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Reducing Electricity Costs: Understanding Cable Losses.

by 5m Editor
13 May 2004, at 12:00am

By Nick Bird for FarmEx - A factor that is usually little considered in the cost of electricity is "cable loss". All electrical cables have electrical resistance, albeit it small. This means that a small amount of power is lost between one end of the cable and the other. The power is lost as heat in the cables.

Power loss costs energy. Energy costs money. So power loss in the cables costs money. Who pays?

Under-sized cables cost money in the long run
Putting in a smaller cable may be cheaper, but it will cost in the long run.
As an electricity customer, you pay for what goes through your meter. (You might also pay for peak demand, or for when you take the power, but mostly, it is how much power in total.) So, if there are any losses before your meter, you don't pay, it's the electricity supplier's problem.

In much the same way, if you are on a water meter, you don't pay for water leaks out in the road (not directly anyway). That's the water board's loss. But you do pay for any leaks on your side of the meter. You pay for cable losses in your own wiring.

In small installations such as houses, shops and small factories, cable runs between the supply and the appliances or "electrical loads" are short, so cable losses are usually small. But in large factories and - especially - farms, electrical loads may be hundreds of metres away from the incoming supply and electricity meter. Power losses can be quite significant.

Producers tend not to realise that they are paying for this twice. Firstly, voltage drop means that equipment works less well. Fans don't give as much throughput, and luminaires don't give as much light. But secondly, they are paying for the loss in the cable as well in higher electricity bills.

The following table gives the cost of providing 1000 units of energy (kWh) into a load at the end of cables of various sizes that have varying degrees of voltage drop.

Drop in cables Volts drop To deliver 1000kWh Extra Cost
0.0% 0 £50.00 0.0%
0.5% 1.2 £50.50 1.0%
1.0% 2.4 £51.00 2.0%
2.5% 6.0 £52.60 5.2%
5.0% 12.0 £55.40 10.8%
10.0% 24.0 £61.70 23.5%

For example, if there is 2.5% voltage drop (6V loss from a 240V supply) as compared to a 1% voltage drop, then the electricity bill is £2.10 higher. That £2.10 of electricity - 4% of the bill - is just wasted in the cable.

The reason that extra cost is greater than voltage drop is that voltage drop is proportional to load, but power loss is proportional to the square of the voltage drop.

Wiring regulations permit up to 2.5% voltage drop within an installation - 6 volts. However, many pig farms have far higher voltage drop, mainly because they have been added to and modified over the years. 5% voltage drop is common, and 10% (at maximum load) is not unheard of. At 10% volt drop, a whopping £11.70 is wasted in the cables.

With some types of load, there is reduced performance, but not necessarily an increase in cost. For example, if you have a nominal 1000W of lighting and there is a 5V drop, you still use about 1000W, but you get less light for your money.

However, with electrical loads that "do a job" - where it takes a certain amount of energy delivered into the load - there is a significant effect. For example, to boil a kettle (raise a certain amount of water to boiling point) takes so many joules of heating. If the power level at the load is reduced (because it is lost in the cables), then it takes longer to deliver that many joules of heat into the water. With a 1% voltage drop it will take 2% longer; with a 2.5% drop it will take 5% longer. (With a kettle, it's slightly worse than that, because the kettle is also losing heat.)

Checking voltage drop

Unless you know your volt drop, you really don't know whether this is an issue you should pay attention to or not. This is surprisingly rarely done, but it's very easy to do. It takes only a few minutes and needs just a cheap digital voltmeter.

First check out the main layout of the supply wiring. There may be several main supply cables running from the main incoming supply and individual buildings or groups of buildings. It's the voltage drop in each main wiring spur you are interested in.

  • On that main spur, turn everything on - or as much as is reasonably likely to be on at the same time. If you have automatic controls, set them so that the loads are switched on and running.

  • Measure the voltage close to the incoming supply, such as at a 13amp socket near the incoming supply meter. Now measure the voltage at the end of the spur - such as a socket in the furthest building. It's the difference in voltage you are interested in. See table on previous page. Now return all your controls and settings to normal!

Incoming supply voltage may be a lot lower than you expect, and may well make equipment work less well, or less efficiently. But at least the electricity supplier is paying for the losses up to your farm. Losses within your farm, you pay for. Worth bearing in mind that it's voltage drop as a proportion of the original voltage. 6V in 240V is 2.5%; 6V in 220V is 2.7%.

How to minimise voltage drop

  • Use larger cables
  • Spread the load
  • Divide the load
  • Reduce load where it doesn't affect performance
  • Improve control
  • Demand Side Management

The simplest and most obvious way is to use larger cables. Volt drop may or not justify replacing existing cables, but it's certainly worth thinking about going up a cable size when you have new supplies installed.

When you ask an electrical contractor for his "best price" for a job, you can't really expect him to fit larger cables than are required by the regulations. 2.5% drop may be "acceptable", but it's not necessarily the best choice for the farm.

The longer the cable, the bigger the cost difference, and the more tempting to go for the minimum size. If the difference is £2 a metre, it's not a lot of money on a wiring job with a 10 metre cable run, but then there is not a big volt drop in a 10 metre run. But a £400 cost difference looks a worthwhile saving on a job with a 200 metre run. Don't expect an electrical contractor to size up the cable to minimise your power losses and price himself out of the job.

As you can see from the table on the previous page, it doesn't take long for a bigger cable to pay for itself. Or, conversely, it doesn't take long for the "least cost" cable size to cost you all the money you saved on the installation.

