Study On Time-delayed Small Signal Stability Of Power System In The Customer Side

With the development of the power system,the interconnection between different regions,the interaction between the demand side and the power grid,and the informatization level of the power grid are improving continually.The embedded sensors and measurement systems make it possible to gather more and more information from the power system.However,the consequent time delay incurs a great challenge to the security and stability of the power system operation.In this thesis,we focused on the influence of the time delays in the demand response(DR)on the power system stability.Analytical methods for the small signal stability with time delays have been theoretically derived in both continuous and discrete conditions.And,appropriate methods to deal with the time delays during the DR control have been investigated with the different demand side resources,such as the electric vehicle(EV)and thermostatically-controlled load(TCL).It is noteworthy that the proposed methods and criteria in this thesis are able to be applied both in the scenarios of DR control and wide-area application.The main contributions of this thesis are summarized as follows:(1)For the continuous time-delayed power system,linearized time-delayed differential equation(TDE)models with time-invariant parameters,uncertain parameters and time-varying delays are developed.For the TDEs with multiple time-invariant delays,the method to calculate the critical eigenvalues is developed so as to determine the system delay margin effectively.For the TDEs with uncertain parameters and time-varying delays,an improved time-delayed stability criterion considering the delay fractioning and the state trajectory information is proposed based on the Lyapunov direction method.A typical second-order system,single-generator-infinite-bus system and WSCC-3-generator-9-bus system are used to validate the correctness of the proposed methods.(2)A frequency control strategy is proposed to coordinate the traditional power plants and EV aggregators.Based on the load frequency control(LFC)technology,the frequency control strategy realizes the coordinated power control of the aggregated EVs and the traditional power plants.The EVs have the higher priority than the traditional power plants for the frequency control when the frequency deviation violates its acceptable range.Then the imbalanced power is allocated to the EV aggregators and the traditional power plants.Meanwhile,the inevitable time delays due to the data measuring and the control signal transmission are considered during LFC with EVs.With the system inertia uncertainty caused by the large scale wind power integration,an improved proportional integral controller is designed to enhance the system delay margins.Finally,a simplified Great Britain(GB)power system is used to validate the effectiveness of the proposed frequency control strategy and the stability criterion,and the simulation results illustrate the inertia uncertainty and the time delays have significant influence on the frequency control effect.(3)To coordinate the power control of the heat pumps(HPs),EVs and traditional power plants,an improved frequency control strategy is proposed.Firstly,the state space model of HP aggregator is developed based on the HP thermos-electric coupling models.Then,considering the time delays in the HP aggregator and the EV aggregator during the frequency control,the influence of the time delays on the control strategy is analyzed,and improvement to the strategy is deduced.Finally,the simulation results based on the GB system indicate the effectiveness of proposed frequency control strategy.(4)A stability analysis method is derived for the discrete time-delayed power system.Firstly,the continuous Pade approximation is discretized to obtain the discrete version.In addition,the discrete Pade approximation is used to approximate the discrete delay term,and the equivalent order-reduced power system without the time delays is obtained.Then the small signal stability of the discrete power system is investigated with the eigenvalue analysis method.Finally,a discrete second-order system,single-generator-infinite-bus system and frequency regulation system are used to validate the effectiveness of the proposed method.