| Energy is an important material basis for the survival and development of human society,and it is vital to the national economy and the national strategic competitiveness.Realizing the transformation of energy production to renewable energy is an important demand for China and even the whole world to achieve sustainable development of energy and economy.However,with the continuous development of the renewable energy generation,conventional thermal power units have been largely replaced,and the grid form and its operating characteristics have undergone significant changes.The prominent characteristics of power electronics in the system lead to frequent wide-bandwidth oscillations when renewable energy generation equipment is connected to the weak grid,and even force large-scale renewable energy generation equipments to disconnect from the grid and torsional vibration of thermal power unit shafting,which seriously threaten the safe and stable operation of the grid and affect the usage of renewable energy.Therefore,it is urgent to study the work related to the wide-bandwidth oscillation mechanism and the suppression method of the renewable energy power generation connected to the weak grid.This paper is supported by the National Key Research and Development Program of China "Stabilization Control Technology for DC Delivery System of Renewable Energy Power Generation Base" and National Natural Science Foundation of China Youth Science Fund Project "Study on Bus Voltage Fluctuation in Wide Frequency Domain of DC Microgrid and its Suppression Strategies",In this paper,the sequence impedance model of renewable energy power generation equipments are established,the impedance measurement technology and equipment of renewable energy power generation equipment are studied,the wide-bandwidth oscillation mechanism of renewable energy power generation connected to the weak grid is analyzed,and the wide-bandwidth oscillation suppression methods of renewable energy power generation equipment are carried out.The research has solved the key technical problems of wide-bandwidth oscillation mechanism and suppression of renewable energy power generation connected to the weak grid,and ensured the safe and stable operation of high penetration renewable energy generation,providing theoretical basis and practical guidance for large-scale grid-connected absorption of renewable energy.The main technical innovations are as follows:(1)Considering the influence of phase-locked loop,current loop and time delay,wide-bandwidth positive-and negative-sequence impedance models of permanent magnet direct-drive wind/photovoltaic power generation equipment,doubly-fed induction wind power generation equipment and virtual synchronous generator with energy storage in three phase stationary coordinates are established,and the wide-bandwidth impedance characteristics of different types of renewable energy generation equipment are compared and analyzed.The sequence impedance of permanent magnet direct-drive wind/photovoltaic power generation equipment is capacitive in the middle-frequency areas and the impedance amplitude is large;the sequence impedance of the doubly-fed asynchronous wind power generation equipment is inductive in the low-frequency areas;the sequence impedance of the voltage-controlled virtual synchronous generator is basically inductive and the impedance amplitude is small;the sequence impedance of the current-controlled virtual synchronous generator is mainly negative impedance in the middle-and high-frequency areas and the impedance amplitude is very small.The simulation results of impedance measurement verify the correctness of the impedance model.The eatablished impedance model provides a basis for revealing the wide-bandwidth oscillation mechanism of renewable energy generation connected to the weak grid and proposing the method of oscillation suppression.(2)Aiming at the problem that the accuracy of the wide-bandwidth impedance model of the renewable energy power generation equipment is difficult to determine,the system scheme of the high/low voltage wide-bandwidth impedance measurement equipment is proposed,including the impedance measurement method,the topology of the disturbance injection device,the wide-bandwidth linear output control method,the switching control method and the fault protection method were developed.Base on the scheme,a 380V/100 kVA low-voltage wide-bandwidth impedance measurement equipment was developed,whose measuring frequency range can reach 1Hz~2kHz.The hardware and software design of 380V/100 kVA impedance measurement equipment is introduced in detail.Through the developed wide-bandwidth impedance measurement equipment,the impedance characteristics of the renewable energy power generation equipment are scanned online,and the accuracy of the impedance model of the renewable energy power generation equipment is verified.(3)Based on the established impedance model,the instability mechanism of photovoltaic power generation equipment,virtual synchronous generator with energy storage connected to the inductive weak grid is analyzed.The output impedance of photovoltaic power generation equipment can be capacitive,and it is prone to interact with the inductive weak grid;the output impedance of the voltage-controlled VSG is basically inductive and can be stably connected to the inductive weak grid;the sequence impedance of the current-controlled virtual synchronous generator contains a large negative resistance component in the middle-and high-frequency areas,resulting in poor grid-connected stability of the current-controlled virtual synchronous generator.The dominant factors affecting the system stability domain have been identified.The weakening of the grid,the increase in the number of photovoltaic power generation equipment connected to the grid and the increase in the control bandwidth of the phase-locked loop will lead to the deterioration of the system stability.(4)An optimal design method for control parameters of renewable energy grid-connected inverter considering the interaction of phase-locked loop and current loop in the weak grid is proposed.The parameter optimization design method firstly designs the current loop controller parameters according to the pole-zero cancellation method.Then,considering the interaction of active power,grid strength,current loop and PLL,the parameters of PLL are designed.By changing the control parameters of grid-connected converter,the grid-connected converter can operate stably in weak or very weak grid conditions.What is more,a voltage feedforward oscillation suppression method is proposed.By introducing an additional voltage feedforward component,the negative effect of the phase-locked loop on the stability of the system through the grid impedance is eliminated,which can effectively solve the wide-bandwidth oscillation problem of renewable energy generation connected to weak grid.Finally,the experimental results demonstrate the effectiveness of the proposed parameter optimization design method and voltage feedforward oscillation suppression method. |
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