题目：
20千伏配电网线路单相接地与10千伏相比有何区别？为什么要加装零序电流保护？
答案：
参考答案：
20千伏配电网一般是中性点接地系统（经接地变接地），单相接地时有短路电流流过，所以单相接地时应动作跳闸。
由于20千伏配电网经接地变接地，零序阻抗远大于正序阻抗，单相接地时的故障电流往往很小，所以需另外加装零序电流保护。
20千伏线路一般配置反映相间故障的电流保护和反映接地故障的零序保护，零序电流宜取自套在电缆上的穿芯式CT。当不具备条件时，可取自出线CT的自产零序电流。
20千伏配电网线路单相接地与10千伏相比有何区别？为什么要加装零序电流保护？
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