Uninterruptible Power Supply (UPS)

What is an uninterruptible power supply?

An uninterruptible power supply (UPS) is a device that keeps a computer running for at least a short time when the power goes out. As long as utility power keeps running, it also keeps the energy storage full and in good shape. The longer power can be kept on, the more energy can be stored. However, there are practical limits that will be talked about later. The technology that makes each UPS system do its job in a different way is what makes them different.

There are different ways to store energy. Most batteries these days are rechargeable. For ease of use, this article's examples and illustrations will be based on that technology. But kinetic energy can also be stored in heavy, rotating flywheels or as fuel.

Types of uninterruptible power supply

Most people use a type of UPS called a full-time or full double conversion UPS, which is also the most effective. The utility power that comes into a UPS is alternating current (AC), which is also what most IT equipment needs (ITE).

Batteries, on the other hand, use direct current (DC), so all battery-type UPSes must change the incoming alternating current (AC) power into direct current (DC) to charge the batteries. This process is called "rectification." The UPS must still send AC power to the ITE, so a device called an inverter must turn DC power back into AC power.

In a double-conversion UPS, power goes through the rectifier, and then the inverter is on its way to the ITE. The voltage and frequency of the output have nothing to do with the voltage and frequency of the input. They can even be completely different from the input, so this system is technically called "voltage and frequency independent" (VFI).

Voltage and frequency independent

In Figure 1, you can see a VFI system working as it should. There are two ways to deal with problems with the power coming in. The worst voltage spikes are taken care of by a surge suppression device (SPD). Lightning strikes on power lines, large motors like those in elevators or medical electronics, welders, and many other things can cause these. But even the smallest changes, like voltage dips or brownouts, never reach the output of a VFI UPS.

Batteries are great at absorbing electrical shocks, and they also send a steady voltage to the inverter. The inverter completely resynthesizes the voltage and current so that the power sent to the ITE is steady and clean. Connecting air conditioners or other motors to the UPS that powers the ITE could contaminate this clean output power, so it's not recommended.

Note the bypass circuit that goes around the UPS. We'll get to that later.

Uninterruptible Power Supply

During normal operation, the battery is always in the circuit and provides small amounts of power when needed, such as during brownouts, so there is never even the slightest break in power output.

Figure 2 shows that when the power from the grid goes out, the battery keeps sending stored energy to the inverter, which keeps sending clean power to the ITE. When the power from the utility company comes back on, power flows back through the rectifier, into the inverter, and back into the batteries.

Uninterruptible Power Supply

UPS static and maintenance bypass

UPSes are not uninterruptible. They are electrical or mechanical devices, so they need regular maintenance and can have parts break down. Because of this, all UPS systems come with a built-in bypass that lets power go around the system and straight to the ITE when needed.

The high-quality SPD is still in the circuit, but it isn't much better than using a power strip with surge protection to power your home electronics. It won't stop the power from going out or fix brownouts or drops in voltage. If the UPS fails, the bypass works as a static switch right away.

When a technician needs to work on the system, the bypass is turned on by hand to protect the parts inside. If the power from the utility fails while the UPS is in bypass mode, power to the ITE is cut off. This weakness is present in any installation with only one UPS. Figure 3 shows how the UPS looks when it is not being used.

The big spikes have been taken out, but the voltage drop keeps going.

Uninterruptible Power Supply

Economy mode operation

The first law of thermodynamics, called the "law of conservation of energy," says that energy cannot be made or destroyed. No electrical or mechanical device is 100% efficient, so every conversion causes a loss, which gets lost as heat.

UPS systems are much more efficient than they were ten years ago, and they stay pretty much the same from low load to high load. But both the rectifier and the inverter still lose power, which is taken care of when the UPS is in bypass mode. As shown in Figure 4, many VFI UPSes now have a more advanced version of bypass called "eco mode." When needed, an eco-mode UPS can switch back to full VFI mode.

When rectifier and inverter losses are removed, power and money are saved until the power goes out and the whole UPS system needs to be used. Some users set the system to work in VFI mode during the day and have it automatically switch to eco mode at night if the tasks aren't as important. Most of the time, eco mode works well, but many people are afraid to switch between modes. Also, the efficiency of new VFI UPS is within 1% or less of what can be achieved in eco mode, so many users no longer find this mode useful.

Note that eco mode UPSes have high-quality filters that also cause a small loss and that switching modes usually causes a short period of instability. The efficiency of eco mode is based on statistics, but it can be 99% if power outages are rare and don't last long.

Uninterruptible Power Supply

Line interactive UPS

The output frequency of a true line-interactive UPS is the same as the input frequency, which is why it is called "voltage independent" (VI). Except for the size of their rectifiers and the fact that they can't switch to VFI mode, they look almost the same as VFI UPSes in eco mode.

