Overview

Voltage Optimisation is a term now commonly used to refer to the well-known energy-saving technique of reducing the electricity voltage supplied to a site in order to reduce losses in equipment.

Voltage Optimisation works by reducing losses in electrical equipment
– thereby reducing energy consumption, CO2 emissions and your electricity bill!

The lifetime of equipment is also extended (because it generally runs cooler at the reduced voltage), with a consequent reduction in maintenance costs.

Also known as “voltage reduction”, "voltage correction" or “voltage power optimisation”, the technique has been understood for many years, and has recently received increasing interest as an effective means of reducing electricity bills, with savings of up to 15% being realised on many sites, and up to 20% on some sites. (Savings of up to 25% or more are possible, but not very likely.)

The voltage supplied to many premises is much higher than it needs to be, leading to excessive losses in many types of equipment. This is partly because of the need to allow for voltage drops across the supply network, but is also a consequence of the harmonisation of supply voltage throughout Europe.

Supply harmonisation

The voltage of electricity supplies throughout Europe has been harmonised to 230V ± 10%
(See EN 50160:2007 “Voltage characteristics of electricity supplied by public distribution networks”), bringing the statutory supply limits to 207–253 Volts. 

In the UK, the statutory supply limits have not yet been fully harmonised, with a somewhat tighter tolerance still in force: 230V +10%/-6% (216–253 Volts).
(See Statutory Instrument No. 2665 "Electricity Safety, Quality and Continuity Regulations 2002")

Supply voltages remain largely unchanged

The UK for many years had a standardised supply (phase-to-neutral) voltage of 240V ±6%, whereas continental Europe had a nominal supply level of 220V. Some regions used different nominal voltages (varying between 220V and 250V), but the wide tolerance of the new harmonised supply range has meant that there was no incentive for suppliers to make any real change, as the old supply limits lie almost entirely within the new limits! So the actual supply voltages have remained largely unchanged as a result of harmonisation.

The average voltage in the UK is still around 242V. (239V in Northern Ireland, 235V in the Republic of Ireland – see the survey of site voltages.)

This means that on the majority of sites, electrical equipment is running at a voltage much higher than needed, resulting in excessive losses.

Optimum voltage for equipment

The Institution of Electrical Engineers recommended in a 1996 report that for safety all electrical equipment needed to be tested across the range 230V +10%-14%, so all equipment produced since then should be capable of working from 198–253V. This allowed for the new lower and higher limits plus an allowance of 4% for voltage drops within the installation.

The IEE Wiring Regulations 17th Edition (BS 7671:2008), in Appendix 12 "Voltage drop in consumers’ installations", specifies a maximum value of voltage drop of 3% for lighting and 5% for all other uses, where the supply is directly from a public distribution system. (Larger drops, of 6% and 8%, are permissible with a private supply.) The implication is that electrical equipment should be able to operate over the range 230V +10%/-15% (185.5–253 Volts).

It follows therefore that there should be no problem in operating most electrical equipment within the standard supply range of 207–253 Volts. 

Indeed, many items of equipment may actually have been designed to operate at 220 Volts, which was the standard voltage in most European countries until recently.

Operating equipment at a voltage higher than really necessary can (and often does) lead to excessive energy losses in the form of heat.

The optimum voltage for a site may therefore be lower than the voltage actually supplied to the site, although the actual optimum voltage will depend on the type of equipment on the site. The primary purpose of voltage optimisation is to reduce the voltage to this optimum level.
 

Voltage reduction equipment

There are several ways of implementing voltage reduction on a site.

Supply transformer tap-down

This approach can be taken if the site has its own high- or medium-voltage transformer, and if there are suitable tap settings on the transformer that allow the voltage to be reduced by the required amount. All that needs to be done is to shut off the supply for a few minutes while the tap setting on the transformer is changed.

An obvious advantage of this approach is the very low cost involved. However, a significant disadvantage is the risk of the voltage on the site becoming too low if (or when) the voltage on the supply grid drops to a lower level.

The taps of a supply transformer are usually set to allow for the variations in grid supply voltage, so that the site voltage is unlikely to vary outside of the 230V ± 10% range. Changing the taps to give a lower output voltage increases the likelihood of the output voltage becoming too low.

Fixed-ratio voltage-reduction transformer

If tap-down of the supply transformer is not an option, then a specially-designed fixed-ratio voltage-reduction transformer can be installed on the main circuit supplying the site. A number of such transformers are available.

These fixed-ratio transformers typically have a number of tap settings, allowing the degree of voltage reduction to be set at the time of installation (or at the time of manufacture).

This type of transformer usually provides a degree of transient suppression.

Some types also incorporate special harmonic-reduction windings – although many sites that would benefit from voltage optimisation do not have a problem with harmonics so this feature may be superfluous for many applications.

A number of designs also incorporate windings for reducing phase voltage imbalance in 3-phase systems (although the voltage trace shown on the next page, recorded on a site using one of these transformers, suggests that this feature may not be very effective).

A disadvantage of fixed-ratio transformers is that – as for supply transformer tap-down – the site is vulnerable to the grid voltage dropping.

(See the graph on the next page for an illustration of the consequences of this problem.)

Voltage regulator (stabiliser)

Perhaps the best approach to implementing voltage reduction is to install a voltage regulator (also known as a voltage stabiliser).

This type of equipment will ensure that the voltage on the site is maintained at the desired level, irrespective of the changes in the grid supply voltage.

It will also ensure that the phase voltages are properly balanced – helping to improve equipment efficiency and reliability.

Voltage regulators are more complex than fixed-ratio voltage-reduction transformers, although in practice they are often no more expensive. However, they have two significant advantages in comparison to the fixed-ratio type:

• Close control of the output voltage (typically to within ± 1%), coupled with the proper balancing of phase voltages, allows for overall greater voltage reduction – giving higher energy savings and a lower total life cycle cost than typical of fixed-ratio solutions.
   
• The site is not vulnerable to changes in the grid supply voltage, as the regulator maintains the desired output voltage at all times.

An illustration of the benefits of the voltage regulator is shown in the chart below:

Equipment effects

The energy savings are achieved by reducing the losses in the equipment being supplied, and vary according to the type of equipment. If the load is linear, a reduction in voltage from 240 to 230 will reduce the energy consumption by 8%, while a reduction to 220 Volts will yield a saving of 16%.

However, the equipment will still operate correctly at the reduced voltage, as it is still well within the statutory limits, and there will usually also be benefits of increased lifetime. However, voltage reduction does not work with all types of load. Variable-speed inverter drives, high-frequency lighting ballasts and switch-mode power supplies for example will generally not yield significant savings at reduced voltage – because the voltage fed to the load is generated electronically and is not affected by the supply voltage. Temperature-controlled heating is another type of load where no energy saving will be obtained, as the heater will still need to consume the same amount of energy to perform its required function.

Although it may not be applicable to all sites or all types of equipment, voltage reduction is nevertheless a highly effective energy-saving technique that is being used very successfully on many sites, and is well worth considering as part of your overall energy-reduction strategy.