SIR, I was amazed to learn that a Scottish Hydro Electric transmission
spokeswoman said “repairs are being carried out on the faulty relay” that
allegedly caused the power cut on April 16 (“works to fend off blackouts”,
P&J, May 10).
I have been an electrical engineer for over 40 years and have never heard
of anyone “repairing” a hermetically sealed relay switch.
The relay switch operated perfectly on the windy night of April 16 when it
detected a sudden surge of voltage and frequency that fell outside
acceptable parameters.
A relay switch has two states: on and off. All of these relay switches
operated perfectly on the night, independent of the relay switch at
Knocknagael Substation which is, itself fed by at least two windfarms, Farr
and Moy.
This was what is known as a “rolling blackout”. It is ludicrous to suggest
that all lights went out all over the north at 8.30pm exactly. My area went
out at 8.43pm when the blast of wind reached Novar windfarm and toggled the
relay switch to off to protect its local circuit and so on up the coast.
Grid operators can switch windfarms on and off remotely – if there is a
risk of too much wind generating too much “wrong time” low-grade
electricity with what is known in the industry as “flicker”. The grid
cannot handle more than 10% of flicker contaminated electricity at any
given nanosecond and this limit was exceeded on the night.
The operators were caught on the hop. With no electricity, all the
windfarms had to be isolated manually.
The spokeswoman goes on to say that they will be making changes to how the
protective equipment operates. This is code for shutting down windfarms
even earlier in windy conditions so that the operators get more and more
constraint payments for not generating when the wind speed is just right.
Andrew H Mackay, Tain
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