基于故障分量时频突变特征的海上风电直流升压送出线路纵联保护

Pilot protection for offshore wind power DC transmission lines based on the time-frequency mutation characteristics of fault components

直流汇集送出系统具有效率高、体积小和无需无功补偿等优点,是未来远海风电场汇集送出的发展趋势。然而,相较于陆上架空线,送出海缆分布电容明显,区外故障时,长海缆分布电容电流使得线路两侧电流具有较大的差异,从而易导致基于暂态量的传统纵联保护误动作。针对该问题,提出基于故障分量时频突变特征的纵联保护原理。通过分析直流送出线路、换流器的暂态电流频率特征,考虑区内外故障时两侧暂态故障电流的频段差异,选择合适的保护应用频段。利用小波变换对故障电流进行时频变换,通过Wasserstein距离构造区内外故障识别判据,解决了区外金属或低阻故障时基于暂态时域量的纵联保护误动作问题。在半实物仿真平台(real-time digital simulation, RTDS)中搭建海上风电直流汇集送出系统模型,对所提保护方案进行仿真验证。仿真结果表明,所提保护方法能够可靠识别区内外故障,保护动作时间小于5 ms,耐受过渡电阻达300Ω,抗噪声能力达20 dB。

DC collecting and transmission systems will be the development tendency for future offshore wind farms because of the advantages of high efficiency,small size and no reactive power compensation.However,the distinct distribution capacitance of submarine cables,in contrast to overhead lines,can lead to significant differences in line currents during external faults,potentially causing false operation in traditional transient pilot protection.Addressing this problem,this paper proposes a pilot protection method using time-frequency mutations in fault components.This paper analyzes transient fault current frequency characteristics in DC transmission lines and converters taking into consideration the differences of frequency band of the transient fault current on both sides when internal and external faults occur,thereby selecting an appropriate frequency band for protection.Using wavelet transformation for time-frequency analysis of the fault current and employing Wasserstein distance,it develops criteria to distinguish internal and external faults.This removes the transient time-domain quantity-based pilot protection false action during external metallic or low-resistance fault.The offshore wind farms collecting and transmission system model is built on a real-time digital simulation(RTDS)platform to verify the proposed protection method.The results show that the proposed method effectively identifies fault locations and operates within 5 ms,with tolerance to transition resistances up to 300Ωand noise resistance up to 20 dB.