Automatic Transfer System Explained in Details (2024)

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.

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

1. Robustness – it must operate as intended, even under abnormal power systemconditions, without human intervention. Just as importantly, it must be able to distinguishwhen 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 topass the proper signals to the alternate power source if necessary (for example, to signala 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:

In Figure 1, the automatic transfer logic provides the decision-making for what automaticoperations 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 initiategenerator 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 oftheir 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 toinstrumentation levels that can be used by these relays. A user interface allows the adjustment ofcertain 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 systemwill be examined.

Modes of Operation //

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

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 bemanually performed. Conversely, in the automatic mode of operation all operations, with a fewemergency exceptions, are automatic, and the system will respond automatically to changingsystem conditions.

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

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 inputvia 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 userinput via the user interface. Attempting to enter automatic mode if the system conditionsare not correct places the system into Auto Mode Failure.

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

Go back to ATS Operation Modes↑

3. Auto Mode Failure //

No automatic operations occur. For automatically-controlled circuitbreakers, only manual tripping (or trip due to a fault) is allowed. To leave this mode ofoperation, 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 thesystem.

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

Circuit breaker cell switches are a feature which must not be overlooked asthey are essential for the proper function of an automatic transfer scheme with drawout circuitbreakers. For the same reason overcurrent trip switches for low-voltage circuit breakers orlockout 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 bemade into a separate mode of operation, this is usually most expediently handled via voltagefailure 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