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Home / Technical Articles / Substation DC Auxiliary Supply – Battery And Charger Applications

DC voltage 110 V or 220 V

Power substation can have one or several DC systems. Factors affecting the number of systems are the need of more than one voltage level and the need of duplicating systems.

Substation DC Auxiliary Supply - Battery And Charger Applications
Substation DC Auxiliary Supply - Battery And Charger Applications (on photo: Newly completed DC auxiliary power supply of substation in Naramata BC; credit: Paul Chernikhowsky via Flickr)

Today, normal DC auxiliary supply systems in power substation are operating either on the 110 V or 220 V level, though lower levels exist. Some systems at the substation may require lower voltages as their auxiliary supply source.

A typical example of these systems would be the optical telecommunication devices or the power line carrier (PLC) equipment, which normally requires 48 V. If the power consumption of these devices is low enough, their supply can be arranged with DC/DC converters, supplied by the higher voltage level DC system.

Elements of DC Auxiliary System //

Single-battery and charger application

The main components of the system are battery, charger and distribution switchboard including the DC system monitoring relay. Figure 1 shows the main line diagram of a single-battery and charger application.

Typical single-battery and charger application
Figure 1 – Typical single-battery and charger application

In a typical installation, especially with batteries of considerable size, the batteries are installed in a separate battery room. The ventilation of the battery room shall be adequate, considering the type and size of the battery. Temperature level in the battery room should not exceed 25°C, since temperatures above this significantly affect the lifetime of the battery.

The charger and distribution switchboard are normally located in the same room, separate to the battery.

The main fuses of the battery are housed in separate plastic boxes, one for plus connection and one for minus connection. These main fuse boxes should be placed close to the battery itself.

The main fuses are supervised and an alarm is given in a case of a blown fuse (Figure 2). If a main fuse (F1 or F2) is blown, the overcurrent tries to divert its path via paralleled miniature circuit breaker (F1.1 or F2.1). This miniature circuit breaker has a very small rated current and is also tripped immediately, causing the alarm contact 95-96 to close.

Battery main fuse supervision
Figure 2 – Battery main fuse supervision

The cables leading from the main fuse boxes to the distribution switchboard are run separately for both polarities with at least a 10 cm distance between each other. The cables are installed in non-conductive (plastic) pipes for the total length.

Usually at the distribution switchboard there is provided a separate fuse switch output for connecting external battery discharger equipment, as shown in Figure 1. This output can be utilized while making a battery discharge test during substation commissioning or regular maintenance and testing.

Auxiliary power supply switchboard
Auxiliary power supply switchboard (photo credit:

Duplication of the system

Relay protection, control and interlocking circuits //

Since the DC system supplying specially relay protection, control and interlocking circuits is of paramount importance to the substation’s reliable and safe operation, the energy supply has to be always available. The need of this reliable supply becomes even more important during disturbances and faults in the high- or medium-voltage primary circuits.

As a result of these faults, the AC auxiliary voltage may not be available, because the incoming feeders may have tripped. After such situation, the re-energizing of the substation is solely depending on the DC auxiliary power available.

The importance of this reliable DC-auxiliary power is crucial for the substation as such. The higher (more important) role the substation plays from the complete distribution or transmission network point of view, the higher are the demands for the substation’s DC auxiliary power systems.

To meet the increased demands for reliability and availability, the DC system can be doubled (Figure 3). This means that there are two separate systems, at the same voltage level, running in parallel. Both of the systems have their own batteries and chargers.

Auxiliary power supply metering
Auxiliary power supply metering (photo credit:

The distribution switchboard is divided into two separate sections, where both battery and charger sets are supplying their own sections.

There is a bus tie switch connecting the busbars of the different sections together. Under normal conditions, this bus tie switch is kept open. In case of faults or maintenance on one of the battery and charger sets, the bus tie can be closed, thus enabling the other battery and charger set to supply the whole load.

Typical doubled battery and charger application
Figure 3 – Typical doubled battery and charger application

The actual circuits that the doubled DC system is supplying are distributed equally among the two sections in the switchboard. Circuits with doubled functions, like trip circuit 1 and trip circuit 2, are connected to separate sections. This way, the fault in one of the sections does not affect the tripping circuits connected to the second section.

The doubling of circuits, especially regarding protection circuits, should continue all the way to the actual primary devices.

This means that for example with the circuit breaker there should be two separate tripping coils, one for trip circuit 1 and second one for trip circuit 2. The cabling for these two circuits (tripping coils) should be done with separate cables utilizing, as far as possible, also different cabling routes.

Furthermore, a common practice is that the main protection relays receive their auxiliary supply from as well as give their trip commands to the trip circuit 1, whereas the backup protection relays utilize the trip circuit 2 for the same functions. The local and remote circuit breaker control functions (opening command) typically utilize the trip circuit 1.

