Oldies but goodies
Sometimes in the rush to find the latest and greatest solutions to power quality problems, we forget about some of the older technologies that have helped facilities managers tame their cranky electrical systems for years. These tools did their job and they can still be useful in certain situations today. This month, John DeDad, EC&M's editorial director, dusts off the PQ toolbox and gives us all a history lesson.
Q. While conducting an on-site inspection for an IT facility renovation, we ran across a piece of equipment our client called a “magnetic synthesizer.” It was part of the original power distribution system. What is it and what function did it provide relative to the IT room power supply?
DeDad's answer: Magnetic synthesizers were used to protect against oscillatory transients and sags and swells. Some manufacturers still have magnetic synthesizers in the product line. When equipped with a lightning arrestor accessory, they protect against voltage surges. They don't protect a critical load against a complete power outage, but via a limited stored energy capability using capacitors, they can provide ride-through, typically to one cycle. Battery backup capabilities permit additional operating time.
These devices use ferromagnetics (pulsed transformers and inductors) and capacitors to synthesize a high-quality 3-phase output, using only the input as a source of energy. The output is fully isolated from imperfections in the input power. They can also maintain a reduced capacity of 3-phase power if one input phase is lost, but the angle between output phases won't be 120°.
The only moving parts and semiconductor devices are located in their controls. A typical installation would include a synthesizer combined with a static switch and a reliable, synchronized alternate power supply, such as a second utility line feed. The static transfer switch would transfer to the alternate source in about a quarter cycle, providing uninterrupted power to sensitive electronic equipment. This would eliminate the need for a UPS.
A magnetic synthesizer typically has the following operating characteristics:
It can operate with an input voltage range as low as ±40% or more of the nominal voltage.
Output power factor remains in the range of 0.96 or higher from half to full load.
Because it regenerates an output voltage waveform, output distortion, which is typically less than 4%, is independent of any input voltage distortion, including notching (Figure above).
Efficiency at full load is typically in the range of 89% to 93%.
Minimum maintenance is required beyond annual replacement of failed capacitors. Redundant capacitors built into the units allow several capacitors to fail between inspections without any noticeable effect to the device's performance.
Output voltage varies about 1.2% for every 1% change in supply frequency. For example, a 2-Hz change in generator frequency, which is very large, results in an output voltage change of only 4%, which has little effect for most loads.
It accepts 100% single-phase switch-mode power supply loading without any requirement for derating, including all neutral components.
Input current distortion remains less than 8% THD even when supplying nonlinear loads with more than 100% current THD.
Q. We've experienced voltage variations at our facility that have caused equipment to malfunction and our manufacturing process to unexpectedly shut down. The installation of voltage regulators has been mentioned as a possible solution to our problem. Are constant voltage transformers considered voltage regulators? If so, can we use them to counter the voltage variations coming from our utility supply?
DeDad's answer: Although the constant voltage transformer (CVT) represents the oldest form of regulation, it's simple and works well. Basically, a CVT supplies a stable output voltage through a range of current loadings within the rating of the CVT. Sometimes referred to as a “saturable magnetic device,” the CVT is a ferroresonant transformer with a core designed to saturate. In other words, a CVT accepts energy into its core, flooding it in such a way that the output is a flat voltage that's stable across a range of input load currents, from light load to heavy load conditions.
That said, you must be careful with its application because the CVT is a product of its time. In other words, its design and application are based on sine wave load characteristics dating to a time when harmonic interaction wasn't yet a prominent phenomenon, as it is today.
Using a CVT has some major drawbacks. First, you can't apply it in an environment with a high harmonic load spectrum unless you considerably oversize it or have the manufacturer include design modifications that ensure harmonic content compensation.
Also, the load range of a CVT has an upper limit. If you exceed that limit, the CVT may actually slowly reduce its output to zero in order to self-protect the circuit. You can avoid this performance trait, known as current limiting effect, by sizing the maximum load the CVT can handle to as much as 80% of the full-load rating of the unit. This oversizing also helps in handling any harmonics present that haven't been compensated for by the CVT manufacturer.
Finally, a CVT's efficiency will be low if its loading is very light. Remember, a CVT uses energy to saturate its core. Normal efficiencies for fully loaded units should be in the high 80% range, and even as high as 92%. On the other hand, efficiencies under light loads may drop down to between 45% and 50%. If you size the load somewhere in the middle, a CVT will operate under most conditions, giving up some efficiency points to an average of between 75% and 80%.
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