Solar PV to a subpanel

2011

2011 wrote: »

They need a local earth as they are large, conductive and quite some distance away from the earth electrode of the existing installation.

Yes but if it's a metal frame buried in the terra firma does it still require an electrode. Is the support not exactly that or is it concrete we are trying to avoid.

What lightning arresters do you recommend Freddy?

2011

Sir Liamalot wrote: »

Yes but if it's a metal frame buried in the terra firma does it still require an electrode. Is the support not exactly that or is it concrete we are trying to avoid.

Concrete is not a great conductor so a metal frame stuck into a lump of concrete is going to have a high resistance to earth compared to an earth electrode that goes 1.5m into the ground. You want to avoid a person that is touching the frame having a potential difference across them. Remember the MET that the frames are connected to could be at a very different potential than the ground local to the frames.

Perhaps this does not require earth electrodes, but this should be assessed, demonstrated and documented before a call is made. This can be tested by measuring the resistance between the frames and earth when disconnected from the MET. Not that hard to do ( see Wenner test).

Regarding lighting protection:
Normally a specialist company would be called in. They would carry out a risk assessment and a design would be based on this.
Copper tape (for example 3 x 25mm) is normally used for the down conductors for a lighting protection system. These would be secured every foot or two. Air termination points would also be normal. Small cross bonding cables are not comparable.

I'd try to avoid pouring concrete tbh.

I see these things a lot advertised as lightning arresters for PV.



Just wondering if they're mustard cutters?

freddyuk

Sir Liamalot wrote: »

Yes but if it's a metal frame buried in the terra firma does it still require an electrode. Is the support not exactly that or is it concrete we are trying to avoid.

What lightning arresters do you recommend Freddy?

You can use ground anchor posts thus doing away with concrete entirely and having a good earthing of the frame. Takes seconds to install each post with jack hammer/generator and correct kit.

I am not recommending any safety devices as this should come from the supplier. As I have said a direct hit is not going to leave much standing. An arrestor on the din rail at the combiner box is the way I would do it. However I cannot recommend something that I cannot test! It is possible it will save the electronics but not certain and I have never had to find out! As it is a gamble you are simply increasing your odds by having something in the circuit.
Here is one supplier http://www.surgedevices.co.uk/ (I have no connection with them)

freddyuk

freddyuk wrote: »

However I cannot recommend something that I cannot test!

Yet another reason I need a Tesla coil. :pac:

2011

It is important to note that surge protection and lighting protection are two very different things.

2011

freddyuk wrote: »

You can use ground anchor posts thus doing away with concrete entirely and having a good earthing of the frame.

This may be possible, but how the only way to know that is to test it and determine that the connection to earth is satisfactory.

cros13

2011 wrote: »

How is the cable between the solar panel and the inverter protected??

Both are about only a few tens of cm long. it's just the fixed pre-terminated cable at the back of the panel.

freddyuk wrote: »

1. If we are talking grass and hedge shading then arrange the Array in landscape rows thus the top row will be full chat until dark while the bottom row will be maximised by the inverters as far as they can. Shading the bottom of a module in portrait will kill 100% of the output.

True, just giving that as an example. I'll be mounting portrait, because it'll be a bit tidier for mounting and cabling. The array is positioned a best as I can manage but there's all sorts of shading issues with ground mounted arrays in the real world.

freddyuk wrote: »

2. My point about lightning is that a direct hit will wipe out the installation whether grounded or not? Having a nearby strike can put transient spikes into the system which can blow sensitive electronics so how does the system handle this? In a string inverter the inverter can be "isolated" between the DC incomers and the inverters to help prevent damage but this would not happen with micro inverters? Enphase seem to recognise the problem and recommend suppression in addition to the inbuilt protection which suggests the inbuilt protection is not really going to help being attached to the module.

Sure, but I'd consider this to be more of a problem if it was a roof mount without the local earth of the ground mount arrays. The whole lightning protection for solar arrays is a bit... overstressed. There are plenty of arrays out there with all sorts of more serious issues. With lighting protection the only thing I'm worried about would be the shed and house. Common bonding should help avoid a lot of issues.

freddyuk wrote: »

As you have already committed to micro inverters you may need to invest in better protection as they are fragile compared to string inverters with robust protections inbuilt. Having a spare string inverter which can be replaced in a few minutes is a good plan. Changing all the micro inverters and attached comms would be a headache.

I think the opposite, comms for the microinverters is wireless. So to switch out a microinverter I can just cover the panel, pop the MC4s and connection from the AC trunk (with a disconnect tool), take off two bolts and the grounding bolt, swap in the new one and re-pair when I get back inside. A string inverter is at least a two man job even lifting it.

A spare string inverter is ~€1500 sitting there. A spare microinverter is less urgent because in most situations the 49 other microinverters will be working fine. Even if I want to buy a spare they are only €85 a pop.

2011

cros13 wrote: »

Both are about only a few tens of cm long. it's just the fixed pre-terminated cable at the back of the panel.

