SUBSCRIBE To Our Mail List To Get This Article In PDF
IN ANY ELECTRICAL SYSTEM, a transfer switch functions to remove electrical load from one source of supply-usually because of a power failure-- and transfer that load to an alternate source. In some systems, that's done manually. Most often, however, the switch operates automatically. The technology is not new (EA June 1997).
The usual automatic transfer switch (ATS) consists of two contactors or circuit breakers interlocked so that only one can be closed at any one time, with suitable monitoring and control to reverse their respective positions when either source is to be disconnected in favor of the other.
That switching takes time. If the alternate source is a standby generator, several seconds (even minutes) may elapse before the oncoming power source can supply full load. If large motors are running, driving either low-inertia or high-torque loads, the control must lock out transfer while the decaying generated voltage on the "dead" circuit is out of synchronism with the oncoming supply.
Fast Versus Slow Transfer
Such concerns have divided ATS applications into two categories: fast transfer and slow transfer. For electromechanical switches, "fast" has often meant a period of three to five cycles, or, for 60 Hz systems, 50 to 80 milli-- seconds.
Today, with so many commercial and industrial operations (as well as medical facilities) dependent upon electronic devices that are highly sensitive to brief power interruptions, what was once "fast" has become unacceptably slow. Transfer switch technology has now developed what are called high-speed ATS devices that will connect loads to an alternate supply in much less time. Electromechanical switching is now possible within two cycles (0.03 second).
You may come across the term closed transition (or soft load) transfer switching. This describes a high-speed electromechanical transfer from a normal source that has not failed to an alternate source that is already up and running. That may be a standby generator. The switch controls allow transfer only when the generator voltage and phase position are compatible with the load (which, in turn, has been in step with the normal power source). "It's all in the control," explained one product engineer, "to get a seamless transfer between the two sources. The key to a successful application is making sure the two sources are truly independent."
Closed transition therefore simply means that no transient disturbance takes place during switching. Actual time to carry out the transfer, from the time the operating sequence is initiated, may be several seconds-but power interruption to the load is nowhere near that long.
A "soft" transfer switch may be referred to as a non-load-- break device. That name can be misleading. It means only that the load remains carried by the preferred (utility) source until the alternate (generator) source is in full operation and its voltage synchronized with load conditions. The transfer is then completed and the load experiences no significant interruption. Both sources must be "live." Both are connected to the load (and to each other) for "a few cycles." This is sometimes called paralleling load transfer. Obviously, if the preferred source has failed, or is faulted, the load must undergo some "dead time" before transfer completion.
That kind of transfer isn't practical, of course, if a standby generator must be started up as the alternate source. Rather, high-speed transfer is suited to online or floating uninterruptible power supply (UPS) applications. Either batteries or flywheel energy storage makes the alternate power source continuously available. It is always synchronized with the normal source of power. The only "dead time" needed to make the transfer is that required to get the contactors open and closed. Such transfer switches typically use special circuit breakers having large permanent magnets to provide the contact opening and closing forces, instead of conventional springs and mechanical linkages alone.
UPS equipment, typically installed to maintain power to certain critical loads, is seldom sized to keep an entire facility in operation for very long. Also, the UPS alone cannot usually handle fault clearing or high inrush currents associated with transformer or capacitive loads.
Redundant Switching
In critical applications, then, major plant power requirements can be best served by redundant sources of full capacity, with continuity assured by seamlessly transferring between them without power interruption. Large computer systems typically require redundant supply so that neither failure nor routine servicing of one power source will necessitate a computer shutdown.
Such capability is offered by the newest development in transfer switching: the substitution of solid-state devices (SCR's) for electromechanical contactors. They operate much faster, exhibit no arcing, contact bounce, or mechanical wear, and are subject to more precise control. A unit using this technology, having no moving parts, is called a static transfer switch, or STS.
"The primary purpose of the static transfer switch," explains one power quality expert, "is to allow virtually uninterrupted transfer of the critical load from one a-c power source to another." Although it cannot be zero, STS transfer time is normally less than ,4 cycle. It can be half that, for complete loss of primary source voltage, to three times that for a voltage swell. The switching thyristors cannot cease conduction prior to a current waveform zero.
