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PowerLogic
Solutions
Volume 5, Issue 2
Waveform Captures: The
Key to PQ Solutions
In This Issue
we discuss how you
can use waveform
captures to diagnose
problems in your
power system.
The Problem
Like the lines on the EKG at your last checkup, those squiggly
lines recorded by your POWERLOGIC ® Circuit Monitor mean
something. We call those lines waveform capture, and, in the
hands of an expert, they provide important information about the
health of your power system. This issue of PowerLogic Solutions
shows some typical waveform captures associated with disturbance events and provides their interpretation. By following a few
guidelines, you can learn to interpret more information from your
circuit monitors; using this information, you can decrease power
system costs in your facility.
The customer who captured the waveform in figure 1 was trying to
diagnose a problem associated with an air compressor. The
compressor would periodically shut down, often with catastrophic
results to the plant, when nothing else in the plant was affected.
The facilities engineer configured the POWERLOGIC CM-2350
Circuit Monitor on the circuit to capture waveforms during voltage
sag events. The problem soon recurred, and, sure enough, the
circuit monitor recorded the event in a 24-cycle waveform capture.
The waveform proved that the contactor serving the motor was overly
sensitive to voltage sag events. The sag shown in figure 1 is mild; it
has only about 10% reduction in voltage for about five cycles on
phase C. The current confirmed, however, that the contactor opened
and the motor stopped. Note also that the contactor poles did not
separate at the same time, as one might expect them to do. Phase A
pole separated about four cycles before phases B and C. The plant
replaced the contactor, and the nuisance compressor trips ceased.
Phase A Current
Phase A-N Voltage
386
193
0
-193
-386
735
0
-735
-1469
Phase B-N Voltage
386
193
0
-193
-386
Phase B Current
1078
0
-1078
-2157
Phase C-N Voltage
Figure 1: This waveform capture
helped prove that a motor
contactor needed to be replaced.
387
193
0
-193
-387
Phase C Current
2526
1263
0
-1263
-2526
PowerLogic
Solutions
Coordination Error on Electric
Utility System: Waveform
Capture Nirvana
Another example of the power of the
POWERLOGIC system is shown in figure 2.
This event was captured by a CM-2350 at
the 600V service entrance of a manufacturing facility served from an electric utility
overhead distribution circuit. The event lasted
less than one-quarter of a second, (about 15
cycles). It damaged silicon-controlled rectifiers in the plant’s plating process, shutting
down the facility for four hours.
Phase A-N Voltage
Phase A Current
5000
4000
3000
2000
1000
0
-1000
-2000
-3000
750
500
250
0
-250
-500
-750
Phase B Current
Phase B-N Voltage
4000
3000
2000
1000
0
-1000
-2000
-3000
-4000
-5000
750
500
250
0
-250
-500
-750
Phase C Current
Phase C-N Voltage
750
500
250
0
-250
-500
-750
2
PowerLogic Solutions
is a publication of
Square D Company’s
Power Management
Operation. Each issue
presents a common
power system
problem, and offers
guidance on how to
solve it.
2500
caused by a fault on the 23-kV overhead
circuit, but the circuit monitor is connected
at the 600V circuit. Between the fault and
monitoring point is a step-down power
transformer connected delta-delta. For
delta-delta or wye-wye connected transformers, a single-phase event on the primary side
affects only one phase voltage. Figure 3
shows an extreme example of a single lineto-ground fault—single-phasing —seen
through a wye-wye transformer connection.
But most service-entrance transformers
are connected delta-wye. For these transformers, a single line-to-ground fault
on the primary side causes a drop in
voltage on two phases on the
secondary. Transformer connections
between the circuit monitor monitoring point and the event are an
important consideration when
diagnosing voltage sag events.
Event 2. Looking back at figure 2:
the first event lasts three cycles and
resembles the usual single line-toFigure 2: This waveform capture exposed a coordination
ground fault on the utility system.
problem on the electric utility circuit.
The second event is the killer. It
involves
all three phases and shows that
The Square D Power Quality/Energy
the
utility
source voltage was removed
Management (PQ/EM) team engineer
about
four
cycles after the three-cycle fault
proved that the event should not have been
had
cleared.