The smaller rectifier only needs to charge the batteries, which help absorb irregularities and increase power when voltage drops. When the power goes out, the batteries do everything. Figure 5 shows how the battery and inverter run in parallel with the output to help make up for changes in the voltage coming in.

Uninterruptible Power Supply

Figure 6 shows a line-interactive UPS when incoming service fails. Just like in a double-conversion UPS, the battery takes over, but the utility is taken out of the circuit by the bypass. Since the ITE runs most of the time on power from the grid, the inverter doesn't have to do a second conversion until the power goes out. This eliminates one of the efficiency loss factors.

Before a decade, VI UPSes could be more efficient by 5% or more than VFI units, but VFI UPSes have improved so much that the difference is now 1% or less.

Uninterruptible Power Supply

Standby UPS

Figure 7 is called a standby UPS. It is a voltage and frequency dependent device. Power is sent straight to the ITE, like a VI UPS, but the battery and inverter are not in the circuit until the power goes out. The output is filtered, but it isn't as stable as a real VI UPS.

Uninterruptible Power Supply

Figure 8 shows that when the power goes out, the utility is turned off and the battery and inverter are turned on. There is some instability in the switching, but the delay is short enough that most computer power supplies can handle it.

When the power comes back on, whether from the grid or a generator, the inverter is turned off, the line power is turned back on, and the rectifier, which is much smaller than in a VFI or VI UPS, charges the batteries.

Uninterruptible Power Supply

Some standby or VFD UPSes are advertised as line interactive, which is a mistake. It's important to know what kind of UPS you have. The internationally recognized VI and VFD identifiers make it easy to tell the difference, but manufacturers don't always use them, especially for smaller systems.

Mechanical and non-battery UPS systems

There are three main kinds of mechanical UPSes, and two of them don't use batteries. All three are true VFI or double conversion systems, but the intermediate conversion is all mechanical:

  • Motor-generator (MG) sets are made up of both a motor and a generator. In a VFI UPS, the motor is like the rectifier and the generator is like the inverter. Utility power drives a rectifier, which runs a DC motor and charges the batteries. When the power goes out, the batteries keep the motor turning, so the generator keeps giving power to the load. MG sets are usually used to keep power going to other mechanical equipment, like air conditioners, rather than to power actual ITE, though they were often used to power old mainframe computers in the past.
  • Diesel-rotary UPSes (DRUPS) are similar to MG sets, but they don't have batteries except for the ones that start the generator. Instead, they have an internal diesel engine that starts and keeps the power going when the utility fails. A flywheel keeps the generator turning long enough for it to stabilize before a mechanical clutch connects it to the generator. Again, these are more often used to keep air conditioners running than to power the ITE. They are often chosen as a cheaper alternative to separate generators when it is very important to keep the cooling going all the time.
  • Flywheel UPSes are similar to both MG sets and DRUPS, but with one big difference. When utility power is available, an electric motor turns the generator, but the system has only a heavy flywheel to keep the generator turning, usually until an auxiliary generator starts up again. The flywheel moves on nearly frictionless air or magnetic bearings in a vacuum-sealed case that can keep power for up to 30 seconds. Combined units can extend the time to several minutes without making the heat that other UPSes do.

UPS power factor

The difference between real power and apparent power is the power factor (pf). This is very poorly understood, but the buyer needs to know it. Historically, most large UPSes had a power factor (pf) of 0.8. This meant that a 100 kilovolt-ampere (kVA) UPS could only deliver 80 kilowatts (kW) of real power. Most modern UPSes have pfs values between 0.9 and 1.0. This means that the real power in kW is much closer to or even the same as the apparent power in kVA.

Central vs. distributed UPS

Distributed UPS usually means small UPSes that are mounted in each equipment cabinet, but sometimes there is a UPS for each cabinet row. There are some small VFI UPSs, but most are VFD or VI, so it's important to know which technology is being bought. Small, rack-mounted UPSes often have a power factor (pfs) of only 0.7. This means that a UPS that says it has 1,000 kVA might only deliver 700 watts. There are times when these are useful, but usually when there are only one or two equipment racks and a centralized UPS would be too expensive.

Small UPSes that are spread out aren't always taken care of as well as larger systems, so failed batteries are often missed until it's too late.

Considerations for selecting and using UPS systems

There are a few important things to think about when choosing a UPS system, such as:


Most modern UPS systems that use batteries are made up of separate parts. They are made up of several smaller UPS and battery units that can be put together as needed to increase capacity, provide backup, or do both. There's no longer a need to buy too much in the hopes of long-term growth. Just make sure the frame is big enough for long-term goals.

Real modules can be bought and put in place as needed, and an extra module or two can be put in place for redundancy. For example, an N+1 redundant UPS with 100 kW might have six 20 kW modules. The same way can be used to add more battery space. Also, most systems can swap modules without stopping operations. If a module fails, it can be taken out and sent back to the factory, and a new one can be sent overnight to the user for installation.

