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Eco mode UPS for data centres: The case for and against

Thu, 11/15/2012 - 17:36 -- Ruth Williams

All large double-conversion online UPSs are equipped with a static bypass path, which provides multiple features including acting as a source of redundancy for the UPS power inverter.

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In online mode, the UPS continuously regenerates the output voltage and therefore the critical load is not subject to power disturbances, even during a mains power problem (see Figure 1 - right). In the event of failure, the UPS switches to bypass so that even if there is an instantaneous failure it should usually stay within the CBEMA (Computer Business Equipment Manufacturers Association) curve for the power quality of the load and the UPS will transfer the load to the bypass and the load will survive.

In eco mode the UPS runs in reverse. 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. Any mains power abnormality causes the UPS to switch power paths between bypass and inverter. 

Different vendors implement eco mode differently. There are variations in the way the system operates the standby inverter. There are also variations in the way the mode is enabled, where it will revert to normal UPS mode under various conditions. 

Some vendors claim to have special ways to control their transfer switch, but they all have the same basic concept: exposing the IT load to unconditioned power and transfer events in return for a gain in efficiency.

Eco mode can offer some huge advantages. The primary benefit is that the efficiency of the bypass path is typically between 98% 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% in UPS efficiency when eco-mode is used.

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According to Neil Rasmussen, VP of Innovation at Schneider Electric, the energy savings associated with the use of UPS eco mode is approximately 2.3%, corresponding to an energy saving of about US$15 000 per year for a 1 MW rated data centre at 50% load with an electric rate of US$0.10/kWh. 

Eco Mode: The case against

The obvious drawback of eco mode, however, is that the IT load is exposed to raw utility mains power without the conditioning normally provided by the UPS. 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, as Rasmussen explains. Rasmussen, who founded APC (later acquired by Schneider Electric) in 1981, notes that despite most UPS manufacturers offering systems with an eco mode function, few use it. “When APC first offered eco mode in the 1990s on larger UPS systems, we found that no customers actually used it,” he says.

Nevertheless, the recent drive to reduce data centre energy use has renewed the interest in eco-mode for UPS systems. Unfortunately, says Rasmussen, there are consequences associated with operation in eco mode that must be understood prior to implementation in complex data centre systems comprised of a mix of IT equipment, transformers, transfer switches, and other possible non?IT loads that are less predictable in their response to infrequent and unusual power events. 

The primary considerations are: potential loss of electrical protection, reliability impacts and operations issues.

Potential loss of protection

A UPS operating in eco mode must respond to a problem that already exists and switch to the inverter. This means that the problem will get through the UPS to the data centre 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.

The transfer switch is actuated.

In practice these events may take from 1 to 16 milliseconds, during which the data centre 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. 

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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 system. There are also 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.

Furthermore, says Rasmussen, the static switches used in eco mode UPS systems can only transfer when the mains voltage is lower than the inverter voltage or at a zero crossing of the mains voltage. Therefore they cannot protect the load from sub-cycle mains overvoltage, even if they detect it immediately.

System reliability

A UPS operating in eco mode will need to start the inverter and transfer in response to detected power events. The frequency of occurrence of transfer events will depend on a number of factors, including the sensitivity settings on the UPS, the mains power quality, and power disturbances created within the facility by other equipment. 

Whether this happens once a month or once an hour, the step-change in power applied to the UPS inverter creates a thermal event and shock to the system. It is well known that thermal transients are a primary cause of failure of electronic power systems. To make this situation worse, the thermal transient is applied at precisely the time when the UPS is needed the most and there is no alternative. 

Therefore the risk of failure is exascerbated at the exact time when the system is needed the most. In normal UPS operation without eco-mode, there is no thermal shock or transient when the mains power fails. If the UPS inverter experiences a random failure, it will almost certainly be at a time when mains power is present so the UPS will go to bypass with no load drop. 

Battery life

There are two primary factors regarding eco-mode and battery life, which are battery wear and battery operating temperature. The transfers to inverter operation could cause momentary operation on battery, even if mains power is present and the battery charger is able to function.

This means eco mode operation will require a transfer to battery much more often than the same UPS operated in double conversion mode. This may be of no consequence if such events happen only once every few months, but may create unnecessary battery wear if these events happen multiple times per day. This extra battery wear depends on the implementation of eco mode, the local power quality, and the eco mode settings. 

It may be difficult to predict battery wear in advance, so it needs to be established over time with real-world experience at a particular site. In principle, eco mode is more efficient so the UPS creates less heat. 

This could mean that batteries run cooler, and therefore last longer. However, the reality is that this effect cannot be assumed and the battery temperature will be strongly affected by the UPS design. 

For example, if the batteries receive cooling from the UPS fans and those fans are off in eco mode, says Rasmussen, then the batteries may actually operate at higher temperatures in eco-mode. If the batteries are in isolated cabinets from the UPS power electronics, then there may be no effect from eco mode. Therefore, no general assumption of the effect on eco mode on battery life is possible, and eco mode impact on battery temperature must be established on a case by case basis.

Fault clearing 

In its normal setting a UPS detects output faults and switches to bypass to obtain the extra fault clearing current needed to rapidly open downstream protective devices. This is a valuable feature that can prevent downtime of IT loads during a fault condition. However, when the UPS is in eco mode, it can be very difficult to distinguish an output fault from a loss of input power. 

