Simple guidelines for ATS selection: proper transfer switching is essential to ensure a seamless move between incoming powerline and genset. Gary Olson of Cummins explains the technology.

TRANSFER SWITCH EQUIPMENT is available in a variety of types, with a dizzying array of features. Selecting the appropriate transfer switch for a specific application requires a clear understanding of site needs and application restraints. This article discusses the various types of power transfer equipment that are available, along with their advantages and disadvantages, so that a more informed selection can be made.

Transfer switches are at the heart of an emergency power system, providing a dependable power transfer between the utility and emergency standby generator, or between other types of power sources and facility loads. When the normal power source fails (usually the utility), transfer switches detect the loss of power, send a start signal to the standby generator and then connect the generator to the facility's loads when the generator has achieved proper frequency and voltage.

In cases of utility failure when the emergency power source is not operating, electrical service to a facility's loads will be lost for a period of approximately 10 seconds while the generator set starts--unless there is an uninterruptible power supply (UPS) serving loads in the system to bridge the power gap while the generator set is starting and the transfer switch is switching.

When transferring between two live sources, such as when normal power has returned or during generator set test/exercise periods, a system designer can select different types of transfer switches that can minimize or even eliminate the electrical disturbance to critical loads.

One might expect that the type of transfer switch selected is primarily a function of reliability, but in some cases selection is simply based on convenience. In general, minimizing the disturbance that can occur when switching between sources will increase the cost of a transfer switch. It costs even more to completely eliminate the switching disturbance, and the added complexity can have negative impacts on the reliability of the transfer device and even the generator equipment.

So, whether the application is a simple standby power system in a home, a large emergency system in a hospital protecting the lives of patients, or standby service to a data center handling millions of dollars in transactions, a careful consideration of the balances between cost, reliability and the quality of power provided to critical loads is necessary to select the most appropriate transfer switch equipment.

Types of transfer equipment

Transfer switch equipment can be categorized into four general groups:

* Open-transition transfer devices, which always open the connection of the current source before connecting to the new source;

* Fast closed-transition transfer devices, which operate like an open-transition transfer switch when a source has failed, and will close to the new source, then quickly disconnect the original source when operating between live sources;

* Soft closed-transition devices, which operate like an open-transition transfer switch when a source has failed, and which actively synchronize the sources, connect and ramp the load to the new source before disconnecting the load when transferring between live sources; and

* Sub-cycle transfer devices, which open and then re-close on the new source in less than one-quarter of an electrical cycle. The resultant interruption is so short that most load devices aren't affected by it. They are used primarily in UPS systems and only occasionally with a generator set. They are very expensive in relation to mechanical switches and they are typically protected with fuses. They are also more complex than mechanical switches and are considered by some to be less reliable. This type of transfer device will not be discussed in this article.

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Open-transition transfer switches

Open-transition transfer switches provide a "break-before-make" switching action. They are designed specifically to transfer power between utility and onsite power systems. The connection to one source is opened before the connection to the second source is closed. Mechanical interlocks that positively prevent interconnection of sources in automatic and manual modes are commonly used.

Open-transition transfer switches are the most commonly used type, and they are used in all types of applications. By design they neither require nor allow generator set paralleling with the utility service.

Typical customer applications include basic standby power systems for small businesses, hotels, small manufacturing and commercial buildings with non-critical loads that are tolerant of brief interruptions during monthly power system testing and retransfer to the utility after an outage.

The major advantages of using conventional open-transition transfer switches are:

* Lowest-cost automatic power transfer option available, due to the simplicity of the controls and mechanism needed and the simplicity of the interface to the generator set.

* Most reliable option available, again due to simplicity.

* Most designs incorporate a mechanical interlock that positively prevents inadvertent utility paralleling.

The disadvantage of open-transition transfer systems is that during retransfer from the emergency generator to the utility following restoration of normal power--or during system testing--a fixed open time is needed to allow the arc developed on opening the contacts to decay. Typically this time requirement is approximately five electrical cycles. In applications with a high percentage of inductive loads (motors, transformers, etc.), a programmed transition function is used to control the speed of operation of the mechanical switch, and keep the contacts open for one-half to three seconds to allow time for the residual voltage in these inductive devices to decay.

Fast closed-transition transfer switches

Fast closed-transition transfer switches provide a "make-before-break" switching action and utilize a momentary paralleling of both sources (<100 milliseconds) during the transfer period. Closed-transition transfer switches utilize a control system similar to that of open-transition transfer switches, and usually employ an interconnect to the generator set that is also similar.

Closed-transition transfer switches require a mechanism that is capable of being operated in open-transition sequence when switching from a failed source to a live source, and a closed-transition sequence when transferring between two live sources.

Many closed-transition transfer switches do not include mechanical interlocking of sources, making it possible, especially in manual operation modes, to manually parallel the sources. This can be very damaging to the sources or the transfer equipment. Closed-transition switching mechanisms are more complex and expensive than open-transition transfer switches. While fast closed-transition transfer devices switch from sources without a total interruption, there is generally a disturbance in power to the loads due to the sudden load change on the source. This is particularly true when transferring a load from the utility to the generator set. In general, in order to prevent disruptive transients, fast closed-transition transfer switches must be transferred sequentially, and each switch should be limited to less than 25 per cent of the standby rating of the generator set.

Closed-transition transfer switches with passive synchronizing

Closed-transition transfer switches with passive synchronizing typically utilize a sync check device (also called a permissive relay or phase band monitor) to sense the phase relationship between the two live power sources and allow interconnection of the sources only when they are synchronized.

The synchronizing is termed "passive" because there is no direct control over the generator set frequency. Instead, it relies on changes in the loads or difference in the frequency of the sources to induce phase angle matching of the sources.

As the loads on the system change, and the speed of the genset changes, the two power sources will eventually drift toward synchronism. The transfer is timed and signaled to occur so that when both sources are close together, they are synchronized.

Paralleling of the sources occurs for a fixed time (not more than one-tenth of a second). The short duration of the paralleling makes it unnecessary to add more complex controls to control load on the generator set while paralleled with the utility grid.

The major advantages of closed-transition transfer switches with passive synchronizing are:

* Lowest-cost "non-load-break" system, due to simplicity of controls and lack of generator set governor synchronizing controls.

* They prevent momentary breaks in power during generator set testing and when closing back to a restored utility source. Splitting the loads into small portions and controlling the sequence of their transfer will minimize disruptive transients.

Disadvantages of this equipment are:

* The overlap time is very brief, so voltage and frequency transients will be imposed on the system, which may be just as disruptive as a short total interruption, especially upon switching to the generator set.

* If facility loads are very stable, the two power systems may require an objectionably long time to synchronize. This "failure" mode may be intermittent, since it depends on site loading variations to provide the frequency change necessary to induce changes in the phase relationship between the on-site power system and the utility.