As was said above, Flywheel UPSs can also be put together in a modular way to make them bigger, make them last longer, or make them more reliable. These must be added and kept up by trained people, though.

Step function

When large loads are suddenly put on electrical equipment, the power can be unstable for a short time. This can happen when the power comes back on and the lights flicker or when big motors start up and the lights dim for a short time. This is a big problem when you have 2N UPS redundancy because if one UPS fails, the second UPS has to take on the whole load right away.

It's also a problem in VFD UPSes, where the full load is moved to the inverter when the power goes out. It can also be a problem in VI systems or systems that are running in "eco mode." When evaluating large UPS systems, it's important for the electrical engineer to get transient load data from the UPS vendor, compare it, and explain the results to the owner.

Batteries and battery duration

Batteries are a technology that is changing because more and more electric cars use them. Batteries are heavy, so you should always check the floor's strength. There are three main types of batteries used today:

  • Flooded lead acid or wet cells are the most expensive but last the longest, usually 25 years or more. But they need separate, fire-rated rooms with acid drains, hydrogen detection alarms, exhaust fans, eye wash stations, deluge showers, and hazmat protective gear. They are also the heaviest and need to be maintained regularly. They are usually used in the biggest and most complex installations.
  • Valve regulated lead acid (VRLA), which are also called "sealed" cells, use a paste electrolyte instead of a liquid and are sealed with small holes. So that they don't give off hydrogen, they are charged more slowly than wet cells. This means that they can be used in any space without any special building or protection. Most warranties last 10 years, but the batteries usually only work for 3 to 5 years, depending on how stable the power is where you live and how often you use and recharge them. There are VRLA batteries that last longer, but they cost more and usually need to be asked for. Wet cells are about the same weight as VRLAs.
  • Lithium ion (Li-ion) batteries are the newest ones on the market. Most places don't need special rooms or buildings to use them. There may still be cities that think they are dangerous, but their chemistry and construction are very different from those that have caught fire in ultra-compact electronics. Li-ion batteries are smaller and lighter than VRLA batteries. They can be partially discharged and recharged without losing power, and they should last longer than VRLA batteries. Still, they are too new to have long-term data.

Battery duration

No matter what kind of batteries they use, UPS systems give off heat, so they can only run for so long without air conditioning. The exact limit depends on things like room size, other equipment, and the heat load of the building, but 30 to 60 minutes is a good rule of thumb.

At some point, the UPS will get too hot and shut down automatically to protect itself. So, if there isn't a generator to restart the cooling, longer battery life is a waste of space and money. It also makes it very expensive to replace the batteries, especially if VRLA batteries are used. If one battery dies, the whole string must be replaced, or else the other batteries will die too soon. If IT staff want shutdowns to go smoothly, they should use a feature on most large UPSes that sends a signal over the network to shut down ITE when the battery life drops below a certain level.

With generators, UPSes are often set up so that they only have enough battery power for a few minutes. Quality generators should start up and be stable within a few seconds, but sometimes longer times are needed in case the generators don't start. Since there are two generators, this shouldn't be necessary.

Battery strings

A battery is the UPS part that breaks most often. So, to get the required length of time, the best setup uses at least two battery strings.

Battery monitoring and maintenance

Third-party battery monitoring is built into many newer UPS systems. If they don't, it should be added to the list of requirements. Batteries usually die when they are suddenly put to work, which is exactly when they are needed the most. There are different kinds of monitors, and different manufacturers have different ideas about which is best. However, any monitoring system will let you know if a cell is weak or has failed before something bad happens. Wet cells require regular maintenance. When monitoring shows a weak cell, the batteries should be changed.

Transformers and grounding

In the UPS drawings, there are no input or output transformers shown. Transformers used to be common in electronic UPSes, but now they are rarely seen. This is a big reason why the efficiency has gone up so much. Getting rid of transformers could have one more benefit and two possible drawbacks:

  • Advantage: If the voltages coming in and going out are the same, there's no need for a full wraparound bypass, which includes the transformers but not the maintenance bypass.
  • Disadvantage: If the voltages at the input and output must be different, you need not only a transformer at the input or output but also a full wraparound bypass with another transformer.
  • Disadvantage: The load is not separated from the UPS by an output transformer. So, the electrical engineer must be very careful when designing the grounding system and figuring out how to prevent fault conditions, which are often called "short circuits" and can damage the output transistors of a UPS. In large power distribution units, this is often done with distribution transformers that are outside the unit.

Considerations for low power quality and generators

When the power isn't stable, VI and VFD UPSs can have problems. Because power usually flickers a few times before staying on, these UPSes have built-in logic that keeps them from going back to normal until the power is stable.

VI and VFD UPSes shouldn't be used in places with unstable power because they have a lock-out feature that stops them from going back to normal if they switch back and forth too often. This means they have to be reset by hand. The same problem can happen if generators are turned on too quickly and go up and down as they try to take on the load.

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