Research On Control Strategies Of AC Excited Pumped Storage Units To Participate In Power System Frequency Regulation

As ac excited pump storage unit(ACEPSU)uses ac excited machine(ACEM)as a generator motor,it has adjustable speed and power decoupling capability and will be a modern pump energy storage unit of a development trend.In order to better play the important role of ACEPSU in the peak regulation and frequency modulation of the power grid,this paper has carried on the modeling and control strategy research correspondingly to improve the power response speed of ACEPSU and the ability to participate in the power system frequency regulation.The main contents are as follows:(1)For the problem that the slow adjustment of the power of the reversible pump turbine,it is difficult to adapt to the requirement of fast power regulation of the pumped storage unit when participating in the power grid frequency modulation.A fast power response control strategy of ACEPSU is proposed.Firstly,based on the different output characteristics of reversible pump turbine,load optimization flowcharts of both turbine and pump modes are presented,and the calculation method of the operational power ability of ACEPSU under generator mode and motor mode are also put forward.Secondly,combined with the operation conditions of motor mode and generator mode,a fast power response control strategy for ACEPSU is derived,which the power is controlled by the AC excitation,the speed or the guide vane is controlled by the reversible pump turbine.Finally,based on Matlab/Simulink platform,the power response rate and operational power ability of ACEPSU under different operation conditions are simulated and verified.(2)Aiming at enhancing the capability of the network frequency modulation supported by ACEPSU,a kind of improved proportional-derivative(PD)loop virtual inertial control strategy of ACEPSU is proposed.Firstly,the expression and equivalent calculation method of ACEPSU virtual inertia are deduced and proposed.Thus the ACEPSU can obtain a larger virtual inertia than the inherent inertia.Secondly,an improved virtual inertial control strategy with proportional-derivative(PD)is proposed by introducing an additional frequency control links which uses the change of power grid frequency as the active increment.Besides,a control parameters calculation method in the improved PD virtual inertia control is also provided.Finally,the frequency response characteristics of the system under different operation conditions are simulated and analyzed by constructing a 3-machine system model with an ACEPSU.(3)Regarding with the issue of insufficient frequency regulation ability of fixed droop coefficient control,this paper proposes an inertial loop-based variable droop coefficient frequency control strategy of ACEPSU which is based on the traditional inertial control.Firstly,considering the influence of frequency change rate and frequency deviation on the system frequency regulation,an R-df/dt function which associates droop coefficient and system frequency change rate is introduced.Secondly,the droop coefficient which dynamically changes with the change of the frequency change rate is obtained according to the frequency change of the grid and the speed of the unit timely.Finally,the frequency response characteristics of power system are simulated and compared based on the variable droop coefficient frequency control strategy of ACEPSU.The research achievement will be a great helpful to improve power response ability of ACEPSU and the capability of the grid frequency modulation.Furthermore,it can lay theoretical foundations and technical support for further playing the role of adjustable speed pumped storage unit in power grid dispatching and improving the stability of power system.