Your guide to better power quality

Without proper power, an electrical device may malfunction, fail prematurely, or not operate at all.

By Amin Almasi

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Power quality determines how fit electrical power is to run consumer devices. Synchronizing the voltage frequency and phase allows electrical systems to function as they're intended to without significant loss of performance or life. The term is used to describe electric power that drives an electrical load and the load's ability to function properly. Without the proper power, an electrical device (or load) may malfunction, fail prematurely, or not operate at all. There are many ways in which electric power can be of poor quality and many more causes of such poor quality power.

The electric power system, in general, comprises electricity generation (AC power), electric power transmission, and ultimately electricity distribution to electric users (such as VSD electric motors) that are the electric power's end users. The electricity then moves through the end user's hardware and wiring system until it reaches the load. The complexity of moving electric energy from the point of production to the point of consumption – combined with variations in related factors such as generation and demand – provides many opportunities for the quality of supply to be compromised.

And although "power quality" may be a convenient term, it is the quality of the voltage – rather than power or electric current – that is actually described by the term. Power is simply the flow of energy, and the current demanded by a load is largely uncontrollable.

The quality of electrical power may be described as a set of parameter values, such as:

  • Continuity of service
  • Variation in voltage magnitude (see below)
  • Transient voltages and currents
  • Harmonic content in the waveforms for AC power

It can be useful to think of power-quality issues as compatibility problems. To address these, consider: “Is the equipment that is connected to the grid compatible with the events on the grid?” Alternatively, ask: “Is the power delivered by the grid, including the events, compatible with the equipment that is connected?” Compatibility problems always have at least two solutions; here, the options are either clean up the power or make the equipment tougher.

Power quality

Ideally, AC voltage is supplied by the plant network (whether electric power is imported to a plant from a national or local network or generated in a power generation unit) as sinusoidal, having an amplitude and frequency given by standards or system specifications. Further, AC voltage ideally is supplied with an impedance of near zero ohms at all frequencies. No real-life power source is ideal, however, and power sources generally can deviate in the following ways:
Variations in the peak or RMS (root mean square) voltage are important to different types of equipment as well as to electrical power consumers. When the RMS voltage exceeds the nominal voltage by even 10% to 20% (or more) for 0.5 cycles to 30 seconds (or even more), the event is usually called a "swell." A "sag" describes the opposite situation: The RMS voltage is below the nominal voltage 10% to 20% or more for 0.5 cycles to 30 seconds. Random or repetitive variations in the RMS voltage by a relatively large value – between 90% and 100% of the nominal value, for example – can produce a phenomenon known as "flicker" in lighting equipment. Flicker is rapid visible changes of light level.

Abrupt, very brief increases in voltage, called "spikes," "impulses," or "surges," are generally caused by large inductive loads being turned off or by lightning.

Undervoltage occurs when the nominal voltage drops below 90% for more than 60 seconds. The term "brownout" describes voltage drops that land somewhere between full power (say for instance, full load or bright lights) and a blackout (no power). It comes from the noticeable to significant dimming of regular incandescent lights during system faults or overloading etc., when insufficient power is available to achieve full brightness. Brownouts typically reflect a reduction in system voltage taken by the power generation unit, the utility or the system operator in an effort to decrease demand or to increase system operating margins.

"Overvoltage" occurs when the nominal voltage rises above 110% for more than 60 seconds.

Variations in frequency and in the wave shape (the latter usually described as harmonics) also are relevant to power quality discussions. The two following impedances are important in power quality of a plant:

  • Nonzero low-frequency impedance – when a load draws more power, the voltage drops.
  • Nonzero high-frequency impedance – when a load demands a large amount of current and then stops demanding it suddenly, there will be a dip or spike in the voltage because of the inductances in the power supply line.

Each of the above-mentioned power quality problems has a different cause. Some problems result from the shared infrastructure or network. For example, a fault on the network may cause a dip that will affect some customers – the higher the level of the fault, the greater the number affected. A problem on one customer’s unit or facility may cause a transient that affects all other customers on the same subsystem. Problems such as harmonics may¬¬ arise within the customer’s own installation and propagate onto the network and affect other customers. In most general forms, harmonic problems can be dealt with by a combination of good design practice and proven reduction equipment.

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