Solar PV - Impact on rating of supply cables and transformer

There may be some confusion, but these problems usually relate to solar installations that export a lot - and  200kVA, or a fifth of a megawatt, really is a lot. Transformers and street cables are sized assuming that the voltage starts high at the supply end of the line, and slopes down towards the load, and the cables are sized so this all works out with maximum permitted voltage drop at the far end. At least until something changes.

With embedded generation, the sense of that voltage slope is now reversed, as current is flowing against the supply, rather than with it, and so the properties at the far end of town that barely got 220V when it set off at 250V at the transformer is now have to push in quite a lot more than 250V to get any sensible export current.

For very good reasons the over-voltage permitted at the far end of the line while the solar panels are generating has to be limited, so the line, and by the sound of it the transformer in this case have to be re-sized, so that the starting voltage at the transformer is lower and with current in either direction, the voltages due to the drops at all points along the line stay within limits.

One way the DNO can describe this fatter cable requirement, is to consider an equivalent system with a higher load, that needs the same cable size. It is partly a historical hang over, from when the only  reason for cable upgrades was a higher load.
There is also often a  need to re lay out at your end with the rating customer end  switchgear and so on, as in principle it may be that there are parts that carry both current from the external supply and the generated current, and might  no longer be adequately protected for the theoretical maximum total current. But if you are starting from scratch that will not apply to you and will be factored in from day 1.
Mike.

sparkiemike said:

One of the solar PV suppliers has said that we would need to add 200kW to our maximum demand figures when informing the DNO of our requirements which would probably take us to next size of transformer.

While Mike is right that historically, and when roughtimating transformer sizes, people do sometimes look at it this way, the correct approach would be to specify the generation, and of this, the required export capacity, separately to demand, through the G99 application process. It should not be added like this on the maximum demand you have on your connection agreement.

You may, for example, have an export (or, indeed, demand) limiting scheme in place, which might completely mitigate the voltage regulation issue, but would not necessarily save you considering other effects, like voltage step changes and harmonics. Conversely, depending on the equipment installed locally and design of the generator, there may be changes to other DNO equipment (e.g. reverse power or NVD protection)

Your installer, or designer, should be able to assist with the G99 process. Often installers prefer to go through G99 after being contracted, unless you're paying them separately for it or they have preferred bidder status, but bear in mind that it may not be quick for a project of this scale and may even mean additional works beyond the transformer sizing. Noting that you seem to have started DNO works but haven't yet got a PV installer in contract, you presumably already have a contact with your DNO who might at least be able to give you a steer as to whether its liable to affect equipment sizing on-site now or whether you can pick an installer and apply for generation connection "at leisure".

In most circumstances, you wont need to add the 200kva solar production to the load. If the load is say 250kva, and solar production is at the full 200 kva, then the net load is the difference between the two, or 50 kva.

Calculate for the worst case which is probably maximum load at 16-00 on a winter evening with no solar, or it might be maximum solar production on a sunny bank holiday, with very little energy being consumed.

If the installed load is say 1,000 kva, then the worst case is full load and no solar power, as might reasonably occur on a winter evening.

If the installed load is small in comparison to the solar power production, then the worst case is minimum load outside of working hours in bright sun.

If the transformer is on site or adjacent, it might be desirable to have it set to a slightly  lower voltage such as 415 volts rather then the widely used 433 volts, no need to allow for voltage drop in lengthy LV mains if there aren't any.

Caveats, if solar production might exceed the load, make certain that the transformer wont trip on reverse power.