Network integration of wind power - Wind Power Integration: Connection and System Operational Aspects - page 108

It is argued that, if the wind gusts so that the farm is tripped and the system can
survive this loss (as it must), then the system can stand the sudden loss of the wind
farm for any other reason. Therefore, the level of security of connection is generally
a matter of wind farm operator choice, at least where this does not affect the
backbone system.
The issue then becomes one of complexity management. Consider the system
shown in Figure 4.9.
There might be one or two connections to the wind farm. Clearly if there is one
connection only (say A), the loss of the connection causes the complete outage of
the wind farm. This could have serious economic consequences in the case of an
undersea cable, where repair involves reserving a suitable vessel, ordering mate-
rials and awaiting a weather window.
Where there are two connections, the loss of, say, the B circuit reduces the
wind farm output to the rating of A. The problem then becomes nontrivial in that
the rating of the circuits is an economic balance. If no benefit were obtained from
covering the loss of another circuit, each cable would be rated at 50% of the total
requirement. However, in reality, the economic loss resulting from the outage of
one circuit is calculated as

ð the risk of outage Þ the economic consequences ð Þ

Suppose the wind farm in Figure 4.9 has a capacity of 200 MW and an esti-
mated life of 20 years. Cable A and B are rated at 100 MW and are each expected to
suffer a 3 month outage in 30 years, giving an unavailability of 0.25/30 ¼ 0.00833
per annum.
In the event of cable A outage, the loss of wind production will depend on the
probabilities of generation at various levels above 100 MW. The load duration
curve above 100 MW is assumed to be a straight line joining the following points:

100 MW 0.25 8,760 ¼ 2,190 hours
200 MW 0 hours

The annual production above 100 MW is therefore

100 2,190/2 ¼ 109,500 MWh

Main
system

A

B

Figure 4.9 Wind farm with double-circuit connection