Using eco mode in UPS systems
Saving energy is the ultimate goal when implementing UPS eco mode.
The choice of using UPS eco mode operation for critical systems is a widely debated topic in the industry today, as energy costs rise and corporations seek to enhance their green image by showing improved power usage effectiveness (PUE) in their annual reports. As with almost all new energy-saving technology, there are pros and cons, which this article will look to explore. We will focus on typical large-scale data center UPS systems, 3-phase, 300 kVA and larger. We will examine a number of salient issues that one must investigate fully before considering implementation of eco mode in a critical system environment.
Before exploring the functionality of eco mode, we must understand the infrastructure within the data center itself—from the IT load to the mechanical equipment. To do this, we must answer important questions, such as:
What are the power limitations on the critical load being supported by the UPS?
Is the IT load older style equipment or later technology? Older technology, due to its generally oversized and robust construction, is generally more resilient to power disturbances.
What is the ride-through time of the individual power supplies of the critical equipment compared to UPS transfer schemes?
What is the response of the non-IT loads to power disturbances and how can they affect the IT loads?
What is the site’s raw power-quality history?
What is the utility’s history of power surges and sags?
After understanding these key power system elements, we can make an informed decision as to whether using eco mode is appropriate for a particular site. To mitigate the effects of a UPS failure, multiple UPS systems are often paralleled to provide redundancy within the system (see Figure 1). These systems are typically integrated and usually provide a user interface (see Figure 2).
What is eco mode?
There are many different interpretations of what “eco mode” operation means in a UPS system. The reality is that eco mode is a broad term used to describe any UPS mode of operation that improves the efficiency of the system. This efficiency increase comes with a particular trade-off in performance, which varies by vendor. This means that, depending on the manufacturer, UPS systems that use an eco mode may have entirely different modes of operation, affecting reliability and energy savings—again, stressing the importance of understanding the operating characteristics of the specific equipment within your facility.
Before we investigate the different styles of eco modes available, let us first look at the standard operation of a typical UPS (see Figure 3). The typical UPS in normal (double conversion) mode takes the incoming utility power and passes it through a rectifier, which converts the ac power to dc power. An inverter creates the ac waveform and distributes it to the loads. Batteries are connected on the dc side of the system. During a utility interruption, these batteries discharge power to the inverter, which continues to supply power without power interruption to the critical load. UPS systems typically have an internal bypass to allow the equipment to seamlessly transfer the critical loads to the utility or internal generation, allowing the UPS to be deenergized during service without dropping the load. This bypass system can also isolate the UPS from the critical loads in the event of a UPS failure.
UPS systems with an eco mode use the same configuration as double conversion units, but with different operational characteristics that provide an increase in efficiency. When placed in eco mode, the UPS system typically allows utility power to bypass the rectifier and inverter and directly feed the critical load. In the event of a power disturbance, the UPS can provide conditioned power to the load by returning to normal mode. This process can significantly reduce the losses in the UPS system, depending on the manufacturer, and generally improves UPS efficiencies by 2% to 4%. The most modern UPS systems are operating in the range of 94% to 97% efficiency. Many manufacturers are stating that they achieve 98% to 99% efficiency when combined with eco mode. The trade-offs of this efficiency increase will be discussed later in this article.
A common misconception of UPS systems is the difference between offline and line interactive, both of which can be considered eco mode. Offline mode supplies the load through the UPS bypass without interaction from the rectifier or inverter, other than maintaining battery charge. When utility power is disrupted, the inverter is started and the load is supplied from the batteries. Line interactive systems are similar but with one significant difference: the inverter and rectifier never completely go offline. The load is still applied from the internal bypass of the UPS, but the inverter and rectifier remain energized. There are two significant benefits to this, the first of which is transfer time. As the inverter never goes completely offline, the load can be transferred from the bypass to the inverter faster, thereby reducing the exposure of the critical load to power disturbances. The second benefit is that line-interactive setups still provide some—although minimal—power conditioning. These benefits come with slightly reduced efficiency. However, this is still more efficient than operating in normal mode.
