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SILICON SILIC CHIP www.siliconchip.com.au Publisher & Editor-in-Chief Leo Simpson, B.Bus., FAICD Production Manager Greg Swain, B.Sc. (Hons.) Technical Editor John Clarke, B.E.(Elec.) Technical Staff Ross Tester Jim Rowe, B.A., B.Sc Nicholas Vinen Photography Ross Tester Reader Services Ann Morris Advertising Enquiries Glyn Smith Phone (02) 9939 3295 Mobile 0431 792 293 glyn<at>siliconchip.com.au Regular Contributors Brendan Akhurst Rodney Champness, VK3UG Kevin Poulter Stan Swan Dave Thompson SILICON CHIP is published 12 times a year by Silicon Chip Publications Pty Ltd. ACN 003 205 490. ABN 49 003 205 490. All material is copyright ©. No part of this publication may be reproduced without the written consent of the publisher. Printing: Hannanprint, Noble Park, Victoria. Distribution: Network Distribution Company. Subscription rates: $97.50 per year in Australia. For overseas rates, see the order form in this issue. Editorial office: Unit 1, 234 Harbord Rd, Brookvale, NSW 2100. Postal address: PO Box 139, Collaroy Beach, NSW 2097. Phone (02) 9939 3295. Fax (02) 9939 2648. E-mail: silicon<at>siliconchip.com.au ISSN 1030-2662 Recommended and maximum price only. 2  Silicon Chip Publisher’s Letter Domestic solar panels can make electricity grid unstable With all the controversy over solar grid feed-in tariffs and renewable energy certificates, yet another problem with domestic solar panels has arisen. According to a story in The Australian newspaper on 13th October 2011, “The runaway take-up of rooftop solar panels is undermining the quality of electricity supplies, feeding so much power back into the network that it is stressing the system and causing voltage rises that could damage household devices such as computers and televisions. Power distribution lines and home wiring were designed for electricity to flow from power stations to appliances, but households with solar panels do the reverse of this”. This is an interesting concept and one which someone familiar with electronics might initially dismiss. After all, the electricity grid is not like a diode, is it? Why shouldn’t it be able to handle power flow from solar panels into the grid? In principle, if there was a small amount of “solar” electricity being fed back into the grid, it would not cause a problem; the power stations would simply generate less power to compensate and everything would be in equilibrium. And we would saving all those nasty “carbon” emissions, wouldn’t we? But as always, things are not that simple. It neglects the fact that the electricity generated in all the power stations has to travel long distances via high voltage lines and various substations and step-down transformers in the streets before it arrives at the customers’ meter boxes. And it is the substations and street transformers which are the basis of this problem. Basically, the energy retailer is able to compensate for local voltage variations in suburbs and streets as energy consumption varies throughout the day but overall, only a limited range of adjustment is possible by tap-changing on the transformers throughout the system. Then what happens if you have large numbers of domestic solar panels in a suburb generating lots of power during the day when consumption may be low? The voltage will inevitably rise, perhaps to levels which are well above what they are supposed to be. The consequences could easily be wholesale damage to domestic appliances and possibly to the grid-feed inverters which at the very least, should switch off. So what can be done about that? Now, while the electricity retailers can actually “dump” load if the system becomes overloaded, there is presently no way to disconnect domestic solar installations if the system voltage becomes excessive. In the meantime, according to the story in The Australian: “In Western Australia, Horizon Power has set limits on how much renewable energy can be installed in a system without affecting the power supply. Horizon is rejecting applications for new renewables installations in Exmouth and Carnarvon. “Energex spokesman Mike Swanston said it was becoming difficult for electricity distribution authorities to set up the power system to ensure correct voltages when some houses in a street had solar and others did not”. Ultimately, this problem might be solved by a change in the design of grid-feed inverters: once the voltage coming in from the street rises above (say) 245VAC, the inverters would be switched off and would no longer be able to generate power. This would protect other consumers but of course, those people who invested in solar panel installations would not get the full benefit. Worse, they might have to pay for power which, if the system voltage was below the threshold, they would otherwise be generating. This is yet another instance of the impracticality of the Green’s advocacy of Australia generating all its electricity from renewable sources. For this and a whole host of other technical reasons, it just ain’t ever going to happen. Leo Simpson siliconchip.com.au