Network integration of wind power - Wind Power Integration: Connection and System Operational Aspects - page 102

33 kV system (along with demand customers). (Note that 200 MW is much larger
than could normally be accommodated at 33 kV voltage level, so the example
serves to illustrate the point rather than being a practical connection.) The wind
farm was to have multiple 33 kV cable connections and the 33 kV node was a direct
connection point to the transmission system. In the first graph, the worst case
voltage step on variation of output is explored by losing all the wind farm output. It
can be observed that overall a unity or leading power factor is preferred. The sec-
ond curve shows that the tap changer varies over a narrow range and thus operates
infrequently at unity power factor generation. Separate studies have confirmed this,
showing that tap change operations are reduced by orders of magnitude.
It is therefore likely that the objective of such a wind farm's automatic voltage
regulation software will be to achieve unity power factor within an acceptable
voltage band. Outside this band, the wind farm should endeavour to control voltage
as the wind farm connected at 400 kV would. In this case, the normal control is
power factor, reverting to out-of-range voltage control and finally emergency
actions, e.g. load reduction and ultimately a wind farm trip. It should be noted that
for some types of wind farm (e.g. DFIG) it may be inappropriate to trip the wind
farm, since, by operating even at low output, it contributes positively to voltage
control.
An alternative strategy would be to target voltage at the 33 kV busbar of the
connection with the transmission system, but that voltage will already be under
pseudo-dynamic control by the transformer tap-changers (see discussion below),
creating a possible conflict between two control systems. It is to avoid such conflicts
that network managers may seek to target different parameters. (See Appendix 2,
Clause CC.S2.3.2, which requires wind farms to prove the three control objectives
of voltage control, power factor control and reactive power control.)

4.3.1.5 Rural network connected wind farms
The network design does not lend itself easily to incorporation of significant gen-
eration at lower network levels. Voltage control is a significant issue.
Consider first generation connected directly to a substation lower voltage
busbar, as shown in Figure 4.5.
Transformer automatic voltage regulators at this system level are likely to have
an objective of adjusting the lower voltage busbar voltage to take account of:

the stochastic absolute level of the higher voltage

●

the voltage drop through the transformer and to some extent the voltage reg-
ulation on connected circuits, both of which are likely to be stochastic in that
they depend on time-varying demand

●

There are a minimum of three settings on such systems, one to take account of
an average load target voltage on the lower voltage busbar and two to represent the
real and reactive components of impedance for line drop compensation (LDC).
The measured quantities are voltage and current flow through the transformers.
It can be seen that adding generation to the lower voltage bus causes the load
delivered from a normal source to decrease and thus fools the control system into