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Spinning-reserve costs could be high if wind penetration rises steeply

6th November 2015

By: Terry Mackenzie-hoy

  

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At Chernobyl, following an emergency shutdown of all power, diesel generators were needed to run the cooling pumps. These generators took a minute to attain full speed, which was an unacceptably long time for the reactor to be without cooling.

It was suggested that the rotational momentum of the winding down steam turbine be used to power the pumps in the time between shutdown and the generators being ready. A test was devised.

If the experiment had gone as planned, the disruption and danger to the plant would be minimal. First, the reactors would be brought down to low power, between 700 MW and 800 MW. Then the steam turbine would be run up to full speed and then turned off. The power generated by the winding down generators would be measured to determine if it was sufficient to power the cooling pumps before the diesel generators got up to full speed.

However, a series of operator errors resulted in an explosion. Did I mention that this has to do with wind turbines? Oh, sorry. Let us go back to the Russians.

What they were trying to do was to determine the “H Constant” of the system – the degree to which, without input, the system can supply energy stored in the system and for how long. In South Africa, this is about 500 MW per 0.1 Hz – in other words, the gain or loss of a 500 MW load will change the power system frequency by 0.1 Hz, say from 50 Hz to 49.9 Hz.

Back to wind power. In this age, when every fool with a smartphone thinks they are an energy expert, they should remember Chernobyl. Not the radiation bit, the bad assumption bit.

The daily weekday demand of the South African power system varies from 30 000 MW to 35 000 MW. Of this, about 1 000 MW is supplied by solar (photovoltaic (PV) and concentrate solar plants) and 800 MW by wind. We can be reasonably sure that PV will be available every day (unless a dark shadow wipes out the sun, in which case we will have other, bigger issues). However, wind is not always available. Wind shifts can occur in less than an hour.

In a landmark paper, Professor Phillip Lloyd examined the magnitude, frequency and rapidity of shifts in wind power supplying the UK power system.

He writes: “The number of five-minute periods when the shifts in wind power were about 40 MW to 50 MW rose from 1 600 per annum when the installed power was 6.5 GW to 3 500 per annum when the installed power was 12.4 GW. About five to ten times per year there were shifts greater than 500 MW over five minutes regardless of the installed capacity . . . the number of shifts of between 200 MW to 300 MW grew from 10 to 80 as the installed capacity nearly doubled. Indeed, the frequency of these relatively large shifts in wind power supply rose quite rapidly as the installed wind power increased.”

The South African wind power magni- tude is small in comparison to the system H-Constant and so the loss of the whole wind power merely requires the ramping up of some spinning reserve, or the starting of a gas turbine. However, if we were to up the wind power supply to say, 10 000 MW, then potentially the South African power system could easily collapse for a loss of 5 000 MW over a short period (remember that the UK power system/European Union link means large power swings are accommodated more easily than is the case in South Africa).

Lloyd further writes: “The implications of this are that it is essential to increase the spinning reserve as the proportion of climate-dependent renewable energy feeding a grid increases . . . the increase in spinning reserve should be proportionately greater than the increase in the renewable energy. As spinning reserve is more costly than other sources of energy, the costs can mount rapidly. This needs to be taken into account in planning the expansion of renewable sources of energy.”

Should we worry? I think we should.

The uninformed demands for “more wind power, more . . !” must be tempered with the costs of equipment to power those still, windless nights and days when the wind drops suddenly.

Not Chernobyl exactly. But to ignore this would be foolish.

Edited by Creamer Media Reporter

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