Depending on the sensitivity of the load and its critically, differing configurations of solid-state UPS systems can do the job.
In last month's session, we completed examining rotary power conditioning equipment, including the extended use of the electromechanical technologies to perform "uninterruptible" power system functions. We covered the "conditioning" aspect - simply using the motor alternator to "buffer" the input to the output, complete with voltage regulation and noise rejection. We added a few twists to the techniques and were able to obtain longer time protection. Finally, we saw the addition of the engine generator and dual feed electrical service, which provided true continuous power.
In this session, we will review the solid-state alternative in the UPS field so that we can catch up on the developments'/applications' progress to date. Here we encounter a "non-moving part" answer in ultimate power conditioning. This technology uses power electronics to convert AC energy into DC, store it in a battery plant for long term, and then send the DC power into an inverter, changing the DC back to AC. Fig. 1 shows a basic block diagram of these elements, along with a high-speed switch on the output of the entire assembly. This output switching device, or "static switch," is an integral part of most UPS equipment and makes the unit able to protect itself and the load from any internal failures.
"On-line" versus "standby" designs
As you look at the basic unit, you see two configurations in operating the system. The first configuration allows the output switch to remain normally closed, thus powering the load straight through the input rectifier to the inverter and out through the static switch to the electrical load device. The second configuration switches the static switch to the bypass line so the load can be supplied by the power line for normal operation, with the inverter "standing by" for fast switching when needed to replace the power line.
In the first configuration, there is a limited amount of switching of the static switch, as the load is served through the rectifier and inverter combination all the time. This is known as an on-line design and has certain pluses and minuses.
In the second, the load is served by the bypass line for normal operations, and it is switched to the battery support when the normal line has a difficulty.
Many recognize the on-line design as being more "uninterruptible." The reason for this is the powering through the rectifier/inverter continually without switching. In fact, this design uses the static switch only when the system is in danger of overload or internal failure. In this design, when the input power is interrupted, the DC supply to the inverter simply takes the place of the rectifier DC output, without switching anything, since the DC bus is directly connected to the inverter input. It's as if the inverter looks on-line and asks, "Who will continue to send me energy?" The battery says, "I have your energy stored here; I will continue your supply."
This design makes use of the static switch, only for sudden inrushes from the load. This might occur when a fault hits the downstream circuit, causing a large attempt to increase the output current in the inverter. The static switch recognizes the inverter cannot handle such a sharp increase, and would shut itself off if it tried to raise its current in such an abrupt fashion. In this case, the static switch moves the load to the utility bypass line, which is a "stiff" or strong source of power, to handle the inrush. The switching takes place in 4 ms, thus not disturbing the operation of the sensitive load. When the inrush is cleared or dies down, the switch transfers back to the inverter output, again without disturbance to the operations. Such would be the function of the switching for external conditions.
If there's an internal failure of the inverter system - say for the loss of one phase - the switch performs the same transfer and locks itself onto the bypass line, sending a signal to indicate the problem and remaining on the bypass line until the problem is fixed.
There is, of course, a price to pay for this on-line design: Energy cost - efficiency - of power flowing through the UPS continually. Total efficiency, input to output, can be in the range of 80% to 85% for the on-line system and sometimes lower based upon light loading.
The standby method has lower cost and higher efficiency on its side, but it gives up continuous conditioning as found in the on-line system. It's certainly less costly for the initial expense, as well as less expensive in operating costs, to power the load straight from the power line. Yet, there are differences in protection provided. Consider the function of the battery support in the standby design: When the utility power line has a disruption, the static switch, with its 4-ms speed, transfers the load to the battery support. When the line is restored, the switch passes the load back to the power line with the same high speed. Notice that there is no "buffering" in the power line, (unless you add an additional regulator), no separation from any outside influences. For example, if the voltage fluctuates on the power line beyond the range of acceptable voltage, the switching may take place repeatedly, and perhaps lead to mishaps or early problems with static switch operation. In addition, the load device may need some kind of separation from the outside power, to keep out other disturbances. The choice of this design may depend on the needs of the load. If you're powering small systems that can stand the power line as it is, the standby design may be a great way to save dollars all around. As an example, certain PCs or networks may not be so sensitive as to require complete separation, as in the on-line design. They may not be bothered by swings in voltage, and can be served well by standby units.
In both of these designs, there are numerous configurations by different manufacturers utilizing their elements in various ways to enhance the flexibility or reliability of the finished product. While we cannot cover all of these different concepts, one of the classic configurations is worth mentioning.
Since the prime concern for UPS technology is uninterruptibility, UPS equipment designers have long studied how to make their equipment more reliable, should any single part or element fail. Designs that incorporate redundancy are well worth considering, especially for very sensitive loads or operations, where downtime is extremely costly or not tolerable at all.
Notice the configuration of rectifiers, inverters, and battery lineups as shown in Fig. 2. In this extension of the basic design, there are several elements operating in parallel so as to compensate for any single failure. In this diagram, you will notice there is no need in normal operations to transfer to the unconditioned bypass line, since the additional elements in the system keep the load running by themselves. The bypass is certainly available for catastrophic failure and with a 4-ms static switch just as before.
The key in this arrangement is to have one more rectifier/inverter package than is needed to supply the load rating. For example, consider the rating of 100kVA redundant. We might design such a system to include three sets of rectifiers, inverters, and batteries, with each set to be operated in parallel. We would count on any two of these sets to be able to furnish rated power to the load at any time. All the sets would be running and sharing the load, but no harm would come to the system if one set failed or required service. It could be disconnected from the output bus using an equally fast method of static switching known as a static interrupter. No disturbance to the system would be seen as that part of the UPS was serviced and restored to the system. This sequence could continue until all three of the sets were serviced or examined, and the load would still be powered by fully on-line UPS.
Another form of redundancy is seen in Fig. 3 (on page 22). Here, a second complete UPS unit is installed in the bypass line. Should the first system require transfer to bypass, as we saw in the first diagram, the bypass itself would be a "conditioned supply" with battery backup.
When budget restraints limit the extent of redundancy, the designer should ensure the UPS has at least two strings of batteries. The weakest link in the system is the battery, and having the battery power in two strings of cells gives protection against a single cell failure that will disable the UPS entirely.
Don't forget to ask your supplier for the equipment's ability to handle current harmonics on the load side of the UPS without making voltage distortion disturbance across the entire output bus. Be sure to ask what the harmonic current distortion is on the incoming line to the rectifier, to avoid interaction problems with your plant or building power supply.