| Renewable energy sources(RESs)are collected within the power plant and then sent out by the transmission line,and this has become an important form of renewable energy utilization in our country.At present,the transmission line is mainly equipped with distance protection and the ratio braking differential protection as main protection.However,the weak infeed of RESs will amplify the influence of the fault resistance,so the traditional distance protection faces the risk of refusal.In addition,since the inertia at the outlet of RESs is low,the current phase angle difference on both sides is easy to swing for internal faults,so the ratio braking differential protection is also facing challenges.Meanwhile,the fault current of doubly-fed induction generators will deviate from power frequency,which will cause a big error in the extraction of the power frequency components by the full cycle Fourier algorithm,so all the protection principles based on the power frequency will have the problem of sensitivity decline.Therefore,it is urgent to propose new principles of distance protection and pilot protection with good performance.The voltage drop at the fault point is similar to a step signal,and has full frequency domain information after Laplace transform.When the selected high frequency signal is higher than the current loop bandwidth and the speed frequency component,the high-frequency fault component based distance protection will not be affected by the weak infeed and frequency offset of RESs.Therefore,the high-frequency impedance models of the inverter-interfaced renewable energy generator(IIREG)and the doubly-fed induction generator(DFIG)were established,and the frequency selection principle that made them have stable phase angle was given.On this basis,the internal and external faults can be determined by comparing the magnitude of the high-frequency operating voltage and the high-frequency voltage at the fault point.Simulation results show that the method has a strong ability to withstand the fault resistance,and field-testing data also verify this method.However,the distance protection cannot quickly cut off faults in any point,so the main protection must be strengthened to ensure the safe and stable operation of the renewable energy system.Based on this,a pilot protection method based on current amplitude ratio on both sides was proposed.At the same time,the adaptability of the method with a teed line was analyzed and an improved criterion was put forward.The method only needs to transmit the scalar signal which reduces the requirements on the communication system and synchronizing ability.It can operate correctly in the case of reclosing with permanent faults and power-lossing of power plants.In addition,its principle is simple and reliable,so it has engineering application value.The above pilot protection is only applicable to IIREGs,instead of DFIGs.Therefore,in order to solve the problem of sensitivity decline of differential protection caused by deviated fault current of DFIGs,a new protection principle based on correlation was proposed.When the system operates normally or an external fault occurs,a traversing current is flowing through the outgoing transmission line,so the current waveforms on both sides are completely opposite.When an internal fault occurs,there are huge differences in the transient current waveforms on both sides.Therefore,the Spearman rank correlation coefficient can be used to measure the correlation degree between the two sides of current waveforms to distinguish the faults inside and outside the zone.The simulation has been perform considering the fault resistance and noise,and field-testing data further confirm the effectiveness of the proposed method. |
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