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Abstract
Flicker is defined as fluctuation or wavering in visible light intensity due to voltage variations. These variations in voltage are caused by fluctuations in the load through the system source impedance. Individuals perceive flicker differently depending on their sensitivity.
The challenge in flicker investigations is to quantify the problem, and then find the underlying source of the voltage variation - either loads with excessive current, or abnormally high impedance causing excessive voltage drop with normal loads.
The GE Flicker Curve (IEEE Standard 141) was developed by General Electric. It was based on testing human responses to light fluctuations. The GE Flicker Curve defined two curves, one based on the threshold of perception and one on the threshold of irritation. While useful, this method is only well defined for simple voltage variation types from a single source.
A newer flicker measurement method is presented in IEEE Standard 1453 which defines PST, PLT, and IFL flicker values.
PST Flicker is short-term flicker, hence the ST in the name, and is defined for an interval of 10 minutes. PST is used for determining the severity of the flicker. A PST of 1.0 corresponds to the threshold of irritability. Values of greater than 1.0 are an indication of flicker problems.
PLT Flicker is long-term flicker, hence the LT in the name, and is defined for an interval of two hours. PLT is a root mean average of the PST value over 12-period sliding time span. The interpretation for PLT is the same as for PST. PLT is most often used for long term studies, especially at distribution and transmission levels.
IFL stands for Instantaneous Flicker Level and quantifies voltage fluctuations at a given instant. IFL is very useful for locating the source of flicker, rather than quantifying how severe a flicker problem may be.
See the whitepaper “Flicker Standards Used by PMI Recorders” for more information about flicker and the standards used.
Recommended Recorder Setup
Analyzing Flicker starts with collecting the data. Revolutions, Guardians, and Eagles (including the Eagle 120) can all measure the newer IFL and PST flicker values, and also the older GE flicker curve values. Both the severity of Flicker and the direction from the point of monitoring can be determined by recording the correct data.
PMI recorders can be configured to collect the required information. Figure 1 shows a Provision Interval Graphs configuration tab in the Settings dialog. The PST Flicker and IFL Flicker graphs are selected, with a ten minute PST interval. The PST interval may be set independently of the normal stripchart interval used for voltage, current, etc. This allows for a 10 minute standard PST reading even with a shorter interval for other data types. The IFL value is computed every second, and uses the regular stripchart interval.
Figure 1. ProVision interval graphs configuration tab
When configuring PMI recorders for collecting flicker data, make sure that these flicker checkboxes are selected. RMS voltage and Current should also be selected. Use a small stripchart interval, if possible, such as ten seconds or less. The time resolution is useful for correlating current spikes with IFL values.
Figure 2 shows a Provision Flicker Settings tab in the Settings dialog. These flicker settings are for the GE Flicker Curve (IEEE standard 141). PMI’s recorders default to the threshold of irritation curve from the GE Flicker Curve.
Figure 2. ProVision Flicker Settings tab in setttings dialog
Monitoring Locations
There are several choices of monitors locations. It is good to have a monitor at the service entrance to determine if delivered voltage is within regulatory specifications. A Guardian or Revolution is a good choice for a service entrance monitor.
A second recorder electrically close to a light fixture is also useful. An Eagle 120 plug-in receptacle monitor a good choice for monitoring, especially inside a residential location. A recorder inside can help determine if flicker is due to voltage drop across internal building wiring, or coming from upstream.
Analyzing data file
When sufficient data has been recorded to capture the flicker events, the recording can be downloaded to ProVision for analysis. Look at the PST to determine if flicker is a problem. Typically a PST greater than 1.0 is considered a problem. For planning purposes, especially on medium voltage lines, a value of 0.7 may be targeted.
Figure 3 shows a one day PST graph from an Eagle 120. This graph shows flicker values that exceed 1.0 which could indicate that there will be customer complaints.
If flicker is present, then the next step is to locate the load causing the voltage variations. To determine if the source is upstream or downstream from the recorder, compare maximum current versus IFL. If spikes in the IFL happen at the same time as spikes in the maximum current, then the issue is likely caused by something downstream from the recorder.
Figure 3. One-day PST graph from an Eagle 120
Figure 4 shows the IFL flicker graph from the same period as Figure 3.
Figure 5 shows the RMS Voltage and Current graph for the same period and Figures 3&4. You will see a correlation between the voltage changes and the PST and IFL flicker graph. There is no correlation between the current and the flicker. This shows that the device plugged into the Eagle 120 sees the flicker but is not the cause of the flicker.
Figure 4. IFL Flicker graph from the same period as Figure 3
Figure 5. RMS Voltage and Current Graph from the same period
A correlation of high IFL times with loads can be used to identify the load causing the flicker problems. If the IFL values are not clearly spiking with current 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 I max during periods of elevated PST then the source of flicker is downstream from the recording.
If the low points of Vmin do not have accompanying spikes of Imax then the source of flicker is upstream from the recording. When the direction of the flicker causing load is determined, the PMI recorder can be moved to further isolate the source. See the whitepaper “Strategies for Investigating Flicker” for more information about analyzing recordings containing flicker. See the whitepaper “New Graph Templates for Flicker Analysis” for useful ProVision templates for flicker analysis and more information about analyzing flicker using these templates.
Conclusion
Flicker is caused by voltage variations due to fluctuations in the load through source impedance.
Individual tolerance to flicker varies but industry standards such as IEE 1453 were created to help quantify flicker and determine reasonable tolerances.
The PMI Revolution, Guardian, and Eagle (including the Eagle 120) recorders can all measure flicker to multiple standards. Strategic recorder placement can help isolate where offending loads are located, along with an analysis of the recorded flicker data. PST and PLT quantity the flicker level and provide an industry guideline for whether flicker at a location should be considered a problem, and the IFL reading may be used to help correlate particular loads with flicker changes.
David Horning
Software Developer
dhorning@powermonitors.com
http://www.powermonitors.com
(800) 296-4120