Transfer time is the time it takes for the UPS system to supply power to the critical load when the primary source fails—switching from one source to another. This is a paramount concern when it comes to operating in eco mode. The important question is, will the UPS system support the critical load? The idea that a UPS in eco mode must switch the load from bypass to the inverter sounds threatening to most users, and explains why many people do not take advantage of this feature. However, it is possible for many users to implement the eco mode in their power system without noticing any difference in system stability or reliability. There are many variables that can affect the transfer time of a UPS in eco mode, and many of these variables can be adjusted by the user.
The capability of the UPS system to support the critical load during an event (e.g., 120% overvoltage) is based on two critical elements: the speed at which the UPS can sense and transfer, and the minimum operating requirements for ride-through of the IT load. This information can be obtained from specific equipment manufacturers and should be consulted thoroughly before considering using eco mode. The historical reference tolerance curve for IT equipment is the ITI curve (see Figure 4). This curve represents the areas of operation when a load may be judged to be protected based on a voltage disturbance, and also shows when the load will generally suffer a power loss due to extended transfer time. Figure 4 includes an overlay indicating a range of transfer times of typical UPS systems equipped with eco mode. The quickest transfer times are found in units that use a static switch, while the slowest transfer times are seen in UPS systems that employ mechanical switches. As the eco mode transfer times coincide with the power loss region, it is evident that a thorough evaluation should be performed to determine if eco mode is appropriate for a particular application. It is important to note that these data were based on equipment specifications from 10 years ago and may not be absolutely compliant with the newer IT equipment.
Power quality is another major concern among UPS users. When a UPS is in normal mode, the utility power is continuously regenerated through the double conversion process. As the UPS creates the waveform, it ensures that the loads will not be exposed to substandard power quality, such as voltage surges, sags, and frequency deviations, which may be delivered by raw incoming utility power. In eco mode, this is not always the case. Most eco modes provide little or no power conditioning when using the bypass. They do, however, monitor the incoming utility power quality, and during the event of a variation outside the set parameters, the UPS automatically switches from bypass to the UPS to provide the same quality of power as normal mode. After the incident has passed and the logic has determined the acceptable power source has been restored, the UPS returns to bypass mode. During the time of the incident, the UPS is not running at the higher efficiency of eco mode.
It is important to review the power quality history of the raw utility power before implementing eco mode. If the incoming utility power quality is extremely poor, the UPS will spend very little time in bypass mode. This will cause the system to continue to run in normal mode, creating little to no increase in efficiency. Fortunately, most modern-day utility supplies, especially in the U.S., are regulated with minimal disturbances in power quality.
Another aspect of power quality that must be examined before implementation of eco mode is load-side harmonics. The double conversion process not only filters the power quality of the incoming utility power, but it also isolates critical loads from harmonics. Most data centers have large motors fed by drives for HVAC equipment. If these motors and their associated drives induce a large amount of harmonics into the system, the harmonics have the potential to flow to the sensitive IT loads when the UPS is in bypass. It is possible to reduce harmonics present in the system with harmonics mitigating equipment (e.g., specialized transformers). However, a harmonic analysis should be completed before using any eco mode features to verify beforehand if there will be issues with the critical loads.
Downstream device coordination
When determining if the eco mode function should be used on a UPS system, many users are only concerned whether it can react quickly enough to prevent dropping the load. However, there is more to consider than just the load. When a power failure event occurs, the brief lapse in time during the transfer may not affect the IT loads but may affect other downstream electrical equipment. A static switch, which can transfer in approximately 0.25 of a cycle, inside a PDU may be affected by the power loss and cause it to change state. Full system coordination is essential to determine the effects the transfer time may have on all critical equipment supported by the UPS.
