Page 264 - Fluid Power Engineering

Deploying W i nd T urbines in Grid 231

The old induction generators are becoming a rarity; doubly-fed in-
duction generators, synchronous generators, and permanent magnet
generators are able to control the reactive power demand.
Utilities around the world have started requiring wind plants to
maintain a power factor of 0.95 or higher. In 2005, the US Federal En-
9
ergy Regulatory Commission (FERC) proposed a requirement that
“large wind plants maintain a power factor in the range of 0.95 lag-
ging to 0.95 leading to be measured on the high voltage side of the
wind plant substation transformer.” The method of providing reactive
power was left flexible: Power electronics or switched capacitors.

Low-Voltage Ride-Through
Thus far, the discussions of connection of a wind farm to a grid have
involved disconnect or islanding of the wind plant in case of a fault
on the grid. Specifically, sudden change in voltage on the grid is one
of the conditions to cause a circuit breaker to trip, thereby disconnect-
ing the wind farm from the grid. Often, the fault is temporary (for
duration of less than about 3 s) because of lightning strikes, equip-
ment failure on the grid, or downed power line. Taking a large wind
plant offline in such situations can cause further instability in the grid
by causing other cascading tripping of generators and loads. Start-
9
ing in 2005, FERC recommended that a large wind plant stay online
and connected to the grid during temporary voltage drops. FERC has
proposed a voltage profile that serves as low-voltage ride through cri-
teria (see Fig. 11-5): A wind plant should stay online as long as the grid

Minimum Required Wind Plant Response to
Emergency Low Voltage
Voltage at point of interconnection 0.8 Wind plant required Wind plant not required
1.1
Beginning of emergency
1
low voltage
0.9
0.7
to remain on line
0.6
0.5
0.4
0.3
0.2
to remain on line
0.1
0
0.625
1
2
−1
0
Time (seconds) 3 4
9
FIGURE 11-5 Low-voltage ride-through specifications by US FERC y-axis units
are ratio of actual to nominal voltage.