Power quality: the electrical network's or the grid's ability to supply a clean and stable power supply

High power quality ideally creates a perfect power supply that is always available, has a pure noise-free, sinusoidal wave shape, and is always within voltage and frequency tolerances.

With increasing and varying energy demands from various industrial processes, many loads regularly impose disturbances on the grid, making deviations from ideal conditions a frequent occurrence in many heavy industries. This is known as poor power quality.

Power Quality Units and Definitions

SkVAApparent power. Given in kVA.
QkVArReactive power. Fundamental only. Given in kVAr.
PkWActive power. Fundamental only. Given in kW.
Cos φPower factor. Fundamental only.
IA RMSTotal current, including harmonics.
IharmA RMSTotal current distortion in A RMS
ITHD%Total Current Harmonic Distortion. Given as a percentage (%).
VVVoltage. Given in RMS.
Vunb%Voltage unbalance. Given as a percentage (%).
VTHD%Total Voltage Harmonic Distortion. Given as a percentage (%).
PstFlicker, short term (10 min).
PltFlicker, long term (2 hours).


Poor power quality and how to avoid it

The power within any grid is usually flawed to some extent. The result? Equipment underperformance, breakdowns, and energy losses—just to name a few issues!

The seven most common poor power quality problems

reactive power

Reactive power: The phase angle between the current and voltage waveforms in an AC system. Used to develop magnetic field in motors, causes low power factor.


Harmonics: Multiples of the supply frequency, i.e. the fifth harmonic would be 250 Hz if the supply frequency is 50 Hz. Caused by e.g. power electronic loads such as variable speed drives and UPS systems.

network unbalance

Network unbalance: Different line voltages. Caused by single-phase loads, phase-to-phase loads and unbalanced three-phase loads like welding equipment.


Transients: Rapid change in the sine wave that occurs in both voltage and current waveforms. Caused by switching devices, start- and stop of high power equipment.

voltage variations

Voltage variations: Includes dips, sags, swells, brown-outs. The line voltage is higher or lower than the nominal voltage for a shorter period. Caused by e.g. network faults, switching of capacitive loads, and excessive loading.


Flicker: Random or repetitive variations in the voltage. Caused by e.g. mills, EAF operation (arc furnaces), welding equipment and shredders.


Oscillations (resonances): The flow of electrical energy, e.g. between the magnetic field of an inductor and the electric field of a capacitor, changes direction periodically.

You may be facing poor power quality if you experience several of the following symptoms:

  • Reduced production speed
  • Increased energy consumption
  • Fees for reactive power consumption
  • Stalled production
  • Damaged or malfunctioning equipment
  • Idling personnel, stalled production
  • Decreased equipment lifetime
  • Loss of data

In addition to the direct impacts above, poor power quality often negatively impacts business costs. Such as with:

  • Rework
  • Increasing start-up costs
  • Postponed revenues
  • Reduced cash flow
  • Poor reputation among customers
  • Loss of market share

What are you Paying for Poor Power Quality?

Improving power quality reduces risk, raises throughput, decreases the cost of doing business, and improves your bottom line.

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