May/June 2010

Power conditioning: Learn methods, techniques, tradeoffs for effective implementation

By Michael A. Stout

The term power conditioner has been used to describe such a wide range of devices it has been rendered useless. Researched on the internet, the price of a power conditioner can range from less than a dollar to more than the price of a new car. Selecting the right power conditioning device requires understanding the potential power pollution level of the power source and amount of protection the connected equipment demands. As a general rule, the price of the conditioner selected will be dictated by the level of power conditioning provided.

This broad topic can be simplified by describing the various conditioning and protection elements that might be incorporated into differing classes of power conditioning products. The elements may be employed as the sole function of the device or be present in differing combinations. This article will keep the scope limited to power conditioning products under 10kVA, however much of the discussion will be applicable to larger capacity products as well.

Surge protection

When most home computer users think surge protection, they envision a surge protected plug strip. Inside the plug strip a simple electronic component called a Metal Oxide Varistor (MOV) has been installed across the line and neutral wiring. When the MOV detects a short, millisecond range AC voltage that is too high, it turns on in an attempt to clamp the voltage to protect the connected equipment. When the voltage returns to an acceptable level, the MOV turns off. Should the high voltage condition be sustained too long, the MOV will become damaged, typically becoming shorted or burned. This opens its protection fuse or circuit breaker. The surge protected plug strip represents the most basic level of power protection.

MOVs are also used in other power conditioner topologies and uninterruptible power supplies (UPS) to supplement their level of protection. They are available in differing clamp voltages and energy clamp ratings, typically stated in Joules. The higher the Joule rating, the more energy that can be clamped without the MOV failing. The Joule rating alone can be misleading, as it is not the only parameter that determines how effective the MOV will be at protecting equipment from voltage surges. An MOV will not protect against a direct lightning strike or major power events. To protect against these events, high level Transient Voltage Surge Suppression devices must be installed inside the building's main electrical panel. 

Passive filtering

Passive filtering, when employed in a power conditioning product, is used to remove high frequency noise coming from the utility power source. It may also be generated by other equipment operating nearby, emitted from their power plugs conducted through the buildings electrical system. The operation of sensitive scientific measurement equipment is often adversely affected by this type of electrical noise. Unless the amount of high frequency noise is very high, most computer-based equipment is not affected. Should the level become high enough to cause a problem, it is best to eliminate it at the source of the problem.  Low-cost power conditioning devices often incorporate some form of passive filtering along with MOV surge protection. In these low-cost power conditioning devices, it is often determined the surge protection provides more value than the passive filter circuit. When used in higher level power conditioners, passive filtering reduces the level of high frequency noise.

Galvanic isolation

When a power conditioner specification states it incorporates galvanic isolation, it has an internal isolation transformer. This transformer typically has a primary winding connected to the incoming utility power. Its secondary winding is magnetically isolated from the primary. The secondary winding provides power to the output of the conditioner. Both windings are wound around a steel core. The magnetic isolation breaks the path for unwanted common mode currents typically created by other equipment being powered from the same building electrical panel.

Because there is isolation, one side of the secondary winding can be connected to ground, creating a new or "derived" Neutral at the conditioners output.  The common model noise level at the conditioners output will be very low to nonexistent. The transformer is also excellent at blocking unwanted high frequency noise. Galvanic isolation adds a desirable layer of power protection to the conditioner. It also adds to its cost, size, and weight.

Voltage regulation

A power conditioner that does not incorporate some form of output voltage regulation gives very limited protection. Microprocessor-based equipment can become unreliable when operated from utility line voltages that are too low or high.  For example, the domestic nominal utility line voltage found in most offices, labs, and hospitals is 120 VAC. As such, computers and equipment have been designed to perform at their optimum level when powered from 115-125 VAC and have a reasonable amount of immunity against momentary power disturbances. Should the voltage drop to a sustained 100-105 VAC, the amount of stored energy in their internal power supply circuits can become too low and make the equipment sensitive to a much smaller magnitude power disturbance. Internal power supply circuitry can also overheat, resulting in premature equipment failure. Sustained or intermittent high utility voltage levels may also result in equipment malfunction or failure. 

Tap-switched voltage regulation

A power conditioner that incorporates tap-switched voltage regulation typically uses a transformer having multiple taps on its primary winding. It also includes voltage sensing and switching circuitry. The transformer used may or may not have galvanic isolation. When the voltage sense circuit detects low incoming utility voltage, it automatically switches to another transformer tap and increases the output voltage to the connected equipment.  Other taps are selected to reduce the output voltage should the utility voltage become too high. 

Tap-switching is a lower cost method of implementing voltage regulation into power conditioners and UPS. The output voltage regulation specification for the conditioner may range from ±25% to ±5%. The more taps, the better the regulation. Voltage regulation is not executed in a smooth continuous manner, but made in abrupt steps. Inexpensive tap-switched designs can create 4 to 25 millisecond voltage drop outs at the conditioner output during tap changes. High voltage transients may be generated during tap changes. When tap-switched voltage regulation is included with some or all of the previously discussed elements, a medium grade power conditioner results.  The conditioner would be effective when used in locations having good power quality, as long as equipment is connected that is not sensitive to momentary power dropouts.

Double-conversion

A double-conversion power conditioner takes a unique active approach that is very effective in protecting against a wide range of power problems.

First, the incoming alternating current (AC) utility power is converted to direct current (DC) and heavily filtered. This immediately removes unwanted high and low frequency noise, harmonic distortion, and frequency drift problems associated with generator power sources. The filtered DC is then regulated, giving the conditioner a wide operational input voltage range (±25% typical). The regulated DC is then used to power a pulse-width modulated inverter stage, which regenerates new, clean, true sinewave AC power with less than 3% total harmonic distortion. The new AC power voltage regulation is smooth and continuous (≥ ±2% typical) over its entire wide input voltage range.

MOVs are typically installed across the incoming and outgoing AC power for added protection. This type of conditioner may have galvanic isolation as an option. In this design, there are multiple stages of electronics between the incoming utility power and the connected equipment. In the event a major destructive power event does occur, the connected equipment is typically undamaged.  

The level of power conditioning and protection with this type of design is impressive, as they eliminate the greatest number of power problems, providing the highest level of protections. For example, some models can even act as a 50 or 60 Hz frequency converter or provide UPS battery backup. This is far beyond the capability of many power types of power conditioners.

Power conditioning needs are as broad and diverse as the number of products on the market. If you know the exact cause of the problem, select the level of conditioner that will best address the problem. Frequently, the causes of power-related equipment malfunctions and failures are illusive. A trial-and-error approach to these types of problems leads to unnecessary frustration, cost, and lost productivity. The best solution is selecting a conditioner designed to solve the widest number of problems out of the box.

ABOUT THE AUTHOR

Michael A. Stout, vice president of Engineering at Irwindale, Calif.-based Falcon Electric, Inc. (www.FalconUPS.com), is an authority in the computer and industrial automation, power conversion, and UPS industries with nearly two decades of experience in critical power systems. In his current position, Stout specifies and designs new UPS and critical power system products and evaluates emerging technologies. Contact him at mstout@FalconUPS.com or by calling 800-842-6940.