Eco-mode: Benefits and risks of energy-saving modes of UPS operation
This paper focusses on a particular technique for improving the operating efficiency of a double-conversion UPS by operating it in eco-mode. The author also discusses situations where eco-mode is recommended or is contra-indicated.
Eco-mode is a method of operating the UPS at reduced power protection in order to obtain improved electrical efficiency and save energy, and is marketed by vendors under a variety of names. In his article (full copy downloadable here) Neil Rasmussen covers the following subjects: What is eco-mode, and how does it work? What efficiency gains are possible and expected? Loss of protection and reliability associated with eco-mode; and operational considerations.
Eco-mode will save energy, although the amount saved is surprisingly small. In addition, the energy saved comes at some reduction in electrical protection and reliability. This penalty will vary depending on the design of the data centre and on the UPS vendor’s specific design approach used in implementing eco-mode functionality. Some data centre operators will decide the energy savings are worth the risks and potential problems, and some will not.
Eco-mode is very similar to the basic mode of operation used in an offline UPS where it is called “standby” or “line interactive” mode1. In these traditionally smaller, single-phase UPSs, the inverter/rectifier circuits are “offline” or are not part of the normal power path. In normal operation, therefore, the load is exposed to essentially raw mains power from the Utility. This mode of operation is similar to what occurs with a double-conversion online UPS that is operating in traditional eco-mode.
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. (Fig. 1)
Fig. 1: Simplified diagram of power flow of a double-conversion online UPS.
There are two main paths that can supply the load, the online (double conversion) path and the bypass path. Note that the load is connected to the raw, unconditioned mains power when the bypass is active.
In online mode, the UPS continuously regenerates the output voltage. 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 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 as explained in the full paper.
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 to 5% in UPS efficiency when eco-mode is used.
The cost 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. 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 explained later in this article. An example of an output voltage waveform of an actual UPS with eco-mode responding to a power failure is shown in Fig. 2.
Fig. 2: Output voltage waveform of a 275 kVA UPS in standard eco-mode responding to a power failure. Upper waveform is output voltage, lower waveform is output current. Note that this vendor claims a detection and transfer time response of 1,2 ms to a power failure – performance that is clearly not attained in this example.
It is important to note that different vendors implement eco-mode differently. There are variations in the way the system operates the standby inverter. There are 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 patented ways to control their transfer switch. But they all have the same basic concept of exposing the IT load to unconditioned power and transfer events in return for a small percentage gain in efficiency. 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 minimised.
Advanced eco-mode
In standard or “classic” eco-mode, as described above, the UPS 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 as shown in Fig. 2. 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 UPS. 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.
Advanced eco-mode can offer other advantages over standard eco-mode. A known feature of double-conversion UPSs operating in online mode is the ability to shield the utility mains supply from harmonic currents generated by the loads. This reduces the harmonic injection back into the utility mains or other source such as a standby generator. In the case of generators, lower harmonic currents will reduce voltage waveform distortion, and may allow the generator to operate reliably at a higher kW load. This “filtering” function of a double conversion UPS is not present in standard eco-mode as the load current (including any harmonics) is passed directly back to the input. However, in advanced eco-mode the inverter is on and connected to the output and can be controlled to absorb the harmonic currents of the loads even though it is not processing the load power. In principle, this harmonic filtering function could filter harmonics to nearly the same level and quality as the filtering provided by a double-conversion UPS not operating in eco-mode.
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% to 1,0% range typically. So while advanced eco-mode efficiency is less than standard eco-mode, it is still better than traditional online mode.
In the paper the author discusses PUE, responding to power problems, harmonics and typical UPS subjects, including battery life.
Contact Ntombi Mhangwani, Schneider Electric, 011 254-6539,
ntombi.mhangwani@schneider-electric.com
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