70
3
Selection and application
3-8 Selecting an MCCB for capacitor circuit
If no reactors are connected in series with the capacitor, then
the R, L, and C defined by the power supply transformer
capacity, percentage impedance and capacitance will cause
wild fluctuations in the inrush current factor (first wave peak/
effective rated capacitor current), oscillating frequency and
damping constant. The amount of fluctuation is especially
significant when it comes to selecting a rated current for the
MCCB. This is why inserting reactors totaling up to 6% of the
impedance into capacitor circuits is highly recommended for
improving the power factor.
Series-connected reactors are needed because the inrush
current from other capacitors is added to the current from
the power supply if capacitors are inserted in parallel using
multiple banks without reactors.
(3) Selecting an MCCB for phase advance capacitor
circuits
Table 3-20 shows the rated current (In) for applicable MCCBs
at various capacitances. Since the conditions for selecting
MCCBs are aimed at preventing mistripping, first find the
effective current (Ict), that is, the transient current plus the
steady state current 0.01s after power is turned on. If that
current (Ict) is less than 1/10 the instantaneous tripping current
of the MCCB (10 times the rated current of the MCCB) or is
more than 1.5 times the rated current of the capacitor (Icn),
then use the main current approximating that value.
In > k
u
Ic Ic > Ict/10 or Ic > Icn
Icn: Capacitor rated current (effective value)
(Single phase: Icn =
Z
C·V, three-phase: Icn =
Z
C·V/
√3
)
In: MCCB rated current (effective value)
Ict:
Inrush current 0.01s after power is turned ON (effective
value)
Ic: Ict/10 or Icn min
k: 1.5 (margin coefficient for the allowable fluctuation error)
V: Line voltage (effective value)
Z
: 2
S
f (f: frequency (Hz) of the applicable circuit)
.OTES s4HEVALUEOFTIMESISTHESUMOFTHEMAXIMUMALLOWABLECURRENT
for the capacitor (1.3 times the rated current) and the allowable
capacitance error plus 15%.
s4HEOSCILLATINGFREQUENCYOFTRANSIENTCURRENTISMUCHHIGHERTHANTHE
fundamental harmonic. It ranges from several hundred hertz to several
kilohertz with no series-connected reactors, or less than several
hundred hertz (200 to 300Hz max.) regardless of the power supply
capacity with reactors totaling 6% of the impedance connected in
series.
s4RANSIENTCURRENTATTENUATIONISRELATIVELYFASTWITHOUTREACTORS
connected in series and is fairly slow with reactors connected in series.
(4) When capacitors are connected in parallel with
individual motor circuits to improve the power
factor
(See Fig. 3-28.)
When selecting the rated current of an MCCB, choose one
where startup inrush current-time characteristics for the
motor will not cause the MCCB to malfunction. If capacitance
less than 30% of the motor capacity is used here, then the
rated current of the MCCB should be at least three times the
rated current of the capacitor. This will prevent the MCCB
from malfunctioning even without series-connected reactors
because the capacitor is installed on the secondary side of
the magnetic motor starter. Refer to the Technical Information
for the magnetic motor starter for more details on available
models and durability characteristics.
Fig. 3-28 Capacitors connected in parallel with the motor
M
MCCB
Capacitors
MS
