What is power quality?

Electric power quality (EPQ) is the degree of compliance of electrical energy parameters at a given point in the electrical system with a set of normalised EPQ indicators.

In other words, the quality of electrical energy is a set of characteristics in which power consumers can operate normally and perform the functions inherent in them.

EPQ at the place of production does not guarantee its quality at the place of consumption. EPQ before and after switching on the electric receiver at the point of its connection to the electrical network may be different.

Main Indicators of Electricity Quality

According to GOST 13109-97 there are 11 indicators of the quality of electricity:

  • steady state voltage deviation
  • voltage swing
  • dose flicker
  • voltage curve distortion factor
  • the ratio of the nth harmonic component of the voltage
  • negative stress unbalance factor
  • zero sequence voltage imbalance factor
  • frequency deviation
  • duration of voltage failure
  • pulse voltage
  • overvoltage coefficient

Voltage deviation

One of the parameters of power quality is voltage deviation.

The voltage deviation is determined by the value of the steady-state voltage deviation. For the value of voltage deviation, the following standards are established:

Normally permissible and maximum permissible values ​​of the steady-state voltage deviation at the terminals of the electricity receivers are respectively +5 and +10% of the rated voltage of the electrical network.

The value of voltage deviation is determined when the process duration is more than one minute.

Normal voltage tolerances. Normally, the tolerance of the voltage is considered to be in the range of 5%, that is: +/- 5% (from 209 V to 231 V).

Maximum allowable voltage deviations. The maximum permissible voltage deviation is considered to be a range of 10%, that is: +/- 10% (from 198 to 242 V).

The following standards apply for the electricity quality indicators defined above: positive and negative voltage deviations at the point of transmission of electrical energy should not exceed 10% of the nominal or agreed value of the voltage during 100% of the time interval of one week.



Important: do not confuse the requirements of GOST to the quality of electricity in the network (GOST R 54149-2010 "Electrical Energy. Electromagnetic Compatibility of Technical Means") and GOSTs describing the quality of power supply for electrical appliances (eg. GOST R 52161.2.17-2009 "Safety of household and similar electrical appliances”). GOST of power quality imposes requirements on the essence of the electricity supplier, and it is this GOST that you can rely on if you need to make demands on the supplier if the power supply is poor. Moreover, the requirements for the quality of the power supply in the instrument documents determine the requirement for the instruments to operate normally over a wider range of current parameters. For devices, as a rule, the voltage range from -15% to +10% of the nominal is set.




Properties of Electrical Energy

EPQ Indicator

The Most Likely Culprits of CE Deterioration

Voltage Declination 

Steady State Voltage deviation dUy

Power supply organisation

Voltage Fluctuations

Scope voltage change dUt Dose Flicker Pt

Variable load consumer

Non-sinusoidal voltage

The distortion factor of the sinusoidal voltage curve KU

The ratio of the n-th harmonic component of the voltage KU(n)

Non-linear load consumer

Asymmetry of the three-phase voltage system

 Voltage unbalance factor by reverse sequence K2U

Voltage unbalance factor for zero sequence K0U

Unbalanced load consumer

Frequency deviation

DF Frequency deviation

Power supply organisation

Voltage failure

The duration of the volatage drop Dtp

Power supply organisation

Voltage pulse

Impulse voltage Uimp

Power supply organisation

Temporary overvoltage 

Time overvoltage coefficient

Power supply organisation