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Exporting Solar
- Size of your solar array and your inverter in kw
- MCS certificate for your installation
- Letter from your DNO showing the approval for export
- Export MPAN for your smart meter
Typically, the size of your solar array will be larger than the size of your inverter. The size of the inverter determines the maximum amount of power you can generate to power your house and/or export and therefore get paid for. You need this to work-out your pay back / value for money.
To be paid for export, your installation needs to be MCS certified. Your installer should have provided an MCS certificate as part of their handover pack. This will detail your Total Installed Capacity (TIC), Declared Net Capacity (DNC) and your estimated annual generation.
To export to the grid, you need DNO approval to connect generation equipment (see ENA for more info for your region). There are several categories of approvals:
- G98 approval can be applied for post-installation and allows you to export up to 3.68kW. This is the most common approval that installers go for as it does not delay installation. However, it limits the amount you can export (and get paid for).
- G99 approval is applied for prior to installation and can take some time. The amount you can export will normally be linked to the size of your planned installation. Export approvals for 6kW with single phase supplies or 10kW with three phase supplies are not unusual.
- G100 is a special form of G99 approval. In addition to an export limit, this approval requires a physical export limiting device to be installed.
Your inverter will also need to be on the ENA type test register. The DNO will make a charge for approval and provide a signed letter that states what approval has been granted.
If you have G98 approval but your solar array / inverter is larger than 3.68 kW, an export limit may have been set on your inverter by your installer. Once you hit this limit, the inverter will back-off the power it generates from your solar array regardless of the size.
Once you have the relevant documents, you can apply to an energy supply company for an export tariff under the Smart Export Guarantee scheme (SEG). If you don’t already have an export MPAN, they will apply to your DNO for one to be allocated. This is different from your import / supply MPAN that appears on your bills. It allows your energy company to get smart meter readings for export from Smart DCC. This process should only take a week or so, but recently, people applying for export have been reporting longer delays. You can’t be paid for export until you receive this.
When everything is setup, your energy supplier will pay you for each kWh you export, normally by crediting your energy account.
Once you are setup for export, you may find your inverter stops for 10 minutes with "10 Minute Grid Voltage Fault" displayed. It may then reconnect but drop off again a few minutes later and keep repeating this. Why does this happen?
In the UK, the grid voltage specification is 230v AC 50Hz RMS, -6% + 10%, giving a voltage range of between 216.2v and 253.0v. To export power to the grid, the inverter has to increase it's output voltage to push power back down your supply cable into the grid. There is a small but significant resistance in the inverter wiring, circuit breakers, meters, fuses and the grid supply cable that runs to your house that the inverter has to overcome.
So, for example, if your normal grid supply voltage is at the upper limit of 250v, the inverter will need to generate a higher voltage than this. The more power you export, the higher the voltage needs to be. This can also be made worse on sunny days if other houses in your street have solar as the sun shines on all your houses at the same time and this makes the grid voltage in your shared supply cable rise even further.
Your inverter constantly monitors the grid voltage via it's connection, including the voltage it generates. For safety reasons, if this voltage exceeds a threshold, the inverter shuts down. There are many reasons for this, for example, if your grid supply cable developed a fault, the inverter might generate a dangerously high voltage that would damage the appliances in your house.
A 10 minute grid voltage fault occurs when the inverter detects the average AC grid voltage is higher than allowed. It disconnects itself from the grid for 10 minutes and then tries to reconnect again. However, often the cause of the high voltage is the inverter itself pushing up the local voltage to deliver export power.
If this happens, there are number of things you can do:
- Go to foxesscloud, Devices, Inverter, More Options and check the history for RVolt. Look for peaks and troughs outside the range 220v to 250v
- Get the cables to your inverter checked to ensure they are properly connected and tightened down
- Contact your DNO and ask them to check your supply voltage. Ideally, this should be between 230v and 240v.
- Change the settings on your inverter so it tolerates a higher grid voltage: Settings, On-Grid, Grid Para. Do not exceed 262v.
- Set a limit on the export power the inverter is allowed to generate: Settings, On-Grid, Export Control
When you export, your inverter has to push the export power back into the grid. In order to do this, it has to get the power from inside your inverter, through you isolators, circuit breakers, meters, henley block, supply meter, main fuse and back down your supply cable. All of these items add resistance to your export path and mean the inverter has to crank up the voltage above your incoming grid voltage to reverse the power flow.
Because your inverter sits on top of all your supply stuff, the internal voltage that it senses when exporting is higher than your grid voltage and this is what leads to the 10 minute grid volt fault. In practice this means, if your grid voltage is the upper end of the range, around 252v, your inverter has to go higher than this.
If you contact your DNO, they are likely to tell you they will not do anything unless you have already set your inverter to 262v. In effect, they are allowing 10v headroom for export.
This is higher than the grid supply specification that your DNO has to meet when delivering power to you but, in practice, only your inverter will see this. By the time it gets to your appliances, the voltage will have dropped a bit and by the time it gets to other grid users, it will have dropped even further.