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How Electricity Is Transmitted

Eskom produces electricity at power stations, most of which are grouped near coal mines in Mpumalanga and the Northern Province. This is not where most of the electricity is used, however. The big load centres are in places like Gauteng, the Western Cape and KwaZulu Natal.

So the electricity must be sent from the power stations to the load centres in the most reliable and cost-effective way. Much research has shown that this is best done by sending the power over high voltage power lines. As it leaves the power station, the electricity is boosted by a device called a step-up transformer to voltages such as 132 000 volts (132 kV) or 400 kV or 765 kV. When the electricity reaches its destination - a substation near a load centre - it is "stepped down" to voltages used for distribution to customers.

In South Africa, we have a total of 27 770 km (as of March 2007) of high voltage transmission lines and 325 000 km of distribution lines, a formidable distance to inspect and maintain. All overhead lines (those not buried under ground) are vulnerable to natural phenomena such as lightning, flooding, veld fires and strong winds, not to mention man-made disturbances such as cane fires lit under the lines and cable theft. All of these cause technical problems that must be fixed so that power can be restored.

All the high voltage lines plus the big transformers and related equipment form the transmission system, also known as the National Grid. This is the domain of Eskom�s National Control Centre.

National Control carries the responsibility of matching supply and demand. This high-tech hub is manned without a break by trained staff who constantly monitor the demand from load centres all over southern Africa and ensure that enough power is being generated to meet the demand. They compare demand levels with load forecasts developed over months and years, adjusting and refining supply in line with new connections and weather patterns. And they play a big role when Eskom develops its long-range forecasts and its maintenance schedules.

National Control has many strategies in hand when demand doesn't match supply. When there is more power than needed, they instruct the pumped storage schemes to pump water up to high-level dams in readiness for the next surge in demand. When there is not enough power to meet the demand, they instruct peaking stations like gas turbines and pumped storage schemes to add their contribution to the grid.

When they have called for emergency generation and there is still not enough, then they engage a pattern of load shedding, a system contractually pre-arranged with some very big power users like steel mills and aluminium smelters. These customers agree to have their power switched off for short times in an emergency so that everyone else doesn't have to experience a power interruption, or blackout.

National Control must sound the alarm when things are going adrift. And when there's a fault at a station like Koeberg or on a major transmission line, they are the ones searching urgently for other sources of power or other lines to carry it.