Electrical - Wind Energy: Renewable Energy and the Environment - page 142

Type1 Type 2

Plant
feeders

Plant
feeders

Generator

Generator

PF control
capacitors

PF control
capacitors

ac
to
dc

Slip power
as heat loss

Type3 Type 4

Plant
feeders

Plant
feeders

ac
to
dc

dc
to
ac

Generator

Generator

ac
to
dc

dc
to
ac

Full power

Partial power

FIGURE 7.12 Four types of dynamic models, wind turbine connection to the grid.

however, the wind speeds are so variable it is improbable for self-excitation to last very long.
Fluctuations in voltage and frequency need to be kept within ranges acceptable to the utility at the
point of connection to the utility grid (Figure 7.13).
The faults on the utility grid will also cause a reaction from the wind turbines. A wind farm
was monitored for 1 year [11], and there were 215 faults. At the monitoring node the voltage drop
and spike in current describe the fault ( Figure 7.14 ). The fault events mostly occurred far from the
wind farm, and most were cleared within ten cycles. Therefore, voltage ride-through capability of
the wind turbines is important. For the doubly fed induction generator, the rotor currents increase
very rapidly and should be disconnected from the grid within milliseconds to protect the con-
verter. When constant-rpm wind turbines come back online they need a lot of reactive power, which
impedes the voltage restoration.
The loss of generation from the wind farm during fault varies from 0 to 100% of the wind farm
capacity. In terms of loss of generation, the benefit of wind power generation is the amount of power

Pad
transformer

Main
substation

Monitoring
node

Wind
turbine

Collector
system

Transmission
line

Infinite
bus

FIGURE 7.13 Typical network topology of a large wind farm.