Commentary on Distributed Energy
The UK’s electricity grid was
started in 1928 with the erection of 6,400 kilometres of mostly overhead cables,
running at 132 kilovoltage and 50 Hz frequency and linking the then 122 most
efficient power stations.
The 132 kV grid is now subsidiary to
a 400 kV super grid run by National Grid’s central control room with the task
of maintaining stability of supply.
The introduction of renewable
sources of electricity has created a distributed energy sector predominantly
with solar PV on houses, industrial premises and large farm buildings. There are
small and medium-sized wind turbines on farms and on some industrial sites.
There is anaerobic digestion on farms, on sewage works and for food factories
Large, on- and off-shore wind farms
augment central generation, but the stability of the national electricity supply
is questioned by the introduction of the increased size of new nuclear power
Stability of supply also relies on
net imports/exports of electricity from the EU.
The national grid consists of 400 kv,
132 kV overhead and intercontinental interconnectors.
Below the 400 kV and 132 kV power
lines in the hierarchy are 33kV and 11 kV high voltage underground cables and
overhead lines for town and rural distribution, followed by 415 V and 240 V for
local distribution, to which solar PV, small wind and anaerobic digestion are
connected as distributed energy, beyond the control of the national grid.
This makes better use of the
existing power lines as self-generation reduces the input from the mains.
The national grid has to balance
inputs with demand. If a major source of power drops out of action, such as a
power station or offshore wind farm, other sources have to be brought on line to
maintain voltage and frequency.
The bigger the size of the source
dropping out, the speed of response in bringing in alternative sources is
critical in avoiding a national blackout.
The loss of a big source may mean
the assembly of several smaller sources. The distributed sources on the local
distribution lines are not available to the grid controller.
Also the closure of big coal
stations, together with the soon to be closed AGR nuclear power stations, has
reduced and will reduce the ability of the controller to maintain stability.
Two power sources lost in quick
succession led to the closure of a large part of Wales to avoid a complete
Also the contribution of the
distributed sources in the isolated section was lost.
To maintain the generation of the
renewables in an isolated section closed to maintain national supply poses a
complex problem. The grid controller would need to isolate sections of the 400
kV and 132 kV lines to relieve the national load sufficiently in which there
would be 33kV, 11kV, 415 and 240 V sections in which renewables are connected.
The question is whether it would be
possible to maintain local supplies from renewables in the closed sections with
standby generation for reference voltage and frequency? With the introduction of
>300 MWh battery storage systems, the complex control systems needed appear
to be resolved.
The electronics in the dc/ac
inverters might be adapted to work when severed from the mains to supply a local
need and to reconnect once central power is restored. If this is the case, then
it would mean an immediate and total severance of all the renewables in an
isolated section, which would otherwise give them an excessive load and they
would be shut down.
Of the list of nuclear power plants
currently in operation only Sizewell B will remain until 2035.
While still in operation the coal
and the wood fired boilers can offer flexibility of output.
The consumption of 385 TWh in 2010
fell by 88 TWh or 23% to 297 TWh in 2021
Central generation of 382 TWh in
2010 fell by 131 TWh or 34%, to 251 TWh in 2021, showing the move to distributed
The UK’s central generation, i.e.,
without the distributed energy has declined from 2016 to 2020 by 11% to 251 TWh
The UK’s consumption has declined
from 2016 to 2020 by 7% to 297 TWh
The contribution of net
imports/exports and solar power of 23 TWh is significant.
The 2020 day GW averages are
taken from the Gridwatch website.
At national minimum power of
17.151 GW, Hinkley Point C’s transmission to the grid of 3.2 GW will provide
19% of the central load, which if it tripped with lightning would certainly be
With SZB, HPC SZC and perhaps two
Hualong 1 GW plants, nuclear would provide 55% of central generation.
These ratios will be more extreme
with more distributed energy and the closure of the current nuclear plants,
The owners of nuclear power plant
have to run them 24/7 at 90% load factor to maximise revenue. If all those
planned are in operation, unless considerable load is added, they will dominate
55% of the minimum load at night.
Just HPC’s or SZC’s transmission
transfer of 3.2 GW would be 19% of minimum power. Either would not be available
to fill the gap if one shut down. Both would need to run 24/7 for an even
The national electricity grid is
soon to be not fit for the purpose.
It needs re-profiling to suit the
increasing distributed energy and decreasing central generation. The potential
new load for electrical vehicles might need a special grid extension for battery
charging and part of the new nuclear could be diverted for this purpose,
improving the stability of the current grid. The loss of a major nuclear power
plant would then just stop vehicles being charged.
Although small modular reactors are
apparently more likely to fit into a distributed system, they are more likely to
be uneconomical with the same technical staff costs and security. Also the
automation and controls would be the same as for bigger plants.
As uranium supplies peaked in 2016,
increased nuclear power fuel prices will rise and affect adversely the viability
Rivers in the UK are suffering low
flow with water abstraction and SMRs would have to be sited on major estuaries.
There would be concerns for radioactive outflows.
Otherwise, they would have to deploy
fan air-cooled condensers, noisy and inefficient.
They would need to be sited in town
industrial estates and there would be considerable local opposition.
With central generation currently
declining it is doubtful that Sizewell C is required. It is too big for grid
stability and unacceptably expensive.
Domestic bills are rising and
forward debiting for a future nuclear power station for over ten years will be
unpopular, if not unaffordable.
SMRs are unacceptable to most
communities and insecure.
Grid stability needs study, but
major inputs are too big.
John Busby 26 August 2021