The Nuances of Backup Power Systems

The Nuances of Backup Power Systems

What Type of Backup Power is Appropriate?

November 09, 2018 by Greg Pink

Designing power systems can sometimes feel like a gamble. When a building has systems other than life safety that must run, such as security, data centers, or chemical/manufacturing processes, relying on the city to provide consistent power without interruption can feel like a loss of control. While city power providers often have excellent reliability, outages happen and the impact is left completely in your court. Luckily, methods of providing backup power have been around for over 60 years and have grown more robust with time.

BACKUP POWER BASICS

Backup power is a supply that is used when the main source has been lost. Backup power may supply the entire building load or a portion of the load, and is most commonly provided by generators, UPS (Uninterruptible Power Supply) systems, or a second connection to the local utility. How much load needs to be supplied and for how long are the important questions when determining what type of backup power should be utilized.

Generators are capable of supplying all sizes of loads and are able to run as long as fuel is available. However, generators are not an instantaneous source of backup power, taking several seconds to start up and get to the point of being able to take on load. This is not ideal for any system requiring constant power (ex. computers) but can be ideal for systems that can have their process interrupted (ex. pumps or fans). Generators can be inexpensive, but are loud, large, and require additional equipment and space.

Like generators, UPS systems can also supply a wide range of loads but are usually suited for short run times depending on the batteries and how much power is being used. UPS systems often transition to battery power in milliseconds, fast enough to make a seamless transition during an outage. Unfortunately, they can be quite expensive depending on the battery capacity and features, but are relatively easy to implement in a building.

Secondary power connections are ideal for an indefinite backup and are extremely reliable when they are supplied from a different location than the main source. Similar to generators, secondary power connections are not immediate backups with transition delays of around five seconds to keep from switching during normal operation. Secondary power connections are difficult to price and depend on availability from the utility, the building size, type, and quantity of loads. If backup power is required for everything within a building, this is often the most cost effective option.

MULTIPLE OPTIONS

Some facilities, such as hospitals, utilize layers of backup power. In a big city, a hospital will often have a primary source of supply, and one or more secondary connections. The small essential loads (life support) are backed up by small UPS systems, and the major life safety loads (fire alarm, smoke exhaust, elevators, lighting) are backed up by generators. When power to the primary source goes out, the UPS systems carry the essential loads through a brief outage before connection to the secondary source is made. If the secondary connection then goes down, the UPS systems again carry the essential loads through a brief outage before the generators start up and supply backup power. Non-essential services (phone charging stations) are not supplied with power and remain off. In this way, the UPS and generator systems are minimized, reducing the cost and electrical infrastructure required while maintaining as close to full building functionality as practical for an emergency power situation.

ELIMINATE OR MITIGATE?

The hospital situation is ideal, but it is also excessive for many other applications. The IEEE Gold Book (Design of Reliable Industrial and Commercial Systems) has analyzed the downtime of electric power systems based on surveys and computer models. It is an important resource we use when considering what is the most likely point of failure in the system and how likely multiple sources are to fail at any one time. For example, at the point of use, a building with a single source of supply is likely to experience a total outage of around 4.5 hours a year. That number is nearly cut in half when a second source of utility supply is added, even though the total time that both supplies will be out at one time is expected to be close to 10 minutes a year. The reason? The majority of the electrical distribution system is the same for both sources, and it is more likely to experience failure than it is likely that both supplies fail. It is not until the electrical distribution system is duplicated almost to the point of use (a considerable expense!) that the outage time is expected to drop under 30 minutes a year. The graphics below illustrate this reduction with redundancy [1].

By attempting to eliminate outage time, it is very easy to throw additional infrastructure and money at the problem, but what is really needed is a careful evaluation of the building and the perceived need for constant power. Often, the importance of wide-spread uninterrupted power is exaggerated and power loss is more likely inconvenient than world ending. This means the system can adopt smaller, more practical solutions, which balance the system cost, future maintenance costs, and outage costs with the chance of a power loss.

There is no universal solution when it comes to designing backup power systems. By evaluating the system and determining where downtime is affordable, a backup solution can be designed to be robust and cost effective. The risk of a power outage can never be eliminated, but careful design can mitigate the extent of affected systems while staying within the project budget.

Overall, periodic maintenance is cheap insurance against unplanned, costly, and lengthy power outages and service disruptions, especially in critical buildings or applications. Testing and scheduled maintenance should be performed frequently on high-energy equipment such as switchboards, distribution panels, transformers, and associated equipment. Visual and mechanical checks, along with a history of detailed testing sheets from qualified technicians, allow building owners to schedule equipment replacements at convenient times and budget accordingly.