| The grid connection of large-scale photovoltaic units will have a certain impact on the stable operation of the new power system.Especially after the system is disturbed by faults,there will be problems of active power shortage and frequency drop.The low-voltage ride through control strategy of photovoltaic units can realize the unit operation without disconnection when the system suffers a fault.The existing fault ride through control methods focus on the reactive power support during the fault ride through,but pay little attention to the active power output and frequency regulation during the fault process.In addition,there are few studies on the quantitative index of frequency stability of high-permeability photovoltaic grid connected system.Therefore,this paper carries out the following research:1)Under the condition of meeting the equipment overcurrent capacity and the dynamic reactive power support capacity of the unit specified in the national standard,the transient simulation model of photovoltaic unit with low penetration function is established;An example system with high permeability photovoltaic is constructed to analyze the change mechanism and influencing factors of transient active power.Through mathematical analysis and graphic analysis,a method to improve the output active power in the fault crossing process of photovoltaic unit is proposed,that is,on the premise of no current of photovoltaic inverter and giving priority to ensuring the output reactive current of photovoltaic unit,the active current during fault is calculated by orthogonal decomposition method,the active current during fault recovery is output by immediate recovery method,and the maximum allowable output current(Imax)of inverter is increased.Through simulation verification,combined with the maximum active power of photovoltaic unit during fault,power improvement gradient and power surge after fault removal,it can be seen that setting Imax to 1.1IN can reduce the cost-effectiveness ratio of transient active power output of photovoltaic unit,and outputting active current according to the active current calculation method selected in this paper can significantly improve the active power output of photovoltaic unit during low penetration,The effect of improving the transient active power shortage of the system is the best.2)According to the idea of dividing frequency bands and giving weight coefficients according to the binary table criterion,combined with the safe operation requirements of system frequency and the protection measures of photovoltaic units,refine the weight coefficients of each frequency band,and construct the frequency stability margin index considering the instantaneous frequency offset of disturbance.In the simulation platform,build the photovoltaic power station model equipped with energy storage devices and different grid connection protection strategies,construct the power system model with different photovoltaic proportion,set 10%and 20%load sudden increase disturbance,and evaluate the frequency response under each scenario with the frequency index proposed in this paper and the existing frequency index respectively.The evaluation results show that this index can distinguish the frequency response with small offset and long duration and large offset and short duration,verify that the proposed indicators can be used to evaluate the transient frequency stability of photovoltaic system with different grid connection protection measures,and increase the accuracy of frequency stability evaluation of photovoltaic system with different proportion.3)The frequency dynamic characteristics and frequency modulation requirements of photovoltaic grid connected system under fault or sudden load increase disturbance are studied,and the frequency modulation control strategy of super capacitor energy storage assisted photovoltaic unit is proposed,which is suitable for both power fluctuation and frequency change caused by fault disturbance.The strategy is that when the frequency is in the frequency modulation dead zone or the photovoltaic unit is in failure,the photovoltaic unit will not participate in the system frequency modulation;When the frequency offset exceeds the frequency modulation dead zone or the recovery stage after fault clearing,the energy storage system assists the photovoltaic unit to provide primary frequency modulation power and inertia support power to the power grid.Through the simulation analysis and the evaluation of the improved frequency index proposed in this paper,it is verified that the control strategy can improve the ability of the system to deal with various frequency fluctuations and enhance the frequency stability of the photovoltaic system with high proportion. |
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