**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.

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 – (X_{s}+X_{t})I_{dc}] - α
cos µ = 1 – (2E

_{x}/E_{do})
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

E

_{x }= direct-voltage drop caused by commutating reactance
E

_{d0 }= 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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