devices to restrict export

[solarsavings]

We are having trouble getting any G59/3 applications passed around here now I'm not talking Gavin or Jason stylee but 6-12KWp

National Grid has informed us that we cannot connect any further generation to our site at Burwell Main Grid, which is the point at which we connect with National Grid, without submitting a statement of works. This statement of works attracts a non-refundable fee of £7500 which would need to be paid by the customer in advance. The current guidance from National Grid is that any additional generation connections would require a new Super Grid Transformer. This would also mean substantial reinforcement work on our network.

We do allow the use of devices to restrict export, but in those cases we will want to witness test the installation to ensure the device is effective for which there is a fee of £900 + VAT. If you wish to install a device to restrict export the export would have to be set to zero for the entire system. I realise this may seem illogical but there are reasons for us adopting this position.

Is there a reasonably priced way to prevent export and comply with the above?

 

[Gavin A]

erm sort of, 0W export is pretty much impossible though, you'd need to get a bit of leeway on that to at last allow a few hundred watts. SMA only goes down to 10% of inverter capacity for example, and no CT clamp really works below about 20W.

All depends on the on site consumption patterns, not a problem at all if they're using minimum 20kW continuously in daytime and you want to put 15kWp on it, but would be a problem if they only had 10kW consumption levels and you wanted to put 15kWp on.

But then you could get into battery storage etc.

first step is to get the half hourly consumption data from the site, assuming they have it.

 

[Worcester]

The answer we've had back for your location is that they effetively can;t refuse 3.68kw / 16A per phase export....
So in practice limiting it to below that is straightforward.

SMA are currently developing their own product - it;s going through DNO approlval at the moment - based on what they currently do in Germany so 90% of the work is already done, and costs (compared to other solutions) are very low.

 

[Gavin A]

my issue with the way both SMA and ABB kit works is that it's not fail safe. So if the control unit or communications cables fail then the inverter reverts to full power.

I think they should do it the other way round, so the inverter is restricted to it's export limit unless it gets the signal from the controller to raise the generation level, that way it should be pretty much impossible for it to ever go wrong (outside of someone resetting the firmware etc).

 

[SibertSolar]

Various DNOs are now catching up with the available solutions in the marketplace, and there are efforts by the likes of the BRE to put together a standardised document/guide for designing/implementing export management and commissioning (not easy!). Worcester is correct in that Germany have great experience in this but they do it slightly differently over there as their FiT structure is different - they tend to work on a max 70% export by default. As such, most German inverter brands will already have the capacity to be throttled back to a fixed peak output value. Most of those same brands will have some form of PMU (power management unit) that talks to the inverters over their RS485 comms bus. Stepped throttling tends to be 0%/30%/60%/100% via digital inputs to the PMU from an external monitoring/measurement source (Elios4You for example) that is set up to trigger at a defined level of exported power or current. In some cases (our Advanced Energy units for example) this level of % throttling by the PMU can be custom programmed with 10% granularity.

Simple methods include just switching off one or more inverters. More complex methods involve infinitely variable closed loop monitoring and threshold triggers, leading to controlled and variable throttling of the inverters from 0%-100% as required.

The trick is satisfying the DNO that the system is fail-safe, as well as being able to test/commission the system in-situe. This may involve additional electro-mechanical components such as current/power relays, relays/contactors, timers and additional CT clamps.

Load-shedding is another string to your bow, when considering how to meet DNO's export restriction requirements. Devices such as power diverters (immersion controllers) and battery stores are also useful when trying to limit export to the grid. Understanding the client's load profile is key here though, which is why we promote a decent monitoring solution be put in place prior to assessing/quoting for such a solution.

Zero export is difficult but not impossible, given a little flexibility by the DNO in the amount of time lag they will permit in the solution. In Germany I believe they work to an EU standard of 10 minutes maximum response time but I could be wrong on that one.

Most export limitation solutions we have worked on in the past 12months have been to a 16A/phase limit, ie: G83...

 

[Gavin A]

the issue one dno engineer raised with us recently on the whole 16amp thing was the point that the inverter specified wasn't actually type tested to G83......

 

[SibertSolar]

Hi Gavin, fair point but it's still a G59 installation though, isn't it? The inverter is simply being limited, it will still respond to/protect the grid based on its G59 settings, the export limiting solution shouldn't interfere with that. Or was the engineer you spoke to suggesting that the imposed limit of 16A/phase meant that the generator HAD to be certified to G83? First I've heard that being flagged up tbh.

 

[Gavin A]

Hi Gavin, fair point but it's still a G59 installation though, isn't it? The inverter is simply being limited, it will still respond to/protect the grid based on its G59 settings, the export limiting solution shouldn't interfere with that. Or was the engineer you spoke to suggesting that the imposed limit of 16A/phase meant that the generator HAD to be certified to G83? First I've heard that being flagged up tbh.

it's only a G59 installation if they allow it to be, if you're relying on the 16amp G83 automatic connection allowance, then the DNO can scupper this by insiting you use a type tested G83 inverter if they really don't want it installing.

 

[jason121]

I am still waiting for reply from WPD regarding the SMA version. Minimum reaction time is 5mins.