Research On Adaptive Recovery Technology For Line Fault Of Hybrid HVDC Transmission System

The hybrid-HVDC technology,which combines the advantages of LCC-HVDC and VSC-HVDC,is an effective solution to the large-scale grid-connected of clean energy.However,the reconvery technology adopted by the existing hybrid-HVDC project is the same as that of LCC-HVDC and MMC-HVDC,which lacks a pre-identification of the temporary fault and permanent fault.When the system recovers under a permanent fault,a severe impact on the converter will be caused.On the basis of fully considering the particularity of the hybrid-HVDC topology and the flexibility of the control strategy,the research ideas including passive detection,active injection,intelligent recognition,and flexible recovery are constructed.The main research work and innovative achievements are as follows:(1)Based on the analysis of differences between the fault clearing strategies of LCC and MMC,a scheme of independently setting the fault clearing strategies of LCC-MMC and MMC-MMC system is proposed.Meanwhile,the time and frequency domain features of voltage and current signals that can be used to identify the fault property are revealed.In the time domain,the reflection characteristic of voltage travelling wave at different converter and fault point are analyzed.The difference of the propagation path of the voltage travelling wave on the healthy line and faulty line are also analyzed.In the frequency domain,the natural frequency of the voltage travelling wave on the line before and after the fault disappears is analyzed.The above research lays a foundation for the adaptive recovery strategy proposed later.(2)A fault property identification strategy based on the Pearson correlation coefficient for the LCC-MMC hybrid HVDC system which can differentiate temporary and permanent faults is proposed.To achieve this,a voltage signal is injected into the faulty line by decreasing the firing angles of LCC-based rectifier.Based on the established OHLs model and the injected signal,an estimated backward travelling wave can be accurately calculated.The fault property is identified by comparing the wave difference of the real backward travelling wave and the estimated backward travelling wave.The proposed strategy has a high accuracy,since all influencing factors including the fault resistance,fault distance,and line parameters have been taken into account.The proposed method can bear 800 Ω grounding resistance and has low requirements for system sampling frequency(10 k Hz).For a 1000 km line,the fault property can be accurately identified after the signal injection for 7ms.(3)For the MMC-MMC system,when the fault resistance is smaller than 100Ω,an adaptive reclosing strategy by detecting the dominant frequency of the residual voltage on the line is proposed.Firstly,the discharging process of the fault point is analyzed,and the dominant frequency characteristics of the fault voltage under different fault properties are revealed.Then,the fault property and the fault disappearance time are identified based on the change time of the dominant frequency.Since this method has a limitation on the fault resistance,a fault resistance estimation method is also proposed to adaptive adjust the amplitude of the injected signal.Moreover,the influencing factors of the proposed algorithm,including the fault resistance and the dead zone,are also analyzed.The principle of this method is simple and reliable.For faults that exist for a short time,this method can speed up the reclosing time of the circuit breaker and shorten the interruption time of power transmission;for faults that exist for a long time,this method can prolong the reclosing time of the circuit breaker and avoid misjudging it as permanent Fault.(4)For the MMC-MMC system,when the fault resistance is larger than 100Ω,an adaptive reclosing strategy based on the signal injected by the modified DCCB is proposed.By modifying the snubber circuits of the insulated gate bipolar transistor(IGBTs)belong to the transfer branch of the DCCBs,the energy stored in the snubber circuits can be used to inject a voltage signal into the faulty lines.The fault property(permanent fault or temporary fault)will be still identified by comparing the wave difference of the real backward travelling wave and the estimated backward travelling wave.A physical experiment platform was built to verify the effectiveness of the proposed scheme.The proposed strategy will inject the voltage signal repeatedly to avoid misjudging the temporary faults existing for a long time.Finally,the algorithm is verified under different fault conditions in PSCAD/EMTDC.