The voltage after that fourso severe, and that it exposed a coordinacycle
respite
gradually decreases in
tion problem on the electric utility circuit
magnitude,
and,
if you look carefully, in
serving the plant!
frequency. This effect is also common. It
happens when source voltage is suddenly
On careful inspection, the event in figure 2
removed from three-phase induction
is actually two events:
motors. They instantly become induction
Event 1. The first affects phase A-N more
generators, supplying a gradually decreasthan the other phases, obviously the result
ing voltage to the plant power system
of a single-phase event on the utility
while the rotors coast to a stop.
system. Most sags in facility power are
caused by single line-to-ground faults on the
In this case, the problem is that they were
electric utility overhead system. Because the not allowed to stop. The utility voltage
events involve only one phase on the overreturned just as suddenly as it left. The
head circuit, the waveform capture shows a
voltage produced by the induction motors/
drop in voltage on one or two phases.
generators was out of phase with the
returning voltage. There was a brief “battle
One phase affected seems straightforward, of electrons,” but the overwhelming shortbut why would two phases drop during an
circuit power of the utility electrons won out
event that involved only one phase? That
and quickly snatched the motors back to
phenomenon has to do with transformers
normal speed. This struggle created a large
between the monitoring point and the
voltage surge, however, that damaged
actual fault. The event in figure 2 was
0
-2500
-5000
60 cycles of waveform capture are
available, you may decide that
fewer cycles are acceptable.
Phase A Current
Phase A-N Voltage
681
341
0
-341
-681
460
230
0
-230
-460
Phase B Current
Phase B-N Voltage
383
191
0
-191
-383
385
192
0
-192
-385
Phase C Current
Phase C-N Voltage
492
246
0
-246
-492
802
401
0
-401
-802
Figure 3: This single-phasing event lasted for two hours. The
resulting 24% phase-to-phase voltage imbalance caused
damage to the facility’s three-phase induction motors.
plant equipment and resulted in the fourhour shutdown.
So what caused the twin events? The
electric utility had two pieces of evidence.
First, they had to dispatch a service
person to replace a fuse in an overhead,
single-phase circuit served by the same
three-phase circuit that served the
affected plant. Second, they determined
that the substation circuit recloser had
operated once since the last time its
counter was read.
Both pieces of evidence confirmed the
Square D PQ/EM engineer’s suspicion:
one fault had occurred, yet two overcurrent devices operated. The fuse
replacement showed that a short circuit
had occurred beyond the fuse, on the
single-phase “branch” circuit. The substation circuit breaker had operated also —
unnecessarily so—with catastrophic
results in the plant. Further investigation
by the utility confirmed the coordination
problem, and they eventually replaced
the single-phase fuse with a fuse that
coordinated better with the three-phase
substation circuit breaker.
Setup Is Important
Circuit monitors (model 2350 or above)
can capture up to 60 cycles of instantaneous voltage and current on all channels
simultaneously, based on a high-speed
event on any channel. High-speed events
need to be set up in advance, however, to
ensure that the right amount of information is captured for your facility. Although
Why limit the cycles? Memory, for
one thing. Your circuit monitor can
be equipped with over one megabyte of memory, but you may have
purchased less. And you may
want to share that memory
between on-board data logs, fourcycle waveforms, and event logs,
in addition to high-speed waveform captures.
Additionally, more than 60 cycles may be
unnecessary because you may rarely
experience events that long. Many customers served from an electric utility transmission system experience events that last
fewer than eight cycles. A 24-cycle, or even
12-cycle, waveform capture setting is
enough to capture the event.
Your electric utility representative may
provide some guidance in determining how
many cycles to select. The representative
can determine the protective device settings
on the electric utility distribution system
serving your facility. Many utilities use
3
The Square D
Power
Management
Operation offers
complete power
quality consulting services to ensure that power
problems do not impact your operation. Contact
our power management experts for information
about the following:
• Power Quality Consulting
• Energy Management Consulting
• Harmonic Filters
• Power Factor Correction
• Power Management Training and
Technical Support
• Digital Simulation Studies
• Remote Monitoring Services
• Data Collection and Analysis
Our number is 1-888-PWR-MGMT.
PowerLogic
Solutions
substation circuit breakers that
are set to sense, open, and
reclose in about 15 cycles. For
these circuits, a 24-cycle setting
is usually adequate.