Fair enough. I guess a short is unlikely but it would be bad news for the solar panel.

Sure, but I'd consider this to be more of a problem if it was a roof mount without the local earth of the ground mount arrays. The whole lightning protection for solar arrays is a bit... overstressed.

I agree and I think a risk assessment would come to the same conclusion.

The only time I had lightning protection installed on a metal structure that was at a comparably low level was a tank farm which stored flammable solvents.

freddyuk

2011 wrote: »

It is important to note that surge protection and lighting protection are two very different things.

Aren't we concerned with surges caused by lighting? If there is lightning about it does not have to touch the installation to cause potential damage via voltage spike. That is my understanding.

2011

freddyuk wrote: »

Aren't we concerned with surges caused by lighting? If there is lightning about it does not have to touch the installation to cause potential damage via voltage spike. That is my understanding.

I agree with all of the above, but my point still stands.

air

2011 wrote: »

My question is does the DC output from the solar panels have a reference to earth?
When you say that the voltage output from a string is up to 600V is that with reference to earth?

The DC output of the panels is totally isolated from the panel frames and earth normally. String inverters continuously monitor resistance to earth and shut down if it reduces below an arbitrary figure - usually a few M ohms.
So someone would have to expose and grab both conductors in order to receive a shock from the wiring if the system was in operational condition.

air

2011 wrote: »

Fair enough, but until that day arrives why bother syncing? Life becomes simpler when you don't have to sync. You can design it so that it can be synced at a future date. You won't need it certified until it is synced.

You can't do anything useful with solar panels unless you either sync them to the grid to offset import or install a battery system. The micro inverters that the OP is installing won't do anything at all without a grid connection.

2011

air wrote: »

The DC output of the panels is totally isolated from the panel frames and earth normally. String inverters continuously monitor resistance to earth and shut down if it reduces below an arbitrary figure - usually a few M ohms.
So someone would have to expose and grab both conductors in order to receive a shock from the wiring if the system was in operational condition.

Well I suspected that this may be the case until the OP posted this schematic in post #24 where it describes the negative wire on the DC output as "grounded conductor white":



The reason I asked the question is because I am working on the design of a DC system coupled to an inverter at present as part of a project at work.

You can't do anything useful with solar panels unless you either sync them to the grid to offset import or install a battery system. The micro inverters that the OP is installing won't do anything at all without a grid connection.

Yeah, I see what you mean. I have no experience with PV arrays.
I was just thinking aloud.

air

No worries, my comment was with reference to string inverters, not micro inverters.
There are actually string inverters that can operate with either side earthed, I think it's a requirement for some thin film amorphous solar panels.
You may as well just ground the negative on the system at work, it's the common convention.
Just beware if you ground the positive for some reason that the wiring colours reverse!

2011

air wrote: »

No worries, my comment was with reference to string inverters, not micro inverters.

Fair enough.

You may as well just ground the negative on the system at work, it's the common convention.

Well as it happens I decided not to, it is an isolated system

Just beware if you ground the positive for some reason that the wiring colours reverse!

Interesting, I never heard that.

air

Isolated battery systems are well and good at extra low voltage but I wouldn't be a fan of them without monitoring above that, especially if the battery system is reasonably accessible / open. Too much risk of the system becoming inadvertently earthed somewhere leading to unexpected results for those maintaining it.

2011

air wrote: »

Too much risk of the system becoming inadvertently earthed somewhere leading to unexpected results for those maintaining it.

What do you mean by "unexpected results" ?

This is common practice in certain applications such as marine and regenerative braking systems on vehicles. For me the advantage is that the system can sustain a single fault, making it more robust.

air

Without monitoring you're relying on that isolation being maintained for the single fault tolerance to be preserved. Submarines etc used lamps to indicate isolation faults for this reason.
If the battery system or any DC load becomes inadvertently grounded at any point due to a fault or miswiring you'll have no way of knowing. Then you could have someone touch an exposed battery terminal for example and get a shock where they previously got none. Not such a big issue for low voltage batteries obviously but at higher voltages it's very significant.

On the wiring colours, it makes sense. Blue is 0V regardless of whether the system if positively or negatively earthed. Gives people half a chance if they start mixing up AC and DC supplies I guess.

2011

air wrote: »

Without monitoring you're relying on that isolation being maintained for the single fault tolerance to be preserved. Submarines etc used lamps to indicate isolation faults for this reason.

Agreed.

My understanding is that isolated systems are used on lots of other marine craft (such as ferries and trawlers) for critical systems (such as navigation equipment) due to its single fault tolerance.

If the battery system or any DC load becomes inadvertently grounded at any point due to a fault or miswiring you'll have no way of knowing.

In our case we would check this during commissioning.

On the wiring colours, it makes sense. Blue is 0V regardless of whether the system if positively or negatively earthed. Gives people half a chance if they start mixing up AC and DC supplies I guess.

Yes, it makes sense. I just hadn't seen this in any standard, can you point me to the document that states this?

