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Home / Technical Articles / Substation AC Auxiliary Supply For Inessential Loads

Auxiliary power supply systems

The purpose of auxiliary power supply systems is to cater for the necessary energy for the operation of primary and secondary devices at the substation. The auxiliary power systems are normally divided in two categories, namely the AC system and the DC system(s).

Substation AC Auxiliary Supply For Inessential Loads
Substation AC Auxiliary Supply For Inessential Loads (on photo: AC auxiliary switchgear 400/230 V; credit: ZPAS Group -

The AC system normally operates with the country’s standardized utility low voltage level, for example 400 V 50 Hz. The DC systems are already described in one of the previous technical articles, so they won’t be mentioned here.

The loads fed by the AC auxiliary supply system are normally so called inessential loads, which are not crucial for the substation operation.

These loads would typically include the following:

  • Substation building(s) climate control and lighting
  • Outdoor equipment and indoor panels desiccation heaters
  • Power transformer cooling fans
  • Driving motor for on-load tap changer of a power transformer
  • Station battery (DC system) charger(s)
  • Normal wall socket outlets

AC Auxiliary Supplies

Usually the AC-power distribution at a substation utilizes the same voltage levels and principles as the normal household electrification of that country. Depending on the practice and the legislation in the target country, the AC distribution system can either be 4-wire (TN-C) or 5- wire (TN-S).

Elements of AC Auxiliary System

The main components of AC auxiliary supply system are:

  • Station auxiliary transformer(s),
  • AC main distribution switchgear,
  • AC sub-distribution board(s) and
  • The cable network

As with the DC auxiliary system, the AC auxiliary system can be also doubled. The doubled system would utilize two station auxiliary transformers, each supplying their own section in the main distribution switchgear. The doubled system can be also constructed so that the second supply is coming from an external source, often the surrounding aerial low voltage network.

The supply for the station auxiliary transformer is arranged from the medium-voltage level side. If there is no medium voltage available at the substation, like a 330/132 kV substation, normally the main power transformer’s tertiary (stabilizing) winding is utilized.

The following figure shows an application where the combined station auxiliary and system- earthing transformer is supplied directly from a main power transformer’s (220/33 kV) low- voltage side.

Combined station auxiliary and system earthing transformer connected directly to 33 kV
Figure 1 – Combined station auxiliary and system earthing transformer connected directly to 33 kV

Figure 1 shows the auxiliary transformer’s physical placement as a part of the bay layout. Since the 33 kV side of a 220/33 kV power transformer is delta-connected, thus not offering a point for system earthing, the station auxiliary transformer is also serving as a system-earthing connection point.

The station auxiliary transformer has a connection group of ZNyn. The zigzag-connected primary winding provides the 33 kV system earthing point. By its nature, the zigzag connection provides also means to limit the earth fault currents into desired level.

The zero-sequence impedance of a zigzag winding can be influenced, within certain limits, by the transformer design.

Station auxiliary and system earthing transformer as a part of the bay layout
Figure 2 – Station auxiliary and system earthing transformer as a part of the bay layout

When the ZNyn-connected earthing transformer is used for low voltage AC-power supply, the magnitude of the available earth fault current on the low voltage side has to be checked, since the zero-sequence reactance of the transformer can limit the current to an unacceptably low value.

Reserve auxiliary transformer
Reserve auxiliary transformer (photo credit:

AC Auxiliary System Earthing //

The primary AC auxiliary system typically uses effectively direct grounded system, having all the supply points connected to earth. Affected by the legislation in each country, the system is either utilizing the 4-wire or 5-wire principle.

The use of a 5-wire (TN-S) system enables also the use of residual current protection in the whole system or in selected feeders.

The residual-current protection measures the sum current in all of the phases and the neutral. The sum current must be zero during normal operation. During a system insulation fault, the sum current will rise and the residual current protection switch will open, separating (de-energizing) the section behind the measurement point from the rest of the system.

A typical operating current for residual switch is either 30mA or 300mA, the first being used for human protection and the latter for fire protection.

Principle of TN-S system with double supply
Figure 3 – Principle of TN-S system with double supply

Reference // ABB’s Distribution Automation Handbook – Elements of power distribution systems

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Mahboob
    Nov 08, 2017

    very good explanations.
    May God Bless you.

  2. praveen Gupta
    Apr 20, 2016

    Good Morning Mr Edvard , I want to know that what is the probability of generating the switching surge during the operation of VCB/ACB or ATS .Whether the surge can be travelled up to LV side even after transformer at 220/400 V .
    What precaution should be taken to protect the loads & AC auxiliary power supply .

  3. Asemokhe Genesis
    Oct 17, 2015

    Very informative. Thank you

  4. sam
    Oct 02, 2015

    Thank you for all your work and your willingness to share it with us. Few months ago it was easire to download all was OK but now it is becoming very difficult. please correct this.

    • Edvard
      Oct 02, 2015

      Thank you Sam! Why difficult downloading? What happens?

  5. Mohd Mounir
    Oct 01, 2015

    thank you Mr Edvard

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