5.2.3 Compatibility with Other Equipment
on the Same Line Power
Most problems are associated with the circulation of high
frequency switching current harmonics, generated by
active input devices through leakage capacitance of the
power distribution system components, like power cables,
supply transformers, and so on. The circulation of high
frequency current harmonics can produce interaction with
other equipment connected to the same bus, increasing
the amplitude of neutral currents and activating the
operation of zero-sequence relays.
Problems associated with ground protections (ground fault
relays: ELCB, RCD, or GFCI)
Normally, ground faults are eliminated with zero-sequence
relays connected through ring transformers or to the
neutral-to-ground connection. With an active filter
connected to the power distribution system, high
frequency switching current harmonics sink into the
ground across parasitic grid capacitances. This results in
improper operation of ground fault relays.
Avoid this problem by replacing the fault relay with non-
sensitive high frequency relay. To ensure effective
protection and unintended tripping of protective relays, all
relays must provide protection of 3-phase equipment with
active current input and brief discharge at power-up. It is
recommended to use a type with adjustable trip amplitude
and time characteristics. Use a current sensor with a
current sensitivity of more than 200 mA and not less than
0.1 second operating time.
Problems associated with UPS units
A UPS unit can become distorted by active filter switching
noise in the line power supply. The power failure detector
of the UPS unit can be irritated by high frequency
switching harmonics in the AC line voltage. As a result, the
UPS could remain on battery power, unable to reconnect
the line power supply voltage.
An option to avoid this problem is tuning the power
failure detector of the UPS unit by changing setup
parameters. Another option is replacing the UPS with a
unit not sensitive to high frequency switching harmonics.
5.2.4 Line Power Resonances
In most common cases, active filters do not affect the load
in the form of a resonance condition. The active filters are
capable to operate in a resonance condition to a minimum
of the 31st harmonics order.
With CTs on the load side, resonant conditions occurring
within the electrical power system between the active filter
and the load don't interfere with active filter functioning.
At light grid loads, the grid resonance frequency changes
with grid loads and can interfere with the active filter.
Filters with CTs installed on the PCC side (light loaded)
might get unstable or experience runaway (uncontrollable)
compensation. To avoid this, use either the sleep mode
function to deactivate the filter at light loads or use
selective harmonic compensation to omitting harmonic
compensation near the light load resonance point.
In the case of line power resonances, the following trips
can occur:
•
WARNING/ALARM 7, DC overvoltage
•
WARNING/ALARM 302, Cap. overcurrent
•
WARNING/ALARM 304, DC overcurrent
•
ALARM 311, Switch. freq. limit
In general, power supply grids with long cables (above
1640 ft [500 m]) have a higher probability of resonance
issues compare to grids with short cables.
5.2.5 Control Logic Problems
Problems with control logic can often be difficult to
diagnose, since there is usually no associated fault
indication. The typical complaint is simply that the filter
does not respond to a given command.
The filter is designed to accept a variety of signals. For
troubleshooting, first determine what types of signals the
filter is receiving. There are six digital inputs (terminals 18,
19, 27, 29, 32, 33) and two analog inputs (53 and 54). (See
Filter Inputs and Outputs.) Using the status information
displayed by the unit is the best method of locating
problems of this nature. By selecting within parameter
group 0-2* Display, line 2 or 3 of the display can be set to
indicate the signals coming in. The presence of a correct
reading indicates that the desired signal is detected by the
microprocessor. This data also may be read in parameter
group 16-6*.
If there is not a correct indication, the next step is to
determine whether the signal is present at the input
terminals of the filter. This can be performed with a
voltmeter or oscilloscope in accordance with the Input
Terminal Signal Test (see
). If the signal is
present at the terminal, the control card is defective and
must be replaced. If the signal is not present, the problem
is external to the filter. The circuitry providing the signal
along with its associated wiring must then be checked.
Active Filter and the Power...
VLT Advanced Active Filter AAF006 D and E Frames Service Manual
5-2
MG90Z122 - VLT
®
is a registered Danfoss trademark
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