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Home / Technical Articles / Automatic Transfer System (ATS) Explained in Details

The Role of the Automatic Transfer System

Using the basic building blocks of utility power, system topology, on-site generation, and uninterruptible power supplies, the basic role of the automatic transfer system may now be defined.

Operational Requirments of Automatic Transfer System
Operational Requirments of Automatic Transfer System (photo credit:

In this role, the Automatic Transfer System must display the following characteristics:

  1. Robustness – it must operate as intended, even under abnormal power system conditions, without human intervention. Just as importantly, it must be able to distinguish when a system condition does not warrant transfer to the alternate power source.
  2. It must be able to control the switchgear as required and, additionally, must be able to pass the proper signals to the alternate power source if necessary (for example, to signal a generator when to start.

Simply stated, the role of the automatic transfer system is to provide the automatic transfer of power for its associated load group from a normal power source, such as a utility service, to an alternate power source, such as standby generation, in the event the normal source fails.

Example System Description

To fully illustrate the operational requirements ofa typical automatic transfer system, a more detailed representation of the system is required. For this purpose, the main-main topology arrangement used, but with the details of the automatic transfer system shown:

Main-Main Automatic Transfer Scheme Detail
Figure 1 – Main-Main Automatic Transfer Scheme Detail

In Figure 1, the automatic transfer logic provides the decision-making for what automatic operations are to happen, and when. It controls the operation of the two transfer circuit breakers, CB-UM and CB-GM, and receives status inputs from those breakers. It also can initiate generator startup for the alternate power source.

Undervoltage (device 27) and negative sequence voltage (device 47) relays on each power source give the transfer logic indication of their condition. In addition, a frequency relay (device 81) is present for frequency indication of the
alternate power source.

Voltage transformers, or VT’s, step the system voltages down to instrumentation levels that can be used by these relays. A user interface allows the adjustment of certain operating parameters of the system, and updates the user onthe status of the system.

Using this example system, the operational requirements of a typical automatic transfer system will be examined.

Modes of Operation //

An essential requirement of anyautomatic transfer system is the ability to have different modes of operation. In a given mode of operation, the transfer system will respond in a given way to changing system conditions. For a different mode of operation, the transfer system will respond differently.

Two basic modes of operation, which any automatic transfer system must have, are:

  1. Manual Mode
  2. Automatic Mode

In the manual mode, the automatic transfer system does not perform any automatic operations, i.e., it does not respond to changing system conditions. All circuit breaker operations must be manually performed. Conversely, in the automatic mode of operation all operations, with a few emergency exceptions, are automatic, and the system will respond automatically to changing system conditions.

On the surface, this appears to be a simple arrangement, and to some extent this is true.

Good ATS design //

However, good automatic transfer system design has well-thought-out mode logic that answers the following questions:

  1. Can the system be placed into automatic mode if system conditions are not correct (for example, if an automatically-controlled circuit breaker is in the withdrawn position or not present in the circuit breaker cell)?
  2. What manual operations are allowed in automatic mode (for example, manual opening of circuit breakers)?
  3. What happens if an allowed manual operation is performed on an automatically controlled device (for example, if an automatically-controlled circuit breaker is manually tripped or trips due to a fault)

Such questions are not always easy to answer. In fact, they necessitate, in a well-designed automatic transfer system, the inclusion of a third mode of operation, typically known as auto mode failure.

The three modes of operation then typically work as follows:

  1. Manual mode
  2. Automatic mode
  3. Auto mode failure

1. Manual Mode

Selected via a selector switch position or other pre-determined user input via the user interface. No automatic operations occur.

Go back to ATS Operation Modes ↑

2. Automatic Mode //

Selected via a selector switch position or other pre-determined user input via the user interface. Attempting to enter automatic mode if the system conditions are not correct places the system into Auto Mode Failure.

In Automatic Mode, operations for certain circuit breakers (such as main and tie circuit breakers) are automatic, however manual tripping (or breaker trip due to a fault) of automatically-controlled circuit breakers is allowed. Such manual or fault-driven operations will result in the system being placed into Auto Mode Failure.

Go back to ATS Operation Modes ↑

3. Auto Mode Failure //

No automatic operations occur. For automatically-controlled circuit breakers, only manual tripping (or trip due to a fault) is allowed. To leave this mode of operation, the system must be placed into manual mode.

This arrangement provides a high level of security for the transfer scheme, i.e., undesired or “nuisance” operations are minimized, enhancing safety, maintainability, and reliability of the system.

Of necessity, to make this mode logic arrangement function properly the breaker status must consist both of breaker open-closed indication and, for drawout circuit breakers, circuit breaker cell switch indication.

Circuit breaker cell switches are a feature which must not be overlooked as they are essential for the proper function of an automatic transfer scheme with drawout circuit breakers. For the same reason overcurrent trip switches for low-voltage circuit breakers or lockout relays for medium-voltage circuit breakers are also required.

Another question that frequently arises is that of a “test” mode of operation. While this could be made into a separate mode of operation, this is usually most expediently handled via voltage failure simulation test switches whilethe system is the automatic mode.

Go back to ATS Operation Modes ↑

Read the next part 2 //

Reference // Critical-Power Automatic Transfer Systems – Design and Application by Bill Brown, P.E., Jay Guditis, Square D Critical Power Competency Center

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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. Frank Lugo
    Nov 30, 2021

    hello, kindly produce an understable sequence of an ATS with relay, phase failure, auxiliaries,… all components . I am total blank on how to handle that suppose am given.

  2. demewoz
    Mar 05, 2021

    i need to know the ATS detail components and functions.
    thank you

  3. Sivasankar
    Aug 18, 2018

    Please Explain about Combined differential stability for Transformer and Generator, Like power transformer stability test procedure in MS Excel.
    Generator Testing also

  4. Hammad
    Apr 23, 2018

    can we ensure supply during ATS because once incoming cut off, then ATS need some time to close other breaker (likely Bus Tie or other incomer) as well. so in that period we do not have power for our load. what should we do that period.

    • Alex
      Feb 18, 2022

      In this case to maintain continuity during Load Transfer there is a Protocol of Momentary paralleling of Bus Tie Breaker and Both incoming breakers for a specified fraction of a second. That time and this condition if is required( generally for super critical instrumentation and process control loads) is generally specified in the Design basis criteria.

  5. gen82
    Jul 11, 2017

    Hello folks, Koen have right, this site must show details about Single-Line Diagram.That diagram is nothing . thankyou

  6. Koen M
    Jun 15, 2016

    This is a basic article about a automatic transfer system:
    Such a system isn’t easy to build.

    Difficulty of the system are the needs for it to work:
    1) No break criteria needed (no interruption)
    2) an automatic switching operation (brief interruption allowed)

    Secondary back up system spec’s:
    Generator (synchrocheck, synchronised)
    Other LV net (transformer) (synchronised?)

    Principle of switching:
    U<40% ?
    No break principle ( first zero crossing)
    No break principle (predection of vector)
    What about load shedding?

    Siemens and ABB have those systems. Schneider also I guess.

  7. Rahed
    Mar 21, 2016

    I want to know properly diagram about of ATS system.thanks

  8. srinivas
    Nov 23, 2015

    I need improve my knowledge in schnider electric products and basic information

  9. sinnadurai sripadmanabn
    Nov 20, 2015

    please provide list of components,brand names,model,type,ratings etc used in this ATS

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