Why, as per the Power Quality Standards, only +6%, –6% voltage variation is allowed for voltages less than 650 V, whereas voltage variation of up to 9% is allowed in case of higher voltage?

Answer by Roman Targosz (PCPC)

The subject should be divided into 2 areas; the first one covering responsibility of Distribution System Operator for performance of public network up to the point of connection to user side installation where the maximum voltage drop should be limited by proper design and installation for selected loads.       

Starting from the first area, the European Standard EN 50160 “Voltage characteristics of electricity supplied by public electricity networks” specifies voltage value as follows:

Under normal operating conditions excluding the periods with interruptions, supply voltage variations should not exceed ± 10% of the nominal voltage Un.

In cases of electricity supply in networks not interconnected with transmission systems or for special remote network users, voltage variations should not exceed +10 % / -15 % of Un. Network users should be informed about the conditions.

Exactly the same applies for medium voltage, while it is not specified for high voltage as for such voltage levels there is usually a precise contract specifying voltage value and its tolerance.

In the second area, the international standard IEC 60364-5-52 Low-voltage electrical installations – Part 5-52: Selection and erection of electrical equipment – Wiring systems specifies maximum voltage drop in user installations as to be maximum 3% for lighting circuits and 5% for all other circuits in case of the supply from public low voltage systems. In case of supply from private network the maximum values are allowed to be 3% higher.    

The main reason to limit voltage value variation is to assure proper functioning of electrical equipment as it may not tolerate voltage beyond its immunity limits. Most of such electrical equipment particularly sensitive to voltage changes is the one operating at low voltage and thus it should be protected against voltage value beyond tolerance.

According to European standards the tolerances in public supply voltage for low and medium level are the same while international voltage drop requirement on user’s side is applied for low voltage only. However there are some premises to have more severe tolerances for low voltage levels:

  • voltage value depends basically on load reactive power changes and at higher voltage levels such changes related to short circuit power may be more dynamic and larger in magnitude.
  • for given loads, the most universal and efficient way to limit voltage drop in both areas is to increase shorts circuit power, mainly by reduction of resistance of feeders and installation cables. At higher voltage levels the X/R ratio is much higher than for low voltage systems and thus voltage stability is larger challenge and is controlled by more sophisticated methods.