AUTOMATIC TRANSFER SWITCH

An automatic transfer switch (ATS) is a self-acting, intelligent power switching device governed by dedicated control logic.

Low-voltage automatic transfer switch assemblies provide a reliable means of transferring essential load connections between primary and alternate sources of electrical power. Data centers, hospitals, factories and a wide range of other facility types that require continuous or near-continuous uptime typically utilize an emergency (alternate) power source such as a generator or a backup utility feed when their normal (primary) power source becomes unavailable. 

Working of ATS :

The control logic or automatic controller is typically microprocessor-based and constantly monitors the electrical parameters (voltage, frequency) of primary and alternate power sources. Upon failure of the connected power source, the ATS will automatically transfer (switch) the load circuit to the other power source (if it is available). As a general rule, most automatic transfer switches seek connection to the primary power source (utility) by default and will only connect to the alternate power source (engine-generator, backup utility) when required (primary source failure) or requested to do so (operator command). transfer switch (ATS) is a self-acting, intelligent power switching device governed by dedicated control logic.

Low-voltage automatic transfer switch assemblies provide a reliable means of

transferring essential load connections between primary and alternate sources of electrical power. Data centers, hospitals, factories and a wide range of other facility types that require continuous or near-continuous uptime typically utilize an emergency (alternate) power source such as a generator or a backup utility feed when their normal (primary) power source becomes unavailable. 

Open transition

An open transition transfer switch is also called a break-before-make transfer switch. A break-before-make transfer switch breaks contact with one source of power before it makes contact with another. It prevents backfeeding from an emergency generator back into the utility line, for example.^[1] One example is an open transition automatic transfer switch (ATS). During the split second of the power transfer the flow of electricity is interrupted. Another example is a manual three position switch or circuit breaker, with utility power on one side, the generator on the other, and "off" in the middle, which requires the user to switch through the full disconnect "off" position before making the next connection.

Closed transition

A closed transition transfer switch (CTTS) is also called a make-before-break transfer switch.

A typical emergency system uses open transition, so there is an inherent momentary interruption of power to the load when it is transferred from one available source to another (keeping in mind that the transfer may be occurring for reasons other than a total loss of power). In most cases this outage is inconsequential, particularly if it is less than 1/6 of a second.

There are some loads, however, that are affected by even the slightest loss of power. There are also operational conditions where it may be desirable to transfer loads with zero interruption of power when conditions permit. For these applications, closed transition transfer switches can be provided. The switch will operate in a make-before-break mode provided both sources are acceptable and synchronized. Typical parameters determining synchronization are: voltage difference less than 5%, frequency difference less than 0.2 Hz, and maximum phase angle between the sources of 5 degrees. This means the engine driving the generator supplying one of the sources generally must be controlled by an isochronous governor.

It is generally required that the closed transition, or overlap time, be less than 100 milliseconds. If either source is not present or not acceptable (such as when normal power fails) the switch must operate in a break-before-make mode (standard open transition operation) to ensure no back feeding occurs.

Closed transition transfer makes code-mandated monthly testing less objectionable because it eliminates the interruption to critical loads which occurs during traditional open transition transfer.

With closed transition transfer, the on-site engine generator set is momentarily connected in parallel with the utility source. This requires getting approval from the local utility company.

Typical load switching applications for which closed transition transfer is desirable include data processing and electronic loads, certain motor and transformer loads, load curtailment systems, or anywhere load interruptions of even the shortest duration are objectionable. A CTTS is not a substitute for a UPS (uninterruptible power supply); a UPS has a built-in stored energy that provides power for a prescribed period of time in the event of a power failure. A CTTS by itself simply assures there will be no momentary loss of power when the load is transferred from one live power source to another

Soft loading

A soft-loading transfer switch (SLTS) makes use of a CTTS, and is commonly used to synchronize and operate onsite generation in parallel with utility power, and to transfer loads between the two sources while minimizing voltage or frequency transients.

Static transfer switch (STS)

A static transfer switch uses power semiconductors such as Silicon-controlled rectifiers (SCRs) to transfer a load between two sources. Because there are no mechanical moving parts, the transfer can be completed rapidly, perhaps within a quarter-cycle of the power frequency. Static transfer switches can be used where reliable and independent sources of power are available, and it is necessary to protect the load from even a few power frequency cycles interruption time, or from any surges or sags in the prime power source.^[

Blog Posted By : Sri J.Jagan Rupchand. M.E., Lecturer / EEE