5 Active Filter and the Power Grid
5.1 Grid Variations
5.1.1 Grid Configurations
Active filters operate with all typical grid configurations
such as:
•
3-phase, 3-wire
•
3-phase, 4-wire
•
Grounded wye
•
Ungrounded/isolated wye
•
Delta wire
•
50 Hz +/-10% tolerance
•
60 Hz +/-10% tolerance
5.1.2 Grid Impedance
The short-circuit impedance or percent impedance of the
power supply represents the grid impedance. In supply
systems with short cables (below 1640 ft [500 m]), the
short-circuit impedance (impedance voltage) of the
transformer or the power supply generator corresponds to
a minimal value of the grid impedance on the point of
common coupling (PCC). The maximal value depends on
low voltage grid wiring type, length, and upper voltage
level grid impedance. In the case of unknown values, the
maximum is estimated as double the supply transformer
short-circuit impedance value.
The correct current of the filter depends on the grid
impedance. For higher grid impedance, the 10% filter
correction current is reduced.
Active filters have no limitations to the lowest grid
impedance. But from the installation point of view, it is
important that the available short circuit current of the
grid is less than potential capacitor overcurrent of 3% of
the filter rating.
5.1.3 Voltage Pre-distortions
Active filters are suitable for operation under non-
sinusoidal voltages. A total harmonic voltage distortion of
up to 10% should not affect the active filter performance.
If active front end based drives or other active input
devices are present on the same grid, the high switching
noise can overload the damping resistor of the LCL filter.
The amplitude of voltage harmonics above 25th order
should be not higher than 3%.
WARNING/ALARM 302, Cap. over current usually indicates
high voltage pre-distortions or high grid impedances.
5.2 Basic Troubleshooting Background
5.2.1 Line Phase Loss and Unbalanced
Phase Trips
The active filter monitors phase loss by measuring the AC
capacitors currents. If phase loss is detected, the filter trips
with ALARM 4, Line phase loss after a time. The time
response of the phase loss detection is approx. 0.5 s
When the input voltage becomes unbalanced, no phase
disappears completely. ALARM 4 is not issued. However,
the following trip alarms may occur:
•
WARNING/ALARM 7, DC overvoltage
•
WARNING/ALARM 302, Cap. overcurrent
•
WARNING/ALARM 304, DC overcurrent
•
ALARM 311, Switch. freq. limit
•
WARNING 321, Volt. imbalance >3%
Severe imbalance of supply voltage or phase loss can
easily be detected with a voltmeter by measuring the line
to line voltages.
5.2.2 Voltage Dips and Flickers
Active filters are suitable for operation on grids with
voltage dips and flickers. The active behavior depends on
the duration, depth and affected phase number of the
voltage dips. When voltage dips threaten possible damage
to active filter components, the active filter stops operation
with following faults:
•
WARNING/ALARM 4, Mains phase loss
•
ALARM 300, Mains cont. fault
•
ALARM 305, Mains freq. limit
Active Filter and the Power...
VLT Advanced Active Filter AAF006 D and E Frames Service Manual
MG90Z122 - VLT
®
is a registered Danfoss trademark
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