Split-Flow Path Of Harmonic Current - Mitsubishi Electric FATEC FR-A800 Manual

7
REVIEW OF INSTALLATION ENVIRONMENT
7.1.3

Split-flow path of harmonic current

When harmonic current is used in the power distribution system, the power supply of the harmonic
current is not a general commercial power supply but the source of harmonics (the converter section
for the general-purpose inverter). The commercial power supply (low and high voltage power
transformer) becomes a load on harmonics. Therefore, as shown in the example of Fig. 7.4, the
harmonic current "In" generated from the inverter (I
number)) splits and flows in proportion to the impedance of power transformer (
the devices connected in parallel with it (in the example of Fig. 7.4, the motor B and the capacitor), in
other words the reciprocal ratio 1/Z between the impedance of the motor B (
impedance of the capacitor (
Since the frequency of harmonics is higher than that of the power supply, the impedance of the
capacitor decreases and thus the harmonic current tends to flow into the capacitor.
As a result, the capacitor may be overheated or damaged.
• Current tends to flow into the smaller impedance, and the harmonics may increase when
there are capacitive impedance elements (such as a power factor correction capacitor).
Therefore, in harmonic problems,
1) Since the impedance at the power supply side
capacity of the power supply system, the larger the power supply capacity is, the less the
influence of other elements is. 2) Inductive loads can be ignored since it is high
impedance for harmonics. 3) Only capacitive loads such as a power factor correction
capacitor should be considered.
7.1 Power supply of inverter (harmonics and instantaneous power failure)
= j X - jX / n).
Ｚ
C n r C
Commercial power supply
Power transformer
X
L
R
L
I
Ln
I
n
I
Mn
Inverter
M M
Motor A Motor B
Fig. 7.4 Power distribution system
= I + I + I +..... (n stands for the ordinal
n 2 3 4
I
Cn
X
r
X
c
Capacitor
X
M
R
M
S is represented by the short-circuit
Ｚ
L = RL + jnXL) and
Ｚ
= R + j X ) and the
Ｚ
M M n M
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