Memorandum submitted by the Institute
of Physics
The Institute of Physics is a scientific charity
devoted to increasing the practice, understanding and application
of physics. It has a worldwide membership of over 36,000 and is
a leading communicator of physics-related science to all audiences,
from specialists through to government and the general public.
Its publishing company, IOP Publishing, is a world leader in scientific
publishing and the electronic dissemination of physics.
The attached annex highlights the key issues of concern
to the Institute which have been linked to the specific issues
raised in the call for evidence. This response was prepared with
input from the Institute's Energy Sub-group, which includes a
range of leading physicists working across the energy sector.
The sub-group reports to the Science Board of the Council.
What should the Government's vision be for Britain's
electricity networks, if it is to meet the EU 2020 renewables
target, and longer-term security of energy supply and climate
change goals?
1. The recent report of the Electricity
Networks Strategy Group (ENSG) provides a valuable input to this
vision. It is based on a detailed evaluation of the reinforcements
and extensions judged to be necessary throughout the British transmission
system to accommodate up to 34 GW of onshore and offshore wind
capacity and 3.3 GW of replacement nuclear capacity by 2020, whilst
maintaining sufficient transmission capacity to ensure that "appropriate
generation can get access to demand". We note that the study
included an investigation of the potential for new or previously
unused technologies on the transmission system such as series
compensation, high-voltage DC (HVDC) technologies and developments
in subsea cables.
2. However, it appears that the ENSG report does
not address the range of new technologies and new operating regimes
that can and should contribute to the vision at the level of distribution
networks, local communities and individual consumers of all kinds.
Among these are combined heat and power (CHP) schemes, microgeneration,
intelligent load management and smart metering. At this level
the task will also require the resolution of difficult and challenging
engineering problems including those of reverse power flows, stability
and control.
3. Overall, the changes envisaged to the
whole electricity landscape by 2020 and beyond are unprecedented
both in scope and in pace. Much will depend on the adoption and
integration of new technologies and operating regimes as yet unproven
on the requisite scale. The attendant risks will be reduced to
the extent that other measures are taken to minimise the technical
changes to the network needed to meet CO2 reduction targets both
for 2020 and beyond. We draw three such measures to the attention
of the Committee:
— the network vision should not be presented
separately from the drive to reduce electricity consumption at
the point of use, by greater efficiency, changed practices, etc.
Every improvement in this field reduces CO2 emissions and potentially
eases the demands on the new networks;
— one fundamental, but little mentioned,
approach to easing the problems of variable generation is the
provision of energy storage, whether electrochemically as electricity
(i.e. batteries and supercapacitors) or in other forms from which
it can readily be converted, for example, stored water in barrages,
pumped water and compressed air. An additional approach might
be termed "pseudo-storage" in which amenable loads such
as heating, refrigeration and possibly air-conditioning are time-shifted
to accommodate short-term variations, possibly assisted by the
addition of thermal capacity to the installation. This last approach
could be applied at various scales—from commercial and industrial
to domestic and could include a contribution from time-shifting
the recharging of electric vehicles. We believe that all these
fields are amenable to considerable development, many without
major technological breakthroughs. All should be part of an integrated
vision; and
— existing Grid technology and operating
practices have proved themselves well capable of supporting large-scale,
low-carbon generation. It follows that a substantial acceleration
of the installation of new and replacement nuclear capacity on
a conventionally reinforced Grid will minimise the technical risks,
speed-up the reduction in CO2 emissions and will be essential
to meet longer term CO2 targets. This too should be part of an
integrated vision.
How do we ensure the regulatory framework is flexible
enough to cope with uncertainty over the future generation mix?
4. Renewables will be variable in their
electricity output, particularly those that are dependent on weather,
sea state, etc. These technologies must have an obligation to
have alternative standby in the event that they cannot deliver,
which should be allowed for in regulation.
What are the technical, commercial and regulatory
barriers that need to be overcome to ensure sufficient network
capacity is in place to connect a large increase in onshore renewables,
particularly wind power, as well as new nuclear build in the future?
For example issues may include the use of locational pricing,
or the availability of skills.
