In the last column, you were introduced to types of backup power, including dual AC feeders, UPS systems and generators. We then reviewed the three basic types of UPS systems:
• Passive-standby (offline, relay-switched and battery-powered, available in .5kVA to 5kVA sizes)
• Line interactive (provide voltage regulation and filtering, available in .5kVA to 5kVA sizes)
• Double conversion online (capable of 10kVA and more, similar as above except that the primary power path is through an AD/DC and DC/AC converter)
Finally, the article provided some guidance on the selection of UPS technology based on application. There are three basic levels of UPS backup protection: Level 3, Level 5 and Level 9. Each level provides increasingly clean, regulated and sophisticated power for the load. With this as a background, let’s continue the discussion by looking closer at what is inside a UPS.
Offline UPS
The simplest UPS technology is the offline system. (See Figure 1 to the right.) Under normal circumstances, AC power travels from the mains through a transfer switch, through a simple low-pass filter and to the load. To support the batteries, the AC power also flows to a charger circuit, which keeps the battery fully charged.
When the AC fails, the switch operates, moving the load from the main AC power feed to that provided by the DC/AC inverter. This typically requires from 1.5ms to 4ms. The battery powers the inverter, which creates 120VAC and passes the power through the filter to the load.
Offline UPS systems are also known as single conversion systems. This is based on the technology’s single conversion of DC to AC power.
There are three points to remember with these types of UPS systems. First, there will be a brief power outage as the device switches between the AC mains and the inverter. Second, there will be little or no mains voltage regulation. And third, the output power will be dirty. In other words, most offline UPS systems provide minimal filtering with the result that, when operating, there is typically significant electrical hash (>7 percent and in some devices up to 30 percent) on the AC output. If you have equipment that is sensitive to either over/under voltage, power line noise or cannot tolerate brief power interruptions, this may not be the best backup power solution.
The upside for an offline UPS system is that it is inexpensive and efficient. These systems are often best utilized for workstation applications. Also, they are seldom compatible with three-phase power and rotary backup systems, which we will discuss in a later article.
Line interactive UPS
The next step up the technology ladder is the line-interactive UPS. A simplified block diagram is shown in Figure 2 to the left. The AC power is passed to the system through a buck-boost transformer. This transformer or inductor provides some amount of over/under voltage regulation.
With a line-interactive UPS, if the input voltage sags or rises, the delta inverter inserts a compensating voltage to stabilize the output voltage. The main inverter provides the compensating voltage/current to help stabilize the output power. Some line-interactive UPS solutions may also provide load harmonic current and input power factor correction, but don’t depend on this without first checking.
Some line-interactive UPS systems use what are called naturally-commutated utility disconnect static switches. (Basically these are SCR/Thyristors.) Be aware that these UPS systems are particularly susceptible to failure under certain input source faults. While the switch typically operates within 1ms for faults like an input short circuit or input open circuit, if the switches do not operate quickly enough, the inverter may attempt to feed back into the fault. If this happens, the UPS will simply shut down, and there goes your backup protection.
One drawback of the line-interactive solution is the frequent use of the battery. These systems adjust to varying input voltages by switching to battery power. The result is often short battery life.
A second drawback of this solution is typical lack of effective power line isolation. Any common-mode noise on the AC mains will be passed directly to the load. Also, harmonics, spikes and surges may get passed directly to the load with little filtering by the UPS.
Recently some vendors have introduced what they call line-interactive three-phase UPS systems with power factor correction. These new designs provide solutions to many of the aforementioned problems. While some vendors call these online solutions, they do not provide all the performance of a true online UPS.
Double-conversion online UPS
This type of UPS is often called a true online or double-conversion solution. As shown in Figure 3 to the right, the key difference between this and other solutions is that the load is fed continuously from the inverter. The incoming AC power is first converted to DC. This power then feeds a charging current to the battery and full power to the inverter. The inverter converts the DC back in to AC (hence double-conversion), which connects to the load.
This solution fully isolates the load from the AC mains. One key benefit is that the load never sees a power drop if the AC mains fail because the load is continually powered by the UPS. AC feeder sags, frequency changes or noise have no effect on the quality of the output power.
Many of these UPS systems have output transformers with a separate derived neutral. This supports common-mode nose rejection. They can also work with three-phase power.
One potential drawback is that online UPS systems tend to be noisy. A well-designed system may generate a noise level of 50dB-60dB at 6ft. Try locating one in an office environment and see how well you are liked.
Generators and UPS
Getting UPS solutions and generators to work together well is often like getting the Hatfields and McCoys to play nice. Both offline and line-interactive solutions require a stable input frequency source and constant phase shift. This is because the output inverters must track the supply frequency and phase to provide any needed voltage or current correction and switch points. The output frequency will be the same as the input frequency unless operating on batteries.
However, what often happens, when a generator begins the startup process, is that the UPS begins to cycle to and from battery operations. If the generator takes a while to get started and stabilized, the UPS batteries may be completely discharged. Also, this means the UPS must rely on its internal 60Hz reference while on battery power.
Sometimes, even once the generator is online, when the UPS shifts to generator-provided power, the additional UPS load will cause the generator output voltage to sag, which forces the UPS back to battery operation. Then the generator output stabilizes, the UPS switches back, again generator falters under the additional load ... Well, you get the idea.
One solution is to use a UPS with a wider input frequency tolerance and a slower df/dt. Unfortunately, if this is allowed in the UPS design, the overall system performance may fall below accepted critical load tolerances of +1Hz and +1Hz/second for slew rate. Some vendors specify that line-interactive products be set to synchronize in a +4 Hz to 8Hz window with a +4Hz/second slew rate. Unfortunately, this can be problematic if you are trying to power a frequency-critical load. A double-conversion UPS does not suffer from this type of service disruption.
Summary
So, what’s the best type of UPS for your installation? It depends on many factors. However, Table 1 to the left was developed by Liebert and will help you visualize key UPS characteristics by design. You will need to plug into the decision process local factors as size, cost and maintenance capability.
The bottom line is that today’s sophisticated video and production gear is highly dependent upon clean and reliable power. In addition, the need to maintain that power in times of severe weather, power outages and equipment failure make it important to have some type of backup capability — but choose your solutions carefully.
Next time we’ll look at hybrid-rotary UPS systems and generators.
Additional resources
High availability power systems, Part I UPS internal technology
