Selection Of The Power Cable Size - ABB PQFS Installation, Operation And Maintenance Instructions

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.
The following steps have to be followed to determine the section of the power cables
feeding the filter:
1.
2.
38 Electrical design and installation ç Manual Power Quality Filter PQFS
Master
Main earth
connection
point in
filter
cubicles
Secondary
earth
connection
point
Figure 30: Earth connection guidelines for a multi-unit PQFS
Determine the RMS current rating of the enclosure for which the cable has to be
rated (Irms).
The rating is marked on the enclosure label.
Each enclosure has to be individually connected to the supply and bottom cable
entry has to be used. If required an optional cable connection box can be added to
allow for multi-cable termination.
The minimum cable section to be used for the power conductors is 16 mm².
The RMS current for which the cable has to be rated equals the current rating of the
unit to be connected to the supply. Note that the neutral connection has to be able to
carry three times the unit current rating.
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.
Slave 1
PE PE
Slave 2
PE