Reviewing data files from power quality (PQ) monitors to find evidence pointing to the cause of a customer’s problems may sound simple enough for someone who has worked in the industry for more than 35 years. However, it is often a challenge to have the customer provide the information necessary to determine what is what.


The amount of harmonic current distortion may be a problem for one site but not for another. The voltage notching may damage the components on one piece of equipment over time but have minimal effect on another. A production line might tolerate sags to 75 percent of nominal for 25 cycles, while another line trips off with just a sag to 85 percent for four cycles. 


To analyze the data specifically from your facility or relative to a particular piece of equipment, the PQ expert will need information only you can provide. Knowing what the problem was, when it occurred, how often and what else was going on at the time will point the analysis in the right direction. Otherwise, the audit report will be a generic type analysis of “typical” PQ phenomena levels. 


A PQ site audit form, similar to the sample above, is a good starting point to help the person analyzing the data know what to look for and whether what he or she sees is a problem. NFPA 70B, Recommended Practice for Electrical Equipment Maintenance, Annex H, provides similar forms. 


A one-line diagram of the facility wiring and monitor location is extremely helpful. In the site audit sample, the monitor location was unknown. It seems highly unlikely that whoever connected the monitor didn’t know where they connected it. Whether it was at the point of common coupling (PCC), at a distribution panel on second floor, or only on the circuit that fed a single adjustable speed drive can make a big difference when interpreting the data.


For example, it is generally unusual to find a significant amount of higher order (above 20th) harmonic current at the PCC in a large facility. This is because the phase angles of the different harmonics produced by different pieces of equipment are not in phase with each other, and they cancel each other out when summed together at the service entrance. However, it is not unusual to find if monitoring the feed equipment that has a 24-pulse converter in its power supply.


The type of equipment on the circuit(s) being monitored is more key information. It could be assumed that, if significant harmonic current for the 5th and 7th, 11th and 13th, 17th and 19th are found, there are one or more six-pulse converter power supplies. But what if such values show up in the harmonic voltages and not the currents? This might indicate such loads are present in the facility but not on the circuit monitored. If these harmonic-producing loads are not in the facility, yet the harmonic voltage distortion is there, that would indicate it is coming in from the utility system and that a neighboring facility may be the polluter. 


The instrument setup is important information to determine if the instrument would have captured the needed information. Just because the instrument didn’t capture it doesn’t mean it didn’t happen. If the transient limits are set too high, a potentially damaging transient, such as severe notching, could occur over and over again, and it wouldn’t be recorded. If the voltage root mean square limit for a sag is set to 80 percent and there is a photocell on a hand intrusion protection circuit of a stamping machine that is susceptible to a sag at 90 percent, nothing would be recorded when the machine shut down inadvertently.


When it comes to providing information to the person analyzing your data, the more the merrier. If there is data recorded from a previous PQ audit, this also is invaluable at determining what is normal and if power quality is improving or deteriorating. Help them to help you get the answers you are looking for, even if it turns out there was no problem with the power that caused your problem.