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 SILICON 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 David Maddison B.App.Sc. (Hons 1), PhD, Grad.Dip.Entr.Innov. Kevin Poulter 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, Warwick Farm, NSW. Distribution: Network Distribution Company. Subscription rates: $105.00 per year in Australia. For overseas rates, see our website or the subscriptions page 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. E-mail: silicon<at>siliconchip.com.au ISSN 1030-2662 Recommended and maximum price only. 2  Silicon Chip Publisher’s Letter Generating your own electricity during blackouts The Publisher’s Letter in the June issue certainly tickled the fancy of a number of readers, as they postulated how the “anti-islanding” feature of grid-tied inverters could be circumvented. We have published a few of their letters in the Mailbag pages this month. Of course, the reason for wanting to circumvent the “anti-islanding” feature is to trick grid-tied inverters into generating power when the grid is blacked out. But the grid-tied inverter “knows” when the grid is up and only generates power at that time, provided of course, that the solar panels are in sunlight. Well, the anti-islanding feature is pretty much bullet-proof, as it is meant to be, so that the inverter cannot feed power to an otherwise dead grid and possibly be a hazard to people working on the power lines. It cannot be tricked by simply connecting it to the output of standard DC-to-AC inverter with a sinewave output. Part of the anti-islanding feature is to measure the impedance of the grid and also test whether its frequency can be “pulled” slightly high or low. When the grid is connected, this isn’t possible. Some readers think that perhaps the anti-islanding feature of a grid-tied inverter is provided by a little module which can be disabled. We think that is highly unlikely and instead, it is part of the overall software. The only way to get around it would be to get into the software and modify those lines of code which provide that feature. Sounds simple but I will bet that even getting into the software would take some doing. All of which means that those grid-tied inverters which are available quite cheaply via the internet are pretty much useless for this purpose, unless you are a software guru. Still, the fact that the topic appears to be of considerable interest has us thinking as well. Why not produce an inverter which will run from the same solar panels as a grid-tied inverter? This would have to cope with the same high input voltages as the grid-tied inverter but be completely independent and would generate power when the grid-tied inverter was effectively disabled. Really, that is not too hard and we just happen to have the basis of a such a design already in our project archive. Which one is it? The answer is the 230VAC Induction Motor Speed Controller that we featured in 2012. Apart from some initial teething problems which led to the H-bridge module and current sensing resistor failing in a rather noisy fashion on some pump loads, it has now proved to be quite a reliable design, especially when driving 3-phase induction motors. So how does that help us? In effect, the Induction Motor Speed Controller contains a complete high-voltage DC-to-AC inverter, albeit one in which the output frequency can be shifted over quite a wide range to enable induction motors to be controlled. As it stands, it can accept around 340V DC (eg, from a solar panel array) and it will produce around 2kW at 230VAC. It would be relatively simple to configure as a free-standing general-purpose inverter. Unfortunately, producing such an inverter is only part of the story if you want it to power your household. You would have to able to completely isolate your household wiring from the grid and then decide which circuits would be powered and so on. That would all need to be done by a licensed electrician and the whole exercise is not likely to be simple or cheap. However, there is now a better solution: a hybrid grid-tied inverter which has battery back-up. This enables you to control the amount of power you export to the grid and instead use it to charge batteries which can power the inverter when the sun goes down and more importantly, let you generate power when the grid is down. So that clearly is the answer but it means that all those thousands of existing solar panel installations can only run at night or when the grid is blacked out by having the inverter changed to a hybrid type. At the moment though, that is a really expensive solution. Leo Simpson siliconchip.com.au