Hybrid Transmission Line
The First Hybrid Overhead/Underground Line Protection System using Flexible Optical CT application in Indonesia
In October 2021, PT Guna Elektro has successfully installed and commissioned the first hybrid overhead/underground line protection system for 150 kV lines between Pasar Kemis Lama and Gajah Tunggal substation, Tangerang, West Java.
In the future, underground cables will be more and more an essential part of transmission systems and many transmission lines will consist of combined overhead line and cable sections, especially in Indonesia where planning constraints may restrict the construction of new overhead lines.
A major challenge for utilities is how to ensure these underground lines or sections of underground lines are reliable and safe. In conventional system, one of the major ways to increase availability has been to allow the protection system to auto-reclose on momentary fault. However, with underground cable, reclosing on an underground fault can cause damage to primary equipment and endanger anyone or anything that might be in proximity to the fault.
With combined overhead and underground line, the challenge is to determine if the fault has happened in the overhead or underground section of the line and only allow the auto-reclose if the fault is in the overhead section of the line.
Transmission lines are protected by line differential or distance protection schemes. Distance protection algorithms are not well suited for hybrid line and are not accurate enough to determine if the fault is in the underground section of the line. Differential protection is more precise, but requires a CT at either end of the underground section. And there is usually no substation or power supply at the underground transition point.
Therefore, PLN have installed hybrid overhead/underground line protection system for 150 kV lines between Pasar Kemis Lama and Gajah Tunggal substation. GAE has successfully delivered this project by integrating several brands of IEC 61850-9-2LE sensors and IEDs, i.e. GE Micom Agile protection relays (P645), Arteche flexible optical CT (SDO FCT) and merging units (SDO MU), synchronized using Tekron PTP GPS time server and connected using Hirschmann Ethernet switch in Parallel Redundancy Protocol (PRP) network. This protection system will send signal to the existing line differential protection relay, to block auto-reclose when fault happen at the underground cable section. GAE also configured these IEDs to be monitored by the existing ABB HMI.
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