问题21、电缆护层保护器的作用。
某GIS变电站220kV电缆出线中间接头发生了短路击穿事故，经排查发现该电缆金属互层保护器已失效，且保护器从未按照要求开展过相关试验。 分析认为，由于互层保护器缺陷未能及时发现，导致电缆投运后金属护层形 成两端接地。护层中产生非常大的异常电流，导致中间接头接地线连接处发 热严重，使该位置交联聚乙烯主绝缘过热软化及绝缘迅速劣化，最终导致主 绝缘击穿故障。电缆护层保护器的作用是什么?
Question 21 Function of cable sheath protector.
A short circuit breakdown accident occurred in the middle joint of 220kV cable outlet in a GIS Substation. After investigation, it was found that the metal interlayer protector of the cable had failed, and the protector had never been tested according to the requirements. The analysis shows that the defects of the interlayer protector are not found in time, which leads to the formation of grounding at both ends of the metal sheath after the cable is put into operation. A very large abnormal current is generated in the sheath, which leads to serious heating at the ground wire connection of the intermediate joint, resulting in overheating softening and rapid deterioration of the XLPE main insulation at this position, and finally leading to the breakdown fault of the main insulation. What is the function of cable sheath protector?

【参考答案】

 
单芯电缆金属护层常见的接地方式有:一端直接接地，另一端经保护器接地;两端经保护器接地，中间直接接地;两端直接接地，中间经保护器 接地;两端直接接地，中间交叉互联接地;直接、保护及交叉互联组合接地。
当电压超过35kV时，电缆一般都采用单芯电缆，随着电压等级的升高，电缆金属外护层的感应电压问题也越加明显。为了减轻电缆外护层感应电荷的影响，可将电缆按照品字形敷设，而由于实际原因（如电缆沟过于狭窄、电缆过硬难以弯曲)，很难将其按照品字形敷设。所以由于电磁感应，长线路高压单芯电缆金属互层产生较高的感应电压，护套上的感应电压叠加起来可达到危及人身安全的程度。如果此时电缆两端金属护套同时接地，会产生环流并导致过热，严重时危及电缆主绝缘。如果电缆外护层一端接地，另一端不接地，在系统短路或受到雷电波冲击时，不接地端会产生很高过电压，危及电缆运行安全。因此单芯电缆应采用一端直接接地，另一端用护层保护器接地的形式，这样当护层上的电荷逐渐积累，电压达到一定值时，护层保护器瞬间动作，释放电流，达到安全运行的要求，保证人身设备的安全。
Answer:
the common grounding methods of metal sheath of single core cable are: one end is directly grounded, and the other end is grounded through protector; Both ends are grounded by protector, and the middle is directly grounded; The two ends are directly grounded, and the middle is grounded through the protector; The two ends are directly grounded, and the middle is interconnected and grounded; Direct, protection and cross interconnection combined grounding.
When the voltage exceeds 35kV, the single core cable is generally used. With the increase of voltage level, the induced voltage problem of the metal sheath of the cable becomes more and more obvious. In order to reduce the influence of induced charge on the cable sheath, the cable can be laid in a zigzag shape. However, due to practical reasons (such as too narrow cable trench, too hard cable to bend), it is difficult to lay in a zigzag shape. Therefore, due to electromagnetic induction, the metal interlayer of long line high voltage single core cable produces higher induced voltage, and the superposition of the induced voltage on the sheath can endanger personal safety. If the metal sheaths at both ends of the cable are grounded at the same time, circulating current will be generated and overheat will be caused, which will seriously endanger the main insulation of the cable. If one end of the outer sheath of the cable is grounded and the other end is not grounded, the ungrounded end will produce high overvoltage when the system is short circuited or impacted by lightning wave, which will endanger the safety of the cable operation. Therefore, one end of the single core cable should be directly grounded, and the other end should be grounded by sheath protector. In this way, when the charge on the sheath gradually accumulates and the voltage reaches a certain value, the sheath protector will act instantaneously to release the current, so as to meet the requirements of safe operation and ensure the safety of personal equipment.