its voltage regulation method are analyzed.Secondly,by modeling the AC-SOP interconnected two-feeder system,the active power and reactive power control gains corresponding to their equivalent duty cycles are analyzed.The mathematical models of AC-SOP in abc three-phase stationary and d-q two-phase synchronous rotating coordinate system are established.The AC-SOP closed-loop control strategy of the power flow mode and power restoration mode,as well as the transient switching mode are proposed.According to the modulation characteristics of AC-AC converter,the equivalent modulation strategy of AC-SOP based on the general vector transformation method is proposed,which makes the closed-loop control of AC-SOP rapidly and accurate.Finally,on the basis of theoretical analysis,an AC-SOP interconnected 10 kV feeder loop model was built in the condition of the PSIM platform.Based on the PSCAD/EMTDC simulation platform,the AC-SOP is configured in the IEEE-34 network model.It is verified that the AC-SOP can achieve bidirectional decoupling control of power flow and improve the voltage profile of the system.By building a single-phase submodule experimental prototype of AC-SOP with a rated capacity of 1 kW,the correctness of design criteria of the proposed topology is verified.