Environmental Engineering Reference
In-Depth Information
cleared. The loss of a single wind farm, while exacerbating the position, is not seen
as a problem. That loss will be met from the response of other plant. The worry is
the widespread loss of generation that threatens the system's ability to supply all its
customers.
For a transmission fault, as stated above, the voltage will be severely depressed
over a wide area but the fault clearance time will be short due to the need for fast
protection in order to maintain angular stability of existing generators. It is common
on the highest level transmission systems to fit double main protection schemes so
that there is practically no additional time lag if one scheme fails. Total fault clear-
ance times of about 70-150 ms are to be expected (see Clause CC.S2.6.1 of the
sample grid code in Appendix 2). On the next voltage level, below the super-grid,
worst case clearance times of 300-450 ms are common. These times assume that the
main protection fails to operate and the backup is required to clear the fault. (Note
that for the sample grid code in Appendix 2 the 33 kV system also has double main
protection and is therefore fast.) On the other hand, faults in distribution systems may
take seconds to clear but they do not depress the voltage severely over a wide area.
Figure 4.12 shows how TSOs have expressed the fault ride-through requirement.
Many intend to keep the requirements under review as wind penetration grows.
It should be noted that, because in places much of the wind farm capacity is
connected to distribution systems, TSOs often require the above performance to be
mandated by Distribution System Operators.
4.6.2.6 Dynamic modelling
To understand transient behaviour, validated models are required. In the case of any
FACTS device, it is likely that high order models will be needed to design the
device. However, power system transient models consist of algebraic representations
of the circuits and transformers and, typically, third-order representation of the
synchronous generators. High order models require short integration time steps, and
therefore generally accommodate only a few network nodes, whereas 10,000 nodes
may be present in a network model. The challenge is to reduce the high order model
100
70
Lower value of
the voltage band
45
15
0
150
700
1,500
3,000
Time (ms)
Time fault occurred
Figure 4.12
Typical fault ride-through requirement
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