Last month, we covered the processes of site power quality (PQ) benchmark or compliance audits and facilitywide energy surveys. Let’s go back to the original usage of PQ monitors and what they were designed to do: troubleshoot possible PQ-related problems that result in process interruptions, whether they shut down a single PC, a paint-spray booth or an entire production line. Before we break out the monitor, we need to review the process.
The overall troubleshooting process follows these steps, whether it is being done on a facilitywide or individual equipment basis: plan/prepare, inspect, monitor, analyze, determine and implement solution, and repeat process if necessary.
It is important to identify what problems the facility’s equipment is experiencing prior to starting a monitoring program. Is the problem specific, such as motor failures, equipment or breakers tripping offline, capacitor or filters being damaged by harmonics? Or is the problem more unknown, such as problems with the power-conditioning equipment; the old equipment ran great, but the new equipment has been nothing but one headache after the other; or the equipment has intermittent resets when running off the standby generator? Sit down with the facility manager, find out who the various operators and staff electricians are with knowledge of the process and problems, and let them describe what they know. What things happened at the same time, and did they happen during the same day or weather conditions?
When multiple locations in the facility have exhibited problems that may be attributed to PQ phenomena, it usually is best to start monitoring at the point-of-common-coupling (PCC), where the utility service meets the facility circuits. The monitor should be programmed to gather a wide range of phenomena until the source can be narrowed down. Monitoring should last at least one business cycle, which is how long it takes for the different processes to cycle around and is usually one week.
If possible, put multiple monitors at various distribution panels or power distribution units throughout the facility to help track the source faster. Instruments often have the ability to sync their clocks together using network protocol or global positioning satellites’ time sources. This will help get a better picture of how things propagate through the facility.
Did the PQ problem originate upstream toward the source of electricity or downstream toward the loads? This usually is one of the first questions that must be answered. By starting at the PCC, Ohm’s and Kirchoff’s Laws can be used to determine directivity of disturbances. To determine directivity if a sag is recorded, compare the voltage decrease to the change in the current. Generally, if the current increases significantly when the voltage decreases, the origin is downstream toward the load. A large increase in current will result in a larger voltage drop across the source impedance, leaving less voltage for the load. Observing what happens after the initial change in state can further narrow down the source. If the current decreases steadily in an exponential fashion to the steady state value, and the voltage recovers somewhat also exponentially toward the nominal value (rather than changing abruptly or linearly), then a large motor was likely the source.
If it is upstream, then call the PQ engineer at the electric utility, who will have access to the PQ monitoring systems at a distribution substation. If downstream, the voltage sag usually is accompanied by little change in the current, or the current reduces significantly or increases slightly. Then the origin is probably upstream toward the source of electricity. A linear load, such as a resistive heater, will have the current decrease proportionally to the voltage drop. A constant power device will increase the current slightly to make the watts constant. A rectified input switch mode power supply, such as those found in most electronic equipment will have the current reduce significantly, even to zero, since the direct current (DC) filter capacitor will have a higher voltage on it than the alternating current (AC) input, and no current will conduct through the semiconductor devices (diodes) until that DC voltage on the capacitors draws down from its load, as shown in the figure, occurring at nine cycles into the 16 cycle sag.
If the direction has been determined to be downstream, the monitor should be moved to the next distribution panel closer to the suspected source. Though it may seem somewhat contrary, most problems originate from within the facilities, as shown in sources, such as an EPRI paper, “Investigating PQ Problems” by Allen Morinec, at the PQA conference: “Wiring and ground errors and disruptive loads within a facility cause up to 80% of all power quality problems reported by customers.”
Next month’s article will take us further downstream to troubleshooting at the equipment itself.
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.