輸電地下電纜一般為具有遮蔽銅線之單心電纜,若電纜遮蔽銅線採單端接地,則在電纜另一非接地之開路端,將因導體電流之電磁效應產生感應電壓,且此感應電壓必須限制於安全容許值以下,以確保維護人員之安全。倘若電纜遮蔽銅線兩端均接地,雖無感應電壓產生,但遮蔽銅線將形成封閉環路而產生循環電流,致使電纜產生回路損失而發熱,進而可能減少電纜之送電容量及加快電纜被覆之老化速度。
另當地下電纜各區段之長度均無等長時,則無論使用何種排列方式,當電纜區間長度越長,其遮蔽銅線產生之感應電壓值與循環電流值均會增大。但當電纜被覆保護裝置內之銅板交錯方向錯誤時,則遮蔽銅線最大感應電壓遠較交錯方向正確時為高,故電纜被覆保護裝置銅板交錯接地方向正確與否,將對遮蔽銅線感應電壓值造成甚大之影響。由此可知,當電纜系統之排列方式、區間長度及電纜被覆保護裝置方式正確與否,均對被覆感應電壓及循環電流造成影響。有鑑於此,本計畫將探討適合電纜實際運轉條件之計算方法,以確認電纜被覆感應電壓及循環電流計算值與實際測量值間之差異,同時探討不同接地系統對被覆感應電壓及循環電流造成之變化,期有助於電力公司運轉維護人員確切掌握電纜系統運轉狀態,進而提升電纜系統之運轉維護效益。Underground cable transmission lines generally have single core cable with metallic shield layer. The metallic shield layer of power cable
allows the fault current flowing to the earth once the shielding system of cable line is grounded at any end, yet a high voltage is often induced at the other end due to electromagnetic effects of current in power cables. In order to protect the security of utility engineers, this induced voltage should be limited to a safety value as low as possible. In this case, both ends of shielding system are earthed with no induced voltage, yet there is a circulating current flow in the shielding system. Under such a scenario, it may cause cable heating loss, resulting in capacity reduction and making the cable worn out with a higher speed. Therefore, in order to reduce the induced voltage and circulation loss in a shielding system, software simulations and practical measurement analysis are applied to probe into different earthling systems of transmission underground cables under various changes of induced voltage and corresponding circulating current.