I have been looking at IEC60445

air

2011 wrote: »

My understanding is that isolated systems are used on lots of other marine craft (such as ferries and trawlers) for critical systems (such as navigation equipment) due to its single fault tolerance.

I think it's more to do with avoiding galvanic corrosion. The same reason the postive is referenced to earth in telecommunications DC systems - this eliminated the disappearance of buried copper cables

2011 wrote: »

In our case we would check this during commissioning.

This won't help with faults that occur following on from commissioning and is the reason why detection systems are used in isolated DC systems such as submarines and normal PV string inverters.

2011 wrote: »

Yes, it makes sense. I just hadn't seen this in any standard, can you point me to the document that states this?

I don't have anything to hand at the moment, it's mentioned here but doesn't quote the relevant IEC standard. Note that blue is always earth in earth referenced systems.
https://en.wikipedia.org/wiki/Electrical_wiring_in_the_United_Kingdom#Wiring_colours

2011

air wrote: »

I think it's more to do with avoiding galvanic corrosion. The same reason the postive is referenced to earth in telecommunications DC systems - this eliminated the disappearance of buried copper cables

Edit: Corrosion is an important consideration, but not the only one.

When I went through this with our resident expert I was told that the risk posed by loosing critical systems on marine craft was in many cases considered greater than the risk of a single fault to earth.
Besides the risk of a single fault being undetected is mitigated as the resistance to earth is generally monitored.

This won't help with faults that occur following on from commissioning and is the reason why detection systems are used in isolated DC systems such as submarines and normal PV string inverters.

Agreed. I am not arguing that there should be no detection system.

I don't have anything to hand at the moment, it's mentioned here but doesn't quote the relevant IEC standard. Note that blue is always earth in earth referenced systems.
https://en.wikipedia.org/wiki/Electrical_wiring_in_the_United_Kingdom#Wiring_colours

Thanks. I doesn't apply in my situation, I'm just interested in seeing it.

air

You hadn't mentioned monitoring which was why I didn't think you had included one.

A further motivation for a floating system in a submarine would be to reduce the chances of an arc fault. It would take two concurrent faults to create an arc with an isolated system. The risk of explosion would be greatly reduced and submarines aren't very tolerant of internal explosions for obvious reasons.

It's horses for courses really. An EV's battery system is totally sealed in normal operation and there is very little switchgear involved between panels and inverter for a high voltage PV system.
Well encapsulated systems like these lend themselves towards isolation. Submarines and vessesls similarly but for altogether different reasons.
Conversely in the telecoms environment a floating system wouldn't be suited. Lots of exposed conductive elements of the systems and multiple independent voltage sources which could sum if left floating. Isolated 380V DC is beginning to be used but only for distribution and it's converted back to 48V at point of consumption for compatibility with existing equipment.

Getting back to the OP, on the battery systems you should check out the Growatt SP2000 system for "phase 2". It's a 5kWh (4kWh usable) lithium based battery system with integrated inverter. They seem to be about £3500 but some have sold for around £1400 on eBay of late.
Personally I can't see much financial justification for storage in Ireland just yet.

cros13

Tesla are selling the powerwall 2 for £5400(€6300) in UK incl VAT ex works retail. I have an installer in the UK who's a friend, has provided the official training materials and is willing to provide the powerwall and hardware at cost ex works.

The AC varient is 13.2 kWh (DC has a 13.5kWh DoD) and comes with​ bidirectional inverter built-in. Capable of supplying 5kW continuous or 7kW peak.
In this particular case it gets a payback time under the 10 warranty, bridging the time between production hours and the nightsaver kicking in with the remaining power dumped into the cars.

The whole thing is a bit of a hobby project for all involved, so it doesn't have to break even. But it's surprisingly close to doing so in 5-10 years, proof will be in the eating of it... but its looking like it.

air

Well the Growatt is DC coupled so no good for you if you go ahead with micro inverters. Personally I think the micro inverters and DC optimisers are a waste of money, especially in your case with no significant shading. String inverters are way cheaper than they used to be.
Finally I think that the Tesla is over sized for a 4kW array, you'll rarely get it charged. I'd put in the second 4kW of panels first. What's your daily average consumption at present?

Not to mention that inversion losses at every panel, string conductor/co-ordinator losses at the installation, losses to DC battery charging ina AC coupling inverter followed by inversion losses for demand filling is a right wasteful way to do things. The system idle is gonna be massive.

PV panel -> battery -> load..keep it simple.

The Growatt looks interesting...nearly got a luddite like me swayed.
You'd have to pay me to take a powerwall. That's like arguing PC or Macintosh though.

freddyuk

Recent news reports highlight the moral hazard of lithium based batteries and there component supply lines. There are cleaner/healthier options out there.

Do you mean that using ~ 900 cells to make 48V could be considered a mite unsustainable Freddy?

Heavens to Betsy...

air

What are the cleaner / healthier options freddy?

If I might be so bold (I think Freddy has better things to do than instigate Holy Wars over which battery chemistry is best...application subjective anyways).

I believe he's a proponent of AHI.

I'm firmly rooted in the lead acid camp.