Lately, the inverter connecting my Powerspout to the grid has been signalling an error message. I've got to the bottom of it now and cured the issue but the journey has been an educational one which I thought might be helpful for others to know about. The inverter is an SMA Windyboy, which is the same as the SunnyBoy, and what I describe only applies, I think, to these two SMA inverters.
In this situation, SMA's trouble-shooting guide instructs you to exclude a genuine earth fault by inspection and testing. If this first-off approach doesn't reveal what's wrong, the next step is to test the 2 varistors housed within the inverter because, the guide says, a failed varistor can cause the Earth Fault warning to be displayed.
In an SMA inverter, the varistors look like this:
The trouble-shooting guide says to test for continuity between B and C, ...if there is no continuity the assembly needs to be replaced; if continuity exists, ...look for a fault elsewhere in the inverter.
It was only sometime later, after I had removed the insulating shroud and seen there were two components beneath it, that I realised testing in this way only tests for continuity across the thermal fuse; it cannot test the functionality of the varistor itself, which is the blue disc-shaped component.
When I did the continuity test, both the varistor-cum-fuses showed continuity, and so I concluded, as the SMA literature had led me to believe, that they were OK. But a week later, having exhausted all other possible causes of the fault condition, I replaced both with new ones and, hey presto, no longer was Earth Fault displayed: I had got to the bottom of it 😊. (** - but see post scripts below)
So what's the 'science' behind all this ? The purpose of a varistor is to eliminate voltage surges which might damage the equipment the varistor is protecting; they are 'sacrificial' devices, meaning they can be destroyed by the excess energy they absorb, and they are also 'wear' components, meaning they gradually lose their function from the cumulative effect of absorbing energy from lesser voltage surges which are not great enough to destroy them.
In extreme situations, varistors can catch fire, either because of the magnitude of the energy passing through them or because of the duration the energy flow exists for. In this situation they pose a fire risk which might destroy the very device they are meant to be protecting and to mitigate this, a thermal fuse is sometimes included in series with a varistor, - a fuse which will 'blow' and terminate the supply through the varistor if its temperature exceeds a set point. This is the arrangement of the two components found in what SMA call 'their varistors'.
SMA state that 'their varistors' are specially manufactured and are not commercially available, - except, of course, from SMA. The cost (in 2017) is €15 plus shipping and VAT (total €29.75), for a pack of two (part code SB-TV3, with insertion tool).
The two components that make up 'their varistor' can however be found on the open market, and with a soldering iron to connect them in series, they can be made up more cheaply. The varistor is an Epcos S20K320 (Vrms 320v, VDC 420v, Imax 8000A, Wmax184 joules Pmax 1 W) and costs 0.55p from Farnell (order code 100-4305).
The thermal fuse is more difficult to find but I managed to track down 6 on eBay: it is a Tamura E3F 250v, 3A~, 115℃ and each one set me back €2.20, inclusive of p&p. But it can also be purchased direct from China for less than this.
Why should the varistors have failed in my inverter to cause the Earth Fault warning? I figure that with 12,492 hours of continuous operation, at ~300v DC, the natural 'wearing' process going on in a varistor is accelerated and probably accounts for their failure. After all, an inverter handling power from a water turbine operating all the time sees a lot more use than one connected to a wind turbine or PV array.
12,492 hours is about 17 months and so I'm thinking this is the interval at which I should expect to see the Earth Fault warning recurring, - and therefore that I should have enough stock of the varistors and thermal fuses to meet that sort of replacement frequency.
It was nice to have gained a little more understanding of how the technology was designed to work.
** note added 3rd September 2017: the fix turned out to be not long lasting. Within 2 months, the inverter was displaying Earth Fault again, initially intermittently and then permanently. Replacing the varistors yet again, with new SMA ones, failed to get the fault light to extinguish***. I conclude that either there is truly an earth fault somewhere or the sensing circuitry within the inverter has gone wrong. I think the latter more likely. It being an issue which does not affect performance, I plan to do no more about it.
