| Integrated into the grid by the power converter,fault current of the renewable energy generator(REG)is influenced by controls and featured with limited amplitude,non-power-frequency component and controlled phase angle.With the increasing scale of the REGs,these features deviated from that of the synchronous generator bring great challenges to the traditional protections based on steady-state power frequency phasor.In this dissertation,after the protection configuration of the transmission line connected to renewable energy power plant(REPP)is advised,new protection principle for such a transmission line is studied.The research work and innovative results are as follows.(1)The configuration and setting of protections for the delivery system of REGs are proposed.Fault features of two types of REGs are analyzed and a simplified calculation method for the maximum fault current is introduced.Taking the actual REPP as an example,the best scheme of protection configuration and setting for the collection line is determined by quantitative calculation.Combined the basic function of protective relaying and short-circuit current features of REPPs,the weakness of the recent protection configuration for the transmission line connected to the REPP is discussed and a new scheme is proposed.(2)The influence mechanism of REG on traditional pilot protections is revealed.The analytical expressions of fault current and equivalent positive-and negative-sequence superimposed impedance of the inverter-interfaced REG are deduced.According to the criteria of three commonly used pilot protections,the behaviors of these relays are analyzed under different controls,fault conditions and normal operation conditions.Results show that these protections are not the best choice for the transmission line and new protection principle should be proposed.(3)A transient current waveform similarity based protection principle is proposed.By analyzing the fault response of the REG's control system,reasons for the irregular waveform of REG's transient current are realized.Based on the obvious differences of the double-ended transients waveform similarity between internal faults and external faults occuring on the transmission line,a new pilot protection principle is proposed.The principle takes advantage of the messy time-domain transients induced by dynamics of the control system and its performance is not influenced by controls,which makes it available for lines connected to all types of REPPs.Accompanied with common sampling frequency,the protection is relatively robust to high level of noises and high fault resistance.(4)Countermeasures for performance promotion of the proposed protection is proposed considering practical situations.According to the analysis of double-ended transient current waveform features when saturated current transformers and multi terminals appear on the transmission line,the detection method of CT saturation is proposed by using the difference between the fault time and the time when the correlation coefficient is larger than the setting value.After the waveform similarities of different combinations are analyzed,the best operating equation for the proposed protection applied on the multi-terminal transmission line is constructed.These improvements make the new protection more feasible for the industry application. |
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