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SOLAR POWER HOMES: UNINTENDED CONSEQUENCES

Every new technological development is bound to have some unforeseen consequences and the move to install grid-tied home solar power (PV) systems is unlikely to be an exception.

One such example came up recently in a discussion group* where one contributor posed the (as yet) hypothetical question in the following terms: 

There is a community with 10,000 homes on an island, which has one power plant to feed electricity to those homes.  Along comes a solar power sales agent, and soon over half the homes have installed grid- tied solar systems.  Each one produces an excess of power during the day.  How would this work? 

There are two answers.  The first is that if the grid-tied output is a small fraction of the total grid generation (as it is in most countries now) there isn’t really a problem.  However, and here is the second answer, if the grid-tied inverter-supplied power were to grow to a significantly large proportion of the total generation, then there would be problems with stability and control. 

Why would such problems emerge? 

Assuming all the individual homes’ grid-tied inverters were pumping power back into the grid, the power plant would be reducing output to try and maintain system frequency.  If that single power plant can regulate fast enough when the clouds pass overhead and then clear, system frequency could be maintained. 

However, if the variation caused by these sudden changes is too wide, then the inverters might sense that the grid is lost momentarily and shut down.  The consequence of this would be to dump the whole generation load on to the power plant.  Assuming it regulates the frequency quickly and restores the normal grid operating parameters, then the various grid-tied inverters would sense a restoration of grid power and resume sending power to the grid. 

A pattern of shut down and restoration might occur, depending on the sensitivity of the inverters, the responsiveness of the power plant and the architecture of the distribution network.  It might work satisfactorily, but it might not. 

A further unintended consequence would occur if the total inverter output is greater than the total load – that is, if all the home solar power systems generate more power than the grid requires in terms of output – then the excess power will start motoring the power plant and raising system frequency.  The power plant will trip on reverse power and the frequency will become very unstable.  Once the power plant trips, the frequency will swing around and the inverters will shut down, thinking the grid is lost.  This time, it really will be lost and the power plant will be off-line, so everyone will be without power. 

For these and other reasons, utilities will set an upper limit on the amount of grid-tied energy production – solar or wind – they are willing to connect.  This whole area will require further study before home solar power systems become too popular. 

Jack Callon 

7 November 2007

*For those with a technical mind, you might be interested to read the blog discussion leading to this article.  Go here.