Application Note

Power Quality Troubleshooting Fluke Corporation 19
Transients
Transients should be distin-
guished from surges. Surges are
a special case of high-energy
transient which result from
lightning strikes (see section 5,
“PQ Troubleshooting of Light-
ning Protection Systems”). Volt-
age transients are lower energy
events, typically caused by
equipment switching.
They are harmful in a
number of ways:
•
They deteriorate solid state
components. Sometimes a
single high energy transient
will puncture a solid state
junction, sometimes repetitive
low energy transients will ac-
complish the same thing. For
example, transients which
exceed the PIV (peak inverse
voltage) rating of diodes are
a common cause of diode
failure.
•
Their high-frequency compo-
nent (fast rise times) cause
them to be capacitively
coupled into adjoining con-
ductors. If those conductors
are carrying digital logic, that
logic will get trashed. Tran-
sients also couple across
transformer windings unless
special shielding is provided.
Fortunately this same high
frequency component causes
transients to be relatively
localized, since they are
damped (attenuated) by the
impedance of the conductors
(inductive reactance increases
with frequency).
•
Utility capacitor switching
transients are an example of
a commonly-occurring high-
energy transient (still by no
means in the class of light-
ning) that can affect loads at
all levels of the distribution
system. They are a well
known cause of nuisance
tripping of ASDs: they have
enough energy to drive a
transient current into the dc
link of the drive and cause
an overvoltage trip.
Transients can be categorized
by waveform. The first category
is “impulsive” transients, com-
monly called “spikes,” because
a high-frequency spike pro-
trudes from the waveform. The
cap switching transient, on the
other hand, is an “oscillatory”
transient because a ringing
waveform rides on and distorts
the normal waveform.
It is lower frequency, but
higher energy.
Causes
Transients are unavoidable.
They are created by the fast
switching of relatively high cur-
rents. For example, an inductive
load like a motor will create a
kickback spike when it is turned
off. In fact, removing a Wiggy (a
solenoid voltage tester) from a
high-energy circuit can create a
spike of thousands of volts! A
capacitor, on the other hand,
creates a momentary short cir-
cuit when it’s turned on. After
this sudden collapse of the ap-
plied voltage, the voltage re-
bounds and an oscillating wave
occurs. Not all transients are the
same, but as a general state-
ment, load switching causes
transients.
In offices, the laser copier/
printer is a well-recognized
“bad guy” on the office branch
circuit. It requires an internal
heater to kick in whenever it is
used and every 30 seconds or
so when it is not used. This
constant switching has two
effects: the current surge or in-
rush can cause repetitive volt-
age sags; the rapid changes in
current also generate transients
that can affect other loads on
the same branch.
Measurement and recording
Transients can be captured by
DSOs (Digital Storage Oscillo-
scopes). The Fluke 43 PQ
Analyzer, which includes DSO
functions, has the ability to
capture, store and subsequently
display up to 40 transient
waveforms. Events are tagged
with time and date stamps (real
time stamps). The VR101S Volt-
age Event Recorder will also
capture transients at the recep-
tacle. Peak voltage and real time
stamps are provided.
Figure 4.3 Fluke 43 can capture and save up
to 40 transients.
Cursor moves to display peak Min/Max values.
Real-time stamp. Date:hr:min:sec