FM Issue: Backup Power Design

By Dennis Bouley
Published in the July 2010 issue of Today’s Facility Manager

There are five principal uninterruptible power supply (UPS) system design configurations that distribute power from the utility source of a facility to the critical loads within that facility. Each of these designs addresses a different set of requirements, and the five approaches include:

• Capacity;
• Isolated Redundant;
• Parallel Redundant;
• Distributed Redundant; and
• System Plus System.

Capacity (N)

A capacity (N) system is comprised of a single UPS module, or a paralleled set of modules, whose capacity is matched to the facility’s critical load projection. This type of system is by far the most common of the configurations in the UPS industry. A common single module UPS system configuration is shown in Figure 1 on the opposite page. (ATS is automatic transfer switch; PDU is power distribution unit.)

A capacity design is simple to deploy and inexpensive to maintain, but it does expose facility managers (fms) to a number of risks. For example, in the event of a UPS module breakdown, the load will be transferred to bypass operation, exposing it to unprotected power. The load is exposed to unprotected power during maintenance. Multiple single points of failure are present in such a design.

(Typical wraparound bypass is a circuit used to change the path of the electrical power so it goes around, or bypasses, its normal path. Bypass operation would take place when a UPS is being worked on for maintenance, for example.)

Isolated Redundant

An isolated redundant configuration is sometimes referred to as an “N+1” system. (However, it is different from a parallel redundant configuration, which is also referred to as N+1.) The isolated redundant design concept does not require a paralleling bus, nor does it require that the modules have to be the same capacity or even be from the same manufacturer. (A wire or bus duct connects UPSs together on the output.)

In this configuration, a main or “primary” UPS module feeds the load. The isolation or “secondary” UPS feeds the static bypass of the main UPS module(s). (Static bypass is a means of quickly bypassing the components of a static UPS so utility power can be directly connected to the load.)

This configuration requires that the primary UPS module have a separate input for the static bypass circuit. This is a way to achieve a level of redundancy for a previously non-redundant configuration without having to replace the existing UPS completely. Figure 2 illustrates an isolated redundant UPS configuration.

An isolated redundant design provides UPS fault tolerance, requires no synchronization, and is cost-effective for a two module system.

However, the design also has limitations. For example, the system relies on operation of the primary module’s static bypass to receive power from the reserve module. Also, both UPS modules’ static bypass must operate properly to supply currents in excess of the inverter’s capability. The secondary UPS module has to be able to handle a sudden load step when the primary module transfers to bypass.

Switchgear becomes complex and costly when the “catcher” UPS supports multiple primary UPS. Higher operating costs also result due to a 0% load on the secondary UPS, which draws power to keep it running. The system requires at least one additional circuit breaker to enable multiple bypass sources (i.e., the utility and the other UPS). Two or more primary modules need a special circuit to enable selection of the reserve module or the utility as the bypass source (static transfer switch). A single load bus per system also results in a single point of failure in the design.

Parallel Redundant

A parallel redundant configuration consists of paralleling multiple, same size UPS modules onto a common output bus. The system is N+1 redundant if the “spare” amount of power is at least equal to the capacity of one system module; the system would be N+2 redundant if the spare power is equal to two system modules; and so on. Parallel redundant systems require UPS modules identical in capacity and model. Figure 3 depicts a typical two module parallel redundant configuration.

The parallel redundant design offers a higher level of availability than capacity configurations because of the extra capacity that can be used if one of the UPS modules breaks down. The probability of failure is lower compared to isolated redundant because there are fewer breakers and because modules are online all the time (creating no step loads). The parallel redundant approach is also expandable if the power requirement grows.

The downside of the parallel redundant design is that the load may be exposed to unprotected power during maintenance if the service extends beyond a single UPS module or its batteries. In addition, a single load bus per system implies a single point of failure.

Distributed Redundant

Distributed redundant configurations employ three or more UPS modules with independent input and output feeders (see Figure 4). (A feeder is a copper conductor feeding a UPS.) The independent output buses are connected to the critical load via multiple PDUs. Some versions of this design include static transfer switches (STS) in the architecture.

A distributed redundant design and a system plus system design (discussed next) are quite similar; both provide for concurrent maintenance and minimize single points of failure. The major difference is in the quantity of UPS modules required in order to provide redundant power paths to the critical load, and the organization of the distribution from the UPS to the critical load. As the load requirement “N” grows, fewer UPS modules are needed when using the distributed redundant approach.

Overall, distributed redundant systems are usually chosen for large, multi-megawatt installations where concurrent maintenance is a requirement. UPS module savings over a system plus system design also drive this configuration.

System Plus System Redundant

The system plus system design (see Figure 5) allows for each piece of electrical equipment to fail or be turned off manually without requiring that the critical load be transferred to utility power.

This is the most reliable and most expensive design in the industry. The high cost of constructing this type of system within a facility can only be justified if national security is at risk or if the cost of downtime is exorbitant. Organizations that choose system plus system configurations are generally more concerned about high availability than the cost of achieving it.

Matching System And Situation

Power infrastructure is critical to the successful operation of a facility. Fms can consider capacity, isolated redundant, parallel redundant, distributed redundant, and system plus system configurations. By understanding the organization’s availability requirements, risk tolerance, and budget capability, an appropriate UPS design can be selected.

Bouley is a senior research analyst at the APC by Schneider Electric (www.apcc.com) Data Center Science Center. He holds a B.A in Journalism and a B.A. in French from the University of Rhode Island, and a Certificat Annuel, from Sorbonne, Paris, France. Bouley has been published in multiple global journals, authoring articles focused on data center physical infrastructure, and has authored several white papers for both APC by Schneider Electric and The Green Grid.