Research On Two-level Control Strategy Of Distributed Static Series Compensator

With the national goal of "Emission Peak and Carbon Neutrality",the construction of new power system with renewable energy as the main part has become a new direction for the development of China’s energy and power industry.Flexible regulation and active control of power grid is the key to build new power system.However,with the large-scale integration of renewable energy into the power grid,traditional power grid can not adapt to the flexible operation of new power system and the demand of efficient accomodation of renewable energy,and it is difficult to adjust the power flow and operation mode frequently and quickly.Distributed Static Series Compensator(DSSC),as a distributed flexible AC transmission device with the characteristics of decentralized installation,low cost and convenient maintenance,has broad application prospects in the transition process from “rigid” to “flexible” in modern power grids.From the perspective of the actual demand of the new power system,this paper studies the control strategy of DSSC at the device level and system level respectively.This paper first briefly summarizes the main types,characteristics and development status of distributed flexible transmission equipment at home and abroad,sums up the current research on DSSC control technology at home and abroad,analyzes the current deficiencies in DSSC control,and puts forward the necessity of studying the DSSC two-level control strategies.Then,the flexible installation characteristics and topology structure of DSSC are compared with traditional flexible equipment.The basic working principle of regulating the power flow of the line and its own various compensation characteristics according to the actual demand of the line are studied.Based on the above content,the line voltage,transmission power and line impedance various adjustment characteristics of DSSC is analyzed.After that,the two-time-scale theory is introduced,and the decoupling methods of conventional linear and nonlinear systems are analyzed.Based on the DSSC device topology,the DSSC AC and DC side dynamic equations are analyzed,and the two types of dynamic equations are combined to establish the third-order dynamic mathematical model of DSSC,then utilize the two-time-scale theory to analyze the model,and extract the fast and slow variables of the model to construct the DSSC two-time-scale model.According to the obtained model,the fast and slow variables extracted from the model,base on the principle of exact linearization of nonlinear system feedback,the device-level control strategy of DSSC is designed from the perspective of fast variables by fixing slow variables;and the PSCAD/EMTDC software is used to verify the correctness of the DSSC device-level control strategy in three application scenarios of comprehensive power flow regulation,promotion of new energy consumption,and three-phase asymmetry compensation.Finally,in order to make the DSSC series system better coordinate with each other in the line,the various types of losses included in the operation of the DSSC device are classified and analyzed,the output allocation and losses of the DSSC series system are studied based on the current conventional output allocation method.For the purpose of improving the utilization rate of series unit device from two aspects of device output voltage and use capacity,as well as minimize the loss of the series system as far as possible,this paper proposes a coordinated output allocation control strategy for the DSSC series system,and uses PSCAD/EMTDC software to invest 10 series units with two different capacities for simulation test verification.The simulation results show that compared with the traditional output allocation method of DSSC series system,the proposed system-level coordinated output allocation control strategy of DSSC series system can significantly reduce the total device loss of DSSC series system,and ensure that the DSSC unit in operation has a higher device utilization rate.