Well it’s been a little while since I typed one of these.There are a few reasons (including a broken leg!), but mostly it’s due to being occupied with other projects – most significantly finishing the new IET Solar PV code of practice, which I am pleased to say should be published later this month.
Another of the projects that has kept me occupied is an export limitation scheme on a new-build school in Scotland. The original school design incorporated a 150KWp PV system – but as the school progressed (and more generation got added in the local area), the ability of the local network to accept all of the 150kWp became an issue. During normal operation, loads in the building are such that any export from the PV array would be unlikely; however a system needed to be in place to limit export to a maximum of 50kW. Following discussions with the DNO, a robust control mechanism was agreed that would monitor the flow of electricity into and out of the building and, when necessary, control the power output of the inverters.
I will describe in a future blog the technical design and commissioning of this control system. However, one of the challenges in designing, commissioning and gaining DNO acceptance of export limitation systems is that there is no technical standard to refer to. This is something that the BSi PV committee that I chair has been aware of for a time. Hence, last year, we drafted a technical document that we hoped would act as a bit of a straw man to get the ball rolling. We circulated this around the industry and to several of our DNO contacts. Since then, I am pleased to say there has been a DNO committee assembled to discuss the issue and start work on a formal DNO document. This group has had a few meetings, is developing the draft document, and I am due to attend another technical meeting later this month. I will report back once there is a little more to report
I was testing this export limiting system last week – a process that involved simulating grid export and checking to ensure the system reacted appropriately. As we increased the simulated building export past the 50kW export limit, the system started to react and reduced the output of the inverters. However, I was disappointed to find that the system simply kept reducing the inverter output in stages until all the inverters were sat at zero. This obviously wasn’t what we wanted and a bit of head scratching ensued. However, it then dawned on me what was wrong – the system was expecting a feedback loop: as it decreased inverter power, it was expecting to see a corresponding decrease in building export. As our simulated export was fixed, nothing happened, so the system decreased inverter output a little more. This kept on occurring until system output was zero. As soon as we added a little feedback into our control loop, everything worked fine
…and the FiT review
Without an appropriate feedback loop our control system fails. As I was driving back from our test visit, the parallels with the government’s current feed-in tariff review seemed apparent. The industry has always agreed that a gradual, staged and clearly defined process for reducing the feed-in tariff was correct. However, what is now proposed is a sea-change, a damaging and abrupt change to a feedback loop we had all bought into and could make business decisions upon. As an engineer, I naturally find myself using engineer’s terminology to describe things. A feedback loop can be a tricky thing – get the control parameters wrong and everything can collapse. Right now I feel like the government has looked at a well-functioning feedback loop and rather than adjust it, is proposing a change that has the potential to act like my faulty loop – simply spiral the result to zero.
I care passionately about PV and have watched the UK industry grow from the start (my company Sundog Energy reached its 20th anniversary this year). I would like to see this growth continue and the UK PV industry prosper in the years ahead. PV is a fantastic technology and has a significant role to play in providing clean, renewable electricity for the UK. It can be deployed almost anywhere and there are still many, many more rooftops that should be turned into solar power stations. Please, if like me you care about the industry, do everything you can to engage in the debate over the proposed feed-in tariff changes – talk to your MP, talk to the trade bodies and engage in the debate.