Network integration of wind power - Wind Power Integration: Connection and System Operational Aspects - page 96

Whilst in this section the discussion is about the potential deficiency in reac-
tive power, it needs to be noted that the active gap may also be a serious issue.
Section 4.6.2 deals with dynamic modelling.
How the steady-state and dynamic increased reactive demands are to be met is
another matter, and the subject of an optimisation study taking account of the costs
of increased wind farm converter size, provision of centralised versus distributed
reactive compensation, mechanically switched technologies and even rotating syn-
chronous compensators (which also add to system inertia and support fault level).
Traditional generation has an availability of about 98% and is thus expected to be
delivering full active power output when a system outage requires maximum reactive
contribution. By contrast, wind turbines may not be rotating for 20% of the time.
It has been reported (Gardner et al. , 2003) that some wind farms generate
below 5% of capacity for 50% of the time. The system must therefore be designed
and operated to do without some or all of their output. Intermittency suggests that
even when the wind is optimal, allowance must be made for output variation. It is
therefore less critical that wind farms can deliver rated reactive output con-
temporaneously with rated real power output. It is a matter of control to prioritise:

reactive contribution when voltage falls ●

reactive absorption when voltage rises ●

over the active power output. (See Appendix 2, Clause CC.S2.3.2 Figure 2 - Type
B units.)
In the United States, NERC have said in the IVGTF Task Force 1-3 Report,
under Specification of Reactive Range : 'The reactive range requirement should be
defined over the full output range, and it should be applicable at the point of con-
nection.' The report further states under Impact of System Voltage on Reactive
Power Capability : 'It should be recognized that system voltage level affects a
generating plant's ability to deliver reactive power to the grid and the power sys-
tem's requirement for reactive support.'
There is reported to be little surplus cost in wind farm construction for a power
factor range of 0.95 lagging-0.95 leading at full output, as this has become the
standard for most manufacturers. Irrespective of what is available, the generator is
following a circle diagram (Figure 3.17), albeit with limits represented by chords,
and therefore the machine is being oversized by the manufacturer to supply reactive
power. To extend the range beyond the above it is necessary to either produce
further oversized plant and converters or to forego some active output at the time.
This has cost implications which become hard to justify.
Two approaches have been considered regarding the capability and capacity
of the reactive power range of wind farms. In some places it is deemed sufficient
for the proportion of reactive power to active power to be constant (i.e. constant
power factor). In other places a fixed amount of reactive power is required
throughout the active power range. These philosophies are encapsulated in
Figure 4.1. It is likely that this type of approach will be followed for all types of
renewable energy generators connected to public supply systems, whether wind,
ocean or solar energy.