Surge protection is also a crucial element of data center equipment protection, especially when implementing eco mode. Due to the bypass configuration of eco mode, it is possible for a surge to propagate through the UPS system in bypass to the critical loads. Some UPS models have an integral surge protection system, but it is typically recommended that this be used only as supplemental surge protection and that primary surge protection measures are also in place, unless otherwise stated by the manufacturer.
Whether your facility is attempting to achieve an Energy Star certification or simply cut costs on utility bills, saving energy is the ultimate goal when implementing eco mode. Most UPS manufacturers boast eco mode efficiencies of 98% to 99%, while best-in-class UPS systems in normal mode achieve efficiencies of only up to 97%. The difference of 2% can lead to substantial energy savings annually as UPS systems are running 24 hr/day, 7 days/week, and 52 weeks/yr. The savings, however, can vary substantially depending on the cost per kWh at the facility location.
It should be noted that efficiency ratings published by the manufacturer are measured when the UPS is at 100% load. The efficiency for a typical double conversion UPS system decreases as the load decreases (see Figure 5). In practice, 100% load is almost never the case. Many data centers run a 2N configuration, which is intentionally redundant to ensure reliability. These systems are designed such that in the event one UPS fails, the other can supply the entire load. Because of this, each UPS is typically running at less than 50% load. Although the efficiency drops with this reduced load, the difference between the eco mode and normal mode efficiencies still averages around 2%.
Consider the example of a data center running a 2N system with two 750 kVA, high-efficiency UPS systems. Because the system is in a 2N configuration, it allows us to assume 50% loading for each UPS. With the 2% gain in efficiency of eco mode as opposed to normal mode, a system that runs 24 hr/day for 365 days will save 131,400 kWh annually. At $0.12/kWh, that’s a savings of about $15,768. This savings gets significantly larger for any system that has a normal mode that is less than 96% efficient.
HVAC system savings should not be overlooked when considering eco mode. Because eco mode places the inverter and rectifier in either an offline or a standby state, depending on the manufacturer, the UPS systems produce much less heat than in normal mode, thus reducing the required cooling load.
Other aspects of implementing eco mode in a facility include component wear, battery life, and thermal transients. As eco mode transfers the critical load between UPS double conversion and eco mode configurations, there is increased component wear. Depending on the type of component used to accomplish this, such as a mechanical switch, it may induce a shortened life of the overall UPS system.
However, as the UPS is operating more efficiently, less waste heat is generated. This may extend the life of some components, due to the system running at cooler temperatures, including the batteries. Of course, this depends on the location of the equipment in relation to the UPS rectifier and inverter and associated system cooling and ventilation (see Figure 6).
Thermal transients are created when a large step load is applied to the inverter, which in turn causes a rapid increase in heat within the unit, which can cause the equipment to fail. It should be noted that the data available at this time does not indicate this to be a trend associated with eco mode. However, it has been an issue in the past with other forms of switching.
System design tips
There are many items to consider before using the eco mode on a UPS system. However, it is possible for many users to reap the rewards without experiencing a loss of performance or reliability. The key to becoming one of those users is a thorough analysis of the system currently in place in the facility, along with its energy savings objectives.
Equipment specifications vary by manufacturer and could cause unexpected behavior in an eco mode application if not properly analyzed. It is imperative that all aspects of the system are evaluated, including the transfer time, associated IT load power tolerances, downstream protection devices, HVAC, and incoming power quality. Most data centers are continuously monitoring the incoming utility main, which can provide easy access to the power quality of the site. If no utility data are available for the site, there are regulating bodies to which the utilities must report their reliability data that can help make power quality decisions.
Although the eco mode UPS may not be right for all applications, with the proper analysis and coordination of electrical systems, it offers the potential for some users to save a large amount of energy.
Robert Corson is a senior electrical engineer at Triad Consulting Engineers with a focus on full design and construction of critical system projects, ranging from Tier III and Tier IV data centers to earth satellite stations. Scott C. Carlson is a staff engineer at Triad Consulting Engineers specializing in high-efficiency system upgrades, with a focus on critical system applications.