Strategies for Investigating Flicker

Published by Wayne LaFleur, Senior Engineer, Power Monitors, Inc.

Email: wlafleur@powermonitors.com, Website: powermonitors.com, (800) 296-4120, February 2014. White Paper.


ABSTRACT

Investigating f licker issues with PMI recorders can be straightforward if the right data is collected, and analyzed with ProVision. By enabling and examining Pst and IFL flicker data, in addition to RMS voltage and current, f licker can be quantified, and possible direction of the flicker source identified. Often the recordings will show that a potential problem exists but then comes the more difficult process of identifying the possible causes. PMI recorders can also be used to aid in the investigation of these causes. It is usually just a matter of knowing what data should be captured and how to interpret the results via Provision. Investigation into the possible source of flicker falls into this category of issues, and is described here.

THE PROBLEM

Flicker is an interesting issue. The name is unique in the sense that it perfectly describes the actual problem. A simple definition of flicker is that it is the visible fluctuation in light intensity due to voltage variations. Often the first indication that there is a problem is from direct observation although by default, PMI recorders will produce a flicker report which may indicate a problem as well. The default flicker report is designed to follow the industry standard GE Flicker Curve and the report will only trigger on events that fall outside of the curve. Flicker is the type of problem that can crop up suddenly based on changes in installed equipment or deterioration of components or connections. The problem may also be transitory and only manifest itself at certain times of the day. Since flicker itself is quantified based on measurements over time, a flicker investigation may require a recording covering many hours and sometimes days.

THE RECORDING

Once flicker has been reported (or is suspected) then the next logical step is to collect the data needed to identify the severity of the flicker and hopefully the location of the cause. Both the severity and direction from the point of monitoring can be determined by recording the correct data. The severity of f licker is captured with a Pst (Perceptibility Short Term) interval graph while the direction from the point of monitoring is captured using voltage minimum, current maximum and IFL (Instantaneous Flicker Level) interval graphs. See the white paper Flicker Standards Used by PMI Recorders for a more in depth definition of Pst and IFL themselves. Provision can be set up to capture each of these graphs in a single recording session via the Interval Graph tab when initializing a recorder. In some cases there may be saved recordings that do not contain Pst or IFL charts. These may still be useful for comparison based on the older GE flicker report contained in all recordings and/or the presence of voltage and current stripcharts. The Revolution, Guardian, and Eagle series (including the Eagle 120) all measure IFL and Pst flicker, and these stripcharts should be enabled in any recording where flicker is suspected. All PMI recorders also measure the older GE flicker curve, including older legacy devices, and this can be used as a fallback if needed.

THE ANALYSIS

As previously stated, a recording of sufficient time duration to encounter and capture any flicker issue is needed. This time will vary on a case by case basis. However, once the data has been captured and downloaded into Provision it can now be evaluated. A quick method to determine if a flicker problem is unlikely is to look at the Vmin, Vavg and Vmax stripchart. If the stripchart shows these three values tracking close together then flicker is probably not an issue. The Vmin and Vmax values are one cycle RMS readings, and if they are close to the average at each interval, then the voltage is not varying much, and thus flicker is unlikely. However, examining the voltage values alone only gives a clue about f licker but it cannot be ruled out on this basis alone. If the voltage variation continues at the most sensitive flicker frequency (8.8Hz), it only takes a 0.2% change to produce significant flicker. Having the additional stripcharts showing Pst or IFL will reveal any periods of voltage disturbance and accompanying flicker. An example of this can be seen in Figure 1 which shows voltage values along with both Pst and IFL to highlight this relationship. Typically a Pst greater than 1.0 is considered a possible problem but this can occur without any other indicators (such as customer complaints or equipment problems). The example shown indicates a severe flicker problem based on the indicators above.

Figure 1. Periods of voltage disturbance and accompanying flicker

Once a flicker issue has been confirmed then the next step is to determine where it is being introduced. The location of the cause of a flicker issue can be determined relative to the point of the recording. A problem upstream is toward the service hook up while a problem downstream is toward the load on the circuit. To discern the direction (upstream or downstream) a comparison of maximum current versus IFL is used. Being an instantaneous value, the IFL is useful when comparing against voltage or current fluctuations. The Pst value is computed over a time interval (typically 10 minutes) and therefore may not register a problem with momentary flicker. If spikes in the IFL graph happen at the same time as spikes in the maximum current then the issue is likely caused by something downstream from the point of the recording. Figure 2 shows an example of IFL compared with Imax and it clearly shows that in this case the current spikes correlate to the spikes in the IFL reading. This is a positive indication that the cause of the flicker is downstream from the point of the recording. Sometimes the IFL values are not clearly spiking with current as in the example so a comparison of Vmin with Imax can also be used to confirm the location of a flicker problem. If the low points of Vmin correlate to the high points of Imax (as seen in Figure 3) during periods of elevated Pst then the source of flicker is downstream from the point of the recording. If the low points of Vmin do not have accompanying spikes of Imax then the source of flicker is upstream from the point of the recording. Once the direction is determined it may be necessary to repeat the above steps to further pinpoint the cause of the issue, so well chosen recording points are crucial to an effective flicker investigation.

Figure 2. Example of IFL compared with Imax

Figure 3. Low points of Vmin correlate to the high points of Imax

CONCLUSION

Flicker can be a minor annoyance or quite literally a major headache (for some people). PMI recorders can be used to not only identify the severity of perceived flicker but also help to pinpoint the cause. When possible, enable Pst and IFL stripcharts, along with RMS voltage and current to capture flicker issues. Use the simple techniques outlined above to aid in your next flicker investigation.

Published by PQTBlog

Electrical Engineer

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