Mechanism Analysis of the Influence of Direct Drive Wind Farm Integration on SSTI of Thermal Generator Caused by LCC-HVDC
Gao Benfeng1, Dong Hanxiao1, Lu Yajun2, Zhong Qidi2
1. Hebei Key Laboratory of Distributed Energy Storage and Micro-grid North China Electric Power University Baoding 071003 China; 2. State Grid Economic and Technological Research Institute Co. Ltd Beijing 102209 China
Abstract：The sub-synchronous interaction between the thermal generator and the LCC-HVDC (line-commutated-converter based high voltage direct current) will cause the sub-synchronous torsional interaction (SSTI) of the thermal generator shaft system, which in turn causes the fatigue accumulation of the thermal generator rotor shaft system. When the thermal generator is connected to the DDWF (direct-drive wind farm) through the LCC-HVDC sending end, or the new LCC-HVDC sending end AC bus is near the DDWF and thermal generator base at the same time, it will constitute a scene with a close electrical distance between the three. At this time, the influence of DDWF on the SSTI of thermal generator caused by LCC-HVDC can not be ignored. Firstly, based on the mathematical model of DDWF and thermal power bundled through LCC-HVDC transmission system, the small signal model of the system is established in Matlab/Simulink by block modeling method, and the correctness of the small signal model is verified by step response. Secondly, the eigenvalues of the small signal model under the two conditions of LCC-HVDC sending end access or no access to DDWF are compared and solved, and the SSTI mode of the system is separated. The influence of DDWF grid connection on SSTI damping of thermal generator is clarified, and the theoretical analysis is verified based on time domain simulation. Finally, the influence of system operation mode on SSTI damping characteristics is analyzed for wind speed, wind farm capacity, electrical distance between wind farm and LCC-HVDC rectifier station and LCC-HVDC control mode. When the DDWF is connected to the LCC-HVDC sending end, the interaction between the thermal power unit and the LCC-HVDC will cause the DDWF output power to change. When the output power deviates from the given value, the GSC controller of DDWF will respond quickly to the power deviation, adjust the active and reactive power output quickly according to the system operating conditions, and alleviate the power fluctuation caused by external disturbance. In this process, DDWF will share the power fluctuation of LCC-HVDC with thermal power units, weaken the sub-synchronous interaction between thermal power units and LCC-HVDC, and enhance the SSTI damping of thermal power units. The simulation results of typical systems show that about 27 % of the sub-synchronous current in LCC-HVDC is absorbed after DDWF is incorporated into the LCC-HVDC transmission system. The main results of this paper are as follows: (1) The PSCAD / EMTDC time domain simulation model of the system is established for the DDWF and thermal power units bundled by LCC-HVDC transmission system, and the small signal model of the system is established by block modeling method. The correctness of the small signal model is verified by comparing with the step response of the electromagnetic transient model. (2) When DDWF is not connected to the system, the sub-synchronous interaction between the thermal power unit and LCC-HVDC will aggravate the external disturbance, and the thermal power unit has SSTI risk. After DDWF is connected to the sending end of LCC-HVDC, the fast response of GSC can replace the thermal power unit to bear part of the DC power change, reduce the sub-synchronous interaction between the thermal power unit and LCC-HVDC, indirectly improve the system damping and reduce the risk of system oscillation. (3) When the capacity of DDWF increases and the wind speed increases, the damping of SSTI increases. When the electrical distance between DDWF and LCC-HVDC rectifier station increases, the system damping decreases. The SSTI damping of LCC-HVDC rectifier side constant current control is larger than that of constant power control, and the SSTI damping of inverter side constant DC voltage control is larger than that of constant turn-off angle control.
高本锋, 董涵枭, 卢亚军, 钟启迪. 直驱风电场并网对直流输电引起的火电机组轴系扭振影响机理分析[J]. 电工技术学报, 2024, 39(7): 1971-1984.
Gao Benfeng, Dong Hanxiao, Lu Yajun, Zhong Qidi. Mechanism Analysis of the Influence of Direct Drive Wind Farm Integration on SSTI of Thermal Generator Caused by LCC-HVDC. Transactions of China Electrotechnical Society, 2024, 39(7): 1971-1984.
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