The magnitude of sag and swell
pickup and dropout settings is
important, too. For most facilities,
Square D Company’s PQ/EM
consulting team recommends a
sag pickup (when the threshold is
exceeded) setting of about 418V
on a 480V system, or 241V if the
nominal voltage is 277.
800
600
400
200
0
-200
-400
-600
-800
1000
750
500
250
0
-250
-500
-750
-1000
8000
6000
4000
2000
0
-2000
-4000
-6000
-8000
Phase B-N Voltage
Phase A Current
Phase B Current
1500
1000
500
0
-500
-1000
-1500
Phase C-N Voltage
800
600
400
200
0
-200
-400
-600
-800
8000
6000
4000
2000
0
-2000
-4000
-6000
-8000
Phase C Current
Figure 4. This waveform shows a downstream fault. The
fault current saturates the current transformer and results
in an unusual waveform.
We refer to three-wire systems as system
30 and four-wire as system 40 or 41. The
dropout setting (when the voltage returns
to a usable level) is typically 432V or 250 V.
See PowerLogic Solutions volume 1,
number 5, for additional information about
sag and swell setup.
Power Metering vs. Fault Recording
4
Phase A-N Voltage
One customer suffered damage to a cable
lug on a circuit breaker serving a power
factor correction capacitor bank (figure 5).
The waveform capture from the event
shows a three-cycle voltage sag, and a
very irregular current waveform. The voltage
waveforms show how the voltage on that
bus was affected before, during, and after
the event. The current waveform during the
brief fault, however, reflects a trade-off
between accurate power metering and
fault recording.
conditions. A fault (or short circuit) draws
much more current than normal loads, often
many times full load current of the circuit.
Fault current magnitudes can cause strange
current waveforms, like the one in figure 4.
The high fault magnitude can saturate the
current transformers in the circuit monitor,
thereby distorting the waveform capture for
current. Whenever you see extremely high
current magnitudes, especially if one or
more cycles is flat-topped, a downstream
fault has occurred.
What About Motor Starting?
Another waveform capture that sometimes
resembles a downstream fault is associated
with starting an induction motor. An induction motor requires many times its full-load
current during starting. This event may
resemble a downstream fault, but there is
usually a clear distinction: current associated with a motor starting gradually
Your circuit monitor system was designed
to provide highly accurate readings of 200
power system parameters at normal
loads. In order to do this to 0.2% accuracy, the circuit monitor usually receives
its voltage and current inputs from instrument transformers sized for normal circuit
loading. In particular, current transformers
for circuit monitor systems are typically
sized to provide a 0-5 ampere output
signal under normal loads.
The circuit monitor was not optimized to
measure currents associated with fault
Figure 5: The waveform capture in figure 4 resulted
when the center leg of this cable lug failed.
decreases, while a fault usually changes
current magnitude almost instantaneously.
This abrupt change in current is due either
to the operation of a load-side or sourceside protective device. If the overcurrent
device is load-side of the fault, the current
returns to a nominal level quickly. If the
overcurrent device is source-side of the
fault (and the circuit monitor), the current
drops to zero.
Phase A-N Voltage
Phase A Current
178
543
89
272
0
0
-89
-272
-178
-543
But there’s more. Note the sudden half-cycle
drop in voltage followed by a ringing transient that begins the sag. This is the signature of a lightning strike and the subsequent
response of the distribution system. In
particular, the lighting strike caused a utility
silicon-carbide lightning arrestor to conduct.
A silicon-carbide arrestor appears as a
short-circuit when a lightning strike flashes
over an insulator. The short-circuit causes a
sudden collapse in voltage that is
restored at the next zero crossing of
voltage. The ringing transient that
follows the event is the typical response
of the electrical system to a sudden
change in voltage.
