Study On The Strategy Of Reactive Power Compensation Of An UHVDC Transmission Line Based On A Wind-thermal Bundled System

The "Three North(3N)" region of China has a large amount of wind power resources, and both of the building size and the capacity of grid connected of wind power is large. However, most of wind farms were built in remote areas which is far from the load center, and these places are characteristics with low power intensity which will lead to a weak AC grid when disturbance resistance happened. Therefore a wind-thermal bundled system by UHVDC transmission system is proposed to transport wind power to the eastern regions. The wind-thermal bundled system will make the voltage level of sending side complicated. Besides, because of the complicated grid structure and high permeability of wind power, it is significantly important to study methods to improve the the security and stability of the siding end of the bundled system.This paper mainly studies reactive power compensation scheme of the wind power, thermal power and UHVDC transmission system. The contents of this paper are following: firstly, establishing a mathematical model of the doubly fed induction generator(DFIG), the mathematical model of thermal power unit(synchronous generator) and the mathematical model of unipolar UHVDC transmission system.Then the wind-thermal bundled system is proposed and the transient stability of the bundled system is analyzed.Secondly, the different compensation scheme of SVC and SVG are analyzed.According to the analysis results, different control strategies are proposed and are testing in DIg SILENT software to verify the transient voltage stability under different schemes. Finally, the optimal reactive power compensation scheme is selected with the simulation results.Thirdly, the static voltage stability of bus bars of built wind farms of bundled energy system by UHVDC transmission is analyzing by using DPL language of DIg SILENT software to draw the curve of PV/QV, respectively. This can get the change values of reactive power and active power of wind farms, then the values will combine with comprehensive evaluation index to calculate the weakest buses of the established models of wind farms.Finally, the static voltage stability and transient voltage stability of the calculated weakest buses are analyzed, and the measures to improve the static stability and transient stability are studied. The ways to improve the static stability is to installing reactive power compensation devices while the ways to improve the transient stability is also installing reactive power compensation devices aspects by analyzing the characteristic of transient voltage stability under different types of faults.