What is Power Quality

By John Houdek

Power quality is the comparison of measured voltage and current characteristics to the nominal values of pure fundamental voltage and current. Ideal power quality is achieved when voltage and current waveforms are sinusoidal, of proper amplitude, balanced among phases and without disturbance or distortion. An example of good quality voltage and current is shown in Fig. 1.


Fig. 1  Power Quality represented by sinusoidal voltage and current

Black = Voltage; Grey = Current


Both waveforms shown are purely sinusoidal with proper magnitudes and are directly in phase with each other.  Ideally, the phase voltage and current waveforms should cross the zero axis at nearly the same time (as these do). When they do, power factor is high (near unity). When they do not, then power factor is low, current increases and kVA is wasted.

Power quality is a concern today because there is more electrical equipment in use that is sensitive to power quality and there is more equipment in use that causes power quality disturbances. Poor power quality can cause electrical or electronic equipment to malfunction, shut down unexpectedly, interfere with other sensitive electronic equipment, or fail prematurely. Poor power quality can be responsible for either immediate or latent damage to equipment and can affect the performance and lifespan of electrical and electronic equipment.

Power quality can be affected by both internal and external events. Disturbances originating on the utility distribution system may be due to electrical device failure, switching events, animals, accidents, weather or possibly neighbors. While some of these may be unavoidable, measures can be taken within buildings (such as using a voltage regulator) to make electrical and electronic equipment less vulnerable to external disturbances. Disturbances originating from within buildings are often due to motor starting, interaction of power factor capacitors, and the operation of electronic equipment. These disturbances are often avoidable through the use of common methods or equipment for improving power quality.

Power quality can refer to a myriad of disturbances of the voltage or current: too high, too low, distorted, contains high frequency "noise", transients, harmonics, low power factor, etc. If voltage is high or low, it can cause higher or lower current to flow. If the voltage contains frequencies besides the fundamental frequency (harmonics), then the current will also contain those harmonic frequencies. Examples of common power quality disturbances are shown in Fig. 2.


Fig. 2 Power Quality Disturbances


The quickest way to assess a facility's power quality is to look at the voltage.  If voltage is low or distorted, then looking at downstream linear load currents can help to identify the root cause. Electric motors draw high peak current when started, known as inrush current, which can be responsible for voltage sags and equipment shut-downs. Some electronic equipment, such as motor drives, battery chargers, welders, etc., draw distorted current (often in pulses), resulting in both harmonic current distortion and voltage distortion. Harmonic current distortion increases the true RMS current and disproportionately increases the peak current.  This can lead to voltage sags as well as increased conductor and transformer heating. In temperature controlled environments, this extra heat may add to the air conditioning load, thus decreasing energy efficiency. Over a period of time, some power quality issues and power quality problems can be solved with harmonic filtering.

With the extensive use of motors and power electronics equipment, power quality has become a growing concern in commercial, industrial and municipal facilities, office buildings, data centers and medical facilities.  Normal operation and life expectancy of electrical and electronic equipment begins with good quality voltage. Suitable goals for power quality are sinusoidal voltage maintained within -/+5% of rated voltage along with harmonic distortion that is less than 2.5% for voltage and less than 5% for current.