Page 260 - Power Electronic Control in Electrical Systems

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248 Power electronic equipment

Other advantages include increased capacity for power transmission within a fixed
corridor and power transfer between two AC networks that is frequency and phase
independent. Finally, due to non-mechanical parts in the power conversion, a fast
modulation and reversal of power can be achieved, and due to the nature of the DC
link, the fault currents between the two systems are not transferred from one network
to another.
Higher order HVDC systems based on 24-pulse or even 48-pulse converter
arrangements are also possible. In this case the harmonics can be shifted around
the 24th and the 48th harmonics respectively. This means the most significant
harmonics will be the 23rd and the 25th harmonics for the 24-pulse converter and
47th and 49th for the 48-pulse one. Clearly such harmonics are easier to filter when
compared with the 11th and 13th harmonics of the 12-pulse system. However, in this
case, the transformer must be designed to have phase-shifting properties other than
the star±delta connection (30 ). Being a high voltage transformer, this makes it quite

costly to the point that AC side and DC side filters are an easier and cheaper way to
use to filter the low order harmonics (at least the 11th and the 13th harmonics due to
the 12-pulse converter). A bipolar HVDC system based on 24-pulse converters per
pole is drawn in Figure 6.77.

Fig. 6.77 Bipolar HVDC power transmission system based on 24-pulse converter for each pole.