Is Customer Electrical Equipment Tripping Off? Voltage Trend Data Does Not Show the Whole Picture.

Power Monitoring has made great strides from the ink pen chart recorders used 30 years ago. Utilities started installing SCADA systems and voltage trend data could be populated using various SCADA communication protocols such as DNP3 and this is still a common valuable practice today.

Power quality monitors have come down in price dramatically now making them a viable option to use the data for more advanced power monitoring capabilities and the following example shows that voltage trend data alone does not show the whole picture.

In Figure 1 below we have a 3-phase voltage trend chart showing 10-minute minimum/mean/maximum values and we can see that at approximately 8:30 AM on November 7 2017 the C phase minimum voltage dropped to 99% of nominal for a minimum of 1 second in the 10-minute window. If a customer calls to ask why their equipment tripped off during the absence of a power outage, you would be challenged to explain to them why.

Figure 1: 3-Phase Voltage Trend Chart Showing Min/Mean/Max Values

Advanced power metering would provide more valuable information. The power monitoring software application used is collecting data from a power quality/revenue meter and captured events can be plotted on the trend chart as shown as a blue hyperlink in figure 1 above. We now know that there was a voltage disturbance at 8:30:01 AM

A user can click on the event hyperlink and now view the voltage and current waveform event shown in Figure 2 and the ½ cycle RMS calculation shown in Figure 3

Figure 2: Blue Phase Voltage Disturbance Event Waveform

Figure 3: Blue Phase Voltage Disturbance Event 1/2-cycle Calculated RMS Plot.

More valuable information is available to determine the true magnitude of the event to explain why the customer’s equipment tripped off. The data in Figure 3 shows that they experienced a voltage sag to 49% of nominal voltage for 0.076 seconds. The 49% voltage sag on the C phase was the maximum deviation for the event but the customer’s equipment also experienced a voltage swell of 129% of nominal on the A phase.

After plotting the event on a reference voltage tolerance curve such as the Information Technology Industry Council (ITIC) Power Acceptability Curve chart below, it becomes apparent that customers would have experienced 1 A phase voltage swell above the tolerance envelope and 1 C phase voltage sags below the tolerance curve.

The customer now have better information that they can take to their equipment vendor to determine if there is a cost effective solution to prevent these disruptions in the future and the utility has improved their relationship with their customer.

Figure 4: 3-phase Sag/Swell Magnitude and Duration plotted on ITIC Chart

To wrap up, voltage trend data from any source is valuable to ensure that steady-state voltage levels are maintained within industry standards however if you don’t have advanced power monitoring, keep an open mind that voltage disturbances may be happening behind the scenes that may impact your or your customer’s equipment

Written by: Gary MacLeod, Product Manager, CPS Current Power Services,

CPS is the Canadian distributor for PQView® and provider for PQ-OnSite™ power monitoring services.

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