power quality monitoring, PQube, Power Standards Lab
With aging infrastructure and renewables joining the supply network, the stability of the electric grid is becoming more and more unpredictable. This reality is affecting plants and manufacturing facilities across the country, causing particularly bad headaches for facilities maintenance supervisors and electrical engineers who manage power-sensitive equipment.
When a fast-growing composites manufacturer in Pennsylvania experienced persistent power problems, the answers were not easy to find. For years, the plant had been plagued by disruptive and dangerous electrical problems that blew fuses/amplifiers and burned circuit boards on equipment across the plant. The anomalies came very fast. In fact, by the time the plant’s electrical engineers became aware of them, tracing the cause was next to impossible.
After checking every panel and service supply, Dan Claxton, plant maintenance supervisor at the facility, hired an outside firm to search for answers — but had no luck. The cost of burned equipment, including circuit boards, amplifiers, fuses, and electronic modulating valves, amounted to $50,000 a year, plus that much more in downtime.
Suspecting an external power problem, Claxton complained to his electric utility. Utility engineers came out and plugged in a power monitor for a week. Noting a problem on the electric utility side of the meter, they replaced the transformer. About a month later, the transformer was replaced again. Nevertheless, problems at the plant persisted. Something bigger was going on, but the electric utility engineers had no good answer as to what it was.
Frustrated but determined to solve the mystery, Claxton began looking at power quality instruments himself — something he could install permanently and use to monitor the power on a continuous basis. Upon the suggestion of one of his team members, he looked into a PQube from Power Standards Lab (PSL), Alameda, Calif. The PQube is a compact, low-cost power quality monitor with a DIN-rail mounting design. The device proved to be perfect for snapping inside the electric panel without much need for extra accommodation.
In addition to the low price and easy installation, the features that attracted Claxton to the PQube were its ability to monitor both AC and DC voltage and current at a 256 cycles-per-second sampling rate coupled with its ability to capture voltage dips and swells (both as waveform and RMS). In addition, the PQube monitors and records a number of other parameters, such as harmonics, unbalance, VARs, high frequency impulses and more, allowing for a detailed picture of power quality to be generated over time. The PQube’s data is delivered to an email or a server directly, in the form of graphs and comma-separated data files. No software is needed to open the files.
Most importantly for Claxton, the PQube could be monitored remotely, by installing an optional Ethernet module. It could also be set to send email notification alerts to up to five recipients, every time a power event occurs.
The combination Claxton purchased for the plant included the Ethernet module, as well as a power supply module and a current transformer interface. This configuration was installed at the primary service panel where it could monitor all the power coming into the plant. Installation and setup took less than 2 hr — and the monitor emailed its first event notification to Claxton that same evening.
The PQube captured a 53% voltage sag, accompanied by a dip in amperage on L1 (see Graph below). Typically, a large drop in voltage accompanied by a drop in current inside the facility indicates a supply-side problem. Automated equipment will typically go offline with a voltage sag of that magnitude.
Claxton continued to accumulate and forward data records of upstream (electric utility-caused) voltage sags to the utility engineers. The pictures left no room for argument. Soon thereafter, service trucks were crisscrossing the roads surrounding the facility and repairing the power lines servicing the business area. At the same time, the plant’s electric mysteries began to disappear.
Claxton insists the pictures he was able to show the utility representatives have improved not just his image as a maintenance supervisor, but his relationship with the electric utility as well. “I believe the engineers were happy to see the reports because it helped them fix the problem,“ he maintains. “They have gone out of their way to fix all of the problems. No one ever wants to take the blame; all we wanted were answers so we could fix these issues.”
RMS Voltage, Frequency, and Current Graph Generated by the Monitoring Device
1. Around 5:30 pm on March 9, Dan receives an email on his smartphone, with a chart, alerting him that a voltage sag occurred. The graph coming directly from the PQube shows that voltage fell to 80% of nominal voltage on the L1-L2 and L3-L1 lines.
2. L1-N voltage sags even more, down to 53%, indicating a L1-Earth fault somewhere upstream from the monitoring location.
3. As a result of the sag between L1 and N, current on L1 drops, causing L2 and L3 current to shoot up.
4. A protection device upstream takes 1.8 sec to trip and remove the fault, then re-closes about 3 sec later but the fault is still there, so we see the 1.8 sec sag again.
5. An unrelated event. Most likely a motor upstream starts up, and is caught on the voltage snapshot. It is unrelated to the trouble-causing sag.