I. Understanding the terminology

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

Q. What does accuracy mean?

A. Accuracy is a margin limited by the maximum deviations that you may have, above or below the nominal pre-set value of a certain parameter, such as the input voltage, and frequency of your stabilizer.

Specifications should always quote the accuracy achieved, for all parameters that may be affected simultaneously, such as the variation over the full input voltage range when the load changes from no load to full load, and the corresponding output voltage stability; not to forget the input frequency and other parameters.

Three phase systems must specify the possible effects on the accuracy of the output voltage’s due to unbalanced loads and/or unbalanced input line voltages.

Q. What dose Spikes, Surges and Surge rating mean?

A. Spike is a sudden instantaneous rise in the supply voltage (Transient Pulse lasting for a small portion of a cycle)

Spike	Surge

Surge refers to transient over-voltage, i.e. pulse increase in voltage that lasts longer than the spike (could be a few cycles) but with a smaller amplitude than the spike has

Surge rating is the maximum peak current the equipment will withstand for a given period, without being damaged.

Q. What do we mean by r.m.s. and mean values?

A. Alternating voltage and current have a sine wave waveform shape and are -unless other wise stated- measured in terms of r.m.s.

For a pure sine wave electric power supply, the peak value is 1.41 times the r.m.s. value, and the mean value is 0.9 times r.m.s. value.

The energy dissipated in a resistance depends directly on the r.m.s. value. The DC voltage of an AC to DC converter depends on the peak value, and the force of a solenoid or the torque of a motor depends on the mean value.

An equipment designer assumes that the electric power supply is of a pure sinusoidal wave form, also it is known that the wave form of a good power supply seldom distort by more than 1% although a distortion up to 3% is acceptable. It makes a little difference whether you measurere the voltage of such a supply with a true r.m.s. or a mean voltmeter, but the load  may draw a very distorted current waveform, and in rating the supply, an r.m.s. reading ammeter should always be used.

Q. What dose the term Speed of response means?

A. The speed of response is the time required for the output of a system or circuitry in response to a certain variation on its input, For the automatic voltage stabilizers, response speed measures in volts per second typically (0.02 – 0.04 V/s). As for the solid-state stabilizers and ferro-resonant stabilizers witch have a faster response, this response identified in terms of time constant.

Q. What is the input voltage range?

A. It is the total permissible variation range of the supply voltage within which the electric devise will operate properly and accurately. This range is usually expressed as a percentage of the nominal input voltage, plus and minus.

For a voltage stabilizer this is the rang within which the stabilizer will maintain its stated output accuracy and if this output voltage is set to a different value, the input voltage range will alter in proportion.

Q. What is the transverse/common mode interference?

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

Q. What dose Soft Start means?

A. When an electric device or equipment is starting it will draw a high inrush current, this current can damage the supplying source or the equipment.

Soft start is a means by which the high input inrush current is reduced, this is important to provide an essential extra protection for the voltage stabilizers as it have to handle wide voltage variations at the input (also at the output) .

If the stabilizer is switched off when mains supply is low or off, and switched on when it is normal or back, this may allow a momentary over voltage of around 50 % which can damage the equipment connected to the output of the stabilizer before the stabilizer has enough time to correct it. With soft Start fitted, this damage can be avoided.

II.  BASIC FACTS

What are the input variations do you have to cope with?  As you may know well, the answer to this issue comes from experience. If not, it must be ascertained by monitoring the supply voltage for sometime. Under many circumstances, voltage is likely to drop largely or rise above the nominal value. While a low voltage can cause system malfunction, a high voltage may cause severe damage to your equipment and must be prevented at any cost.

Wherever there is a good mains supply, a standard stabilizer that can cover an input variation rang of –15% to +5%, will usually be adequate, but a variation rang of –25 to +10% or even more can be encountered in a locations with poor mains supply.

Determining the right needed input variation range for the required stabilizer is extremely important, because if the input voltage range is exceeded, then the output voltage will increase or decrease by the same ratio by which its input has gone out of limit.

Q. What about the frequency of my power supply?

A. Your nominal mains supply frequency is unlikely to vary by more than about 2% and this will normally be within the capability of most voltage stabilizers. However, some stabilizers cannot cope with the smallest of variations.

Q. How can I determine the ratings of the required stabilizer?

A. You need to check the ratings of the equipment to be supplied through the voltage stabilizer, whether quoted in Amps or KVA, and whether being single or three phases. The nominal voltage, frequency and power factor, are also required.

This information usually written on the nameplate attached to the equipment (most properly at the backside), otherwise you will need to consult the manufacturer or take some measurements. Note that if the current drawn by your equipment is of non-linear behaviour, such as AC to DC power supplies, it is important to measure its true “r.m.s.” value.

