Research On Pilot Protection Of The Tie-Line Of Photovoltaic Power Generation

With its unique advantages,photovoltaic(PV)power generation has become a hot spot in the development of new energy.However,The integration of large-scale photovoltaic power generation has also brought adverse effects on the power grid.Due to the influence of natural conditions,the output of PV power generation has randomness and fluctuation,and the fault characteristics of PV system are affected by the inverter control,which makes the sensitivity of the line protection principle based on the traditional synchronous machine system decline,even the problem such as incorrect action.Therefore,it is necessary to study the protection of the tie-line of PV power generation.Firstly,the current and voltage characteristics of the tie-line of PV power generation are studied.Considering the control requirements of Low Voltage Ride Through,the control strategy is adopted to suppress the grid-connected negative sequence current during asymmetric faults.The simulation model of PV power generation grid-connected system is established,and the fault characteristics of the tie-line of PV power generation are analyzed and simulated.Secondly,a pilot protection method of the tie-line of PV power generation based on the correlation discrimination of model identification is proposed.The differential voltage and differential current of the tie-line of PV power generation accord with the capacitor circuit model when the fault is outside the tie-line area.When the fault is in the zone,the differential voltage and differential current of the non-fault phase line conform to the capacitor circuit model,but the fault phase line does not.The correlation analysis of differential voltage derivative and differential current is carried out.If the correlation coefficient is about 1,it conforms to the capacitor circuit model and is judged as out-of-zone fault.If the correlation coefficient is less than 1,it is judged as a fault in the zone,and fault discrimination is carried out accordingly.Finally,the integrated impedance characteristics of the differential voltage/current ratio between the two ends of the tie-line of PV power generation in case of internal and external faults are analyzed,and the pilot protection method of the tie-line of PV power generation based on the integrated impedance is proposed.When the fault is outside the zone,the integrated impedance of the tie-line shows the characteristic of capacitance of the tie-line,its imaginary part is negative and its absolute value is large.When the fault occurs in the region,the integrated impedance of the non-fault phase line is still characterized by capacitive reactance and the imaginary part has a large absolute value,while the integrated impedance of the fault phase line shows the characteristics of the system impedance on both sides and the tie-line impedance,the absolute value of the imaginary part is small.Based on this,the fault within and outside the zone and the fault types can be identified.The simulation results of PSCAD/EMTDC show that the two pilot protection methods are not affected by the inverter control strategy of PV power generation,and have strong ability to resist transition resistance and high reliability.