Voltage Notching is described by IEEE
as a recurring power quality disturbance due to the normal operation of power
electronic devices (i.e. rectifier), when current is commutated from one phase
to another. Conventionally, the current waveform is used as the starting point
for harmonic analysis, and voltage notching is simply derived from the IZ drops
of the harmonic currents.
In addition, voltage notching
characterizes an unusual case that falls between harmonics and transients. This
is because notching takes place in steady-state, thus, it can be distinguished
by the harmonic spectrum of the affected voltage. On the other hand, the components
of the frequency related to voltage notching are somewhat high and may not be promptly
categorized with a measurement device commonly employed for harmonic analysis.
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| Voltage Notching |
Sources
Voltage Notching is primarily caused
by three-phase rectifiers or converters that generate continuous DC current. As
mentioned, the voltage notches happen when the current commutates from one
phase to another. Subsequently, a momentary short circuit between two phases
will occur during this period.
Moreover, the depth of the notch at
any point in the system is influenced by the source inductance, the isolating
inductance between the rectifier/converter, as well as the point being examined.
Consequently, the width of the notch is the commutation angle.
µ = cos-1[cos α – (Xs+Xt)Idc]
- α
cos µ = 1 – (2Ex/Edo)
where:
µ = commutation angle
α = delay angle
Xs = system reactance in per unit on
converter base
Xt = converter transformer reactance
in per unit on converter base
Idc = DC current in per unit on
converter base
Ex = direct-voltage drop
caused by commutating reactance
Ed0 = theoretical direct
voltage
Effects and Solution
Voltage Notches introduce harmonic and
non-harmonic frequencies that are much higher than those found in higher
voltage systems. Usually, these frequencies are in the radio frequency range, that cause negative operational effects, such as signal interference
introduced into logic and communication circuits. Also, when of sufficient
power, the voltage notching effect may overload electromagnetic interference filters, and other similar high-frequency sensitive capacitive circuits.
Furthermore, the solution for voltage
notching typically involves isolation of the critical and sensitive equipment
from the source (i.e. rectifiers) of the power quality problem.
Synopsis:
Duration: Steady-state
Source: Electronic Devices (e.g.
converters)
Symptoms: Malfunction
Occurrence: Very Low
Mitigation: Isolation
References:
IEEE 519-1992. Recommended Practice
and Requirements for Harmonic Control in Electrical Power Systems
IEEE 1159-1995. Recommended Practice for
Monitoring Electric Power Quality
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