Research On Fault Current Analysis And Limiting Strategies Of Grid Forming Converter

With the development of energy strategy in the new era,building a new power system with new energy as the main body is an inevitable choice for China to achieve the goal of "double carbon".The control methods for grid connection of new energy generation units can be divided into grid-following(GFL)and grid-forming(GFM).This article takes GFM converters as the object,with the goal of improving the system frequency and voltage support ability during the new energy grid connection period.The control structure and parameter design of grid building type converters are studied,and measures for overcurrent suppression of converters under three-phase short circuit fault conditions in the power grid are proposed.The research content is summarized as follows:Aiming at the problem of inertial demand of new power system,this paper explores the control structure of grid converter with inertial support ability.Firstly,the main sources of inertia of the traditional power system are analyzed,the stator electrical characteristics,rotor mechanical characteristics and primary voltage regulation and frequency regulation characteristics of synchronous generators are studied,and the mathematical models of each part are established.Compared with the fixed physical parameters of traditional synchronous generators,virtual synchronous generator(VSG)can realize flexible changes in inertia,damping and other parameters to adapt to different power grid operating conditions.In view of the cumbersome steps in the current parameter debugging process and the difficulty of achieving the ideal control effect,this paper proposes a parameter design method considering the phase stability constraint,which provides a theoretical basis for the research of overcurrent suppression strategy later.Firstly,based on the derivation of the small signal model of the control link,the influence of the control effect of the main parameters of each part is analyzed.For the power control link,according to the cutoff frequency of loop gain and phase stability margin constraints,the control parameter selection area that meets the above constraints is obtained;For the voltage and current inner loop control,considering the sampling delay of the voltage and current signal,considering the dynamic response and phase margin requirements,the theoretical reference value of the control parameters is obtained through the design of the control bandwidth.The simulation results show that the selected parameters can realize the stable control of the GFM converter,which can effectively reduce the invalid test in the process of parameter debugging.In addition,by analyzing the output electrical quantity of different virtual inertia and different virtual damping converters,it can be seen that virtual inertia will weaken the dynamic response speed of the system while improving the anti-disturbance ability of the system.Virtual damping can effectively slow down system oscillation and improve system dynamic performance.To address the overcurrent problem and other safety problems of GFM converters under the most severe condition of grid voltage three-phase short-circuit fault,this paper proposes an overcurrent suppression strategy based on the additional current loop,which can achieve rapid and effective suppression of overcurrent while ensuring the characteristics of the voltage source of the configuration converter.Firstly,the causes of overcurrent are analyzed from the principle of control characteristics,and two feasible ways to obtain current suppression are analyzed,which are to change the phase(corresponding to the active loop in the control)and the amplitude(corresponding to the reactive loop)of the equivalent voltage source of the GFM converter,and take this as the entry point,and introduce additional current loops in both the active loop and the reactive loop,and improve the damping of the active loop,the power reference value and the control target of the reactive loop.At the same time,in order to alleviate the frequency stability problem that may be caused by the instantane ous impact of the additional current loop,a low-pass filter structure is added after the additional current loop.Finally,the simulation results show that the proposed control strategy can effectively suppress the overcurrent during the grid voltage fault,so as to ensure the safe and stable operation of power electronic devices.The findings of this paper can provide ideas and references for GFM converters parameter design,improve support capability and keep safe and stable operation under grid fault.