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.