5. Because of the natural variability of the
environmental sources of most renewable supplies (e.g. wind power),
strategies are needed for their significant integration with present
supplies. This particularly applies to electricity supply, where
all forms of generation require national "back-up" because
of power plant and grid failures. Presently, the Grid has sufficient
overcapacity for large central fossil fuel and nuclear plant outages
and this same overcapacity is sufficient for UK renewables plant
for the next 5-10 years.
6. However, the present system will not be able
to support the 35-45% of variable renewable generation that is
generally considered necessary to meet the 2020 renewables target.
The location of the most productive projects—wind, tidal
and wave—will require extensive new transmission lines or
undersea cables; for example, significant investment will be needed
to connect wind farms in Scotland or the Thames Gateway into the
Grid. Because the connection has to be capable of taking the full
output, but the load factor of the best wind farms is only around
35%, it follows that the cost of connection to them per unit of
electricity produced is about 2.5 times that of a conventional
generator of the same maximum output and a typical load factor
of 90% or more. Undersea cables will be more expensive than overhead
lines of the same capacity.
7. As the penetration of variable generation
rises to the levels anticipated for 2020 and beyond, it will be
necessary to maintain and possibly increase the capacity of conventional
generating plant to serve the concomitant increased requirement
for system balancing and reserve.
8. A detailed study is required to assess
the impact of potential additional renewables capacity in the
2020 time frame. The study will need to address cost alternative
scenarios for the mix of technologies providing the additional
capacity and, in particular, the issues associated with distributed
resources and the potential "grid connected market".
This concept requires a radically different approach to manage
the transmission network and current trading arrangements. Such
information, together with any additional network associated maintenance
and security costs is a prerequisite for calculating the cost
of electricity produced.
What are the issues the Government and regulator
must address to establish a cost-effective offshore transmission
regime?
9. There are significant cost implications
for transmitting electricity from offshore to onshore. In addition
to the transmission of power, there is the cost of maintenance
of the power generator and the cables. Hence, regulation must
be introduced to ensure the reliability of power supply.
What are the benefits and risks associated with
greater interconnection with other countries, and the proposed
"supergrid"?
10. A supergrid would extend the regulation and
security of supply already provided by existing national grids.
However, the need for synchronisation across it could allow a
cascade fault to spread across the system. The cross-channel link
between the UK and France is an underwater cable. This type of
line has limited capacity for AC transmission. For this reason
it is operated as an HVDC link. HVDC not only reduces power losses
but is an effective means for the bulk transmission of power from
remote sites, particularly for undersea cables, which have a high
capacitance. In these, the current required to charge and discharge
the capacitance of the cable causes additional power losses in
AC systems. Hence, the HVDC link allows the AC networks on each
side to be operated asynchronously, limiting the spread of certain
types of cascade fault. Similar links could be provided on continental
Europe, to segment the network, even where these are not required
for underwater transmission. These could be in the form of back-to-back
rectifier-inverters, or DC lines, as appropriate.
11. In order to assess the extent to which a
European supergrid would be able to make up shortfalls of renewable
generation in the UK it will be necessary to make an exhaustive
analysis of its proposed design, capacity and operating rules
and, in particular, the degree to which it will be truly independent
of the grid systems of member countries.
What challenges will higher levels of embedded
and distributed generation create for Britain's electricity networks?
12. Electrical generators with DC output
are interfaced to the Grid via inverters. They include photovoltaic
arrays and wind turbines with permanent magnet generators. Many
of these have a modified square wave output, which contains harmonics
at low frequencies (150 Hz, 250 Hz, etc). These harmonics are
difficult to filter out from the 50 Hz line frequency. As long
as the total power being fed onto the Grid via inverters is small,
this is not a serious problem. However, with greater penetration
of such generators, the level of harmonics present on the Grid
increases to the point where it can interfere with electrical
appliances. In particular, 3rd, 6th, 9th, etc., harmonics can
cause large currents on the neutral line of three phase systems.
