SMA offers solutions for restricting power exports

One of the major expenses incurred when installing commercial scale PV systems over 16A per phase can be the cost to upgrade the grid infrastructure to accommodate the increased power flowing from the array the grid. When installing a lot of PV onto a roof, the DNO (Distribution Network Operator) has to ensure that the cables and transformers are able to deal with whatever energy the PV system is going to export.

High days and holidays

However, in many cases such large PV systems are installed on premises that normally have high levels of self-consumption such as a factory. Where a business consumes a lot of electricity during daytime hours, then most or all of the electricity being produced will be consumed directly on site with very little being exported. Only on dormant days such as weekends and bank holidays is there the potential that all the power generation will end up exported onto the grid and it is this eventuality that the DNO has to take into account.

Therefore it may be sensible to agree with the DNO that no more than a pre-determined amount of electricity can ever be exported onto the grid. This is a mutually beneficial arrangement, because the DNO doesn’t have to worry about large amounts of electricity entering the grid on weekends and bank holidays (when it is least needed) and the client does not have to pay to upgrade the grid to cope with such large currents.

Restricting the amount you export

Export restriction is now a requirement in Germany where systems are limited in only being able to export 70% of the generation capacity. SMA has developed some sophisticated ways of managing this. By combining the SMA Sunny Home Manager and the SMA Energy Meter, the amount of electricity being exported can be measured. The Active Power Feed-In value can be programmed in, for example to 4kW and if the balance between generation and self-consumption reaches a point where the system might export more than this value, then the Sunny Home Manager can tell the inverters to de-rate and turn DOWN their production. This is more sophisticated than other export limitation devices which simply turn OFF the inverters.

Smart energy management

Take the example of a dairy on a single-phase supply which often has to supplement the electricity it imports from the grid by using a diesel generator. An installer makes an offer to install a 100kW Solar PV system. The DNO assesses the situation and determines that in order to cope with all the electricity being produced the grid will have to be upgraded to 3-phase and heavy costs need to be incurred. However, the energy consumption at the dairy is normally so high that most of the electricity won’t go onto the grid at all. In this case, the installer could come to an agreement with the DNO in which the Sunny Home Manager is installed and programmed to limit the export to only 30kW. Most of the time the system will be producing 200kW and using that electricity on site. However, on a few days a year perhaps the dairy has to shut down for whatever reason, so in this case the Sunny Home Manager will turn the 200kW right down to 30kW. The DNO is happy, because the integrity of the grid is ensured and the client is happy because now he only needs to pay to upgrade the grid for a 30kW system.

Fail-safe operation

There are two fail-safes to the SMA system. One is that the SMA inverters are programmed so that if the Sunny Home Manager malfunctions, they will remain de-rated to the last value received from the Sunny Home Manager. The second and ultimate fail-safe can be a breaker set to trip if the system fails and exceeds the 30kW limit.

The Sunny Home Manager can de-rate the system down to 10% of its nominal value. For instance a 17kW inverter can be de-rated all the way down to 1.7kW. The system works with up to a maximum of 12 inverters where the inverters communicate with the Sunny Home Manager via Bluetooth or Speedwire. The SMA Energy Meter is needed to measure the amount of electricity being exported. The Sunny Home Manager can also be programmed to turn on loads to minimise any losses that such de-rating on dormant days might incur.

It's good to talk...

The financial savings of such a set-up can be significant. This system has the potential to open up commercial PV systems that ordinarily would not be viable because of prohibitive DNO costs and potential stress to the grid. Such systems also lead towards a more harmonious integration of renewable energy into the grid and a better relationship between installers and the DNO.

To determine whether such an application is feasible or not requires a good analysis of the energy consumption of the site and an early conversation with the DNO.

This article originally appeared on the Bay.Wa r.e. website.