Automatic transfer switches are designed to transfer electrical loads between two power sources. The most common power sources involve a commercial utility (normal source) and a diesel-fueled standby power generator (emergency source). While most transfer switches are equipped with basic features that make them suitable for simple installations, larger or more complex projects often require additional features to meet special requirements. One such special requirement is the need to protect sensitive equipment whenever building loads are switched between two live, un-synchronized, power sources. How exactly does a proposed transfer switch execute the transfer of the load between the normal source and the emergency source, and what effect will the transfer sequence have on the connected loads?
These questions tie directly to the automatic transfer switch controller. This device is responsible for executing much of the transfer sequence on a transfer switch. The transfer switch controller handles all power sensing, timing functions and fault monitoring required for fully automatic operation. Beyond its basic functions, the controller can also be specified and factory-configured to operate in one of three “modes of operation”. These modes are commonly referred to as open-transition (default), delayed-transition and closed-transition. This article discusses the main differences between these configurations, and provides some guidelines for selecting the right type for your specific project.
Open Transition – The most basic transfer switch solution for small load profiles.
Transfer switches of the “open-transition” type provide a break-before-make transfer sequence. This means that the load is disconnected from one source, prior to being connected to the alternate source. Most transfer switches will accomplish the transfer sequence very quickly, yet a brief power interruption is noticeable to the building occupants, and is most certainly noticed by sensitive electronic equipment. For a small business or residential installation, without sensitive loads, or large inductive (motor) loads, this solution is generally acceptable.
Delayed Transition – The industry’s choice for medium and large load profiles.
A transfer switch with a “delayed-transition” control logic will still provide a break-before-make transfer sequence. However, this type of equipment will disconnect from one source, and then “pause” (or delay the transition) before it proceeds to connect to the alternate source. Why delay the transfer sequence? In many industrial installations, electric motors are a big part of the load profile. When large motors are de-energized, their gradual slowing down can have the effect of maintaing a relatively high voltage on the line terminals. De-energizing and quickly re-energizing large electric motors can cause current surges that can trip protective devices and, in many cases, be damaging to the motors themselves. Delaying the re-energizing of these motors allows them to coast down to safe voltage levels, before they are again powered-up. A similar issue occurs with power transformers, which must be de-energized long enough to allow their magnetic fields to collapse to safe levels before they are re-energized. One other application issue, of special importance for many mission-critical facilities that utilize uninterruptible power systems (UPS), is the need to allow UPS control devices to properly sense a power outage, and complete a switch to battery power. This should all happen before the UPS is re-energized by an alternate source, and asked to switch away from the batteries. In a nutshell, for applications where the load profile consists of large electric motors, transformers, or UPS-connected appliances, a delayed-transition transfer switch is the correct selection. These switches are by far the most common, and they can be found in municipal water treatment plants, manufacturing facilities, large pump stations and a broad range of commercial facilities.
Closed Transition – The best choice when frequent load transfers are executed between live sources.
A closed-transition transfer switch offers a make-before-break transfer sequence. Here, the source from which the load is being transferred remains closed (connected) until the alternate source, to which the load is being transferred, is also closed (connected). After both have been closed, the source from which the load is being transferred is opened (disconnected). The key to this sequence is that it provides for a continuity of power to the load during transfer (i.e.: no power interruption). Two conditions must exist in a closed-transition transfer sequence. First, both sources can only be closed when they are synchronized. Second, most utility companies in the United States require that the period of time where both sources are closed be very short, usually no more than 100 milliseconds. These two conditions require special accessories on the transfer switch, and depending on the configuration, closed-transition transfer switches can be further divided into three sub-categories:
- “Passive” – the simplest form, whereby the synchronization is left to the slip frequency of the generator, and the two sources connect to the load only when a synch-check relay allows it. Once the alternate source is closed, the primary source is immediately opened (in less than 100ms). It is important to note that when two sources are left to synchronize on their own, the time that it takes to synchronize can vary from one occurrance to the next. If the timing is important, the transfer switch control logic can be programmed to revert to an open-transition transfer if a closed-transition transfer cannot be completed within a predetermined time frame. Likewise, it is standard practice for these transfer switches to revert to an open-transition transfer, should either source completely fail while waiting to synchronize.
