Not Melting the Grid

(when you connect renewable energy sources and energy storage)

Local connections

Most houses – apart from large, posh houses with special arrangements – have an electricity supply designed to be able to deliver a peak of about 24kW, with a main fuse of 100A. You’d rarely hit this peak in practice; it would only happen if you had pretty much everything turned on at once, and everything that’s controlled by a thermostat or similar turned themselves on at the same time. The average consumption is more like 0.5kW per household, but of course it varies up and down all the time. Sometimes it’s actually zero, or very close to that. (There are probably a few things on standby, or indicator lamps showing the power is reaching a set of extension sockets or whatever, consuming a total of a few watts.)

You could have solar panels capable of generating a peak of 24kW attached to your mains supply without damaging your connection, but any more than that would blow your main fuse – and simply upgrading the fuse would risk damaging your connection to the mains cable in the street.

However, if everyone on your street had that amount of solar panels, the fuse at the local substation (usually called a final distribution substation, but I’ll stick with local) supplying your street would blow on a sunny day – and if you upgraded the fuse, the cable in the street could burn out, or the transformer that connects the cable in the street to the high voltage grid might blow up. This is because the energy industry sensibly assumes that not every household will be drawing their peak consumption simultaneously – whereas the sunny days happen at the same time for every house in the street.

Nearly everyone might put an electric kettle on at the same time, during an advert break in a popular program, but they won’t all be taking an electric shower at once, using every ring on the cooker at the same time, doing the vacuuming, and running the washing machine, dishwasher and tumble drier – and using every electric fire. Not all at once, not in every house in four streets all at once. What they do allow for is a combined peak of about 4kW per house in the area served by any one local substation. (That’s in urban areas where there are a few hundred houses per local substation. In rural areas where there may be only a dozen or fewer houses, they have to allow quite a bit more per house, because with fewer consumers, there’s a greater risk of them all using quite a lot simultaneously.)

So as long as only one house in six has 24kW worth of solar panels, all will be well. Or you could say that each house should have no more than 4kW, and you’ll be all right. This is why there’s a limit of 4kW on the basic arrangements for government subsidies on domestic solar panels. You want to have more, you have to make special arrangements, and the electricity company may have to upgrade your connection, or even your local substation. (Unlikely in practice, unless every house in the area has solar panels, and more than the odd one want to have more than 4kW worth.)

You could have a bit more than that anyway, because although some houses might sometimes be drawing zero watts (or nearly zero) at the same time as the panels are generating their maximum, it’s extremely unlikely that they all will simultaneously. Just as the peaks are unlikely to be simultaneous, so the troughs are unlikely to be. You can allow for generating enough to max out the fuse at the local substation PLUS whatever is being consumed in the area.

But the combined trough will be less than that 0.5kW average, probably quite a bit less. So the extra amount over and above the 4kW per house isn’t really worth trying to take advantage of.

Intermediate Connections

Similar considerations apply to the higher voltage connections and intermediate substations that connect the local substations to the National Grid, and ultimately to the power stations. There are various levels of such connections: 240V local connections; 11kV, 33kV and 132kV intermediate connections; and 275kV and 400kV National Grid. (The whole network is three phase and the local connection is actually 415V three phase. Each house is connected to only one phase, which is 240V relative to neutral. Neutral is very close, but not identical, to earth potential.)

Just as the peaks in consumption in all the houses in one street are unlikely to coincide with each other, so the peaks in consumption in all the different areas are unlikely to coincide.

If we’re only looking at domestic consumption, this is a small effect – most of the effect is already seen at the local level, and what peaks and troughs remain at the local substation level mostly do coincide at different local substations, because they have common causes, like loads of people switching on their kettles at the end of a popular TV programme, or during advert breaks (sharp peaks); or loads of people cooking meals at similar times (much broader peaks); nearly everyone being asleep (a broad trough); or a spell of cold weather (long term peak).

But larger consumers such as supermarkets, warehouses or factories, who often have their own local substations, will generally have different peaks and troughs – from each other, and from local substations serving domestic properties. A local substation serving a street of small commercial premises will have different peaks and troughs again, and so forth.

Overall, that spiky graph of consumption in households becomes a somewhat smoother graph at the local substation level, smoother still at intermediate substation level, and even smoother at grid connection level. It still has peaks and troughs, but the peaks are not so big (in proportion to the average), and the troughs are not so deep.

The deepest of the troughs is called the base load.

The amount you can generate anywhere without damaging the circuitry is equal to the capacity of the supply to that point, PLUS the local base load – once the local base load has been taken out of what you’re generating, you can safely push the rest back up to the network. That wasn’t useful at the local level, because the base load was very small compared to the capacity of the supply – but as you go up the system, it becomes quite significant.

If you want to instal more than (supply capacity + base load), you need an upgraded connection – just as you would if you wanted to connect a large additional load, an additional twenty houses on a local circuit, for example.

