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UPS Eco-mode: The Benefits and Risks
Many of the latest models of UPS systems now have an energy saving operating mode, often called “eco mode” or “green mode”, when operating in this way the UPS can achieve high nines efficiency at almost any load.
As good as this may sound, hardly any data centres choose to use eco mode because of a fear that this mode doesn’t offer the same level of protection as a true on-line uninterruptible power supply.
It is true that in order to achieve improved efficiency, eco-mode operates the UPS at a reduced level of power protection, but Eco mode will save energy. The decision that IT, Facilities and Finance departments face is whether the savings made by running in eco mode justify the risks.
So what is Eco mode?
Eco-mode is very similar to the basic mode of operation used in an offline UPS where it is called “standby” or “line interactive” mode. In these traditionally smaller, single-phase UPSs, the inverter/rectifier circuits are “offline” standing by, ready to switch on if the power fails or a problem is detected.
Similarly with an On Line UPS operating in eco-mode the load is normally powered by the bypass path, allowing raw mains power to supply the load, and the UPS inverter is engaged only when the utility mains fails. In normal eco mode operation, therefore, the load is exposed to essentially raw mains power.
Rather than true online mode, where the UPS continuously regenerates the output voltage through the inverter, in eco-mode the UPS inverter operates in a “standby” mode. In principle, this is a simple change in the control software of the UPS. However, the reality is considerably more complex.
Traditional UPS Eco Mode – In standard or “classic” eco-mode, the UPS system inverter is essentially off. If there is a utility power failure when the inverter is in standby, then both the UPS’s output voltage and current waveforms will be negatively affected. It takes time for the UPS to detect the failure, turn on the inverter, and supply clean power to the load from the batteries. This transfer time is what, in large part, allows the input disruption to briefly reach the output of the uninterruptible power supply.
Advanced Eco Mode – However, it has been possible to greatly reduce this and other negative effects of standard eco-mode. Advances in firmware control schemes and electrical design has led to the creation of what is known as “advanced eco-mode”. This form of eco- mode is available in varying degrees from a few UPS vendors today.
In advanced eco-mode the primary power path during normal operation is the same as standard or classical eco-mode: through the bypass path. But with advanced mode, the inverter remains “on”, operating in parallel with the input without actually handling the load current. Since the inverter is already “on”, it can more seamlessly take over the supply to the output where there is a mains failure.
While advanced eco-mode offers advantages over standard eco-mode, it does come with a trade off. Since the inverter circuit is “on” in the advanced mode, the overall efficiency of the UPS is less than it would be if it operated in standard eco-mode. The difference is small, however, having been observed to be in the 0.5% – 1.0% range typically. So while advanced eco-mode efficiency is less than standard eco-mode, it is still better than traditional online mode.
Benefits of Eco Mode
The benefit of eco-mode is that the efficiency of the bypass path is typically between 98.0% and 99%, compared to the base UPS efficiency of 94% to 97%. This means there is a pickup in UPS efficiency of between 2-5% when eco-mode is used.
Risks of Eco Mode
Eco-mode cannot predict the future. It must respond to a problem that already exists and then switch to the inverter. This means that a mains power problem can get through the UPS to the critical load until the following four things happen:
- The power problem is detected
- The UPS determines whether and how to respond
- The UPS inverter is energized (Note: in advanced eco-mode, it is already energized)
- The static bypass switch is actuated (opened)
In practice these events may take from 1 to 16 milliseconds, during which the critical load is subjected to the power problem.
While the loss of 1-16 milliseconds of power may not affect the typical 2U server, the loss of power for 8 milliseconds to a power transformer can cause that transformer to saturate when voltage is restored, tripping breakers.
The loss of power for even a few milliseconds on a circuit feeding a PDU equipped with a static switch will cause a state change on that switch. This can lead to an undesirable state change of the overall power system, including overloads and dropped loads.
The loss of power for less than 16 milliseconds can cause protective devices on pumps and fans to operate, creating an unexpected and undesirable state change of the sys- tem.
There are non-server IT devices, such as switches and other auxiliary devices that may not be as robust as servers are in handling dips and sags in the range of 1-16 milliseconds.
The risk of eco-mode is that the IT load is exposed to raw utility mains power without the conditioning normally provided by the double-conversion, online UPS system. The UPS must continuously monitor the mains power and quickly switch to the UPS inverter when a problem is detected, before the problem can affect the critical load. This may sound simple, but actually is quite complicated and entails a number of risks and has some potentially undesirable side-effects. However, depending on how eco-mode is implemented by the vendor, the degree to which the load is exposed to raw mains can be significantly minimized.
How much is really saved?
The energy savings associated with the use of UPS running in traditional eco-mode (standard) 100% of the time are approximately 2.3%. In financial terms this equates to approx. £10,000 per year for a 1 MW rated da-ta centre running at 50% load with an electric rate of £0.10/kWh.
The savings in advanced eco-mode are likely to be approximately 0.5% less when compared with standard eco-mode, or around 1.8%. If the system operates only part of the time in eco-mode, then the savings are proportionally reduced.
So to sum up, eco-mode represents a potential way to save energy in data centres, hospitals, and other UPS applications and Data Centre operators can expect to see savings on the order of 2-3% in total energy if eco-mode is enabled.
A UPS with eco-mode also qualifies for the Enhanced Capital Allowance Scheme.
The use of eco-mode entails risks. Eco-mode introduces a number of new modes of operation of the data centre, and reduces power protection. Current power supplies and IT equipment are much more resilient to power variations than similar equipment of prior generations, suggesting that this equipment should operate reliably using eco-mode. However, complex systems comprised of a mix of IT equipment, transformers, transfer switches, and other possible non-IT loads are less predictable in their response to infrequent and unusual power events, and their compatibility with eco-mode is less certain.
For this reason, eco-mode should be used in situations where power quality is excellent.
As data center, hospital, and industrial designs become more standardized, equipment continues to improve, and real world experience using eco-mode accumulates, predictability and confidence in eco-mode will improve.
If you would like to discuss how Source UPS can help, get in touch with a member of our team today.
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