Voltage Stabilisers Technologies

Selecting power conditioners

Selecting power conditioners is not simple. It requires a sound appreciation of both the application and the available alternatives.

This article considers four faces of this selection process:

  • getting the basic facts lined up
  • deciding on the required level of protection
  • understanding the terminology
  • appreciating the advantages of the different
  • types conditioner.

Basic facts

With what input variations do you have to cope?

You may well know the answer to this from past experience. If not, it must be ascertained by monitoring the supply voltage over a period of time. In many circumstances, voltage is likely to drop to a greater extent then to rise above nominal, but remember that, while a low voltage can cause system malfunction, an high may cause severe damage.

Where there is a good mains supply, a stabiliser covering input variation of –15 to +5%, will usually be adequate, but a variation of –25 to +10% or more can be encountered in a location with poor mains supply. Choosing a stabiliser with the right input variation capability is important, because if its input voltage range is exceeded, then its output will increase or decrease by the same amount by which it has gone out of limit.

Supply frequency

You nominal mains supply frequency is unlikely to vary by more than about 2% and this is well within the capability of most stabilisers. But note, there are some systems that cannot cope with such variations.

Rating your equipment

You need to check the rating of the equipment to be supplied, whether quoted in A or VA, and whether single or tree phase. The nominal voltage, plus the frequency and power factor, are required. This information will usually be shown on the rating plate attached to the equipment, otherwise you will need to consult the manufacturer or take measurements. Note the if the current draw by your equipment is non-line(for example ac to the power supplies), it is important to measure its true “rms” value.

Decide what level of protection you need

Some types of equipment are more sensitive than others when faced with flaws in the mains supply, so today’s stabiliser technology aims to protect in several different ways. Make sure you know what your equipment needs protection from and the level of voltage deviation it can tolerate, before malfunctions and equipment damage become genuine risks.

How stable must the supply be?

In other words, what level of accuracy do you need from your stabiliser? Most stabilisers will stabilise to about 5% or better, but there is a cost factor to be considered. If a liver accuracy, say to about 5%,is adequate for your equipment needs, then the input voltage range of the stabiliser is extended by –4% and +6% beyond the figures quoted for the higher accuracy. As the cost of the stabiliser is finked to the input voltage range it has to handle, then accepting lower output voltage accuracy may make it possible to select a more economical unit.

Will transients cause problems?

Transient noise is present to some extent in all mains supplies. High energy voltage spikes and surges are introduced into distribution lines by the switching of equipment, especially heavy inductive loads, or by atmospheric electrical disturbance. Some stabilisers are fitted with voltage surge protection which will clip surges or spikes to approximately three times the supply voltage. But will this be sufficient for equipment?

Some systems are little affected by transients, but electronic equipment, especially if microprocessor-based, can be damaged and/or malfunction: in such cases merely clipping high voltages will not be adequate and full transverse mode interference protection will be needed.

Common mode supply interference

Common mode interference can also be a problem and most microprocessor-base equipment will require a supply with this earth to neutral noise factor limited to less than 0,25V peaking.

For this reason, you may be considering installing a clean line from the mains distribution point, but this will not always give the required level of protection. Often this can introduce more problems than it solves!

Line conditioning is the more possible answer, achieved by fitting an isolation transformer with the secondary neutral linked to earth. This eliminates the need for a dedicated clean line and at the point of linking neutral to earth, common mode interference is nil. In other words. The neutral conductor has been solidly referenced to the local earth, eliminating the common mode supply interference caused by neutral currents. Stabilisers with added interference suppression like this are often referred to as line conditioners or power conditioners.

Circuit protection

There are other protective devices you will want to consider:

  • circuit breakers which, in addition to providing over current
  • protection ,will trip if the output voltage deviation is above a present level.
  • soft starters ensure that you stabiliser cannot deliver a momentary over voltage on switch-on.
  • a bypass switch that facilitates inspection and maintenance
  • by isolating the stabiliser and connecting the equipment directly to the mains.
  • Output fuse(s).
Understanding the terminology

Like other advanced technologies, voltage conditioning tends to talk its own language. So it is important to know what the basic terms mean, especially when comparing units from different suppliers, to be sure that everyone is using the same definition and applying the same performance parameters.


By this we mean the maximum deviation you will get, above or below the set output voltage of your stabiliser. Specifications should always quote the accuracy achieved, even if all four parameters are affected simultaneously, a variation over the full input voltage range, change from the off load to full load, plus variation in power factor and mains frequency. Three phase systems must specify the effect caused to the output voltage’s accuracy by unbalanced loads and unbalanced input line voltages.

Surge rating

This is defined as the maximum peak current the equipment will withstand for a given time, without damage to the stabiliser.

Waveform distortion

Alternating voltage and current are, unless stated to the contrary, measured in terms of rms. In pure sine wave supply, the peak is 1,41 times the rms value and the mean is 0,9 times rms. The energy dissipated in a resistance will depend on the rms value. The de voltage od an ca to de convert will depend largely on the peak value, and the force if a solenoid or the torque of a motor on the mean value. An equipment designer assumes that the electricity supply will be sinusoidal and,

A good mains supply is seldom distorted by more than 1% and distortion up to 3% is acceptable. It makes little difference whether you measure the voltage of such a supply with a true rms or a mean voltmeter, but the load (especially when it is a power supply) may draw a very distorted current waveform, and in rating the supply, an rms reading ammeter should always be used.

Speed of response

For electronic servo stabilisers, response speed is defined in terms of the time taken to reduce 10% voltage error to 2%. The faster response of solid state stabilisers and ferro resonant stabilisers are defined in terms of time constant, varying with the kVA rating.