This cascades all the way up the network. Installing 24kW of solar panels won’t hurt your own domestic connection, but if everyone in your street does it, you’ll blow the fuse at your local substation. You can safely instal just enough between you that on a sunny day, your local substation will be working in reverse, feeding as much back into the grid as it could possibly supply when you’re all consuming electricity like mad. In the same way, it’s all very well your local substation feeding that much back up the system, but if every local substation in the area does it, you’ll blow the fuse at the intermediate substation. And so on, all the way up to the National Grid. It’s not quite so likely to be a problem at this level, partly because it’s unlikely that every street really will instal so much, and partly because of that (supply capacity + base load) allowance. But the network engineers have to keep their eyes on the issue, and be ready to instal additional interconnections – and may want to charge someone for the cost of doing that.

If you want to instal a big wind turbine or a large solar farm, that at peak might generate 2.5MW, you need an additional connection for it, probably to the nearest intermediate substation. You’d almost certainly overload a local substation, but unless there’s a very large amount of local generation (solar panels or whatever) you’re highly unlikely to overload an intermediate station, which can probably handle a few such installations.

A wind farm with a lot of big wind turbines will need its own connection to the national grid, just as any conventional power station does.

The National Grid

This is the backbone of the whole system. Most of it’s on overhead cables on pylons, but some of it is underground. Just as with the local and intermediate connections, you have to respect the capacity of each link in the system. If a large town grows, and its consumption grows, its connections will have to be upgraded. If you double the capacity of a nuclear power station, its connections will have to be upgraded. If you build a large wind farm it needs its own connection to the grid – and the connections beyond the point where it connects may need to be upgraded, too.

But that “may” is important. If the point you’re connecting to is part of the supply chain to a large consumer, such as a large town, and the (supply capacity + base load) of that town is greater than the peak output of your wind farm, all is well – no upgrade is required. (Not forgetting that any local generation, such as from solar panels, within the town, will reduce the base load the town presents to the grid. It might even make it negative, so watch out for that possibility too!)

Similarly, if the point you’re connecting to is part of the chain from a power station where you’re trying to reduce the consumption of fuel, you may be able to turn down the output of that power station to avoid overloading the connection, thereby saving fuel. This is after all the point of building renewable power sources! (Saving fuel and reducing carbon dioxide emissions are two sides of the same coin – except in the case of nuclear power, where saving fuel and reducing fission product creation are two sides of that other coin.)

In fact this is really the only place where “melting the grid” is an issue – wind turbine power can rise (and fall…) fairly quickly, whereas a coal-fired or nuclear power station takes some time to change its output. You need to be able to temporarily feather the blades of the wind turbines, or dump energy in some other way. This is not difficult or expensive engineering. (Gas turbines can change their output extremely quickly, as can hydroelectric, including pumped storage, systems. So the problem does not arise with them.)

Interconnection Costs

All this is of course very well understood by the engineers who design and maintain the network. Interconnections are not cheap, nor are they disproportionately expensive – but the costs ought to be more transparent than they are. Opponents of renewable energy within the power industry (and there are plenty of them) are liable to use interconnection costs as an excuse to oppose development of renewable energy sources. I’ve seen this very clearly in anti-wind campaigns. In one case in our district, the National Grid claimed that an interconnect for two 500kW turbines would have to be tens of miles long – when there’s an intermediate substation handling 10MW peak load just four miles from the site of the proposed wind turbines, with no generating capacity attached to it other than less than 1MW of solar panels.

So far, so good...

...but so far we’re only talking about replacing fossil fuels and nuclear power for part of the time. We really need far greater renewable sources of energy, and to store energy when there’s surplus renewables, for when the weather is dull and windless.

Supply Levelling

You can supply at least the local peak power (and in many places peak load plus base load) into any substation on the network, without upgrading the network.

You can do this at any one point, but you can’t do it everywhere at once. Loads aren’t expected to peak simultaneously, so when you connect several together the combined peak can be assumed to be less than the sum of the separate peaks. The same does not apply to supply from wind turbines and solar panels: it’s quite possible that they’ll all peak together occasionally – well, not exactly, but certainly the combined peak could be a considerably bigger proportion of the sum of the individual peaks than happens on the demand side.

But engineers can work out just how much you can put in here and there without needing to upgrade the network, and where it’s worth upgrading it to connect available supply to demand. You can supply the whole of base load demand from wind and sun on a sunny day with optimum wind.

This is all well and good, and is a good first target to aim for. It will save a lot of fuel – and correspondingly reduce carbon dioxide and fission product production. But the wind doesn’t blow constantly at the optimum speed, and the sun doesn’t shine at full strength constantly either.