- “Active” – as the word implies, the synchronization is actively controlled by an automatic synchronizer, and the generator is “driven” to synchronize with the utility. As in the passive mode, once the alternate source is closed the primary source is immediately opened. This mode of operation is not difficult to achieve, but it requires that the engine’s governor control be accessible to the transfer switch controller (some modern engines prevent external devices from controlling the engine’s speed governor).
- “Soft Loading” – this mode can be described as a brief paralleling mode, whereby the synchronization is controlled by an automatic synchronizer. As before, the generator is brought into synchronism with the utility, but it is now closed (connected) to the bus at no load. A load-sharing module will then ramp the load from the existing source to the oncoming source over a short period of time (typically 2 to 10 seconds). Once the load has been fully transferred, the primary source is opened (disconnected). Note that this transfer sequence can be performed in both directions, from normal to emergency, or from emergency to normal. The main benefit of this configuration is the gradual transfer of the load, which provides smoother transitions and less wear and tear to the engine, generator and transfer switch components. This application is commonly seen at larger airports where the air traffic control and other navigational aids are often transferred from a “good” normal source to the emergency source in anticipation of bad weather. Once the threat of bad weather has passed, the load is transferred back gradually, all without a power interruption to the facility. While UPS-served loads also enjoy interruption-free transfers from emergency to normal power sources, some industrial applications may still benefit from a “soft loading” transfer sequence, especially when they are backed up by a single standby generator. When considering this option, be sure to consult with the local utility company as this application involves paralleling with the utility, and strict controls will be needed to maintain the safety of the utility’s service personnel.
I know that cost is always important, so let’s answer a question that may be on your mind… What are the cost premiums for the addition of these “delayed” or “closed-transition” options? The cost of a delayed-transition control logic is negligible and is probably already part of the cost of a transfer switch of suitable size for use in a commercial application. For the benefit of a closed-transition transfer sequence, you may expect to see an added cost of anywhere between 15-20%, depending on whether you require a passive or an active mode of synchronization. For a soft-loading configuration, added costs depend on what hardware might be required to communicate with the engine, but you can expect a minimum 25-30% premium. As always, consult your local utility and your preferred transfer switch vendor to review your project specifics and obtain site-specific budget pricing.
As a final note, I would like to suggest that selecting the right type of transfer switch can contribute to a greater sense of confidence for the facility’s operations staff. When load transfers occur smoothly, with minimal disruption to the building occupants, facility operators may feel more inclined to conduct more frequent testing of their on-site power systems (without the fear of leaving the facility in the dark!). With regular on-load testing, the generator(s) and emergency distribution gear are exercised and, in doing so, become more reliable, and that is always a good thing!
15 thoughts on “Open, Delayed or Closed Transition? Selecting the Right Transfer Switch”
Informative and concise. Thanks.
Thank you for your feedback!
Thanks for this doc. Very helpful indeed.
Thanks, glad you enjoyed it!
Great information, David. It was technical, logical in its presentation and can be used as an explanation to end users.
Thank you, I appreciate your feedback!
This is a pretty excellent explanation, thanks David
You’re welcome! I’m glad that you found it helpful.
Thank you David.
Great information and nice presentation..really helpful.
Thanks for the feedback!
I liked it too; well done and thanks.
Hi David, If normal power is suddenly cutoff would close transition cause momentary intruption?
Yes, under that scenario, there would be a power interruption until the switch transfers to the emergency source.
Hi David, nice to see you again, even if in a small photo–and thanks for the well-written piece. Regarding a transfer switch feeding a 45kVA power transformer supplying power to a six-SCR bridge used to develop DC voltage to apply to a generator main field coil via slip rings, if we use a delayed-transition approach, how long would it take for the residual magnetic flux to die down? The transformer will be approximately 1/2 to 3/4 loaded at the time.
What happens to the transformer secondary voltage and frequency when the Normal source is interrupted–since the flux continues as it was in direction and magnitude (no longer driven by AC power and changing direction 120 times per second), I assume the frequency drops to 0hz and the voltage becomes DC . . . having difficulty wrapping my head around that situation.
Hi David, long time… thanks for reaching out! The transformer’s behavior is outside of my knowledge but I would check with the SCR bridge vendor to see what isolation may be possible between the bridge and transformer (when the primary on the transformer is interrupted), and whether this isolation can be coordinated with the ATS’s delayed-transition. Interesting application… if you find a good solution, please post back!