Environmental Engineering Reference
In-Depth Information
DC Transmission
[1],
[2]
AC cables possess large capacitance resulting in large reactive power currents which are
proportional to cable length. Therefore the useful power that can be transmitted reduces as
length and voltage increase, with viable AC cable lengths limited to only a few tens of
kilometres. Cables when run with DC have no length limit. Additionally, they are cheaper
and less lossy. DC schemes also provide a better level of reliability if a single cable is
lost.
In the longer term, as wind farm sizes grow, perhaps to several GW, and the distance to
shore increases, it is likely that DC transmission may be used to link the wind farms to the
grid network. The cost of a DC line to transmit a certain power is considerably lower
than that of the AC equivalent. However, generators are most effi cient when designed to
provide AC, and AC is unbeatable when used to distribute energy to consumers at
different voltage levels. Hence DC transmission requires conversion from AC to DC and
from DC to AC at the extremities of the DC line. This is achieved through power electronic
converters.
DC transmission is used extensively at present in power networks when bulk power has
to be transmitted over long distance overhead lines or medium distance underground or
undersea cables. It can be shown that if the power to be transmitted is large enough and the
overhead or cable line is above a critical length, the savings in transmission line costs more
than compensates for the extra expense of the converters. Because of the DC based coupling
of the AC networks, a DC link is asynchronous i.e. the two linked AC systems do not have
to operate at the same frequency. This was illustrated in Figure 4.49 where a gearless wind
generator is connected to the network through a DC link. In conventional HVDC (high
voltage DC) links the converters used are the line-commutated converter of Figure 4.26. This
technology has a proven track record but also suffers from the disadvantages listed in Section
4.5.5 .
An HVDC link can be realized using the voltage source converters (VSC) described in
Section 4.5.6 rather than line-commutated converters. This technology has only become pos-
sible through recent advances in high power IGBTs. Unlike line-commutated converters,
VSCs have no need for an AC source commutation voltage, are capable of independently
controlling the active and reactive power and inject negligible amounts of harmonics into the
AC network. Another advantage of the VSC based HVDC is that it can be built on a modular
basis using standardized units to build up a converter station. It is because of these charac-
teristics that utilities are usually in favour of VSC based HVDC technology. Compared to
the AC option the VSC-HVDC would be substantially more expensive in near to shore
wind farm schemes but would become more favourable with increasing scheme sizes and
distances.
Figure 8.2 shows the key parts of a VSC based HVDC terminal. A mirror image of this
terminal is connected at the other end of the transmission cable to complete the system.
The heart of a terminal consists of a three phase IGBT based bridge, as shown in Figure 4.35.
Because of the limited voltage and current rating of the largest available IGBTs each of
the six 'valves' consists of a number of parallel-series-connected IGBTs to increase the
power handling capability of the terminal. IGBT chips and diode chips are connected in
parallel in a submodule. A StakPak IGBT has two, three, four or six submodules. The number
of submodules is based on the required current rating of the application. For high
