Research On The Techniques Of Mitigation Of Sub-synchronous Oscillation In Direct-drive Wind Farm Grid-connected And Its Delivery System

Renewable energy generation(REG)is currently a frontier research hotspot with extensive international attention,involving multidisciplinary and intensive knowledge.REG has important economic and strategic value,which is a key technology for country to achieve energy transition strategy in the 21 st century.In recent years,REG represented by photovoltaic and wind turbine has developed rapidly.The proportion of REG in the world's total power generation is increasing year by year.Due to the factors such as climate and geography,the installed capacity of renewable energy is mainly concentrated in the Northwest,North China and Northeast regions,resulting in obvious power electronic characteristic and weak grid in our country's "Three North" regions.The problem of impedance interaction between REG base and AC/DC transmission network is becoming more and more serious.The subsynchronous oscillation(SSO)accident of wind farm grid-connected and its delivery system frequently occurs,which seriously threatens the safe and stable operation of REG base and external transmission system.Thus,it makes sense to explore the SSO suppression technology of wind farm grid-connected and its delivery system.This paper is supported by the National Key Research and Development Project of China "System Stability Control Technology for DC Delivery system of Renewable Energy Generation Base",Program for Guangdong Introducing Innovative and Entrepreneurial Team,etc.With the support of the Research Center of National Power Conversion and Control Engineering Technology,we have carried out in-depth research on the SSO suppression technology of direct-drive wind farm grid-connected and its delivery system.In view of the SSO problem in the three scenarios of direct-drive wind farm connected to the weak grid,series compensated transmission line and high-voltage direct current(HVDC),different SSO suppression methods are proposed to improve the safety and security of direct-drive wind farm grid-connected and its delivery system.The main work and innovations of this paper are as follows:(1)Aiming at the SSO problem caused by the direct drive wind farm connected to the weak grid and considering the situation where the control system of direct drive wind farm is hard to modify,a static synchronous compensator(STATCOM)based on energy storage is proposed to suppress this SSO.First,a small-signal equivalent impedance model for direct-drive wind farm connected to the weak grid is established,and the SSO mechanism of direct-drive wind farm connected to the weak grid is analyzed.Considering that the grid impedance becomes larger and is inductive when grid becomes weaker,a voltage-type control method is designed to make the impedance of the proposed energy storage STATCOM appear inductive.Then the proposed energy storage STATCOM is incorporated into the AC bus of the direct drive wind farm to form a parallel structure with the grid-connected line of wind farm.Since the impedance characteristic of the proposed energy storage STATCOM is consistent with that of the weak grid,the parallel impedance between the proposed energy storage STATCOM and grid is smaller than the individual grid impedance.From the perspective of impedance,the grid impedance at the grid-connection point of direct-drive wind farm is equivalently reduced,and the grid strength at the grid-connection point of direct-drive wind farm is enhanced.Then,the goal of SSO mitigation can be achieved.Finally,experimental results verify the feasibility and effectiveness of the proposed SSO suppression method.(2)Aiming at the SSO problem caused by the direct drive wind farm connected to the weak grid,a new-type phase-locked loop(PLL)based SSO suppression method is proposed.Firstly,the influence of the control parameter of direct-drive wind turbine on the impedance characteristic of direct-drive wind farm is analyzed.Through comparative analysis,PLL parameter has a relatively large impact on the impedance characteristic of direct-drive wind farm in the sub-and super-synchronous frequency range.Only optimizing the parameter without changing the PLL structure,the capacitive impedance in the sub-and super-synchronous frequency range cannot be effectively reduced.The direct-drive wind farm and weak grid are still prone to cause impedance coupling.In order to effectively optimize the impedance in the suband super-synchronous frequency range,a new-type PLL is proposed,which can significantly reduce the capacitive impedance in the sub-and super-synchronous frequency range,and decline the impedance coupling between weak grid and direct drive wind farm.The proposed PLL can suppress the SSO in direct-drive wind farm connected to weak grid.Finally,simulation and experimental results verify the effectiveness of the proposed SSO suppression method.(3)Aiming at the SSO problem of the series compensation delivery system of direct drive wind farm,a SSO suppression method based on the impedance reshaping of direct-drive wind turbine is proposed.Firstly,a small-signal equivalent impedance model of the series compensation delivery system of direct drive wind farm is established,and the SSO mechanism and characteristic of the series compensation delivery system of direct drive wind farm are analyzed.Due to the series compensation capacitor,part impedance of the series compensation line is capacitive in the subsynchronous frequency range,which can cause the impedance coupling between the direct drive wind farm and series compensation line.Therefore,a impedance reshaping control method of direct drive wind turbine is proposed to enhance the resistance characteristic in the subsynchronous frequency range,which can enhance the oscillation damping ability of direct drive wind farm in the subsynchronous frequency range,and achieve the goal of SSO suppression.Finally,simulation and experimental results verify the effectiveness of the proposed SSO suppression method.(4)Aiming at the SSO problem of the DC Delivery system of direct-drive wind farm,a STATCOM-based SSO suppression method is proposed.First,a small-signal equivalent impedance model of the direct-drive wind farm DC delivery system is established.The mechanism and characteristic of the SSO caused by the interactions among the direct-drive wind farm,the sending end grid and the DC delivery system are revealed.When a SSO occurs in the direct-drive wind farm DC delivery system,the grid voltage at the sending end of the DC delivery system will appear an obvious SSO.Thus,a control method based on voltage feedforward compensation is proposed for STATCOM.The proposed STATCOM is incorporated into the sending end grid of the direct-drive wind farm DC delivery system.The SSO can be suppressed by stably controlling the voltage of the sending end grid.Finally,experimental results verify the feasibility and effectiveness of the proposed SSO suppression method.