Quality time

Never has the need for reliability in the systems that support the fabric of our modern lives been greater. In addition to power cuts, any number of disturbances to the power supply – surges, spikes, sags, harmonic distortion and electromagnetic interference – can affect the performance of sensitive electronic devices and contribute to the premature ageing of components.

Financial implications
Increasingly, the cost of poor electrical power quality is a bottom line issue for industry and service companies, with around 50% of buildings suffering from severe power quality problems. The impact of power quality disturbances can be substantial. Even the smallest variation can have significant implications for organisations in the form of lost time, productivity and revenues.

Electrical disturbances and harmonic distortion can cause damage and premature ageing to equipment and reduce energy efficiency. Harmonic problems within an installation can overload the neutral load, overheat transformers and put too much stress on correction capacitors. Repairing or replacing capacitors and transformers every seven to ten years, when they are expected to function effectively for 30-40 years, can have serious financial consequences.

The Copper Development Association's 2001 report on the costs of poor power quality estimates that such problems cost industry and commerce in the EU about 10 billion euros per annum, while expenditure on preventative measures is less that 5% of this.

Sources of poor power quality
There are three origins of power quality problems: supply, internal distribution and internal loads. Taking supply first, this is the source of power from the utility company to the transformer of a commercial building for example. It is where power coming into the facility can be monitored.

Looking at internal distribution, 80% of all power quality problems occur in a company's distribution and grounding systems caused by corroded connections, defective electrical devices and the like.

High-tech devices and unsophisticated loads such as heaters and lights contribute to the quality of electrical power in a circuit. Internal loads can cause poor power factor, harmonics and disturbances.

Power quality can be broken down into three key areas: power factor, harmonics and disturbances.

Power factor
The measurement of power factor in a facility determines the amount of power that is supplied but not utilised ie power efficiency.

When the power factor ratio is 1·00 it means that every watt of power arriving from the utility company is put to use. A healthy power factor is in the mid to upper 0·90s; anything below that is considered poor. Motors and electronic equipment apply a load to the circuit that can cause a phase shift, resulting in inefficient power use.

It is more cost-efficient to take steps towards improving power factor than it is to live with excessive utility charges. The effects of power factor cannot necessarily be eliminated, but can be minimised if properly diagnosed. A good quality handheld power analyser is the best way to monitor a circuit's power factor. One simple measurement at the transformer can provide true power, apparent power and power factor data.

Poor power factor usually results from a motor causing a phase shift, with industrial operations being especially susceptible. Once power factor is determined and the origin identified, correcting the problem is a simple case of compensating the circuit with additional capacitor banks. While these are a permanent solution, it is wise to continue monitoring for any new problems.

Harmonics
Most simple electronic devices like motors and lighting are linear loads, which means ac impedance is constant regardless of applied voltage and the ac sine wave is unaffected. However, non-linear loads such as personal computers strip off ac power and convert it to dc power. This process adds harmonics to the fundamental frequency.

A harmonic is a frequency, which is an integer of the fundamental UK 50 Hz wave. The total harmonic distortion is the percentage of distortion to the fundamental frequency, which should not exceed 5% of voltage or 20% of current.

The addition of harmonics to a system results in distortion to the voltage and current waveforms. The impact to the electrical systems depends on the total amount of distortion present and on which harmonic the distortion is located.

In the three-phase, four-wire electrical system commonly found in commercial buildings, current flows through each phase conductor and returns in a common neutral conductor. In a balanced system, the neutral currents from each phase cancel each other out. Any imbalance in phase current returns on the neutral at the fundamental frequency (50 Hz). As this return current is typically small, it is generally not considered a problem.

Triplen harmonics are commonly found in commercial settings and disrupt this balance. Rather than cancelling each other out on the neutral, triplen harmonics from all three phases are added together in the neutral conductor. This can result in a higher than expected current that can cause excessive heat in the neutral conductor and transformer.

The electrical loads in an industrial plant can be affected greatly by the negative sequence harmonics. These harmonics counteract the energy created by the electrical power system. For example, in a motor the negative sequence harmonics try to force the motor to turn in a reverse direction. The impact to the rotation of the motor depends on the magnitude of the current harmonics. This has serious impacts to the torque produced by the motor and the heat given off and it affects production being run on the machine and the motor life.

To troubleshoot the problem of harmonics effectively, the harmonic factorisation should be analysed using a quality, handheld, power analyser. In commercial environments, harmonics are not usually removed, as doing so requires filters or devices that minimise the effects on the circuit and these are not always cost-effective.

The alternative to removing harmonics is to minimise the effects they have on the system. Oversizing the neutral conductor or derating the existing transformers is a possible solution for a three-phase, four-wire system affected by triplen harmonics. While derating a motor is possible, it is more common to use filters to reduce the effects of harmonics in an industrial facility.

Disturbances
The term 'disturbances' is actually used to describe any kind of fluctuation in power. The most common types of disturbances are sags, swells, overvoltages, undervoltages, and transients.

Sags are momentary decreases in voltage, caused by the starting of heavy loads such as machinery, and last anything between 0·5 seconds to one minute (a decrease over one minute is an undervoltage). Swells are the opposite of sags ie increases in voltage caused when heavy loads are shut down (any increase over one minute is an overvoltage).

Though they are caused by different factors, sags and swells often follow each other as the system attempts to compensate. This further increases their potential to cause damage.

Transients are caused by a rapid release of energy stored in an inductive or capacitive source in the electrical system, or from an external source such as lighting. They can vary widely from twice the normal voltage to several thousand volts and last from less than a microsecond up to a few hundredths of a second. While the duration of transients is unnoticeable to the human observer, their effect on power quality is considerable. A single lightning strike can result in a transient large enough to destroy electronic devices.

The nature of disturbances is often systematic, but infrequent. By tracking a circuit over an extended period of time with a good quality power analyser, data can be saved and analysed later on a computer. This makes disturbances easier to pinpoint.

Unlike harmonics, reoccurring disturbances are often fairly easy to eliminate once they have been properly identified. For example, devices like surge protectors are a simple way to protect against transients.