Bidirectional power supplies dynamically route power from multiple sources to different loads, some of which are also power sources at other times.

We recently had a brief power outage in the neighborhood due to a storm and a downed tree. Since the outage only lasted about eight hours, it was a minor inconvenience in the bigger picture, but a major reminder nonetheless of how much we depend on electricity at home as a convenience and a necessity.

Not surprisingly, the informal discussion “standing on the street” turned to the desirability of a residential uninterruptible power supply (UPS), which ranges from about $5,000 to $15,000 installed, depending on capacity and installation issues. These home UPS systems (yes, saying “systems” is redundant with the acronym UPS, but they all do it) are typically powered by a generator powered by natural gas, diesel fuel, or propane in an on-site tank (Figure 1). Some have a battery as well, or even just a battery (sometimes supercapacitors are used instead, depending on load characteristics and other factors).

Figure 1: In a typical home generator-driven UPS, the transition of the house’s power source from the grid to the generator occurs about ten seconds after the grid power is applied and the generator starts. (Image: This old house)

Generator-powered UPSs are often referred to collectively as a Generac system, as this supplier is the market leader in the US with about 60% of the market, while Kohler is second. They are not the same as a cheap, small, portable backup generator that you manually hook up and start when the power goes out.

One of the neighbors noted that he did not understand why these generator powered consumer UPS systems had a switchover time of about 10 seconds until they restored power. Commercial battery-backed systems have no switching time, or on the order of 10 milliseconds, depending on the topology. After all, a hospital UPS is usually one that guarantees no power outages.

His legitimate question was, “What’s the problem with making it seamless?” Although it’s an easy question to ask, the answer isn’t obvious. The technical reality is that a UPS designed to guarantee no power interruption is a much more complex, expensive system with many more trade-offs than one designed to allow for a brief interruption.

UPS: the two main types

There are two broad classes of basic UPS systems, with several variations within each:

  • Offline: Also called backup architecture, this is the most common home/consumer approach. The load (house) is usually powered directly from the AC line and grid (Figure 2). When the UPS controller senses a multi-second loss of power (more than a brief interruption or interruption), it starts the generator (which is not necessary for a battery-backed system). A static switch (switch) disconnects the load (house) from the AC line and connects it to the backup generator.
Figure 2: In an offline UPS approach, the load typically operates from the grid-supplied AC line while a backup source (generator or batteries) is on standby if needed. (Image: EtechnoG)

The AC output of this generator is rectified and converted to grid-equivalent AC by an inverter (the battery, if present, obviously does not need this rectification stage). Key to this operation are “static switches” that provide a power transfer function; when the power fails, one static switch disconnects the AC line and the other connects the backup generator (or battery) to the load.

For a backup system equipped with batteries, it is also possible to charge the batteries from the AC line when the power is on. Most systems use an automatic sensor/control to start and switch the generator, although manual systems are available for about $500-$1000 less, but what do you do when the power goes out if you’re not home?

  • Online: To offer continuous operation completely without power interruptions, the online approach (sometimes called “online double conversion”) is used. (Figure 3). The AC grid continuously charges the batteries and even under normal conditions the load runs from the batteries through the inverter. This sounds very simple and logical, but presents some major technical challenges, including inverter design, switching, and battery sizing and management issues to maximize run time and battery life.
Figure 3: For an online UPS, the load operates from the UPS as a source that is continuously recharged through the network; the generator is activated while the system is still running on batteries. (Image: EtechnoG)

Some commercial systems use only batteries and do not have a generator, which trades one set of limitations (battery run time) for another (generator issues). Some home systems like Tesla Powerwall or Generac PWRCell also use batteries exclusively and are charged via the grid or solar panels, but there is significant additional cost and complexity. Additionally, some locales have zoning regulations regarding the installation of charged batteries above a certain capacity in residential areas (electric vehicles are an exception, of course).

In some ways, the best system combines both battery and generator: the battery carries the load through short interruptions with little or no interruption, while the generator is used for longer interruptions. As always, it’s a complex set of trade-offs between cost, complexity and capabilities.

Of course, this is a highly simplified overview of the reality of UPS architectures, sub-architectures and variations (some widely used, some proprietary). However, it shows that the answer to the seemingly simple question of why most home UPS systems have switching failure is neither simple nor obvious. One thing is clear: in these basic UPS systems, power comes from a primary and backup source and flows to the load. When the AC line is available, things are “quiet” in terms of power flow and switching.

The next part of the article examines the role of bidirectional supplies in UPS and other power systems.

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External references

Bidirectional power supplies support new UPS requirements, Part 1

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