Power Conditioning . . . What Does It Really Mean?

Bradford P. Roberts, P.E.
Director of Marketing—Power Quality Products Division, S&C Electric Company

The need for high-quality power continues to grow rapidly. With this growth has come a rapid increase in the types of equipment targeted to improve power quality.

There are significant challenges in choosing the most effective power quality solution for a particular application, so it is critically important to understand the characteristics of power disturbances, as well as the power input requirements of the equipment to be protected.

One of the most effective power quality improvement solutions to date has been uninterruptible power supply systems. There are two main designs of UPSs: on-line and off-line. On-line uninterruptible power supplies are so named because they operate continuously to rectify the ac source voltage from the electric utility to dc and then invert the dc voltage back to ac to serve the critical load. This process is called double conversion, and the power produced is often referred to as “conditioned.” As will be discussed later, early computers required such conditioned power . . . and because of this hard-to-break precedent, on-line UPSs still constitute the majority of three-phase installations protecting large critical loads such as data centers.

Off-line uninterruptible power supplies such as the PureWave UPS™ System operate in a standby mode. The utility source directly powers the load under normal conditions. There is no continuous rectification of the utility source or inverter operation to create an ac voltage. An off-line UPS operates only when the utility source deviates from specified values; under such conditions, the PureWave UPS System supplies a clean sine wave to the load within 1/4 cycle typically.

What is the Role of a UPS?

The primary reason to apply an on-line or off-line UPS is to protect the critical load from significant variations in the voltage supplied by the utility. Voltage sags represent over 98% of all power quality problems according to the Electric Power Research Institute Distribution Power Quality Study. Momentary outages are next most common, followed by long-term outages. Voltage swells can also be a problem but are typically not a significant concern. A voltage sag or swell is defined as a voltage that is more than 10% below or 10% above nominal system voltage for one cycle, respectively.

Just how sensitive to voltage variation is computer equipment? Figure 1 illustrates the Information Technology Industry Council (formerly CBEMA) power acceptability curve. The ITIC curve provides a voluntary guideline to which most electronic devices are now designed. It defines the magnitude and duration of sags and swells in the steady-state voltage which devices should tolerate, and includes a requirement that equipment operate even with a 20-millisecond voltage drop to zero. Both on-line and off-line UPS systems will easily assure that the voltage provided to the critical load is per ITIC guidelines.

Information Technology Industry Council (formerly CBEMA) Power Acceptability Curve
Figure 1. Operating domain of PureWave UPS System.

The crosshatched band in Figure 1 illustrates the operating domain of the PureWave UPS System versus the ITIC curve. If the RMS value of the utility source voltage deviates more than 10% from its nominal value, the PureWave UPS System will transfer the load to the battery source in approximately 2 to 4 milliseconds. Voltage is maintained within ±5% of the pre-sag voltage while the UPS is running.

Both on-line and off-line UPSs provide the same protection from these voltage excursions. The only difference between the two designs in responding to these voltage excursions is in the transition time to switch from the utility source to the battery source. In redundant systems where a PureWave UPS System has been installed ahead of an on-line UPS System, it’s been seen that the PureWave UPS System responds to utility voltage variations that the on-line UPS does not react to. The patented algorithm employed in the PureWave UPS System responds so quickly and the output of the PureWave UPS System is of such high quality that there is often no need to utilize the stored energy in the downstream on-line UPS system. Battery life in the on-line UPS System should thus be significantly increased in these redundant applications.

Power Conditioning

There is a great deal of confusion on the issue of power conditioning. Some UPS users believe they require power conditioning. But rarely do UPS users agree on what power conditioning means or why they need it. Some definitions are in order.

In the most general sense, “power conditioning” refers to control of voltage deviations. Voltage deviations that are typically thought to be addressed by power conditioning include steady-state voltage regulation, voltage waveform distortion due to harmonics, and fast voltage transients. Let’s discuss what all this means.

Voltage Regulation

In the early days of computer systems, gradual changes in the utility source voltage created problems for the computer power supplies. This led to the school of thought that very closes voltage control is essential to computer operation. But times have changed and technology has greatly improved. Computers and servers now use “smart” switch-mode power supplies that are quite tolerant of gradual and small changes in the source voltage. Precise voltage regulation is no longer essential. It’s an expensive legacy to continue to carry forward.

Not only have computer power supplies improved tremendously, but so has the voltage regulation of utility sources. More than 99.99% of the time, the typical utility source is within the range of 114 to 126 volts (120 volts ± 5%). Since ITIC-compatible equipment is required to have a long-term voltage tolerance range of 108 to 132 volts (120 volts ± 10%), the typical utility source is well within this range.

The bottom line is that the voltage regulation provided by on-line UPS systems is not required for critical applications. Nevertheless, some users still believe that on-line UPSs are needed for voltage regulation in data center applications. There is a very steep price to pay for this misperception. The 7 x 24 double-conversion process of on-line UPSs is much less energy-efficient than off-line UPSs. Energy efficiency for on-line UPSs is typically 92 to 94%. But once the energy expended for air conditioning is factored in, the overall energy efficiency of on-line systems averages 90 to 91%. A PureWave UPS System, on the other hand, has a total energy efficiency of 98.5 to 99.5%. The incremental cost savings of a PureWave UPS can amount to hundreds of thousands of dollars over the life of the system.

On-line UPSs also have a price in terms of reduced reliability. In on-line UPSs, the batteries are always “in the circuit.” This exposes the batteries to ripple from the dc bus and also makes it impractical for the batteries to be properly equalized charged. It’s been well documented that battery problems account for up to 70% of all on-line UPS failures. But with its off-line design, the PureWave UPS System does not have this weakness. Field experience has shown a mean time between failure (MTBF) of 500,000 hours for PureWave batteries versus less than 20,000 hours for the valve-regulated lead-acid batteries typically used in on-line UPS systems.

