Voltage perturbations of the electricity supply on Coll

A description of the island's electricity supply can be seen here

If your home's supply voltage is too high then your electrical equipment may run too fast, shine too bright ... there may just be too many toos ... and may burn out prematurely.

Coll suffers atypical voltage fluctuations and there are five factors at play:

1. Coll is close to the end of a long 33kV and then 11kV distribution network. The lower the voltage (Coll has the lowest @ 11kV) the greater the power losses in transmission and the greater the voltage fluctuations during transients.
     ( Power loss in a cable, ΔP = V² / Ω )
   
   
2. Coll has two sub-sea cables, to Tiree ~4km & to Mull ~15km. These generate significantly more capacitive feedback (10-20x more!) than an equivalent overhead cable, called Reactive Power, interesting debate here
   
   
3. Erratic generation from alternative energy sources, both on Coll and Tiree, are accentuated by sub-sea cable feedback and because the 11kV network is a 'cul-de-sac'.
   
   
4. Coll has a minimal electrical infrastructure, less able to absorb power fluctuations.
   
   
5. Many turbine operators have had their supply transformers 'tapped-down' to avoid their turbines disconnecting at high volt limits = a loss of FIT income!
   There's a legal requirement for the electricity supply voltage not to exceed +10%, or less than -6%, of the normal of 230V (ESQC Regulations)).
   This equates to maximum & minimum allowed voltages of 253V & 216.2V respectively.
   An OFGEM requirement, for all self generation, is to disconnect generation when these limits are exceeded and this protection is built into all authorised grid-tie inverters. This is to protect the system, not any individual. Whilst, with a tapped-down transformer, a turbine/solar inverter may well see a voltage below the legal limit, neighbours may have to endure over-voltage.
   This observation, in no way, is to pass comment or judgement on island micro-generators. This is knowledge well beyond their remit. However, I believe this practice may be outside the spirit of OFGEM's rules, protection for others could be compromised.
   
   

Over-voltage on Coll is most prevalent in the summer, on bright, gusty days. The islands often get sunshine during a gale; both solar and wind micro-generation is at a combined maximum - with minimal energy being consumed. I have recorded 265V on occasions!

History of large scale renewables on Coll &Tiree

Over-voltage on Coll started the day (2010) 'Tilly', Tiree's community wind Turbine, started generation. The 'voltage issues' soon got into the local press and I received strong words of denial from TREL (Tiree Renewable Energy Ltd); no "high volts" recorded at the turbine nor at Tiree power station. Reasoning why Coll experienced these episodes was irrelevant as there was no 'official recorded' evidence, SSE had no voltage monitoring equipment on Coll.

I set up (privatively) voltage recording which emailed SSE engineers every time over voltage was recorded. After a thousand or so emails the message had some meaning.
In addition, I'd phone SSE’s emergency number to report dangerously high voltage, showing them the reading from my SSE calibrated voltmeter.
Although unprovable, a Collach’s (newish) washing machine exploded, coincidental to the weekend I recorded long sustained periods of voltages around 265V !
SSE eventually commissioned a STATCOM at Tiree power station, one of the first in the UK.

At the same time SSE had concerns about a large scale wind turbine proposal at Gallanach, Coll. A STATCOM could compensate for the additional voltage perturbations this presented.
Page 1 of most STATCOM installation manuals' say the STATCOM must be CLOSE to the source of the renewables. Even Tiree power station, the other side of Tiree from Tilley, isn't that close. Gallanach is further and has feedback from the 4km sub-sea cable to combat. Even with proximity limitations, the STATCOM has worked well and is vital.

Once Gallanach started generation there was no protection for Coll should the STATCOM on Tiree become unavailable. Coll has no voltage control of its own. Isolation occurred whenever the line from Coll to Tiree was down.