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Under exceptional conditions it is possible that the active filter is showing a 100% load indication whereas its nominal
RMS current rating is not yet attained. This is because the filter has reached an operating limit other than the RMS
current limit. Possible other limits are:
• Temperature limit due to a too high ambient temperature or a failing cooling system.
• Peak current limit due to an a-typical peak current requirement of the load.
• Peak voltage limit due to an a-typical DC-link voltage requirement of the load or due to a high network voltage.
Under all these conditions, the filter will run in limited mode and may not attain 100% of its nominal current rating.
– Harmonics put in 'standby' by the filter system:
When selecting a harmonic that has not been selected before, the filter will identify the network characteristics for
this harmonic. After this process, the filter will launch the filtering process for the component considered. If during
the network identification process for a given harmonic a special (problematic) condition is encountered, the sys-
tem puts the component in 'standby'. In that case the harmonic selected is not filtered for the time being. Special
network conditions include extremely high impedance of the supply network or extremely low impedance towards
the load. When consulting the harmonic selection table of the PQF-Manager, harmonics put in 'standby' can be
recognized by the label 'S' that is displayed in the harmonic selection column (which reads otherwise either 'Y' or
'N'). The following possibilities exist to bring a harmonic out of standby:
• The user restarts a network identification process by changing the 'S' indication into a 'Y' indication in the har-
monic selection table.
• The filter automatically restarts an identification process on all harmonics that were put in standby previously
when a successful identification of another harmonic is made. As a result, the harmonic considered will be au-
tomatically filtered when the network conditions allow for this.
– If plain capacitors (i.e. capacitor banks not incorporating detuning reactors) are present in the network it is recom-
mended to switch them off or change them into detuned banks. Sometimes, the commissioning engineer is faced
with an installation where both an active filter and plain capacitors are present however. While this is an ill-advised
and a technically unsound situation, HITACHI has acknowledged that in this case also the active filter should aim
to give an optimal performance. For this reason the control software of the filter incorporates a Stability Detection
Program (SDP) that aims to increase the filter performance in this type of applications.
In installations where plain capacitors are present and cannot be switched off or changed to detuned capacitor
banks, adhere to the recommendations below for optimal results.
• Install the capacitor banks upstream of the active filter CT measurement location
• Set the filter in Mode 3
– In installations where detuned banks are present, it is recommended not to select harmonic orders below the tun-
ing frequency of the detuned banks.
types of detuned banks.
Table 15: Recommended harmonics to be deselected for different detuned bank types
Detuned bank type
5.67 %
6 %
7 %
14 %
STATUS
Approved
Table 15
indicates the harmonics recommended to be deselected for different
Harmonics recommended to be deselected
2, 3, 4
2, 3, 4
2, 3
2
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