Mitsubishi Electric FR-A700 Series, Техническое руководство

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509
SELECTION
S
E
LECTION
3
(high-voltage and special high-voltage power
capacitors), JIS-C4901 (low-voltage power
capacitors) stipulate their harmonic immunities.
When parallel resonance is produced by
harmonics, an excessive current entering the
power capacitor may overheat the capacitor and
cause dielectric breakdown.
Ordinarily, the power supply impedance is often
small enough (the power supply capacity is large)
and the power capacitor rarely results in a failure.
When a low-voltage power capacitor susceptible
to harmonics is used, it is recommended to use
the one with a 6% series reactor.
Indicated below is the single-wire diagram and its
equivalent circuit where the power capacitor is
connected in parallel with the inverter. The
harmonic current In generated by the inverter is
divided into a harmonic current Isn which flows
into the power supply and a harmonic current (Icn)
which flows into the capacitor. Icn is found by the
following formula:
Icn : Harmonic current flowing into the capacitor
Xs : Power supply impedance
Xr : Impedance of series reactor
Xc : Impedance of power capacitor
n : Harmonic order
If nXs + nXr - Xc/n = 0 in the above formula,
resonance occurs and a very large current flows
in the power capacitor, burning the capacitor. To
prevent this, reduce the capacitor current using
the series reactor, or insert the power-factor
improving reactor in the inverter input circuit or DC
circuit, thereby reducing the harmonic current
from the inverter.
* The power-factor improving capacitor and surge suppressor
on the inverter output side may be overheated or damaged
by the harmonic. Also, since an excessive current flows in
the inverter to activate overcurrent protection, do not provide
a capacitor and surge suppressor on the inverter output side
when the motor is driven by the inverter. To improve the
power factor, insert a power factor improving reactor on the
inverter's primary side or DC circuit.
The harmonic immunity of the capacitor is specified in the
JIS Standards, e.g. the effective current including the
harmonic current found by the formula on the left shall be
within 130% of the rated capacitor current, and within
120% for the one with the series reactor.
Power
supply
Isn
In
Icn
Xr
Xc
Xs
nXs
Isn
In
Inverter
Xc
n
nXr
Inverter
Icn
Equivalent circuit regarding the inverter as a harmonic
current source
Icn = (
nXs
)
In
nXs + nXr - Xc/n
2) Synchronous generator
When the power is supplied to the inverter by
an engine generator or when the inverter is
connected to a line where a synchronous
generator is running in parallel with the
commercial power supply, a harmonic current
generated by the inverter is divided between the
synchronous generator and commercial power
supply line. An induction current develops in the
braking winding and field winding of the
synchronous generator. If the induction current
is too large, heat generated may lead to
increased loss (reduced output), overheat,
shorter life etc.
In a synchronous generator, assume the loss of
harmonic current to be equal to the loss of
negative phase-sequence current. In that
assumption, adjust the equivalent negative
phase-sequence current of the synchronous
generator caused by harmonics to be the 15%
or less of the permissible negative phase-
sequence current provided in JEM1354 (diesel
engine driving land synchronous generators).
Equivalent negative phase-sequence current I
2
In : Harmonic current
n : Harmonic order
When a synchronous generator is used, loss
due to the harmonic current is large. If it
exceeds the permissible value of the damper
winding, select the large-capacity generator or
design the generator which allows the loss due
to the harmonics. Alternatively, insertion of a
reactor in the inverter input circuit or DC circuit
is effective to reduce the harmonic current.
Inverter
MCCB
Power factor improving
AC reactor
Do not provide a power
factor improving capacitor
Power factor improving
DC reactor
Motor
Power factor improvement of the inverter
I
2
=
(
4
n
2
In)
2