Solar PV panes and power cuts

[jolinton]

Hi

I can understand that, during a power cut, an inverter connected to a PV array needs to shut down to prevent maintenance workers on the grid but I don't see why the inverter cannot still continue to provide domestic power. Surely all that is needed is an isolating relay which can distinguish mains power from generated power. Then as soon as the mains power is cut off, the inverter is isolated from the grid. What is the real issue here?

 

[SibertSolar]

On-grid inverters, compliant with G83 or G59 engineering framework are not designed to switch into off-grid/islanding mode when mains connection is lost

 

[snowhead]

The Real, issue is to prevent accidental / unintended connection of P.V to the grid.

With the vast numbers of P.V installations the probability of a failure of the safety interlock is far higher.
The consequence is a potential Death.
Doesn't take much to understand why the rules are as they are.

On the face of it the situation is no different to standby generator installations where generation occours within premises but is blocked from exiting by electrically and mechanically interlocked contactors, the difference is the total numbers.

 

[jolinton]

On-grid inverters, compliant with G83 or G59 engineering framework are not designed to switch into off-grid/islanding mode when mains connection is lost

I quite understand this. My question is - why does it have to be this way? I cannot see any technical objection to the inverter switching automatically into off-grid/islanding mode. Indeed I can envisage advantages to the power companies if ordinary domestic supplies were fitted with automatic isolating relays. They could, for example, be programmed to switch back online only after mains power has been restored for several minutes thereby avoiding massive instantaneous demand when power is switched back on and also avoiding that really annoying thing when a lightning strike causes a temporary power out causing your computer to reboot only to go down again a few minutes later.

 

[billt1968]

It's not that simple. Grid tied inverters are designed to be connected to a load (the Grid) which can absorb all the output of the panels. Your local load will vary continuously and won't be enough to stabilise the inverter output in sunny conditions and will overload the inverter in dull conditions.

There's no reason in principle why GTIs can't be designed to provide a local supply when the grid goes down and there is enough PV generation. In fact SMA are selling such a device in the US, but it only has one local outlet and won't supply the whole house. If SMA wanted to sell it in the UK it would need suitable type approval and I'd bet that the potential market is vanishingly small and not worth the expense.

If you want a maintained system then you need batteries, lots of them and a suitable system design. Something like the Sunny Island. Unfortunately that can't legally be grid tied in the UK and it will at least double the system cost.

 

[jolinton]

It's not that simple. Grid tied inverters are designed to be connected to a load (the Grid) which can absorb all the output of the panels. Your local load will vary continuously and won't be enough to stabilise the inverter output in sunny conditions and will overload the inverter in dull conditions.

Yes, I suppose that makes sense - though the inverter attached to my petrol generator has no trouble coping with either no load or overload conditions. Why is it different for a GTI?

The problem is, I haven't a clue how an inverter actually works. Am I right in thinking that it is an entirely solid state device? (I know that early inverters were simply a DC motor driving an AC alternator) Do modern inverters make any noise? (eg do they have fans?) Why is the life of an inverter limited? (I am told that they usually need replacing after 10 years at a cost of £1000. Is this true?)

Many thanks for your help

 

[sheephat]

The inverters use the main to sync our their AC supply, if you have no mains you have no voltage reference

 

[Gavin A]

Yes, I suppose that makes sense - though the inverter attached to my petrol generator has no trouble coping with either no load or overload conditions. Why is it different for a GTI?

The problem is, I haven't a clue how an inverter actually works. Am I right in thinking that it is an entirely solid state device? (I know that early inverters were simply a DC motor driving an AC alternator) Do modern inverters make any noise? (eg do they have fans?) Why is the life of an inverter limited? (I am told that they usually need replacing after 10 years at a cost of £1000. Is this true?)

Many thanks for your help

that's wrong - the design life for SMA, Power-One etc is around 20-30 years in ideal conditions, obviously this will reduce if placed in particularly hot dusty locations and the heat sinks / fans are left to gather dust, but that's the design life.

the micro inverters are solid state, with design lives more in the 50 year range.

We suggest customers allow for the potential for 1 inverter replacement inside the 20 year fit period so they can be pleasantly surprised if this isn't needed, which, EST etc give this advice on a worst case scenario basis, not on what the actual design life of the kit is.

Many of the inverters actually do have a stand alone mode they can operate in without the grid, but designed to run from batteries - the generation potantial from solar is too variable to be directly powering loads in the house, that's why it's really different to a generator based system - it could be chugging along nicely then a cloud passes over and the voltage and power would drop to a level that caused severe problems for your electrical kit / or the inverter would just trip out. So you need batteries, changeover switches and smart control mechanisms... or you could go without some of these and end up with a really badly performing system like some of the 'midnight sun' type systems that just suck up power to the batteries continuously whatever the generation level as far as I can tell (mostly from reports on here and checking the kit spec out)

 

[sheephat]

So its essentially a UPS with EcoFriendly Charging?

I don't know much about PV other than what I studied, my guess is the inverters I worked with needed a grid reference to work, maybe as a safety feature to prevent back feeding.

 

[Gavin A]

So its essentially a UPS with EcoFriendly Charging?

I don't know much about PV other than what I studied, my guess is the inverters I worked with needed a grid reference to work, maybe as a safety feature to prevent back feeding.

All G83 or G59 inverters need the grid to reference to, or they'll not work while at those settings.

Some of them though, such as some SMA inverters, do have the ability to work in other modes such as standalone, or even turbine mode for wind / hydro turbines, you just have to get into the installer settings and reprogramme them... they may well need sunny island though to enable them to switch between each mode automatically, actually I'm pretty sure they do, unless you're enough of a geek to know how to programme your own controller (which I'm not, but there are some people around who could do this).

 

[sheephat]

I'm assuming your premisis would have to be in island mode to comply with the DNO (if you have one)

Didn't know they could do standalone, guessing it would have to be an ideal day to get the power you needed for you lights and heaters during the day!