Q. What causes transient over-voltage?

Q. What are the possible damages due to transient over voltage?

III.  Levels of protection

Certain types of equipment are more sensitive than others when subject to the fluctuations of the mains supply. Thus, today’s stabilizer technology aims at protecting equipment in several different ways. It is essential that you plan carefully your equipment protection needs based on the level of voltage deviation it can tolerate, before malfunctions and equipment damage become genuine risks.

Q. How much stability does my equipment demand from the power supply?

A. In other words, what level of accuracy do you need from your stabilizer? There are stabilizers can maintain its output stabilises within nominal value ±0.5% 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 stabilizer should be extended by – 4% and + 6% beyond the figures quoted for the higher accuracy. Since the cost of a voltage stabilizer is linked to the input voltage range it has to handle, accepting lower output voltage accuracy may make it possible to select a more economical unit.

Q. Will transient noise create problems?

A. 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 on or off equipment, especially heavy inductive loads, or by atmospheric electrical disturbance such as lightning and storms. Advanced stabilizers are fitted with voltage surge protection that will clip surges or spikes to approximately three times the supply voltage. Nevertheless, the question is, will this be sufficient to protect your equipment?

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

Q. What is the' earth to supply’ interference?

A. 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 peak. For this reason, you may be considering installing a ‘clean line’ from the mains distribution panel, but this will not always guarantee the required level of protection. Some times this can introduce more problems than it solves!

Line conditioning will be the correct solution, achieved by fitting an isolating transformer with the secondary neutral linked to earth. This procedure will eliminate the need for a dedicated clean line, since at the point of linking neutral to earth, the common mode interference is nil.

In other words, the neutral conductor is referenced to the local earth, eliminating the common mode supply interference caused by neutral currents. Stabilizers with added suppression interference like this are often referred to as line conditioners or power conditioners.

Q. How about the other circuit protections?

A. There are other protective devices you need to consider:

• Circuit breakers: which, in addition to providing over current protection, will trip if the output voltage deviation is above a pre-sent level,

• Soft starters: will ensure that your stabilizer cannot deliver a momentary over voltage on switching-on,

• A bypass switch: that facilitates inspection and maintenance through isolating the stabilizer and connecting the equipment directly to the mains,

• Output fuse(s): for current limiting and short circuit protection.

IV. Types of voltage stabilizers

There are several fundamentally different stabilizer 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 for you to look hard at any stabilizer offered and to recognize its basic type and characteristics, and not miss led by vague descriptions. There are four main categories of stabilizers available. These are:

1. Automatic Servo motor stabilizer
2. Electro-mechanical /Solid-state tap changer stabilizer
3. Ferro resonant voltage stabilizers
4. PWM continuous back-Boost stabilizer

Q. What is the Automatic Servo motor stabilizer?

A. This type of stabilizer uses an advance electronic controlled servo motor concept to govern a motorized variable transformer. Because of the motors involved, 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 input voltages 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 stabilizer, periodic maintenance is required.

Q. What is an Electro-mechanical/solid state tap changer?

A. In electro-mechanical devices in which regulation is achieved by selecting one of a number of tapping 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.

Q. What is Ferro resonant voltage stabilizer?

A. Commonly known as CVTs (constant voltage transformers), these devices depend on a technology that is radically different from either tap switched or servo electronic stabilizers. 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, ferrous 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 this type of stabilizers is completely loaded, its efficiency approaches 90%, but when lightly loaded, it can be as low as 75-80%.

Keep in mind that no matter how light is the load, the transformer must be driven into saturation. This uses energy even when unloaded.

Q. What is EVS (PWM continuous back-Boost) stabilizer?

A. The EVS stabilizer 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, it's secondary being connected between the supply in and the load. The controller module can thus add or subtract a voltage to the supply. The two four quadrant bi-directional switches produce the electronic power controller function; these switches consist of IHBT and power diodes that 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 stabilizer 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 alter and supplied to the primary of the buck boost transformer; the secondary voltage then adds 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 IGBT's and allowing fault currents to be cleared by fuse blowing.

Q. How to differentiate between the various types of stabilizers?

A. Well, tap switches are smaller, more efficient, more able to provide for large, short-duration current demands, and can be cheaper in some cases. On the other hand, Servo electronic stabilizers tend to be more suited to the large three-phase installation, although an ongoing maintenance program would be imperative. Ferrous are probably more reliable and have better noise filtration.

In the size required by PCs, the difference between tap switches and ferrous 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 ferrous having 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 ferrous grow. This tilts the scales in favour of tap switches and even more so in favour of electronic stabilizers.

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

1. How much dose your equipment cost?

2. How much can you afford to repair or replace this equipment?

3. How much will be the losses if this equipment stopped?

4. How much dose it worth to protect them by means of a voltage stabilizer?

5. How much can you pay for it?

Recap

To summarize, the choice between various types of stabilizers is a matter of the need of the specification, site, and comfort 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.