Investigating “Blinky” Lights

www.dranetz.com

The HDPQ family of products have a number of trigger mechanisms for capturing different types of power quality phenomena.   Most users are familiar with settings for sags and swells, but those typical limits usually don’t capture the “blinky lights” problems.   While the Pst (perceptibility short term) parameter will indicate whether the voltage supply is likely to produce light flicker when the Pst is close or above 1, it doesn’t help determine where it comes from or what the source is since it is a ten-minute journal value.  The graph below illustrates such.

The voltage and current plots show that it is an increase in current over 100A that results in the voltage decreasing by 5-8V every 15 minutes. The rms variation limit for sags is set to 90% of the nominal 120V or 108V, so no sag was reported.   In above example, the red dotted line is that sag limit.  Yet the residents complained of “blinky lights” while the Pst does increase to 0.8 each time, not over 1.  

The rms deviation transient trigger is one method to capture such as an event.  However, there is a parameter that is included in the IEC and IEEE standards that was defined just for such disturbances.   It is the RVC or Rapid Voltage Change.    It looks for a sudden change in the voltage from one steady state value, waits for stability in any continued variations until stable at another steady state voltage value for 1 second or more.    The delta change limit is typically set to 2-3%.  Since the variation shown in the data above was 4-7%, this mechanism was ideal for capturing and reporting each time that it happened.  An example at 22:189:54 is shown below.   The rms deviations and wave shape triggers also captured the event, but they also triggered on other disturbances that weren’t part of the investigation, which produced extra data to sort through

#39 10/03/2017 22:02:05.297 AV Misc at 0.8 Deg
#40 10/03/2017 22:02:05.297 BV  Misc at -179.1 Deg
#41 10/03/2017 22:02:05.306 AVRmsDev  Normal To High
#42 10/03/2017 22:02:05.306 BVRmsDev  Normal To High
#44 10/03/2017 22:02:05.323 AVRmsDev  High To Normal
#45 10/03/2017 22:02:05.323 BVRmsDev  High To Normal
#48 10/03/2017 22:18:54.874 AV  Misc at 0.7 Deg
#49 10/03/2017 22:18:54.874 BV  Misc at -179.0 Deg
#50 10/03/2017 22:18:54.874 AVRmsDev  Normal To High
#51 10/03/2017 22:18:54.874 BVRmsDev  Normal To High
#52 10/03/2017 22:18:54.883 A Delta V RVC  Rapid Voltage Change 0.175 Sec.
#53 10/03/2017 22:18:54.883 B Delta V RVC  Rapid Voltage Change 0.259 Sec.

The right tool for the right job has always been a wise adage.  Setting the right tool up with the right limits on the right parameters to help solve the customer’s problems is an extension of that idea that can help get the answer quickly and clearly.   By the way, the answer here was a common answer for such problems.   The HVAC unit was causing the blinky lights.