Premium Membership

Study specialized technical articles, papers & video courses in low/high voltage areas. Save 50% on all courses at the EEP Academy with the Enterprise Plan.

Home / Technical Articles / Automatic transfer switch between three power sources (analysis of schemes)

About automatic transfer switches

When a switchboard can be supplied from two or more different sources, transfer between those sources can be done manually or automatically. This kind of configuration is usually designed when reliability and quality of power supply need to be provided.

Automatic transfer switch between three power sources (analysis of schemes)
Automatic transfer switch between three power sources (analysis of schemes)

A typical example is the power supply of a switchboard from two possible sources: regularly from the transformer, i.e. grid, or from a diesel Genset (generator set) as a standby source.

If during transformer operation, interruption of supply, or some power quality disturbance occurs, the transformer will be disconnected, while diesel Genset will be started and connected to a switchboard. In most common applications, RMS value of supply voltage is used as criterion for transfer.

Upper and lower permissible voltage limits are usually adjustable. When the proper value of grid voltage is restored, diesel Genset is disconnected and stopped, while the transformer is connected to switchboard again.

Electrical equipment which serve for automatic transfer of supply between available sources is called automatic transfer switch (ATS).

It usually comprises:

Sensing relay for power quality monitoring

In general, these relays may be of various complexity, or even several different relays can be combined. However, in most cases, a simple phase monitoring relay with potential-free output is used for this purpose.

Typical monitoring values are: voltage RMS (including phase absence), voltage asymmetry and phase sequence.

Control circuit for transfer logic

Control circuit for transfer logic may be realized in relay technique, or by PLC. In both cases, priority among sources, the transfer time(s), and other important parameters are defined by this circuit.

Switchgear for (dis)connection of sources

This function is realized with power contactors, circuit breakers, switch disconnectors, or solid-state switches. Since, practically in all cases, supply sources cannot operate simultaneously, i.e. in parallel, electrical and/or mechanical interlocking between switchgear devices can be often seen.

ATS Automatic transfer switch 1250A Distribution Board
Figure 1 – ATS Automatic transfer switch 1250A Distribution Board

ATS devices are particularly important for following applications:

Premium Membership Required

This technical article/guide requires a Premium Membership. You can choose an annually based Plus, Pro, or Enterprise membership plan. Subscribe and enjoy studying specialized technical articles, online video courses, electrical engineering guides, and papers. With EEP’s premium membership, you get additional essence that enhances your knowledge and experience in low- medium- and high-voltage engineering fields.

Check out each plan’s benefits and choose the membership plan that works best for you or your organization.

Log In »Learn More »

Premium Membership

Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! It helps you to shape up your technical skills in your everyday life as an electrical engineer.
More Information

Miodrag Kokotovic

Graduated from Faculty of Electrical Engineering, within University of Belgrade, in the field of electrical power systems. Expert in electrical part of tender preparation, design, procurement, construction and commissioning of treatment plants and pumping stations, electrical power quality and energy management.

Leave a Comment

Tell us what you're thinking... we care about your opinion! Please keep in mind that comments are moderated and rel="nofollow" is in use. So, please do not use a spammy keyword or a domain as your name, or it will be deleted. Let's have a professional and meaningful conversation instead. Thanks for dropping by!

fifty four  ⁄  nine  =  

EEP Academy Video Courses

Learn to design LV/MV/HV power systems through professional video courses. Lifetime access. Enjoy learning.