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.

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.