By Paul Homewood
h/t Ray Sanders
Stephen Broderick is a Doctor of Engineering at Southampton University. He submitted this written to evidence to Parliament in 2017.
Written evidence from Stephen Broderick (EVD0062)
Basis of Opinion: This is my doctoral research area at Southampton University. The work is as yet incomplete (I am in my final year). To research the topic, I have developed a system able to simulate, study and manage networks with EVs undertaking various trip duties. Further, the below is informed opinion based on observing / modelling likely UK situations but not proven in practice.
Comments are the authors own and relate to home charging of EVs on existing Low Voltage (LV mains) distribution networks.
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Summary: The UK Distribution network "as is" is adequate for immediate needs, but will be substantively overtaxed by unconstrained EV home charging (by up to 7:1), for EVs draw c. 7 kW for hours. These issues follow uptake of EVs i.e. minimal at first then overwhelming as years pass.
Consequences will depend on local circumstances, but have potential to include:
• power cuts (overloads of supply equipment => power equipment "blowing fuses")
• brown-outs (loss of sufficient voltage) potentially causing:
◦ home appliance damage and
◦ household fires
• potentially, a move to restrict EVs to (say) 1 in 7 homes.
Two general methods are seen to alleviate these situations:
- reinforcement of the networks (asset replacement) – expensive, slow, disruptive
- use of a control system capable of managing EV home charging.
Further, the EVs would need to obey the issued commands; this is not assured.
Note that the ICT / SG method does not provide a complete solution, but is expected to defer major costs for decades.
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Background information to assist the reader:
- A brief summary of technical terms is appended.
- The UK “electricity grid” is a set of millions of connected components, individually selected to affordably perform a given goal. These parts have been installed in an as-needed / piecemeal manner since c. 1900. Many parts date from the 1950 / 1960’s
- The grid can be broken into 3 main sections:
- Supply (the making of electricity)
- Transmission (sending electricity at high voltage around the country e.g. National Grid)
- Distribution (of power at medium and low voltage to customers e.g. "mains" 230 V)
- Many papers have been published concerning EVs and "the grid"; however most relate to US networks – similar to the UK only at higher levels. The lower Distribution level is different, so to meet the challenges of different situations.
• US: adequate (occasionally challenged) Generation and Transmission. Strong Distribution which can provide 8 – 14 kW to each home simultaneously, the major load being Air Conditioning.
• The US Distribution system typically uses many “near-home” local transformers each supplying 1 – 4 houses;
• UK: adequate Generation, strong Transmission. Adequate Distribution (for present loads) able to provide 1 – 2 kW to each home simultaneously.
• The UK Distribution system typically uses a small number of “substations” with a transformer, each supplying 1 – 100’s of houses.
Reiterating the US / UK difference in Distribution capability:
◦ US: 8 – 14 kW per home, able to sustain peak loads for long periods vs.
◦ UK: 1 – 2 kW per home on average, able to support occasional higher loads due to averaging over many customers
Most published papers originate from the US so silently assume the US model. However the respective Distribution networks have quite different characteristics.
* * * Overseas studies and experience-based advice may not relate to the UK.
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Some Numbers:
a) there are about 250,000 Distribution networks in the UK, each of individual nature;
b) the design of these has historically been guided by a measure called "ADMD" which has been set variously to c. 1 to 2 kW per supplied home. This is a statistical measure and assumes customers take random loads at random times;
c) to drive an "average" day’s distance (c. 27 mpd) an EV consumes c. 9 kWh at the wheel;
d) (at the time of writing) batteries loose c. 8% on charging and the same on discharging; the inverter electronics lose a similar percentage;
e) the daily average EV power draw (at the home charging point) is then consumed power plus losses i.e.
9 * 1.08 * 1.08 * 1.08 = 11.3 kWh
Other aspects cause losses; 12 kWh is a reasonable "EV supply average daily demand".
f) driving distances are, in general, dependant on location / nearness to a city. RAC studies suggest the following mileage ratios:
City : Urban : Rural of 1 : 1.4 : 2 (i.e. country dwellers drive twice as far as those in the city)
g) a modern EV home charger draws 7 kW.
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A 100 Home Illustration
A 100 home development built in 2017 has an LV distribution network fitted to supply 150 kW simultaneously. An overload of c. 50% is possible for up to 8 hours (following this a period of cooling / low load is necessary). This network includes:
• substation (with transformer and switching)
• in-road cabling (3-phase 230 V per phase)
which has an asset value of c. £ 30 – 45 k.
If 100 EVs arrive in the evening and start to charge, the peak load is 700 kW and the distribution assets go into immediate substantive overload.
However the EVs require (on average) 100 * 12 kWh => 1,200 kWh of energy, which if supplied in a staggered manner over 10 hours is 120 kW continuous load thus doable.
This suggests that a local rationing or management system (able to pace demand intelligently) would help. A simplified version of this was successfully trialled in 2015 (My Electric Avenue).
Yet this has ignored the usual demand of the households; in winter they will need power for home use. In extremis, it may be necessary to upgrade (reinforce) local substations and cables.
Such reinforcement includes:
• replacing the transformer
• digging up the road and relaying cables
• a spend of c. £45 k per 100 served homes i.e.
25 million * £ 450 = £11.25 billion (approx assets costs; a "broad-brush" estimate)
Which, after manpower costs plus profit is added may be perhaps x 2 or x3 as much.
Note that the politics to this falls into three sections:
- the money – who pays?
- the inconvenience, primarily digging up the roads to relay cables (especially in cities)
- the manpower – with the best will in the world, this is a project which may take a decade or more – with present manpower. To achieve this faster requires more hands.
Hence, a new generation of electrical engineers and technicians are needed.
The initial threat though is simultaneous arrival and charging; even with 1 in 7 of homes having an EV the system is at full capacity in the early evening.
NOTE This ignores home-heating by Heat Pumps (HP) scheduled from c. 2040 on as part of the UK’s CO2 minimisation initiative.
** HP alone impose more load than EVs; immediate reinforcement will be necessary **
As he notes, his calculations do not include heat pumps, which overload the grid even further.
Other factors which will exacerbate the problem is the number of 2-car households. His figures, of course, only look at the number of households.
There will be massive variations from place to place. Many inner cities will have very little offstreet parking, so these will have fewer grid problems.
On the other hand though, rural and suburban dwellers drive many more miles than city ones, as he points out. Grid overloads there will be much worse as a result.
And some areas may have grids close to overloads already.
Given that EV drivers will likely to required to only charge during low demand periods at night during times of short supply, this will compress further Broderick’s 10-hour window – maybe to just six.
The CCC recommended years ago that the nations’ distribution grid should be upgraded all in one go, rather than in small increments. It obviously is better to dig up the roads once than ten times!
Meanwhile nobody in Parliament appears to have given this any thought at all, content to kick the can down the road and leave the cost and blame for their successors.
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