Transfer switch
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Transfer switches allow switching from a primary power source to a secondary or tertiary power source and are employed in some electrical power distribution systems. Most often transfer switches can be seen where emergency power generators are used to back up power from the utility source. The transfer switch allows switching from utility power to emergency generator power. The switch is either a manual switch, an automatic switch or a combination of manual and automatic. During a power outage, the transfer switches isolate the emergency circuits from the utility line allowing for efficient operation of the generator without back feeding onto the utility.
Transfer switches continually monitor the incoming utility power. Any anomaly such as voltage sags, brownouts, spikes or surges will cause the internal circuitry to command a generator startup and then a transfer to the emergency generator when additional switch circuitry determines the generator has the proper voltage and frequency. When utility power returns, or no other anomalies have occurred for a set time, the transfer switch will then transfer, back to utility power and command the generator to turn off after another specified amount of "cool down" time.
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[edit] 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. [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.
[edit] Closed Transition Transfer Switch (CTTS)
In a typical emergency system, there is an inherent momentary interruption of power to the load when it is transferred from one available source to another. 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.
When transferring loads in this manner during a test or when re-transferring to normal after power is restored, the switch will operate in a make-before-break mode provided both sources are acceptable and in synchronism. Typical parameters determining synchronism are: voltage difference less than 5%, frequency difference less than 0.2 Hz, and relative phase angle between the sources of 5 electrical degrees. Since the maximum frequency difference is 0.2 Hz, the engine will generally be required to 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 fails, the switch must operate in a break-before-make mode (standard open transition operation).
Closed transition transfer makes code-mandated monthly testing less objectionable because it eliminates the interruption to critical loads, which occur during traditional open transition transfer.
[edit] Applications
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. It should be understood that a CTTS in a system is not a substitute for a UPS (uninterruptible power supply). In addition to providing line conditioning, 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 wil be no momentary loss of power when the load is transferred from one live power source to another. [2]
[edit] Utility approval
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.
[edit] Soft-loading Transfer Switch (SLTS)
An SLTS essentially uses CTTS technology but actively changes the amount of load accepted by the generator. [3]
[edit] References
- ^ [1] Safely Installing Your Standby Electric Generator, Flathead Electric Cooperative, Inc.; Jul. 2006; accessed Dec 2006
- ^ [2] Generac Power Systems Inc., accessed Dec 2006
- ^ [3] Pybus, Dennis; Finding Surplus Electric Power in Traditional sources; Electricity Today High Voltage Power & Engineering e-Magazine; accessed Dec 2006