13. Inverters using pulse width modulation (PWM)
operate at switching frequencies of 10 kHz of more. This switching
frequency is easy to filter from the line frequency, resulting
in a much lower harmonic content in the output. By using a sufficiently
high switching frequency and a simple single-stage output filter,
it would be possible to supply all of the electrical power onto
the Grid via inverters, and still remain within the limits of
regulations on all harmonic bands. Advances in power transistors
allow them to switch at high frequency with very good efficiency.
However, a serious issue of concern is that the UK has virtually
no industry to deliver such invertors.
14. Central power plants generally have
synchronous generators, which have independent control of reactive
power—being able to act as both sources and sinks. Distributed
generators—particularly wind turbines—use induction
generators, which absorb reactive power. This can result in poor
voltage regulation when used in conjunction with inductive loads,
for example, induction motors. As a result, more reactive power
compensation would be required across the network.
15. For central generation, the power flow
along distribution networks is from the transmission network—via
sub-stations—to the load centres. As a result, there is
a voltage drop along the line. If generators are installed on
the distribution network, power can flow in the reverse direction,
resulting in a voltage drop from the loads back towards the sub-station.
As the voltage at the sub-station is fixed (within limits), this
results in higher than normal voltages at the ends of the distribution
network, which may exceed the permitted limits. On-load, tap-changing
transformers can adjust the voltage at the sub-station to minimise
this.
16. Distributed generators connected to
the utility network, for example, 1-5 MW wind turbines connected
to the 11 kV network, are accessible to central control—real
and reactive power, etc. Small generators installed on users'
premises are self-contained units, with their own control systems.
These are designed to maximise the users' generation of electricity,
and are not generally accessible for control by the utility. These
will draw real power, and supply or absorb reactive power, according
to their instantaneous generating capacity and load demands. In
general, these will not coincide with the requirements for network
regulation, and such regulation will need to be provided by systems
installed at other locations.
What are the estimated costs of upgrading our
electricity networks, and how will these be met?
17. According to the ENSG report, an investment
of £4.7 billion is required for the transmission system alone.
How can the regulatory framework ensure adequate
network investment in light of the current credit crunch and recession?
18. No comment.
How can the regulatory framework encourage network
operators to innovate, and what is the potential of smart grid
technologies?
19. A number of Flexible AC Transmission System
(FACTS) devices are already available, or are being developed
for smart grid technologies.
20. On long distance transmission lines, the
capacitance and inductance between lines give the line a characteristic
impedance, which limits the current for a given voltage. Generally
this is lower than the limit set by ohmic heating in the cables.
Smart devices, such as electronically controlled on-load, tap-changing
transformers, can adjust the voltage to match the instantaneous
power demand, thus increasing the capacity of a given transmission
line.
21. Reactive power compensation devices,
such as a Static Synchronous Compensator (STATCOM), can compensate
imbalances elsewhere in the network. Suitable placement of such
devices can reduce the reactive power flowing along transmission
lines, thus reducing the additional resistive losses associated
with this.
22. In addition to smart grid technologies,
different types of transmission line can be used. Six-phase transmission
can use the existing double three-phase lines, and have a 73%
higher capacity. They can be connected to three-phase lines using
delta-star/delta-inverted-star transformer combinations. This
can be achieved by reconnecting existing transformers, rather
than replacing them. Thus, extra capacity can be provided by the
existing network with new installation being required only for
additional demand beyond this extra capacity.
23. HVDC lines have a higher capacity than
AC lines for high voltages (~ 1 MV) and long transmission distances.
Improvements in the efficiency of inverters reduce losses in the
reconversion from DC to AC, increasing the range of applicability.
However, due to the difficulties of switching HVDC (even though
there is increasing UK expertise in developing new HVDC switching
components), these can only be used in single point-to-point lines,
rather than complex networks.
Is there sufficient investment in R&D and
innovation for transmission and distribution technologies?
24. Transmission technology appears to be
efficient and relatively mature. There is scope for more R&D
on more intelligent and flexible distribution techniques. The
biggest gains would come from progress in storage and "pseudo
storage" techniques as mentioned previously.
What can the UK learn from the experience of other
countries' management of their electricity networks?
25. No comment.
March 2009
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