Western Power Transmission Line Protection Design and Philosophy

Author: Dean Sharafi, Western Power, Australia

Western Power Transmission Line Protection Design and Philosophy

Western Power is the state-owned utility of Western Australia and operates various voltage levels in Transmission and Distribution network. Transmission voltages include 66KV to 330KV covering a large area connected through the network (South West Interconnected System-SWIS). It contains around 88000 km of power lines with load around 3600MW.  The complete scheme for 220KV and 330KV lines consists of duplicated, fully independent and discriminative protections capable of providing high-speed fault clearance over the entire line length. These protections may be either unit types, such as differential, phase comparison, or distance with tele-protection signaling (using direct or permissive transfer tripping).

These protections use separate tele-protection signaling equipment. A single communication bearer to accommodate all the signaling channels is considered acceptable except where both protections require information from the remote end for its basic operating characteristics. In this case, each protection has independent bearer.

The complete scheme for major transmission inter-connectors (132kV and below) consists of duplicated, fully independent and discriminating protections capable of providing high speed local fault clearance and high speed remote fault clearance on one protection, and medium speed remote fault clearance on the second protection.

These protections may be unit, interlocked distance or plain distance types. Regional inter-connecting lines at 132kV and 66kV have the same philosophy for protection. Regional transmission feeders from major transmission substations enjoy the same standard of protection with addition of a remote backup protection (of the form of an IDMT overcurrent function) to cover conditions on the regional transmission network outside the scope of normal design. Designing the protection for each line category depends on the length of the line.

Short lines are less than 10 km, intermediate lines - up to 25 km and long lines - more than 25 km.

Fault levels in the major transmission network are high, for example, 20 GVA at 330kV. One of the main limitations in our system design is the speed at which high power faults can be cleared from the system, particularly three phase faults.

The types of protection schemes adopted for transmission lines are:

  • Current differential (comparison over microwave radio/optical fiber)
  • Circulating current/opposed voltage (pilot)
  • Interlocked distance
  • Distance
  • Over-current and earth fault

Unit protection schemes (eg. Pilot protection) and non-unit protection schemes (eg. distance protection) are often used on the same line to take advantage of their complementary performance. Protection No.1 has arbitrarily been chosen for the unit protection, or the protection with the highest speed.

Where both protection schemes on a line are of the same type (eg. Double distance protection) they are based on different operating principles or are sourced from different manufacturers. This is to reduce the risk of common mode protection failure.

Where duplicate unit schemes (eg current differential) are used they use separate communications bearers over different routes. Voltage transformer supervision is used in conjunction with all distance relays.
Earth fault relays are used on all lines (as part of Protection No 2) to help detect high resistance earth faults outside the sensitivity of the main protections, and to provide general system back up protection.

Breaker failure protection is installed with the fastest and most comprehensive protection.

Single shot reclose is used for feeders at metropolitan substations and two shot reclose for feeders at rural substations. On newly designed EHV lines high-speed single phase auto-reclosing scheme is used to improve reliability of the system.

In our transmission network 90% of relays are micro-processor based. Older relays are constantly replaced with new micro-processor ones.

Power. Flexible. Easergy.
BeijingSifang June 2016