Network integration of wind power - Wind Power Integration: Connection and System Operational Aspects - page 90

an anti-paralleling thyristor soft-start arrangement to reduce the fluxing surge experi-
enced by the network when the generator circuit breaker is closed (see Figure 3.16).
When the generator is fully fluxed the anti-paralleling arrangements are by-passed.
If not controlled, the fluxing period can lead to a large drain of reactive power from the
grid, which creates an unacceptable voltage depression and a step change in voltage.
The degree of voltage depression and step will be a function of the size of the
induction machine and the strength of the network at the point of connection. Since
wind turbines tend to be on sites remote from dense population, the network is often
weak at the point of connection, i.e. electrically remote from generation sources and
hence of low fault level.
It is uncommon to have the power factor capacitance switched-in during
starting, which exacerbates the issue, but with accurate control the starting current
can be reduced to between 1.6 and 1.0 times full load current. It is nonetheless at
very low power factor.
In the case of retrofit projects it is possible, but expensive, to install dynamic
reactive compensation. Devices such as SVCs, STATCOMs and DVARs can be
used. These are described in Appendix 1. Such devices may also be justified for
new installations to improve fault ride-through (see Section 4.6). Combinations of
static and dynamic voltage support are possible.
DFIG machines are also accelerated by wind energy and no major starting
voltage dips are evident due to their ability to control reactive as well as active
power generation - see Section 3.6.
Full speed range machines are usually synchronous machines behind a fully
rated converter (Section 3.6) and may be accelerated by the wind energy. The
network effects are entirely dependent on the performance of the converter, which
will be a voltage source device (see Appendix 1), to cope with the absence of fault
level on the rotor side. The converter will draw only active power from the grid
system and will supply the required power factor to the generator during fluxing.
For wind farms the major issue is frequent starting and stopping in gusting or
marginal wind conditions, therefore under adverse conditions the voltage dis-
turbance level could be very high. Wind farms contain a number of turbines, and
can be subject to overall control by a wind farm management system. An objective
of the system can be to control the wind farm so that only one machine may start at
any time, or alternatively the voltage may not be allowed to fall below a threshold
value during starting. This reduces the network impact considerably. In the early
days utilities sometimes imposed a 10 minute gap in return to service of wind
turbines, but that has largely been replaced by the wind park management system
because it is recognised that wind turbines tend not to start or stop together.
An important issue relates to energising the wind farm feeder. Each wind
turbine commonly has an associated network transformer (Figure 3.16). Energising
a large wind farm implies simultaneously fluxing a large number of transformers.
This is similar to energising a distribution feeder, which also has a large number of
transformation points to distribution load. It is common for standards (e.g. P28 in
the United Kingdom or IEC 60868 - see Section 4.5) to refer to a voltage limit for
infrequent operation, which may be twice the maximum amount allowed for