I'm working on a Puget Sound Energy commercial account and their interval data is showing negative demand readings for several 15-minute periods over the past 3 months. We're talking about readings like -127 kW, -89 kW, -203 kW during normal business hours when the facility should be drawing 300-400 kW. The customer doesn't have any generation on-site, so these negative readings make no sense. PSE is billing these as zero demand but I'm concerned there might be legitimate usage that's not being captured. Has anyone dealt with negative demand readings in interval data before? What causes this and how do you get it corrected?
Load profile analysis showing impossible negative demand
Vera, I've seen this with older electromechanical meters that are starting to fail. When the current transformers begin to deteriorate, you can get phase angle errors that result in negative power readings. Central Maine Power had a batch of meters from 2008-2010 that started showing this behavior around year 12-15 of service. The meter thinks power is flowing backwards when it's actually flowing forward normally.
Could also be a wiring issue where one of the phases got reversed during a recent maintenance or upgrade. Idaho Power had a case where a contractor accidentally reversed the polarity on one phase during a meter changeout, causing negative readings. The customer was actually using power but the meter was registering it as generation. Definitely push PSE to investigate the meter and CT connections.
Thanks Pete and Glenda. I'm going to request that PSE do a full meter inspection and CT test. The timing makes sense - this meter was installed in 2009 so it's about 13 years old. If it is a failing meter, I need to figure out how much actual usage might have been missed during those negative reading periods. Could be looking at significant under-billing if the meter has been failing intermittently.
Vera, definitely document everything and keep pushing PSE for answers. I had a similar case with MLGW here in Memphis where negative readings turned out to be a precursor to complete meter failure. The customer ended up getting under-billed by about $18,000 over 6 months before we caught it. PSE should be proactive about replacing that meter if it's showing signs of failure, and they need to do a load study to estimate any missed usage during the error periods.