Power quality - what is it?

Advance Electronics Managing Director Nathan Briggs considers the question "What is power quality?", and examines exactly what can be done to safeguard essential equipment.

Advance Electronics Managing Director Nathan Briggs considers the question "What is power quality?", and examines exactly what can be done to safeguard essential equipment.

In Europe at least, single phase power is nominally 230V, having changed from the 240V system in January 2003.

The applicable harmonised standard is EN50160:2000.

Power standards in the UK are defined by Ofgem the electricity regulator.

However, if you look at their parameters for acceptable performance, you will discover that the tolerances for the old 240V system were +/-6% while the 230V system is +/-10% for 95% of the time (with -15% allowed at other times).

In other words, the UK now has a lower quality of supply than it had before.

Nevertheless, we tend to assume that, being an industrialised country, our power generation and grid is delivering what we as consumers would call "quality" power.

However a brief look at Ofgem's 2003 annual report shows that, not only is the power supply network filled with problems, but an ever more demanding customer base and intense price competition are actually making these worse.

So what does Ofgem regard as unacceptable power quality? The most obvious form of disruption is of course loss of power.

According to the Ofgem report there were just 12 loss-of-supply incidents in the year ending March 2003 - the highest recorded number since 1998.

In isolation, that may seem like fantastic news, but these statistics only apply to faulty equipment in the National Grid itself.

Human error or protective disconnection is not considered to be the fault of the Grid and is unmeasured.

However it does include storm-related damage, which was responsible for five of the 12 incidents.

Also according to the report, there were "no reportable voltage or frequency excursions".

Again this sounds remarkable but, to be reportable, a "voltage excursion" has to be greater than 6% (beyond +/-10%) and be continuous for 15min.

Remarkably therefore, 400V is fine as long as it lasts less than a quarter of an hour, but even 267V would have to be continuous and recorded to be classified as an "excursion".

Frequency excursions are extremely rare in a system as large as the National Grid, mostly because of the number of sources and their inherent slow speed of frequency change.

Frequency in the UK is nominally 50Hz +/-1% - and almost always better.

We are therefore left with no suitably independent definition of a power problem or of what constitutes "quality power".

When problems occur, it is possible to claim compensation from your supplier, but few individuals or organisations realise how little this compensation actually is.

Private consumers can claim GBP 100 if a power cut lasts for more than 18 hours, or GBP 50 for four or more cuts each longer than 3 hours in a 12 month period.

The supplier can however claim exemption for extreme weather, even from the 18 hour rule, by demonstrating to OFGEM that "everything reasonable was done".

Although some large commercial contracts negotiated directly with the supplier call for other forms of compensation, if a company doesn't have a specific contract, the above rules will apply.

In over 20 years of power quality consulting and manufacturing, Advance Electronics has never known of a successful claim against the supplier on "unacceptable power quality".

Therefore, the clear message is "protect your own".

The following are some common power problems and definitions.

A power cut, blackout, loss of supply or supply interruption entails complete loss of power (voltage less than 1% of nominal), ie no current flowing for greater than 20ms.

Power cuts can be prearranged or accidental.

"Long" cuts are more than 3min, and EN50160 "expects" up to 50 annually, depending on the area.

A power sag, brown out, supply fade or voltage dip entails a reduction in voltage greater than 10% for longer than 10ms.

According to EN50160, dips last between 10ms and 1min.

However, our experience is that a dip can be hours in duration, for example during a substation fault.

A power swell or supply surge is an increase in supply voltage greater than 15% for longer than 100ms.

A transient, spike or impulse is a sharp increase in supply voltage (high dV/dt) with an amplitude greater than 50% of nominal supply.

This is frequently followed by a decaying oscillation.

A lightning transient or EMP is a series of high-energy repetitive spikes with an interval between spikes less than twice their duration.

Harmonics or waveform distortion entail a voltage variation from the ideal sinewave caused by harmonics of the prime frequency.

EN50160 quotes a total harmonic distortion (THD) of less than 8%.

Most harmonics are internally generated rather than supply borne.

Normal mode noise or radio-frequency interference (RFI) is a variation of the waveform caused by a superimposed frequency that is nonharmonic.

Frequently this waveform decays or rings.

And common-mode noise or RFI has a similar cause to normal-mode noise, but where the return or source path is the earth conductor.

These definitions are further confused by the fact that power problems normally occur in groups, often with one disturbance cascading several others.

In addition, natural impedances and inductances in the supply wiring and equipment can cause ringing or "echoes" of a power problem, further confusing the demarcation between cause and effect.

In addition, three-phase power can suffer from unbalance, where the RMS values of the phases differs by more than 5%, and/or phase angles are not equal.

"Quality power" can be defined as power without any of these defects.

Several major studies have been carried out to record the types of disturbance and their frequency.

George W Allen and Donald Segall of IBM System Development Division conducted one of the most respected studies and, although based in the USA in 1974, their report is still quoted today.

They monitored AC power to IBM equipment at 200 locations in 25 cities across the USA, and recorded the various AC power anomalies that disrupted the equipment operation during a two-year time span.

In effect, their criterion for quality power was whether IBM equipment was disrupted.

In 1974 most ICT equipment used linear power supplies which are largely unaffected by small transients and completely immune to most forms of noise.

Today, most ICT equipment uses switch mode power supplies whose susceptibility to transients and common mode noise is far greater.

The Allen-Segall study concluded that 88.5% of AC power problems were transient related.

Allen and Segall found that the most disruptive (49%) of power problems stemmed from oscillatory, decaying transients.

These are examples of long-duration non-lightning-related transients.

Lightning-induced voltage spikes or impulse transients were the next most frequent, representing 39.5% of the total number of AC power problems.

Although the USA has a higher incidence of lightning than in the UK, Britain is still subjected to an average of 0.3-1 lightning strikes per square kilometre per year.

In contrast, power outages accounted for only 0.5% of equipment disruptions, whereas sags and swells were responsible for 11% of AC power problems.

Electrical distribution systems have not changed significantly from the mid-1970s, while electrical and electronic equipment has become much more sophisticated.

Now, although power outages are becoming more infrequent, harmonic distortion has become a major concern for equipment users.

Transient activity has also become increasingly threatening to modern state of the art electrical and electronic load devices.

Other studies include Goldstein and Speranza's "The quality of US commercial AC power" which, in 1982, recorded nearly double the frequency of disturbances at Bell Telephone sites.

Key's "Diagnosing power quality-related computer problems" led to the development of the CBEMA curve detailing recommended immunities for a computer power supply.

The IEEE Emerald Book recommends applying surge protection devices in a cascaded or two stage approach - at the service entrance and then downstream at the panel boards and critical loads.

This comprehensive installation creates an effective shield from high-energy transients.

The IEEE Emerald Book also recommends using surge protection devices as a part of a building's transient protection system.

Because the service entrance device can only reduce and not completely eliminate the high-energy transient, a second device must be applied to protect critical equipment.

By placing a surge protection device (SPD) at the service entrance or main incomer (known as Zone C) and branch panel boards that feed external equipment like HVAC units, the building has an effective shield from high-energy transients trying to make their way in from the outside.

A second layer of SPDs sited at key branch locations (Zone B) provides valuable protection against internally generated disturbances, which are prevented from feeding back within the electrical system.

The bidirectional nature of parallel SPDs allows one unit mounted at the panel board to protect all circuits within the panel board, making it a very cost-effective application.

Finally, point-of-use series suppressors are placed at critical loads and delicate equipment (Zone A), such as fire alarm systems, security systems, and in computer rooms.

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