| With the high proportion of PV connected to the power system through the power electronic inverter,the operation characteristics of the traditional power system have been profoundly changed,and the safe and stable operation of the power system has been severely challenged.On the one hand,due to the weak damping and low inertia characteristics of renewable energy,a high proportion of photovoltaic access will lead to a significant reduction of the equivalent inertia of the system,which greatly threatens the frequency stability of the system.On the other hand,due to the influence of the control strategy of the inverter and the dynamic characteristics during failure,the fault characteristics of the renewable energy power system are quite different from that of the traditional synchronous power supply.Therefore,considering the fault characteristics of photovoltaic grid-connected systems under different control strategies and the effects on system stability is an urgent research topic in the field of protection.In this paper,photovoltaic grid-connected system is taken as the research object,the voltage stability and frequency stability of the system during the fault period are taken into account,and the influence of the dynamic response characteristics of phase-locked loop on the fault characteristics of photovoltaic grid-connected system under the scenario of symmetrical and asymmetric faults on the grid side is studied.The main research results are as follows:(1)Aiming at the problem that the equivalent inertia of the system decreases under the high proportion of photovoltaic access,the frequency response model of the photovoltaic frequency modulation system under the virtual inertia control strategy is considered to predict the lowest point of the frequency and voltage during the fault period,and the stability of voltage and frequency can be comprehensively evaluated through the frequency and voltage indexes.According to different fault conditions,an adaptive low voltage ride-through strategy is proposed.Through reasonable distribution of active power and reactive power during the fault period,a better support effect on frequency and voltage is achieved.The comprehensive effect of the adaptive low voltage traversal strategy is further quantified by using the mutation decision method.Finally,the effectiveness of the proposed low penetration strategy is verified by simulation.(2)Aiming at the reactive power priority mode in the adaptive low voltage ride-through strategy,the phase-locking deviation between the output of the phase-locked loop and the actual phase is analyzed when the point of common coupling voltage phase changes under the three-phase symmetric fault on the grid side,and the factors affecting the phase-locking deviation are identified.In the case of ignoring the response time of the inner current loop,the influence path of the phase-locked deviation is defined,and the analytical calculation model of photovoltaic fault current considering the dynamic response characteristics of the phase-locked loop is established,and the harmonic characteristics of the fault current are defined.Finally,the accuracy of the proposed analytical model is verified by simulation,and the effects of different influencing factors are compared and verified.(3)In view of the asymmetric fault scenario of photovoltaic grid-connected system,the influence of the dynamic response characteristics of the phase-locked loop on the second second-order generalized integrator was analyzed,and the fundamental frequency component of oscillation attenuation was introduced into the positive and negative sequence components extracted during the asymmetric fault,and the output phase deviation of the phase-locked loop considering the influence of the separation of positive and negative sequence components was further obtained.Cause the control input of PV inverter is also affected by positive and negative sequence components,an analytical calculation model of PV fault current considering the effect of PLL on separation of positive and negative sequence components under asymmetric faults is established by simultaneous governing equation and physical equation.Finally,the accuracy of the proposed analytical calculation model is verified by simulation. |
