Research On Steady State Control Of Soft Normally-open Point For Distribution Network

This paper discusses the research background,basic topology,regulation principle and typical working mode of soft normally-open point applied to distribution network.In order to provide guidance for the design and optimization of system parameters,a generalized small-signal modeling is established.According to different application scenarios,voltage regulation strategy and power regulation strategy based on soft normally-open point are studied respectively.The small-signal model of the soft normally-open point connected to the distribution network is extremely important for studying the stability of the entire system,and it is also the basis for the overall design of the system parameters.In this paper,a small signal model of multi-terminal soft normally-open point connected to the distribution network is established.The model includes the linearization equations corresponding to the AC system,voltage source converter,internal DC system and control system.The modeling process is not affected by the limit of the number of terminals.State space is used to describe smallsignal model.The state matrix of the multi-terminal soft normally-open pointing system that is not limited by the strength of the AC system is derived.The objective function of control parameter optimization is established according to the characteristic roots of the state matrix.Taking the three-terminal system as an example,the characteristic roots of the state matrix are calculated and the stability of the system can be judged.The accuracy of the model is verified by the comparison of the electromagnetic transient model and the small signal model,and the parameter optimization method is verified by the time domain simulation.Compared with the traditional tie-switch,the soft normally-open point offers more flexibility.The response speed of the soft normally-open point is sufficient to deal with the sudden changes in the output of the distributed power supply,which can realize real-time accurate voltage adjustment.In this paper,the model of voltage over-limit problem of distribution network with distributed generation is studied.taking three-terminal soft normally-open point converter station as an example,the acceptance capacity to distributed generation when soft normally-open point is connected to distribution network are analyzed.On the condition that distributed power output and network topology are unknown,the sensitivity coefficients of the node voltage to active power and reactive power are obtained by applying the power disturbance of the soft normally-open point,and then the power flow through the soft normally-open point is adjusted,which makes the node voltage return to a reasonable range.The effectiveness of the proposed control strategy is verified by simulation,and the problem of the voltage over-limit problem of the distribution network is solved.Soft normally-open pointes are usually installed between two or more feeders in the distribution network.Due to the use of fully-controlled devices,it can realize a decoupling control of the active and reactive power,so it is able to adjust the flow of active and reactive power between feeders.Therefore,it can quickly balance the active power of the feeder and achieve reactive power compensation at the same time.This paper takes the three-terminal soft normally-open point as an example.The method of using soft normally-open point to realize feeder active balance and reactive power compensation is deduced.The real-time digital simulation system RTDS is introduced.The basic composition of the RTDS experimental platform and the connection method of different equipment boards are studied.The control and protection device of regional distribution network docked with RTDS is introduced.A simulation model of the three-terminal soft normally-open point connected to the distribution network is built in RSCAD.The power adjustment module of the control and protection device sends active and reactive commands to RTDS.The effectiveness of the feeder equalization strategy and feeder reactive compensation strategy are verified,respectively.