Harmonic Distortion

The 60-hertz (or 50-hertz) voltage waveform can be distorted by the presence of higher frequencies commonly referred to as harmonics. Sensitive electronic equipment operates best when the total harmonic distortion (THD) of the source voltage is less than 5%, and no single harmonic exceeds 3% of the total voltage.

Utility power sources rarely provide high harmonics. Detailed studies by the Electric Power Research Institute, based on measurements taken at 300 utility distribution sites over a two-year period, show that a typical utility distribution system provides power with an average 1.6% THD. See Figure 2.

Results graph.

On the other hand, on-line UPSs create voltage harmonics in the output due to the characteristics of the inverter. For a 100% linear load, on-line UPS systems are rated to have an output voltage THD of 5%, with no single harmonic exceeding 3% of the total. THD is generally higher for non-linear loads. On-line UPSs thus typically generate more harmonics than are provided by the utility source. With an on-line UPS, the critical load receives a steady diet of harmonics . . . 24 hours a day, 365 days a year.

But the PureWave UPS System generates no harmonics the vast majority of the time, as it is off-line (i.e., not “running”). The critical load is served directly by the utility source with the very low harmonic content noted above. For the extremely brief time that the PureWave UPS System is running (typically less than 1 second), it provides power with a voltage THD of 5% or less, and no single harmonic exceeding 3% of the total. Thus, the combination of utility power and the PureWave UPS System results in the critical load receiving a higher-quality sine wave, with much lower total harmonics, the vast majority of the time.

Transient Events

There are two principal categories of transient events: impulses and oscillatory transients. Severe impulses, or spikes, can reach thousands of volts and last less than ½ cycle—well outside the ITIC voltage envelope. Such impulses can damage sensitive equipment. Oscillatory transients are generally of lower magnitude but can last for several cycles.

For most transients, on-line UPSs provide some level of protection. The battery energy storage, being constantly on-line, provides voltage transient “blocking.” But even though on-line UPSs provide limited transient protection, they are almost never supplied without additional transient protection. Typically a transient voltage suppression system (TVSS) is provided to protect the UPS and the critical load. The obvious question is why pay for a TVSS if an on-line UPS can inherently provide transient protection?

The answer is two-fold. While an on-line UPS can filter transients from the load, the UPS itself is susceptible to significant damage due to transients such as lightning. In the event of a large transient, an “unprotected” on-line UPS can be severely damaged which, of course, places the critical load at risk. There is also concern about the static bypass switch. A transient such as a lightning strike can short the normally open SCR switching devices in the static switch, exposing the on-line UPS to an out-of-phase, short-circuit condition when the utility source voltage returns. To protect against these risks, TVSS equipment is installed ahead of on-line UPS systems.

Off-line UPS systems such as the PureWave UPS System do not inherently provide transient protection to the critical load. It’s therefore general practice to install a TVSS system to protect the critical load from transients. Both on-line and off-line UPSs require TVSS protection. In this respect, there is little difference between the two technologies.

Power Conditioning in Summary

Off-line UPS systems are clearly the future. While in the early days of the computer age there were benefits to double-conversion, on-line UPSs, technological advancements and economics now dictate that off-line UPSs are a better approach.

Off-line UPS systems provide the same level of protection as on-line UPS systems in meeting the primary mission of a UPS . . . protecting the critical load from significant source voltage deviations such as sags, outages, and swells.
The combination of modern computer power supplies and highly reliability electric utility source power has eliminated the need for the voltage regulation provided by double-conversion, on-line UPSs.
The voltage supplied by off-line UPSs is typically of lower harmonic content (much higher quality) than the voltage supplied by on-line UPSs.
Although off-line UPS systems can provide some protection from voltage transients, TVSS is still required. Since TVSS is a requirement of both on-line and off-line UPS, there is no difference in either the quality or cost of the transient protection provided to the critical load.

With the question of power conditioning clearly understood, the advantages of a PureWave UPS System are readily apparent.

PureWave UPS Advantages

The PureWave UPS System offers much higher energy efficiency and lower operating cost than on-line UPS systems. The savings can be in the hundred of thousands of dollars over the life of the system.
The PureWave UPS System provides a major leap forward in reliability by correcting the Number 1 cause of on-line UPS problems: battery failures. With its unique off-line design and carefully engineered battery management approach, the PureWave UPS System provides outstanding battery life and reliability. The batteries are routinely equalize charged and protected from exposure to ripple on the dc bus—a common problem with on-line UPS systems and a major contributor to battery failure.
The documented Mean Time Between Failure (MTBF) for PureWave UPS batteries is over 500,000 hours versus less than 20,000 hours for the valve-regulated lead-acid batteries of on-line UPSs. And PureWave UPS batteries are provided with a full-replacement, five-year warranty.
The PureWave UPS System can be sized for loads from 250 kW to 12,000 kW. It can be applied at voltages from 480 volts through 38 kV and offers the only true medium-voltage UPS. It is well suited for whole-facility protection. It is also easily expandable to provide for future load growth.
The PureWave UPS System is the only UPS truly designed for outdoor application as well as indoor. It eliminates the need for very costly interior space.
The PureWave UPS System does not require an over-sized backup generator. There is no need to size the generator to accommodate the harmonics of the rectifier, higher battery charging currents, and higher energy losses associated with on-line UPSs. The generator can typically be sized at 105% of the load instead of 150% of the load (much less costly), as is necessary for on-line UPSs. The PureWave UPS System also offers true “soft loading” of the generator during an electric utility disturbance.

The PureWave UPS System is thus the power quality protection system of the future. Contact your local S&C Sales Office for more information