In fact, there are many installations around the country where voltage drop hasn't even been considered. Cables have been sized on the basis of nominal maximum current. (Which is why so many pig farms have far greater voltage drop than they should.) It's worth being aware that the maximum rated current of a cable is based on its ability to lose heat. That is, if a cable is operating at its maximum rated current, it will be warm, so it will be losing a lot of power.

Spreading large loads across different phases also pays dividends. This means the current is carried by several conductors, so for any given electrical load, the volt drop is reduced.

Dividing large loads into several stages is beneficial since, in most cases, full power is not needed for most of the time. For example, a particular room might have 10kW of heating capacity (for maximum demand) but for most of the time 5kW or less is needed. Dividing the heating into two stages (of 5kW) doesn't reduce the heating needed - to deliver the same amount of heat, it would be on for twice the time - but it does mean a lower voltage drop when it is on.

Reducing the electrical load where you can - by using higher efficiency equipment - reduces the voltage drop for other equipment where, perhaps, you can't reduce energy use. Low energy bulbs produce more light per unit of electricity than tungsten bulbs. They are more efficient, so they save energy. But resistance heaters can't be made much more efficient. It doesn't matter how you do it, it takes a kWh of electricity to produce a kWh of heat. However, if you fit low energy bulbs, it reduces the total electrical load, which reduces the voltage drop, so more electricity gets to the heater, and less is lost in the cable. So a saving in one place helps with savings in another.

When looking at this sort of change, you have to consider whether it has an impact on production. Fitting smaller heaters, say, doesn't mean lower electrical consumption if it means that pigs struggle to put on weight.

Clearly, the above techniques should be considered, especially in new installations, but major rewiring or extensive equipment replacement can be expensive and pay back - though worthwhile - a long time coming. More immediate and cost effective volt drop reductions may often be achieved by improved control methods.

Most of the heavy electrical loads are to a greater or less extent automatically controlled, and most are not used, or not used fully, for most of the time. Improved control methods can be used to try to reduce the amount of time that heavy loads are on at the same time.

To make an analogy - if you have to boil two kettles, you will get less volt drop, and therefore pay for less electricity, if you boil one and then boil the other (so there is only one on at a time) rather than boil them both together.

For some equipment, this might be quite easy. For example, most ad lib feeding systems run on timers - they are permitted to run at certain times of the day. Instead of having all the feeding systems running at 10 am, you might set one to run at 10 am, another at 10:15, another at 10:30 and so on. Or, rather than setting them so they run just once or twice a day, run them more often. This means they run for a shorter time, so they are less likely to coincide with other electrical loads.

On most pig farms, the biggest users of electricity are fans and heaters. Most makes ventilation and heating controls offer very limited options to influence whether or not loads are on at the same time. In fact, the manufacturers often pride themselves on "keeping it simple".

So simple that they waste power. For example, they offer only on-off control of heating (often the biggest user of electricity). This may be "simple", but it means loads are switched for much longer at a time, and makes it more likely that large electrical loads are on at the same time. (On off control also gives less stable temperatures and tends to lead to higher electrical use as well - as shown in an earlier study - but that is another issue.)

Demand Side Management

A more sophisticated approach is "Demand Side Management". This means that a production site actively manages electrical demand, rather than simply relying on "passive" techniques such as bigger cable sizes.

It means a "joined up" approach to resources. Rather than any piece of equipment taking as much power as it wants whenever it wants it, power is allocated according to need, availability or priority.

"Standalone" control systems can't do this. They act as individual consumers. They are only aware of what they want themselves. Which is why there is a surge in power demand at the end of a Coronation Street special, and why the roads are clogged up on a Bank Holiday weekend. A surge in demand from many consumers is a problem for electricity supply companies, but not for individual consumers, unless the supply companies can't supply enough. It doesn't cost the consumer any more, because he or she only pays for what goes through the meter. Losses up the consumer's meter, or problems in supply enough are down to the electricity supplier.

But it is a problem on farm where the supplier is also the consumer. Losses along the way (in the farm's own cables) or shortage of supply are the consumer's problem and the consumer's cost.

Electricity supply companies can't control the demand to any great extent, but they do have teams of people responding to it. People who adjust generator outputs, switch in and route power as necessary.

Farms can't afford to have people to do it 24 hours a day, but they can have automatic equipment that does it for them. Networked control systems can have additional software that works out better what equipment can be on and when. For example, if there are a number loads that only need to be on for some of the time, the software works out a way to give them all as much as they want, but not to use it at the same time.

It's what we, as consumers would do ourselves - and try to do given the chance. We all have to get from A to B, but there is no need for us all to be at exactly the same point on the motorway at the same time. Many of us try to do it ourselves - by avoiding peak times - but it's not that effective in isolation. If we knew that there was a slot available at exactly such and such a time, and if we used it we would get there faster, more efficiently, and we wouldn't have to pay for building so many more motorways, I guess we'd all leap at the chance.

As they say, it's about working smarter, rather than working harder. Instead of putting in bigger cables or a bigger transformer, we arrange our use so that all the equipment gets as much as it needs, but we avoid everything wanting power at the same time.

Software based demand side management is still in its infancy in pig farm controls, and is only made possible by increasing use of networked systems, but offers considerable potential benefits in both reduced capital expenditure, more efficient operation and reduced running costs.

Source: FarmEx - April 2004

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