An Internet search of the topics covered in power quality courses or seminars will likely show that the vast majority of the information disseminated relates to alternating current (AC) circuits. At first glance, that seems logical; even though Thomas Edison may have wanted otherwise, George Westinghouse won out, and the power delivered from the electric utility companies and even backup generators is AC voltage.
Power quality courses, whether offered by educational institutions, consultants or equipment vendors, usually include discussions on monitoring and recognizing sags, swells, transients, harmonics, interharmonics, flicker and frequency variations, which are all AC phenomena. Some of the more advanced classes cover noise, poor grounding practices and electromagnetic fields and higher frequency forms of interference. Mitigation of these AC problems includes discussion on wiring changes, filters, isolation transformers, uninterruptible power supplies, backup generators, and transient voltage surge suppressors, with all but the last one being AC-only solutions.
But AC does not power all equipment. The telecommunications industry has used direct current (DC) power sources for years. There is even a growing movement within the larger web and data server facilities to distribute the power to the racks as DC. The internal power source of many loads today, especially all of those termed “electronic loads,” convert the AC to DC to be used within the equipment. Most chargers supplying laptops, video cameras, and even flashlights output DC voltage, which is often used to refresh the internal batteries. Since the power quality courses talk about power quality phenomena on the AC side only, does this mean DC is immune?
The answer to that is absolutely not. Often, the power quality phenomena affecting the AC circuits makes its way into the DC power system, which causes a problem with the equipment’s operation. So why isn’t there emphasis on DC aspects of power quality problems? The lack of emphasis can be partially attributed to the general assumption that DC-rated power quality issues are the utility’s fault, even though there is the often-quoted statistic that 70–80 percent of the power quality problems originate on the customer side of the meter. Most people would not take apart a piece of equipment to modify it (even if they had the skillset to do so), as that would void the warranty. However, knowing how the low-voltage DC circuitry operates can help determine the best way to make the equipment be no longer susceptible to what typically occurs in the AC realm of the electrons.
The figure above illustrates the schematic of a typical single-phase rectified input switching power supply. The AC voltage at the 120V outlet (V line-to-V neutral) is reduced down by a transformer to a value (V2) slightly larger than the maximum output voltage (V5, V6, V7) from the DC-to-DC converter, then rectified and filtered (V3) to smooth it out. A three-phase rectifier, used in adjustable speed drives and other similar equipment, would convert the three AC phases in sequence using silicon controlled rectifiers (SCRs) or thyristors. Depending on the equipment of product, either V4 and/or V8 may be referenced to the equipment-grounding conductor.
If a sag occurs, the voltage (V2) out of the transformer will be proportionally reduced. If V2 is less than V3, the diodes will not conduct the electricity, so no current will flow into V3 to recharge the capacitor (C). As the DC-to-DC converter draws more power out of the capacitor, the voltage will decay to a point where the converter can no longer function properly if the sag lasts long enough. The result will be that the output voltages (V5–V7) will no longer be at their proper steady-state levels, and the circuitry that relies on that may start to malfunction. What is a “1” or a “0” for digital circuitry, such as memory and microprocessors, may get confused and not use the proper value.
For analog circuitry, such as the type used in process-control transducers, false signals that don’t represent the true state of the system will be given to the controller. A voltage transient occurring either between V line-to-V neutral or V line-to-ground or V neutral-to-ground, can get through to the output voltages and either cause a misoperation or damage to the semiconductor parts if it is large enough. Likewise, a swell can damage the DC-to-DC converter if it is large enough and lasts long enough.
But there is also a path from the communication and other interfaces on the DC voltage side; noise, transients and electromagnetic interference can get in and cause similar problems with alarm and security systems, communications and microprocessor operation in general, even when the AC side is functioning properly. A future article will cover how they get in and what damage that they can do if not properly protected.
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.