How Cape Town, Joburg prevent Stage 1 load shedding

The City of Cape Town and City Power in Johannesburg announced today (10 July 2015) that their residents would once again not be affected by stage 1 load shedding, thanks to spare generation capacity.

Eskom announced earlier that it would implement the first stage of its rolling blackout schedule from 08:30, which would likely continue until 22:00.

This is not the first time the City of Cape Town has been able to prevent stage 1 load shedding from affecting its residents – it was able to offer the same respite yesterday.

On 21 June, both the Cape Town and Johannesburg metropolitan municipalities were able to avert the stage 1 load shedding.

Asked how it was able to do this, Cape Town said it was thanks to its Steenbras hydroelectric pumped storage scheme; while City Power explained that it has numerous measures in place to try and avoid load shedding.

Steenbras

Steenbras was commissioned on 8 August 1979 and is owned by the City of Cape Town.

Each of its 4 generating units can act as either a pump motor or a turbine generator.

“Surplus national generating capacity is employed to utilise relatively low-cost off-peak electricity available at night to pump water from a lower to an upper storage reservoir,” said the City of Cape Town.

During periods of peak demand in the day, the water is released back to the lower reservoir, generating electricity as a hydroelectric power station would.

“Steenbras is used for the purpose of removing peaks. In winter, these peaks usually fall between 08:00 and 10:00 in the mornings and 18:00 and 20:00 in the evenings.”

The City may have spare capacity at times outside the peak periods and at times when these peaks are low, but it warned it won’t always be able to prevent a stage 1 blackout.

“An assessment is done prior to implementing any stage of load shedding,” it said.

Steenbras pump turbine
Steenbras, photo by Element Consulting Engineers

City of Cape Town’s power stations

Outside Steenbras, the City does not have other power plants that generate excess capacity.

“The City has the Roggebaai Gas Turbines (42MW) and the Athlone Gas Turbine (36MW) – both of which are also dispatched for removing peaks but for much shorter periods.”

However, these use more expensive fuel (Aviation Jet-A1), and after maximising the output of Steenbras are not used as much as the hydroelectric dam.

Joburg’s blackout-beating plans

City Power said it uses a number of “interventions” to avoid load shedding in Johannesburg.

The independent Kelvin coal power plant in Kempton Park, which has an operational capacity of below 200MW, is one of these measures.

City Power’s other blackout-beating interventions involve limiting the power demand of Johannesburg:

  1. Ripple control – which lets City Power remotely turn off geysers.
  2. Load limiting – which is currently being tested in Aspen Hills.

Load limiting will work through a smart meter, and will limit the amount of electrical current a household is allowed to use during load shedding.

When load shedding hits, the smart meter sends you a message to let you know what you need to reduce your consumption to. The meter will also show what your current consumption is.

If you don’t comply, your power is shut off for 30 seconds. This procedure is repeated 5 times. If after the fifth shutdown you still haven’t reduced your electricity usage below the required level, you will be load shed.

Though City Power’s load limiting system is only being piloted in Aspen Hills, the utility said it is busy rolling it out elsewhere.

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How Cape Town, Joburg prevent Stage 1 load shedding