Wind power technology - Wind Power Integration: Connection and System Operational Aspects - page 72

MVAr import

B

Effect of PFC

A

Power import Power export

Figure 3.17 Circle diagram of induction machine (showing effect of power factor

correction)

This can be avoided either by limiting the size of the capacitor bank or by arranging
protection to trip the capacitors rapidly in such an event.
On an electrical power system, network short circuits are usually detected by
sensing fault current from large synchronous generators. Induction generators only
provide fault current into three-phase short circuits during the so-called sub tran-
sient period and this is too short for reliable operation of over-current relays. Hence
induction generators cannot be considered as a reliable source of fault current. Thus
it is conventional practice, in the event of a network fault, to rely on the short-
circuit current from the network to operate protection to isolate the wind farm and
then to use under/over-voltage or frequency relays to trip the wind turbines.
An important limitation of fixed-speed wind generators is that they can over-
speed and lose stability if the network voltage is depressed. Voltage depressions
can occur over a wide geographical area due to short-circuits on the main trans-
mission network. In this case the low terminal voltage of the induction generator
allows the generator to over-speed and draw high values of reactive power. This in
turn lowers the network voltage further and leads to voltage collapse. It is clearly
very undesirable that, just when the power system is potentially stressed due to a
short circuit, wind turbines, over a wide area, will trip. Hence the transmission
system operators, who are responsible for the security of the power system, are
imposing so-called 'fault ride-through' requirements. These require that, in the
event of a fault on the high voltage transmission system (275 kV or 400 kV), which
depresses the transmission network voltage to zero, the wind turbines continue
to operate. These requirements are difficult to meet with simple fixed-speed
wind turbines.

3.6 Variable-speed wind turbines

In recent years the size of wind turbines has become larger and the technology has
switched from fixed speed to variable speed. The drivers behind these develop-
ments are mainly the ability to comply with connection requirements and the