Phase B Current
Phase B-N Voltage
171
86
267
The sag lasts until the utility
overcurrent device, probably a
substation recloser, senses the fault
Phase C Current
Phase C-N Voltage
current and opens. We also know that
this lightning strike was located on an
adjacent feeder sharing the same
utility distribution substation as the
Figure 6: Starting a 3-phase induction motor results in a
high current similar to a downstream fault.
plant where the circuit monitor is
located. This is clear because the
Figure 6 shows a typical motor starting
voltage returns to nominal when the breaker
event. In this case, the circuit monitor was
opens. Had the fault been on the feeder
located at the motor, on the load side of
serving the plant, the voltage would have
the motor starter. When the contactor
dropped to zero, much like the second part
closes, voltage is applied. The waveform
of the event in figure 2.
capture shows this voltage. The current to
the motor quickly increases to about 490
Interpreting Waveform Captures
amperes, then gradually, over about nine
Saves Money!
cycles, increases to a nominal full-load
Learning to interpret the waveform captures
value. Note that the tip about downstream
from a circuit monitor may require a little time
faults applies here as well. Any motor
and training, but it is a skill that can save
starting event that exceeds about 7.5
money for your facility. It can help you quickly
amperes into the circuit monitor (after
diagnose a component malfunction, identify
applying the current transformer ratio) will
appear flat-topped.
0
0
-86
-267
-171
-524
178
521
89
261
0
0
-89
-261
-178
-521
Phase A-N Voltage
What's That Scratchy Event?
The last waveform capture is a bit tougher
to interpret. Sure, figure 7 looks like a
voltage sag on phase C. It was probably
caused by a single-phase fault on the
electric utility overhead distribution circuit.
And the service entrance transformer
between the fault and the plant monitoring
location is probably a wye-wye, since the sag
shows up mostly on one phase.
Phase A Current
469
314
235
157
0
0
-235
-157
-469
-314
419
Phase B-N Voltage
Phase B Current
281
210
141
0
0
-210
-141
-419
402
201
-281
Phase C-N Voltage
255
Phase C Current
127
0
0
-201
-127
-402
-255
Figure 7: The phase C-N voltage shows the characteristic footprint of a silicon-carbide lightning arrestor.
Note the sudden half-cycle voltage drop and ringing
transient on phase C.
5
PowerLogic
Solutions
a problem in your distribution system, and
troubleshoot a motor starting anomaly.
Money savings come from:
• prevention of downtime
• repairing problems once, at the root cause
• reducing equipment damage
• negotiating price adjustments with your
power supplier, for problems caused by
the supplied power
How can you learn more? Attend a
PowerLogic University class soon.
PowerLogic University constantly adds to
its database of typical waveform captures.
The experts at the University can teach
you all the subtleties of capturing and
interpreting voltage and current disturbance events. Then you, too, will be able
to maintain and improve the health of your
electric power system.
In short, the effort spent learning the
characteristics of waveform capture events
is a good investment.
Test Your PQ—Power Quotient
The following true-or-false questions will test your knowledge of the information in this
issue, as well as previous issues of POWERLOGIC Solutions. Answers are in the righthand column.
6.
5.
4.
3.
2.
1.
False. Circuit monitors capture all channels of voltage and current simultaneously when a high-speed alarm is
triggered. This issue.
True. The 87% figure is a good trade-off
value between too many and too few sag
events. Most plant equipment will not be
affected by sags below 87% voltage. This
issue.
True. Even though there is usually a
transformer between your circuit monitor’s
measuring point and a disturbance on the
electric utility circuit, the waveform capture
can provide useful information about utility
events. This issue.
False. Most sags are caused by faults on
the electric utility distribution system.
Volume 1, Number 5.
False. The amount of information you can
store depends on circuit monitor on-board
memory and its allocation. This issue.
True. Savings associated with power
monitoring start at about 4% of your total
power management costs (energy,
downtime, and equipment utilization).
Volume 5, Number 1.
6
1. POWERLOGIC Circuit Monitors capture
only the voltage or current channel
affected by a high-speed alarm like a
voltage sag.
2. Typical voltage sag pickup setting is 418V
on a 480V circuit (system 30), or 241V on
a 277V circuit (system 40 or 41), which
equates to a sag of about 87% of nominal.
3. Circuit monitor waveform captures can
give you useful information about the
quality of power on the electric utility
distribution system serving your facility.
4. Voltage sags are usually caused by
equipment inside your plant or facility.
5. Circuit monitors are limited to one
60-cycle waveform capture per day.
6. POWERLOGIC monitoring systems
typically reduce power management
costs by at least 4%.
Bulletin No. 3000HO9904 10M DL January 2000 © 2000 Schneider Electric All Rights Reserved