Here's the STS operating sequence as described in one supplier's literature :
"The first step in a transfer is evaluation of the alternate source and the status of the power electronic switches. During the first two milliseconds...the control checks the alternate source to make certain it is better than the preferred. At the same time, the power electronic switch status is reviewed to ensure that it is ready for transfer. Once the decision has been made, the actual transfer . . . is accomplished in one to two milliseconds in most cases. For minor voltage disturbances, a programmable time delay is employed before the transfer decision is made, since such dips are usually very short . . . and don't disrupt in-plant equipment.... These transfers will all be within the limits prescribed by the Information Technology Industry Council....."
The emphasis on computer susceptibility to power source disturbances has tended to mask the effects of such disturbances on other apparatus. Solid-state motor starters-even conventional magnetic starters-will drop out during voltage sags that won't disturb most computers, and no standards exist for starter dropout voltage. Power transformers can tolerate interruptions of A cycle or so, but a longer outage can result in disturbingly high magnetizing inrush. Some a-c relays will misbehave on power interruption of only three milliseconds (about 1/5 cycle).
The oncoming power source must not be connected to the load before the off-going source is disconnected. Even though the two sources are synchronized, the reason for making the transfer is often a disturbance or fault on one source, so that even a brief interconnection between both sources could be disastrous.
How static transfer switches (STS) work?
The STS consists of two sets of SCR bridges. The transfer is made by switching one bridge off as the other is switched on. The load is without power for less than one cycle. Whereas conventional ATS equipment is most often used in low-voltage circuits, the STS may be used to transfer feeders from one medium-voltage source to another in critical applications. The power system will include two alternate sources of equal integrity-in effect, a redundant supply. Synchronism is always present. If the transfer can be essentially instantaneous, the effect of internally generated voltage in decelerating motors becomes insignificant. This is particularly well adapted to large data processing installations or complete manufacturing facilities. The chief limitation of the STS is its suitability only to switching between two equally operational but independent power sources. Obviously, the STS is not the proper choice when the alternate supply is a standby generator.
With all that in mind, these are some important features of an STS system:
- Either of the two sources should be selected as "preferred" (a typical ATS has predetermined "normal" and "emergency" contactors).
- Interlocked maintenance bypass circuitry, allowing uninterrupted manual bypass of power to the load from either source, so the STS can be serviced.
- Load current sensing, to prevent transfer onto a downstream fault or overload. Otherwise, both sources could experience failure.
- The two power sources must match in voltage, frequency, and phase angle. Although that seems an obvious consequence of independent feeders from the same utility system, interposed transformations can create an unsuspected phase difference, and load variation can result in a voltage difference.
- The STS requires control power supply redundancy so that failure of either source does not disable the transfer function.
Here's how they work: Load power is normally supplied from the preferred source via mechanical contacts-a switch that is electrically operated and mechanically latched. When a transfer is called for, the SCR's are switched on to connect the alternate source within from 5 to 20 milliseconds (on a "break-before-make" basis), depending upon whether the transfer is planned or automatic.
Power flow through the SCR's is limited to about 15 milliseconds, with the mechanical contacts handling the load at all other times. Control power is available from either source. Because they carry load so briefly, the SCR's can have lower ratings and require little or no auxiliary cooling.
At least half a dozen manufacturers offer STS equipment, most of it for maximum circuit voltages between 480 and 1,000. Current ratings range from 30 to 4,000 amperes. As is so often true for power electronic control equipment, the fast, precise operation of the STS comes at a high price. Figure 4 compares transfer time and relative cost for manual ATS, high-speed ATS, and STS units. Whereas low-voltage STS cost typically ranges from $75 to $200 per rated ampere, medium-voltage units (from 4,160 to 34,500 volts, 300 to 600 amperes) are far more expensive. If the need exists, however, there may be no better alternative.
SUBSCRIBE To Our Mail List To Get This Article In PDF
