An Ounce of Prevention
Richard P. Bingham, Dranetz-BMI
When I went through Firefighter 101 training, the instructors emphasized that the primary purpose
of a fire department is to prevent fires, not put them out. The old adage about “an ounce of
prevention is worth a pound of cure” is applicable to the power quality world as well. Recording
disturbances that shut down processes or disrupt the quality of the product is valuable, but even
more valuable is preventing them from occurring in the first place. The same power quality
monitor used to do the former can provide the information necessary for the latter. It just takes a
little attention to the details.
Many people are too busy in today’s Internet-speed economy to step back and look at things until
they become a crisis. However, one should look at the data with the mind set of determining are
things getting worse, are there marginal operating conditions present, is there “an accident just
waiting to happen”?
One way to quickly view RMS variation data is using a 3-D magnitude-duration chart. The magdur chart plots the magnitude or amplitude of the voltage versus the duration of the sag or swell
versus how many times that value or range of values occurs. Figure 1 shows an example of such
over a one week period. Note that the majority of the sags have a magnitude of 80-89% of
nomimal and last 1-5 cycles. While the severity of these is often not enough to cause a process
interruption, it can be a valuable clue that something is about to happen.
Figure 1. Mag-dur chart
Richard P. Bingham is the manager of technology and products for Dranetz-BMI.
© copyright February 2001 Electrical Contractor Magazine
www.ecmag.com
Closer examination of the information from a timeline in Figure 2 shows that the events don’t
seem to occur at any particular time or pattern. Eventually, an event occurred that was severe
enough to cause computers that weren’t on a UPS to reset themselves.
V
300
275
250
225
200
175
150
125
100
10/11/00
10/12/00
10/13/00
10/14/00
CHA V
10/15/00
10/16/00
CHB V
10/17/00
10/18/00
10/19/00
CHC V
10/11/00 00:00:00.00 - 10/19/00 00:00:00.00
Figure 2 – Timeline
Closer examination of the events themselves found that the majority of them were sags of 1-2
cycles long that only involved one phase and were self-clearing, as shown in Figures 3 and 4.
Eventually, there was a sag that involved all three phases, and caused the problem with the
computers. By looking at the load current at the time of the sags, it is evident that the problem
was caused upstream, back towards the source of the electricity, as the current decreased when
the voltage decreased. Further coorelation to other conditions determined that it was a very
windy day. A likely scenario, given the numerous single phase sags leading up the disruptive
event, was that there are conductors on the poles that were swinging in the wind and brushing up
against a high impedance ground, such as a tree branch. Eventually, two conductors were
involved at the same time, and then all three, as shown in Figure 5.
Richard P. Bingham is the manager of technology and products for Dranetz-BMI.
© copyright February 2001 Electrical Contractor Magazine
www.ecmag.com
CH AV
CH BV
CH CV
500
400
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200
100
0
-100
-200
-300
-400
-500
500
400
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200
100
0
-100
-200
-300
-400
-500
500
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200
100
0
-100
-200
-300
-400
-500
11:40:14.46
11:40:14.44
11:40:14.50
11:40:14.48
CHA Volts
11:40:14.54
11:40:14.52
11:40:14.58
11:40:14.56
CHB Volts
CHC Volts
10/15/00 11:40:14
CH CV
CH BV
CH AV
Figure 3 – SLTG – Phase B
500
400
300
200
100
0
-100
-200
-300
-400
-500
500
400
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200
100
0
-100
-200
-300
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-500
500
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100
0
-100
-200
-300
-400
-500
11:08:34.44
11:08:34.42
11:08:34.48
11:08:34.46
CHA Volts
11:08:34.52
11:08:34.50
CHB Volts
11:08:34.56
11:08:34.54
CHC Volts
10/15/00 11:08:34
Figure 4 – SLTG – Phase A
Richard P. Bingham is the manager of technology and products for Dranetz-BMI.
© copyright February 2001 Electrical Contractor Magazine
www.ecmag.com
500
400
300
200
CH 100
0
AV
-100
-200
-300
-400
-500
500
400
300
200
CH 100
0
BV
-100
-200
-300
-400
-500
500
400
300
200
CH 100
0
CV
-100
-200
-300
-400
-500
11:08:34.44
11:08:34.48
11:08:34.52
11:08:34.56
11:08:34.42
11:08:34.46
11:08:34.50
11:08:34.54
CHA Volts
CHB Volts
CHC Volts
10/15/00 11:08:34
Figure 5 – 3 Phase sag
Armed with this data, the facility manager could have contacted the local electric utility to send a
crew out to find the source of the ground before it became a problem for that facility and others
fed by that circuit.
Remember Smokey the Bears motto, “Only you can prevent PQ problems”.
Richard P. Bingham is the manager of technology and products for Dranetz-BMI.
© copyright February 2001 Electrical Contractor Magazine
www.ecmag.com