Input voltage range

This is defined as the total permissible supply voltage variation range within which the stabiliser will maintain its stated output accuracy. It is expressed as a percentage of the pre-set output voltage, plus and minus, and if this output voltage is reset to a different value, the input voltage range will alter in proportion.

Transverse and common mode interference

Some protection against these two forms of interference are not as effective as you may be lead to believe. Surge clippers and RFI filters will not provide a clean line. Expensive ultra isolating transformers provide little attenuation of high energy transverse spike and will attenuate common mode noise no better than any isolating transformer, provided that the earth and secondary neutral are linked, which is always desirable.

Soft start

Soft starters provide an essential extra protection when stabilisers are having to handle wide voltage variations. If the unit is switched off when mains supply is low, and switched on when it is high, a momentary over-voltage can damage equipment before the stabiliser has had time to correct it. With soft fitted, voltage is low at start-up.

Get to know which type does what

There are several fundamentally different stabiliser systems on offer today. Naturally , each manufacturer will promote the virtues of his own system and be nowhere as keen to reveal its deficiencies. It is important to look hard at any stabiliser offered and to recognise its basic type and characteristics, and not be misled by vague descriptions. There are three main categories of stabilisers available.

Servo electronic stabiliser

This type of stabiliser use an advance electronic servo concept to control a motorised variable transformer. Because of the motorization, there is a small delay in voltage correction. However, output voltage accuracy is usually +- 1% with input voltage changes of up to +-50%. This type of technology tends to be extremely effective when considering large three phase applications, as it is able to maintain its accuracy of all these phases, despite of balance and load balance at any power factor.

They are also able to withstand large inrush currents, normally experienced with inductive loads. However, due to the mechanics of this type of stabiliser, periodic maintenance is required.

Electro-mechanical/solid state tap changer

In electro-mechanical devices, regulation is achieved by selecting one of a number of tappings of a transformer , by means of relays. They usually reduce input voltage variations of +-15% to +-6% of constant load. Load variations will degrade this accuracy to about 10%. However, the physical mechanics of the relays result in spurious switching and possible relay failure. Solid state tap changers use triacs in series with the taps of a transformer. Eight or more triacs may be used; however spurious transients entering the tap changer can cause two triacs to fire simultaneously and this, with over current or over voltage, can cause the triacs to fail.

The incorporation of isolation and surge protection into tap changers results in this type of device being called a power line conditioner or an electronics line conditioner.

Ferro resonant voltage stabilisers

Commonly known as CVTs (constant voltage transformers), these devices depend on a technology that is radically different from either tap switched or servo electronic stabilisers. To keep the explanation simple, these devices use a loosely coupled saturated secondary winding resonated by large capacitor. They are extremely rugged. Other than the capacitors, which may need replacing from time to time, there are no electronic or mechanical parts to fail. They also have an inherent ability to suppress normal-mode and inverse mode noise. Because of the saturation of the transformers, impulses cannot penetrate the secondary winding. Due to its inherent filtering ability, literally any amount of distortion can be supplied to the CVT and its resultant output waveform would exhibit no more than 3 to 5% waveform distortion. Because of its inherent current limiting ability, it is able to withstand overloads and short circuits on its output without any short term damage. Due to this effect, most ferros are unable to service high current limiting demands in excess of 125 – 150% without the secondary current limiting. This can be a disadvantage when supplying high inrush current loads.
Efficiency is another concern. When it is completely loaded, efficiency approached 90%, but when lightly loaded, it can be as low as 75-80% . Remember that no matter how light the load, the transformer must be driven into saturation. This uses energy even when unloaded.

EVS (PWM continuous back-boost)

The EVS stabiliser series consists of an Electronic Power Controller module per phase which supplies an in phase or out of phase voltage to the primary of a buck/boost transformer, its secondary being connected between the supply in and the load. The controller module can thus add or subtract a voltage to the supply. The electronic power controller function is produced by the two 4 quadrant bi-directional switches, these switches consist of IHBT and power diodes which are used to chop the input voltage at 20 kHz, with the pulse width dependent upon the required output voltage. The control PCB compares the 50 Hz stabiliser output used to control that of a stable reference voltage, the error being used to control the two bi-directional switches. The high frequency PWM waveform is then altered nad supplied to the primary of the buck boost transformer; the secondary voltage then odds or subtracts an appropriate voltage to provide a stable output voltage. The speed of responses is very fast typically 20-30 ms.

Two thyristors are used to by-pass the currents in the IGBT components during output ,protecting the IGBTs and allowing fault currents to be cleared by fuse blowing.


So how is one to choose? Tap switches are smaller, more efficient, more able to provide for large, short-duration current demands, and can, in some cases, be cheaper.

Servo electronic stabilisers tend to be more suited to the large three phase installation, although an ongoing maintenance program would be imperative. Ferros, on the other hand, are probably more reliable and have better noise filtration.

To be honest, in the size required by PCs, the difference between tap switches and ferros shrinks to insignificance. Arguments range between rivals in the industry, but the customer will make his or her determination more on price than on any other criterion. This is where ferros have a big edge. To date, it tends to be cheaper to build a ferro than an electronics tap charger for personal computer systems.

As power requirements increase, and particular three phase requirements, the disadvantages of ferros grow. This tilts the scales in favour of tap switches and even more so in favour of electronic stabilisers.

In the final analysis, the decision of which unit to buy can be evaluated by answering a few direct questions:

  • do I need voltage regulation? If so, the choice between the various stabilisers is a matter of the need of the specification site and confort of the buyer with the points we have discussed. Price will no doubt make the choice clear.
  • do I need power conditioning? If so, the choice is between a ferro and a power-line conditioner. The relative strengths and weaknesses of both product types have been described. The potential buyer of multiphase units should obtain evaluation units, and ‘try before you buy’ to measure actual on site performances presented here. Price differences between the two are usually slight.