***yet another note added 12th March 2018, - following my further replacement of the varistors, which failed to extinguish the earth fault warning, I did nothing. After a while, I'm not sure how long, perhaps 2 weeks, the earth fault warning DID disappear, - and it has remained off ever since up to the time of writing this note. Perhaps new varistors just take time to 'bed in'.
****an even more recent note added 4 Aug 2018, - the warning light remained off as related above, until 3rd July 2018; from then it has been on constantly; so today I first replaced the varistors again: no change, - and then removed them completely: still no change. It would seem therefore that the varistors are not the cause, as was surmised above, but if the warning light extinguishes in the next few days, that hypothesis will not be right. I'll post another update if that is the case.
*****this saga runs on and on ! - and since I see from Blogger stats that quite a few people visit this posting, I have written this latest update.- this one is dated 22 Oct 2018.
- As recounted above, on 4th Aug I removed the varistors completely and the earth fault persisted. Being somewhat fearful of operating without the varistors, I re-installed them sometime late in August. Earth fault was still displaying.
- I then changed out the entire inverter for a brand new one (also a Windy Boy 1200) and the earth fault STILL displayed. So at this point I had to conclude there truly WAS an earth fault in the turbine or transmission cable.
- Wanting to keep the brand new WB 1200 with no/few hours of use, having seen that it too displayed earth fault, on the same day as having put it in, I changed it out back to the original WB 1200.
- I then re-started packing the 'dry-side' of the turbine, i.e. the alternator side, with silica gel. I had stopped doing this some months before because the bags never lasted more than 3 weeks before needing re-charging. But this time, I resolved to stick at keeping the dry side below 50% RH. And what should happen after about 2-3 weeks, to my astonishment, the earth fault disappeared. I suppose that must have been about 5 weeks ago (i.e. mid Sep) and the fault has not re-appeared to the date of writing this. No doubt there will be a follow on in due course !!
****** further note added 14 Jan 2019: from mid Sep 2018 until now earth fault has NOT been displayed 😊. In this time, I have kept the silica gel bags doing their drying action and have also instituted steps to minimise water entering from the wet-side (see here). I have also changed the dump load element for a new one because the original was beginning to show pitting of the chrome plating. Which of these interventions, if any, was instrumental in stopping the earth fault displaying I am at a loss to say.
******* further note added 28 Jun 2019: earth fault has not displayed since mid Sep 2018 😊 😊. In this time, use of silica gel has not been constant and I now do not use it at all. My conclusion is that it never played a part in solving the earth fault issue.
The dump load heating element was changed on 15 Nov 2018, as mentioned above. The earth fault warning had extinguished before this change was made so it can hardly have been instrumental in curing the problem, - unless the problem was an intermittent earth fault which has been eradicated by putting in the new element. This is the most credible explanation to my mind. Some time I'll get round to measuring the insulation resistance of the old element to see if it falls below that of a new one.
******* further note added 27 March 2020: the earth fault warning has not returned.
******* further note added 28 Jun 2019: earth fault has not displayed since mid Sep 2018 😊 😊. In this time, use of silica gel has not been constant and I now do not use it at all. My conclusion is that it never played a part in solving the earth fault issue.
The dump load heating element was changed on 15 Nov 2018, as mentioned above. The earth fault warning had extinguished before this change was made so it can hardly have been instrumental in curing the problem, - unless the problem was an intermittent earth fault which has been eradicated by putting in the new element. This is the most credible explanation to my mind. Some time I'll get round to measuring the insulation resistance of the old element to see if it falls below that of a new one.
******* further note added 27 March 2020: the earth fault warning has not returned.
******** further note added 30 Jan 2022: earth fault has not re-appeared since a new dump load element was put in 15 Nov 2018; there can be little doubt the previous element was the cause of the earth fault. Problem solved !
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