3.2.5 Filter Power Section
Line power enters though the input terminals or the
disconnect and/or the RFI option, depending on the unit’s
configuration. If the unit is equipped with optional fuses,
these fuses limit damage caused by a short circuit in the
power section.
The three line phases are fed to a Harmonics Isolation
reactor (HI reactor) that distributes line power to the
inverter (or the Adjustable frequency drive for the LHD). If
the filter is used as a stand alone AAF unit, the HI reactor
is considered a line power-side filter containing only the
line power-side reactor Lm.
The line power will not be applied to the inverter until the
intermediate circuit (DC bus) has been charged and the AC
contactors have cut in. This happens after the soft charge
circuit has charged the intermediate circuit capacitors in
the inverter. By turning the filter on, the inverter connects
to the line power through the inverter side reactor (Lc),
the AC contactors, and the HI (Lm) reactor. At this stage,
the DC voltage is boosted, the amount depending on the
AC line voltage.
3.3 Additional Circuitry
3.3.1 AC Contactor
Because the line power contactor circuit ties into the soft
charge circuit, understanding both is necessary to
understand the working principles of the Line power
contactor. The filters contain two three-phase normally-
open contactors. These are used as single-phase by
shortening all input terminals together and all output
terminals together. This is done to reduce the size of the
contactors. Since the DC link is floating, this guarantees
that there is no current flow when only two phases are
opened. The two virtual single-phase contactors are in
front of phase R and T, respectively.
The line power contactors connect or disconnect the active
filter inverter from the line power. These contactors are
commanded to close after the soft charged period has
passed and before operation of the filter begins. The
contactor is commanded open if the filter stops for any
reason, such as when an alarm condition is detected, or
when the filter is commanded to stop or sleep. It is only
closed when the filter is ON, thereby minimizing standby
losses.
When the line power contactor is open, active filter control
is maintained by the soft charge circuit. The status of the
main contactors is monitored via an auxiliary contact
reporting to terminals 32 and 33 on the FC control card
(PCA1).
3.3.2 Soft Charge Circuit
Because the soft charge circuit ties into the main contactor
circuit, understanding both is necessary to understand the
working principles of the soft charge circuit.
The purpose of the soft charge circuit is to:
•
Limit inrush current when DC link capacitors are
charged
•
Provide control power when the line power
contactor is open due to faults or when the filter
is in sleep mode
The soft charge circuit contains MOVs, fuses, resistors, and
a control transformer. Three fuses on the grid side protect
the circuit. Three delta-connected MOVs suppress transient
when present on the incoming line power.
Resistors in series with the L1-L3-phases limit the inrush
current during start up when the DC link capacitors are
not charged. When the capacitors are charged and the
filter is commanded to operate, the contactors across the
resistors energize and short circuit the resistors. Voltage for
the coils of these contactors is supplied by the soft charge
control transformer.
The soft charge control transformer has one primary and
two secondary sides. The line power contactor is powered
within 110–127 V. Depending on the AC line voltage, the
line power contactor is powered by one of the two soft
charge control transformer secondaries. Control is from
connector FK100 on the power card (PCB3).
When the filter is powered on the soft charge circuit, the
DC link capacitors will be charged to approximately
sqrt(2)*line to line AC line voltage. The soft charge time
depends on main voltage and filter type. Standby current
is 0.3 A.
lists the soft charge time and RMS
current.
Filter size (A)
Imax (RMS)
Soft charge time (s)
342 V
550 V
342 V
550 V
190
3.3 A
5.2 A
1.2
0.3
250
3.3 A
5.3 A
2
0.4
310
3.3 A
5.3 A
2
0.4
400
3.3 A
5.3 A
3.7
0.7
Table 3.3 Soft Charge Electrical Characteristics
Internal Active Filter Oper...
VLT Advanced Active Filter AAF006 D and E Frames Service Manual
MG90Z122 - VLT
®
is a registered Danfoss trademark
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