ESKOM - Transmission Line Protection Issues and Their Solutions

Author: Graeme Topham, South Africa

ESKOM - Transmission Line Protection (Issues and Solutions)

One of the main challenges currently being faced is an increase in the number of high resistance faults which fall outside the capability of the impedance protection. The current solution being considered is to apply direction earth-fault comparison protection as an additional protection function.

Eskom is the South African electricity utility, generating 95% of the electricity used in South Africa. Eskom's total net generation capacity is 40 GW. Rapid growth in the country in recent years has seen the load increase so that the current spinning reserve is less than 8 %. This has put immense pressure on the transmission system in terms of transferring the required power from the generation to load centres.

The 28 000 km transmission system includes lines above 132 kV at voltage levels of 220 kV, 275 kV, 400 kV and 765 kV. Also included is 1 000 km of 533 kV d.c. used for importing power from the neighboring Cahorra Bassa scheme. In the mid-1980s, Eskom commissioned the first 765 kV transmission line and currently has 1 153 km of 765 kV lines in operation. As part of the current 42 billion USD expansion program over the next 5 years, Eskom is building an additional 1 500 km of 765 kV lines to strengthen the transmission capacity between the generation pool in the North East to the Cape (one of the main load centres in the South West of the country).

Lines having a length of more than about 200 km are considered to be long whereas short lines are generally those less than 10 km. However, from a protection perspective, the Source to line Impedance Ratio provides a better assessment in terms of the protection needs. Duplicated protection is used for all transmission lines, with duplicated impedance being applied in the majority of cases. For short lines, duplicated current differential protection is used. In most instances, identical relays are used for Main 1 and Main 2. The decision to apply identical protection systems is based on a number of considered issues (e.g. training, spares holding etc.) and also on doing rigorous type and model power system simulator testing on all relays before applying them to the network. To date, this philosophy has proved successful. All transmission stations are equipped with dual battery systems feeding each Main protection system. Trip coils are also duplicated in all instances. Approximately 30 % of transmission line protection relays applied are of numerical technology.

A number of the 400 kV lines are series compensated, with plans to add additional series capacitors to both the 400 kV and 765 kV networks. Single-pole tripping and reclosing is applied on the majority of the transmission lines. There are some exceptions where operational requirements or limitations of equipment preclude the use of single-pole tripping and reclosing.
On the impedance-based protection schemes, Permissive Overreaching is applied. In addition, a separate channel for direct transfer tripping is employed to facilitate the transfer tripping of the remote line end when required.

All impedance relays are selected to block for power swing conditions. Separate power swing tripping relays are applied at strategic locations to measure out-of-step conditions and to effect system separation at pre-determined locations so that the resultant sub-systems are viable and stable with the minimum of load shedding needing to be applied.

One of the most important issues related to transmission line protection is ensuring that the engineered protection schemes meet the increasingly demanding needs of the Eskom power system.

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