The next target to aim for is having enough wind turbines to supply base load even when the sun isn’t shining, and enough solar panels to supply base load even when there’s no wind. The network will need to be upgraded in some places to allow power generated in one area to be delivered to another – but the grid won’t melt (or fail in any other way) because the engineers will know what’s going on, and will upgrade the network where necessary. It’s got a cost, but it’s not exorbitant, and will be factored into the costings of the projects.

But then there are the days when the sun is shining, and there’s plenty of wind – everywhere ‐ and the demand is low, maybe just base load. You’ve shut off every fossil fuel and nuclear power station. You’ve still got too much power. What do you do?

There are four main things you can do, in order of preference:

(1) If you have any storage capacity, you can store some energy (see Storing Energy).

(2) You can increase demand temporarily, by cutting the price of electricity temporarily. With smart metering this could happen automatically. There are many uses for electricity that don’t mind only getting supplies erratically, as long as the electricity is cheap enough. (This is actually one form of load levelling, not supply levelling.)

(3) If your wind turbine blades are designed to be able to be feathered, you can feather some or all of them.

(4) You can switch off some of your solar panels. Yes, it really is as simple as that.

Option 2 may involve upgrading the network – but probably not in many places if any yet, because we’re still only considering a maximum of about twice base load, which is still well below peak load.

Whether Option 1 involves upgrading the network depends where the storage is. Ideally it will be close to large wind farms, and capable of absorbing an amount of power comparable to that generated by the wind farm. This is REAL supply levelling: you can chop off the peaks of supply, and use them to fill in the troughs. If you have enough storage, you can now have a bigger wind farm, generating as much on average as its connection can take, rather than limiting the peak. You may have to feather some turbines once your energy store is full, but that will be less frequent – and the more storage you have the less frequent it will be.

If the storage is elsewhere, you have to consider the capacity of the intervening links, and there may well need to be some network upgrading (or may not – remember we’re still only dealing with twice base load, which is a fraction of peak load).

Obviously the system would be designed for the feathering and unfeathering of wind turbine blades and the switching off and on of solar panels to happen automatically – you wouldn’t have to have engineers running around doing it!

Beyond Base Load x 2

There’s no practical limit to the amount of wind or solar capacity we can have. If they are delivering too much, we can turn some of them off. At other times they’ll be delivering part of their capacity, and the more we have the more often we’ll be able to meet the whole of demand from renewable sources.

Eventually we want enough wind and solar power to supply all our electricity all the time. To do this, the average power supplied by these renewable sources of energy has to match the average power demand, and we need large amounts of energy storage to match the supply peaks to the demand peaks. If the storage is close to large consumers, we can level the load; if the storage is close to large suppliers, we can level the supply. In either case, the network upgrade needed is only between the energy store and the consumer, or between the energy source and the energy store. If a large energy store is remote from both suppliers and consumers, then connection costs may become significant – but they’re unlikely to be an overwhelming factor in the costings of a project.

How Much Storage??

It makes sense to provide enough storage to smooth out troughs of supply that occur frequently, but the cost of providing enough storage to cover unusually long sunless, windless periods would be very high, and the facility would be of only marginal use. The only thing you can do then is to generate power in existing fossil fuel or nuclear power stations*, which would be idle most of the time, and to minimize demand using those smart meters, or even – horror of horrors – by load shedding or browning out.

Load shedding means summarily disconnecting low priority loads. It’s a supplier’s term – consumers call it a power cut. This is common practice in developing countries. (Hospitals are high priority loads, and only get power cuts as a result of faults.) Equipment is generally designed to tolerate power cuts; any that isn’t should have an uninterruptible power supply.

Browning out means lowering the voltage so that loads draw less power, and is also common in developing countries. Some equipment does NOT tolerate brown outs well, but there’s no good reason why that shouldn’t be changed over a period of a few years as old equipment is replaced, or they can be connected through a constant voltage transformer.

Power cuts and brown outs would be a last resort. We (mostly) avoid them today by having a lot of excess generating capacity, much of which is only rarely used. If we only need to use our fossil fuel stations for a couple weeks once every few years, the amount of carbon dioxide they’d be producing would be a tiny fraction of what they produce today. They wouldn’t even need to be in spinning reserve – you’d only start to spin them up when stored energy began to run low.

How much storage? How long is a piece of string? The more you have the less frequently you’ll need to use fossil or nuclear fuel or suffer power cuts or brown outs. How much do you want to spend? In the past we’ve spent whatever it takes to ensure, as far as possible, that the lights stay on. Should we now spend whatever it takes to ensure we burn no fossil fuels? There comes a point – a moving point – where there’s lower hanging fruit to pick first. Other things that cost less (in resources; money is of no consequence) for greater benefit.

* Worth noting that nuclear power becomes hopelessly uneconomic when used only very occasionally, because its costs are mostly capital, not fuel.

(N.B. Numbers in this piece refer to the UK, but similar logic applies more generally.)

©Clive K Semmens 2015