Voltage perturbations of the electricity supply on Coll

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

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

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

1. Coll is close to the end of a long (~100km) 33kV & 11kV distribution network. The lower the grid voltage (Coll has the lowest @ 11kV) the greater the power losses in transmission. This results in greater voltage fluctuations, as seen in our homes, when there are large, quick, changes in our two islands' consumption or generation levels.
     Power loss in a line, ΔP = I² x Ω
     Transmission power W = V x I
      ∴ Power loss in a line, ΔP = W² x Ω / V²
   
   
2. Coll is connected by two sub-sea cables; to Tiree ~4km & to Mull ~15km. These generate significantly more capacitive feedback,10-20x more, than an equivalent overhead lines. This is known as Reactive Power, interesting debate here
   
   
3. Erratic alternative energy generation, both on Coll and Tiree, are accentuated by all these factors; sub-sea cable feedback, 11kV network and it's a 'cul-de-sac' on Tiree (not a ring main).
   
   
4. Coll has minimal electricity consumption, i.e. less able to absorb power fluctuations.
   
   
5. Many turbine operators have had their supply transformers 'tapped-down' to minimise disconnections during high voltage events. This is done after complaints as the owners were loosing 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 cut-out limit, neighbours may have to endure over-voltage.
   This observation, in no way, is to pass comment or judgement on island micro-generators. The 'tapping' is done by our DNO but I believe this practice is, at minimum, outside the spirit of OFGEM's rules. Protection for others is being 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 (in 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 here", nothing recorded at Tilly nor at Tiree power station. Technical reasoning why Coll experienced these episodes was irrelevant; there was no 'official recorded' evidence, SSE had no voltage monitoring equipment on Coll.

I set up voltage recording in central Coll which automatically 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 reporting dangerously high voltage. My SSE issued, and calibrated, voltmeter showing excesses of 260v was definitive.
SSE eventually commissioned a ~£400k STATCOM at Tiree power station, one of the first in the UK..

Once Gallanach started generation there was no protection for Coll should the STATCOM on Tiree become unavailable, say, the grid connection to Tiree was down. Coll has no voltage control of its own.

During one such period of isolation, a property close to Gallanach turbine suffered severe electrical damage; high, unstable voltages. Only then was a radio interlock installed to disengage Gallanach's turbine from the grid on Coll's disconnection to Tiree's STATCOM.