Environmental Engineering Reference
In-Depth Information
Transmission
cable
Converter
valves
DC capacitors
Converter
reactor
AC bus
Transformer
Filter
Control
system
Figure 8.2
Schematic of a VSC based HVDC system. (Reproduced with permission of Asea Brown
Boveri Ltd)
power ratings requiring a high DC voltage, more than one hundred StsckPacks are connected
in series.
If the desired DC voltage does not match the AC system voltage, a normal AC transformer
may be used in addition to the converter series reactor. The converter has a PWM switching
frequency of 2 kHz. Since there are only high frequency harmonics generated on the AC side,
fi ltering can be done using a simple high pass fi lter. On the DC side there is a capacitor that
also serves as a DC fi lter. The two pole confi guration shown in Figure 8.2 where the middle
of the two pole DC system is earthed, enhances transmission reliability. In the case where a
fault occurs in one pole, the other can provisionally continue transmission with an earth
return.
This type of reliability is not possible in a three-phase AC system. After a transient loss
of the AC supply, the asynchronous nature of the HVDC link allows 'black-starting' by con-
trolling the frequency from zero to the required wind turbine frequency, while ramping up
the wind farm voltage.
The VSC based HVDC technology is advancing fast. Manufacturers will soon be able to
offer such links rated at 1 GW and operating at
300 kV on the DC side. This will substantially
extend the off-shore range and size of wind farms that can be potentially exploited. Whether
such schemes will be realized in the future depends on the shortage of near-shore sites and
on the technical success and reliability of the present offshore schemes, which are operating
or are under construction.
±
Other Demands on Large Offshore Wind Farms
Due to the economies of scale, future offshore wind farms are likely to be an order of mag-
nitude larger than the present onshore schemes. The rating of such wind farms will be com-
parable to that of other traditional generating plant on the grid. As a consequence, they will
