Simulation Techniches,stability Analyses And Optimized Control Strategies Of Converter Dominated Power Systems

Nowadays,the number and capacity of power electronic converters in power systems have reached a certain scale.The operation of power grids is increasingly complicated.The stability mechanisms are changing and the transition process of the rotor angle/voltage stability is shortened.The sub-synchronous and super-synchronous oscillations gradually appear.The problem of stability problem in electrolmagnetic time scale begin to appear.So it is urgent to study the electromagnetic transient(EMT)simulation modeling method and optimized control strategy of converter dominated power systems.This paper focuses on reduction method of large-scale AC-DC power system,fast and efficient EMT modeling method of large power grid,stability limit analysis of convter station connecting to weak AC grids controled by synchorous emylation control and optimized control strategy of devices conecting to AC systems.The main content is as follows(1)A reduction method suitable for EMT simulation of AC/DC hybrid power system is studied.The method stratifies the large power grids according to the voltage level,and simplifies the low-voltage network while retaining the backbone grid.By keeping the short-circuit current provided by the low-voltage network and balancing power flow by the equivalent load,Norton equivalent of the low-voltage network is realized.This method does not need to get the node admittance matrix of the system and is convenient to implement in the commercial software.This method,combined with the dynamic parameter aggregation of generators,can keep the dynamic response of large power grid approximately unchanged.According a case study of planning data of China Southern Power Grid in 2020,the effectiveness of the proposed method is verified.(2)The rapid and effecient modelling method of EMT simulation is studied.In order to unify different formats of electromechanical transient data,abstract models of power system components are introducecd for preparation of object-oriented rapid modeling program;modeling method of generator units in EMT is proposed to solve the diversity of transient stability analysis models of generator units and lack of support of models in EMT software.An efficient modeling method for the HVDC system is proposed,which can simplify the HVDC system as much as possible while maintaining the accuracy of the trancient stability analysis results.It helps to speed up the simulation.According to the EMT simulation example of planning data of China Southern Power Grid in 2020,the superiority of the proposed method is verified.(3)The stability characteristics of MMC by synchronous machine emulation control are studied from three aspects:steady state,small disturbance and transient stability.The maximum DC power of converter station is studied from the perspective of the steady state model.It shows that the greater the short-circuit ratio of the AC system,the greater the maximum power flow between MMC and grids,the maximum power of the coinverter gets larger when the resistance of the system impedance is smaller,proveded the converter operates as a rectifier,while this conclusion get oppsite provided the convert operates as an inverter.The small signal model of converter station which takes into account the dynamic characteristics of controllers and the circuit is established.The analysis shows that using the synchronous machine emulation control the converter can maintain small signal stability when approaching limit when connecting to weak AC system,but the response time becomes larger.Insufficient damping occurs even when connected to strong AC system.Therefore,supplementary control may be necessary at this situation.From the perspective of transient stability analysis,it is found that when the short circuit ratio of the point of commom coupling is slightly larger than 1,the receiving end MMC with synchrous machine emulation control strategy maintain stable in a variety of transient processes.(4)Optimized vector control strategies of converters isproposed.The analysis of this paper shows that the common current vector control can not be stable when the VSC is connected to the weak or passive isolated grid.According to the instability mechanism,the vector control framework is improved by introducing two feedback blocks from the SRF-PLL to the cascaded decoupled controller.Time domain simulations show this enhancement can equip the vector control framework with sufficient damping and stability in weak or even passive grid connection.Similar dynamic behaviors of converters are obtained as vitual synchronous machine control by the control strategy proposed in this paper.Thus this paper provides an alternative way to control the converter in weak and passive conditions.