When most people think of the source and effects of power quality problems, they think of voltage interruptions, harmonics, transients, sags, swells and even flicker, all of which are phenomena that occur on the alternating current (AC) voltage system. In fact, harmonics and flicker can only be calculated on AC systems, but problems aren’t unique to AC. Most electronic equipment today runs internally on direct current (DC) voltage, and some new programs have the goal of powering the entire data center from DC.
Process control systems and alarm systems, especially with battery backup, often run at 24 volts (V) DC or lower. The newer integrated circuits, used inside information technology equipment, such as PCs, keep getting lower in amplitude. What was the standard of +/–12V DC and +5V DC logic inside of equipment just a couple decades ago is now more like 2.8V DC or 1.3V DC for microprocessor and I/O components powered at 3.3V DC. Lower voltage levels can mean disturbances and other power quality phenomena that had no effect on the system’s operation before can now cause misoperation or even damage to newer equipment.
In one instance, an equipment vendor was having problems with particular units in the field that were from second-generation designs. Customers using the equipment were becoming quite unhappy with the lost production and inability of the vendor to find any source of the problem. The vendor had put a power quality monitor on the incoming AC power to the equipment and didn’t find any significant power quality issues. Back in the vendor’s manufacturing facility, they were able to use a power quality monitor that had measuring inputs that were “DC coupled” for voltage and current. This means it could accurately measure either AC or DC signals or both simultaneously. Data showed the internal power 24V DC bus with significant drop-outs or interruptions, even while the line-to-neutral voltage didn’t indicate anything. As it turns out, the designers of this particular equipment had modified the grounding scheme between the power supplies and the AC inputs, which resulted in a shutdown of the power supply despite a valid AC signal.
Another facility had problems with fire alarm system malfunctions that resulted in long process interruptions. In this particular facility, the operation required scrubbers to cleanse exhaust fumes from the process. When the AC power was considered to fall below adequate levels, the system would shut down, as there was no backup power to keep the scrubbers running. Unfortunately, summer thunderstorms were all that it would take for the alarm system to think there was a problem and trip the plant offline. Monitoring of the alarm signal as the trigger to record the AC input signal showed that there were some transients getting through from the AC side into the low-voltage DC side, though the transients weren’t large enough to affect any of the rest of the equipment. An appropriate filter eliminated the problem.
Process control sensors can also be susceptible to power quality phenomena that may not affect other equipment, such as the programmable logic controllers (PLCs). It is often the computer or information technology-type equipment that people think of as being vulnerable to such on the AC side. But disturbances that couple through to the DC side, or have an effect on the grounding system so as to change the reference for the process control signals, can result in misinformation to the PLCs or other control circuits. The controller takes what would be an appropriate response to the stimulus, but since it isn’t a real situation, it may be an overreaction, which can trip the system offline.
Even the phone system is primarily low voltage (except for the ring signal). A –48V DC signal drops down below 10V when a phone or modem goes off the hook. On this DC signal rides the millivolts of the voice or data information. Telephone influence factor (TIF) is a parameter that evaluates the effect of harmonics that are coupled into the phone lines, degrading the quality of the signal. With the increased use of fiber optic cables, this is becoming less of an issue.
Monitoring at the misoperating equipment with the appropriate measurement capabilities for the way that power is used within the equipment can save time and money in getting the process back to running smoothly.
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.