变电站二次电缆屏蔽层电流对二次端口的耦合

Current in Secondary Cable Shielding Coupling to Secondary Ports in Substation

短时电磁暂态作用下,变电站接地体间会产生相对电位差,使二次电缆屏蔽层中流过电流并耦合至二次设备端口,从而造成运行故障甚至设备损坏。鉴于此,通过模拟试验研究获取了二次电缆屏蔽层电流对二次端口的耦合特性,并分析了耦合机理。首先,搭建了模拟开关操作、短路故障、雷击等电磁暂态作用下,电缆屏蔽层电流对端口耦合的试验平台,并开展了试验测量。试验结果表明：当二次电缆的首端和末端接有相同的元件时,各类电磁暂态作用下,首端和末端耦合的端口电压均波形相同,极性相反;双芯电缆的端口电压只有共模电压,没有差模电压。然后,建立了电缆的集总参数等效电路,通过电路分析表明,端口的骚扰电压水平与电缆屏蔽层电流通过屏蔽层电阻产生的压降相当。研究结果还表明,二次端口的耦合系数随频率的上升而降低,耦合阻抗与频率无关,其值接近电缆屏蔽层的电阻值。这些结果为进一步研究二次端口的电磁骚扰防护奠定了基础。

Electromagnetic transient will induce a relative potential difference between earthing conductors and current in secondary cable shielding in substation. The current can couple to the secondary ports through control cables, which may cause operation failure even damage of secondary equipment. Consequently, we obtained the characteristics of the voltage and current coupled to the secondary ports through the experimental simulation, and analyzed the coupling mechanism. First, we employed established test platforms for simulating switching, short circuit and lightning transients to carry out numeral experiments. The experimental results indicate that the voltage waveforms at both terminal ports have the same profile but the opposite polarity, under the condition of the same component at the terminals for all kinds of transients. Then, only common-mode voltage with no differential-mode voltage exists for the twin core cable. Based on the results, an equivalent circuit of secondary cable was proposed. It is revealed that the current in the cable shielding layer will induce the resistance voltage drop, i.e. the voltage at the secondary port. The results also show that the coupling coefficient decreases with the increase of the frequency, and the coupling impedance, which is independent of frequency, is close to the resistance of the cable shielding layer. All these results will be helpful to further research on the electromagnetic protection of the secondary ports.