Bidirectional power supplies support new UPS requirements, Part 1

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

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

Not surprisingly, the informal “standing in the street” discussion turned to the desirability of a residential uninterruptible power system (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 UPS acronym, but everyone does it) are usually powered by a generator fueled by piped natural gas, diesel fuel, or propane in an on-site tank (Figure 1). Some have a battery bank as well or even exclusively (supercapacitors are sometimes used instead, depending on the load characteristics and other factors).

Fig 1: In a typical generator-driven home UPS, the house’s power source transition from grid to generator occurs about ten seconds after grid power and the generator is started. (Image: This Old House)

The generator-powered UPS is often referred to generically as a Generac system, as that vendor is the market leader in the US with about 60% of the market, while Kohler is second. They are not the same as a lower-cost, small, portable backup generator which you manually connect and start when power fails.

One of the neighbors remarked that he doesn’t understand why these consumer generator-powered UPS systems have a switchover time of about 10 seconds until they restore power. Commercial systems with battery backup have either no switchover time or one on the order of 10 milliseconds, depending on topology. After all, the UPS at the hospital usually is one which assures no power interruption.

His legitimate question was, “What’s the big deal with making it so there is no interruption?” While it’s easy to ask that question, the answer is not obvious. The technical reality is that a UPS designed to ensure no power interruption is a much more complicated, costly system with many more tradeoffs than one which is designed to allow for a brief interruption.

UPS: the two basic types

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

Offline: also called a standby architecture, this is the most-common home/consumer approach. The load (house) normally runs directly from the utility AC line and grid (Figure 2). When the UPS controller senses a multi-second loss of power (more than a brief sag or brownout) it starts the generator (which is not needed for a battery-supported system). A static switch (transfer switch) disconnects the load (the house) from the AC line and connects it to the backup generator.

Fig 2: In the off-line UPS approach, the load normally operates from the grid-provided AC line while the backup source (generator or batteries) is on standby if needed. (Image: EtechnoG)

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

For a battery-equipped back-up system, there’s also a provision for charging the batteries from the AC line when power is on. Most systems use an automatic sense/control for generator start-up and switchover, although manual systems are available for about $500-$1000 less, but what to do when the power fails if you are not home?

Online: To offer continuous operation totally without any power gaps, the online approach (sometimes referred to as “double conversion online”) is used (Figure 3). The AC grid power continuously charges the batteries, and even under normal conditions, the load operates from the batteries via the inverter. This sounds very simple and logical, but it brings some major technical challenges including inverter design, switchover, and battery sizing and management issues to maximize run time and battery life.

Fig 3: For the online UPS, the load operates from the UPS as source, which is continuously recharged via the grid; the generator is activated while the system is still running on batteries. (Image: EtechnoG)

Some commercial systems use batteries only and do not have a generator, which trades one set of limits (battery runtime) for another (generator issues). Some home systems such as the Tesla Powerwall or the Generac PWRCell also use batteries exclusively, and they are recharged via the grid or solar panels, but there is significant added cost and complexity. Also, some locales have zoning regulations regarding having installed charged batteries above a certain capacity in residential areas (electric vehicles are a carve-out exception, of course).

In some ways, the best system combines both battery and generator: the battery carries the load through short disruptions with little or no interruption while the generator is used for longer outages. As always, it’s a complex set of tradeoffs among cost, complexity, and capability.

Of course, this is a highly simplified overview of the reality of UPS architectures, sub-architectures, and variations (some widely used, some proprietary). Nonetheless, it does indicate that the answer to the seemingly simple question of why most home UPS systems have a switchover interruption is neither simple not obvious. One thing is clear: in these basic UPS systems, power comes from a primary and a backup source and flows to the load. When the AC line is available, things are “quiet” in terms of power flow and shifts.

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

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