During an output fault a UPS in eco mode may detect a drop in input voltage that causes a switch to inverter, which will extend the fault clearing time and possibly expose the critical IT load to a momentary loss of power. Some vendors claim to have sophisticated control and detection algorithms in their eco mode UPS to reduce this problem. Nevertheless, this is an additional factor that should be considered as the efficiency gains of eco mode are balanced against the various costs and risks.

Not a huge fan of eco mode, Rasmussen likens its operation to the hand-off of a baton in a relay race. “It is critical that it works correctly, each hand-off is a little different, and on 

rare occasions there may be a problem. For this reason, eco mode should be used in situations where the number of handoffs are as few as possible, i.e. where power quality is excellent.”

Eco Mode: The case for

2-20bDr. Ian Bitterlin, Chief Technology Officer of UK data centre developer Ark Continuity, heartily agrees with Rasmussen’s final point. “Where the mains supply is good, I use eco mode and where it isn’t, I don’t,” he says.

“There are some places around the world where you would definitely want to use eco mode and there are some places you definitely do not. I did a job in Trinidad where eco mode was switched on once and never switched on again because they were never more than two hours between mains interruptions.”

Bitterlin believes using eco mode is an issue of mindset. “Eco mode is not necessarily unreliable, it is just a perception. The operation of UPS is simply to keep the output voltage within the CBEMA curve tolerances and I do not see an issue whether this is done by the mains or not. 

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There are no extra components; it is just a software change that says if the UPS fails I am going to switch to bypass and if the mains fails I am going to switch to UPS. It’s the same switch, but it’s the old online/off?line legacy argument as to failure on demand. It doesn’t matter if the UPS failed as long as the load has survived. In the end it comes down to the individual client’s appetite for risk and the depth of his pocket.”

Bitterlin also believes giving the responsibility for data centre system architecture over to contractors and consultants has also hindered the implementation of eco mode operation. “My theory is that eco mode has been slow to be adopted because UPS manufacturers often do not deal closely enough with the end?customer. A lot more end?clients would use eco mode if they had the choice. 

“But there’s no reason why consultants should take the risk of a UPS system failing,” he says. “A contractor has no interest with regards to what happens once the job is finished.”

Energy savings

Despite slow take up, there remains a very good reason why a data centre may want to turn on eco mode: energy costs. Over a period of ten years, the cost of electricity comprises 60% to 70% of the total cost of ownership of a data centre. As it costs no more to build a UPS system with an eco-mode function than one without, the potential benefits are clear to see.

Bitterlin recognizes these advantages of eco mode, particularly in Europe where power prices are relatively high. “It all depends on your market’s acceptability of the price of energy,” he says. 

“There is more talk in the United States about eco mode, but they really don’t want to do it because when you’re only paying US$0.05/kWh, you don’t really care about a few efficiency percentage points. The only green thing about America is the colour of its money. 

“In Europe, however, people are very attracted to eco mode because you’re lifting your fully functional UPS from 95%-96% efficiency at full load to 98-99%. As the cost of power goes up and realism sets in far more people will adopt eco mode than at present.”

“There is paranoia about eco mode and not everyone has to use it. But it’s better to have the ultimate protection of double conversion combined with the energy saving of eco mode than anything else.”

Need for power surge protection

Bitterlin stresses the need for adequate surge protection for eco mode operation. “If you are going to run eco mode then you are going to have to have a proper surge suppression system in place on your incoming mains supply down to the PDUs. This should be in situ anyway, but some people don’t invest in this, even though it’s not very expensive.

2-21“With a proper surge suppression system the mains is fine for the load. There is not a server manufactured in the world that says you have to use UPS. There are servers that are built to the ITIC curve, the old CBEMA curve, which implies you have to have UPS. But with a dual-cord load you can have UPS on one and mains on the other.”

As the UPS control system intelligently manages eco mode operation, degradation of UPS performance should necessarily be a factor. “The UPS system decides if the mains supply is stable enough and if it is, for example after half an hour, then eco mode kicks in. The first time there is a glitch it goes back into double conversion and stays there until the mainstream is stable for a while. You don’t get constant flicking backwards and forwards, which would be bad for the UPS.”

Bitterlin also disagrees with Rasmussen’s belief that eco mode will necessarily lead to a loss of power protection. “By running in eco mode you can switch back to inverter load within four milliseconds. A capacitor at the front of the server gives some ride-through, typically 20 milliseconds rule for the latest ITIC curve and longer at partial load. So for eco mode you’re talking about the difference between 4 millisecond switching and a 20 millisecond immunity.”

Eco mode, however, is not for everyone. A bank or continuous trading organisation, for example, may never use eco mode. For Credit Suisse, for example, the few percentage points gained in efficiency, while not insignificant, pales in comparison to their turnover of £400 million (US$620 million) per minute. 

It may be particularly suitable for companies that trade only during the day and operate only a website and e-mail at night to programme eco mode to operate through the night. “If it was my data centre and my load I would use it,” says Bitterlin. “I believe in the technicalities and I don’t see any reason why it would fail.”