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

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More Information

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. Dk
    Aug 26, 2020

    The supply to the auxiliary DC have failed how to start the process of maintenance and rectification . The alarm and other relay circuit also seems to be not working . Kindly give me the step wise process .

  2. Confirm
    Jul 12, 2019

    Good evening what is possible to happen when DC failed to work in a substations and operation is carried out. Is there going to he an explosion or what??

    • Tony Tech
      Jan 24, 2020

      The relays and associated circuit breakers won’t be able to trip due to the absebse of supply voltage into their operating coils which initiates the relays operation incase of occurance of fault. So yes an explosion may occur when an abnormal condition such a s fault occur in the unprotected zone

  3. Mahir S. Saleh
    Jul 01, 2019

    Dear Sir,
    It is appreciated if you advice us about, which is better solution for battery system in the mobile substation 132/33kV to get 110V DC, using 92 cells with 1.2V or 74 cells with 1.5V?
    Best regards
    Mahir S. Saleh

    • GOKUL E
      Sep 18, 2019

      2 V * 55 cells is the better operation.

    Apr 13, 2019


  5. rafael alvarez
    Mar 28, 2019

    Could anybody recommend a brand of batteries charger, application is for battery bank who is providing energy to protection devices on medium voltage interrupters

  6. Anil kumar
    Mar 09, 2019

    Dear sir,
    Please provide pdf of battery charging and discharging cycle calculation and maintenance of battery bank of 150ah.
    My mail id is [email protected]

  7. srikanth
    Feb 28, 2019

    it’s too good for learners

  8. Ahmad Fahim Rahimi
    Feb 16, 2019

    Dear Sir,
    I am writing this email on behave of DABS (National electricity company of Afghanistan). DABS is responsible for generation and distribution of energy across the Afghanistan. DABS needs to purchase 220 VDC and 48 VDC battery & battery charger backup system. for further conversation please contact us through below email to share the design and technical specification for your quotation.

    Email: [email protected]


    DABS Procurement Team

    Dec 26, 2018

    Dear sir

    Please answer the following

    1. Some times battery cells develop high resistance. This causes battery bank to not to give energy to the equipment in Substations
    2. is there any alternative option for dropping resistar
    3. we want online monitoring of battery condition
    4. Chargers should be lightning proof and switching transient proff. what IEC standards are required?

    best regards

    SEC Riyadh

  10. Willem du Toit
    Sep 27, 2018

    Thank you for this portal. I am in the Technical training environment and your contribution through this portal is of exceptional value. I do not see myself as a Subject Matter Expert but I have learned and confirmed a lot over the past 30 years in this vast field. Your website have given me frequent info and confirmation on training topics needed. I would like to thank you sincerely for this.

  11. PA Shah
    Aug 07, 2018

    What is load in watts of each relay which gets dc power supply. say over current relay, earth fault relay, percentage difference relay, distance relay, etc. This will helpful in battery sizing.

  12. Hamid
    Feb 27, 2018

    Dear sir
    I need site acceptance test form for battery charger in power substations.
    Thank you

  13. Dennis Sharma
    Jan 09, 2018

    Please post an article on Substation Design.
    It is much needed.
    Thank you

  14. Jared jones
    Dec 29, 2017

    What is the difference between float voltage equalization voltage

  15. Vinay
    Nov 04, 2017

    Can i use a switch mode power supply which gives DC output voltage, when it is fed with UPS AC supply in case of Battery bank and battery charger.

  16. Jeevan
    Aug 16, 2017

    If I use 125 DC volt charger, what is the maximum voltage tolerance load (trip coils) can accept. ?

  17. Lacirtcele Reenigne
    Oct 05, 2016

    What is the recommended backup time for the batteries and sizing…

  18. Rakesh Tiwari
    Sep 28, 2015

    i want to know what is the use of auxiliary supply in a substation how its works?

    • Umesha.S
      Dec 27, 2016

      As the name indicates that Power supply which cater the substation auxiliary loads.
      Auxiliary loads are categorized as critical & non-critical loads.
      AC supply used for non-critical loads and DC supply for critical loads as in this article.

      The non-critical loads are such as substation lighting, heating, ventilation, small power, CB/isolators operating mechanism etc.
      The critical loads are trip coil of CB, protection relays, SCADA, communication devices, emergency lighting etc.

  19. Rakesh Tiwari
    Sep 28, 2015

    i want to know, what is the use of auxiliary supply in a substation? how it works

  20. heshmat abdul jabbar
    Sep 23, 2015

    Thank you for these good information

    Sep 21, 2015

    Thanks for your technical articles.
    could you please do articles on substation design.

    I am a project engineer based on substation installation and commissioning

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