Research On Double Loop Additional Frequency Control Of Multi Terminal Flexible HVDC Transmission System

In recent years,the proportion of new energy grid connected is increasing year by year,while the proportion of traditional thermal power units is decreasing.While realizing the strategy of environmental protection and sustainable development,the uncertainty of new energy power generation output leads to greater fluctuations and lower stability of power grid.At the same time,VSC-HVDC technology is used for long-distance power transmission,which makes each AC area independent in frequency,reduces the inertia of Isolated AC power grid,and makes the grid frequency more sensitive to power disturbance.The traditional proportional integral double loop decoupling current vector control method for voltage source converter can not provide frequency support for AC area,so it is necessary to add additional links in VSC controller to make wind farms connected to the grid via multi terminal HVDC system participate in AC area frequency regulation.In order to study the frequency support control strategy,the mathematical models of MT-HVDC components are established.It mainly includes:AC area mathematical model,wind farm mathematical model,MT-HVDC network mathematical model,VSC mathematical model.Through the analysis of mathematical model,it shows that VSC can control active and reactive power independently.At the same time,it can support the frequency of AC area by changing the power injection of VSC.In order to realize the frequency support in AC area,this paper proposes a dual loop additional frequency control strategy for MT-HVDC,which realizes the frequency support by adding active frequency droop control to the active control loop and reactive frequency differential control to the reactive control loop.All the control links are based on local signals and do not need remote communication.The double loop additional control can greatly improve the transient performance of frequency response.In order to solve the problem of excessive DC voltage deviation,an adaptive voltage droop control related to voltage deviation is designed.On this basis,the frequency droop control formula is derived,and an adaptive droop control strategy with additional frequency control is proposed.The control strategy can greatly improve the transient and steady-state performance of DC bus voltage.A simulation model is built on MTLAB/Simulink platform to verify the proposed control strategy.On the basis of adaptive voltage droop control,a frequency additional control link based on consistency is added to AC area side VSC controller,and a proportional integral control based on AC area frequency deviation is added to wind farm side VSC.The control strategy mainly realizes the frequency support of Isolated AC region by sharing the power margin of each region,and improves the frequency performance by relying on the support of other regions when a frequency disturbance occurs in one AC region.Its stability is proved by Laplace transform and Routh criterion.A simulation model is built on MTLAB/Simulink platform to verify the proposed control strategy.The dual loop additional frequency control proposed in this paper can make the MT-HVDC transmission system realize the frequency support for Isolated AC area and improve the frequency performance of AC grid,which has practical significance for the development and construction of MT-HVDC transmission system.