Manual Power Quality Filter PQFM
Electrical design and installation 47
6.6 Selection of the power cable size
Several types of power cable can be used to connect the filter to the network. Local
regulations and habits often determine the user’s choice. Note however that due to the
high frequency output filter of the PQF, there is no radiated emission through the
feeding cables. Consequently, there is no need for special screening of the filter
connection cables (Cf
Section 6.4
).
NOTE: For cULus versions of PQFM, the maximum size of cables that can be connected
to the fuse base is 350 kcmil
.
The following steps have to be followed to determine the section of the power cables
feeding the filter:
1. Determine the RMS current rating of the cubicle/filter for which the cable has to be
rated (Irms). The rating is marked on the cubicle label.
In standard execution each filter cubicle is fitted with its own connection point. Each
cubicle has to be individually connected to the supply and bottom or top cable entry
can be used. In this case the RMS current for which the cable has to be rated equals
the current rating of the unit to be connected to the supply.
Optionally PQFM filters can be provided with a common cable entry cubicle (WxDxH =
600 x 600 x 2150 mm excluding base frame of 100 mm height). In that case the power
cable connections for each power cubicle are centralized on a central bar system in
the entry cubicle. When this is the case, the RMS current for which the feeding cables
have to be rated equals the nominal current rating of the complete filter. The filter
identification label on the master door contains the nominal filter current rating.
2. Determine the factor X and the cable section required taking into account the skin
effect.
The multiplication factor X is a factor that takes into account that the current that will
flow through the filter connection cables is predominantly a harmonic current, i.e. a
current of which the frequency of the most important components is higher than the
network base frequency. Due to the frequency being higher than the network base
frequency a physical phenomenon called ‘skin effect’ comes into play. This effect
implies that for higher frequencies the current will not flow through the complete
cross section of the cable but will have the tendency to flow at the cable surface. The
result is that although one may use a cable of A mm², the section through which the
current flows is only X*A mm² (with X < 1). In order to compensate for this “loss of
section”, the cable has to be oversized such that the total equivalent section through
which the current flows taking into account the skin effect is acceptable.
The multiplication factor X to be used depends on the cable material (e.g. copper [Cu],
aluminum [Al]) and on the base frequency of the network on which the filter will be
installed. For a given installation its value can be determined using the following
process:
Step 1: Determine in a conventional way (e.g. using cable manufacturer’s tables) the
cable section A (mm²) for the RMS current Irms obtained in 1 above.
