IEEE Power & Energy Magazine - November/December 2016 - 32

the side of the machine is connected to the stator, rather
than the rotor. Unlike the crowbar circuit, which is completely separate, the chopper circuit is located in the dc link
of the converter, making it more cost effective than the bigger crowbar circuit.
Crowbar Tests

On-site test waveforms of a DFIG-based wind turbine at
high power output with an active crowbar were successful. Wind turbine active and reactive powers are shown in
Figure 9. In this test, the initial power of the turbine was
at approximately the turbine rating of 2.0 MW, and the
wind speed was in the range of 15-17 m/s. At this wind
speed, the turbine was actively using turbine blade pitch
control to maintain rated power output. The initial blade
pitch was about 10.35°. The three-phase voltage dropped
to 20% of nominal (0.2 Un) for 625 ms. During the voltage
drop, active power (Ppos) declined to 200 kW at maximum. About 60 ms after the voltage drop, the wind turbine
provided 496-kVa reactive power (Qpos). After the fault
was removed, the power fluctuation of the wind turbine
due to the stimulation of the drive-train dynamics continued for about 2.8 s.

750

2.0
Ppos
Qpos

450
300
150

0.8

0.4
0.0

(kVar)

(MW)

1.2

-150
0

(s)

100

0
-50

600
400
0.0

Ppos
Qpos
0.5

1.0

1.5
(s)

2.0

2.5

-100
3

-150

figure 10. The power characteristics of a full-power
converter-based wind turbine.
32

ieee power & energy magazine

(kVar)

(kW)

800

Large Disturbance Experiment-
Staged Grid Faults
To verify the LVRT capability of a wind turbine and the
performance of the reactive power compensators after correcting performance issues, the northwest branch of SGCC
cooperated with several institutions, including CEPRI, in
testing the LVRT capability of the Jiuquan wind power
cluster. By the end of December 2012, there were 18 wind
farms, with ten kinds of wind turbines, in operation in the
Jiuquan wind power cluster with a total installed capacity
of 3,512 MW.

Staged fault tests were conducted in December 2012. Single-phase instantaneous and permanent short circuit faults
to ground were applied on the 330-kV transmission at Dunhuang (see Figure 2) and interphase faults were tested on the
35-kV collector system of a wind farm.

Measured Performance of
Wind Turbines for Staged Faults
Full-Power Converter-Based Wind Turbine

figure 9. A fundamental positive sequence of a wind
turbine active and reactive power.

1,000

The on-site tests of a full-power-converter-based wind turbine with a chopper for LVRT capability on a wind turbine
at high power output showed similarly acceptable results.
In one test, also for a three-phase voltage drop down to
20% for 625 ms, active power decreased to 570 kW immediately upon the voltage drop. Within 40 ms of the voltage
drop, 920-kVar reactive power was provided by the WTGs,
helping to support the voltage during the fault. After the
voltage returned to normal, the output power of the wind
turbine recovered quickly. Both wind turbine speed and
pitch angle normally fluctuate with changes in the wind
speed. When the voltage drop occurs, the pitch and speed
also increase slightly.

Fault Types

600

1.6

Chopper Test

The power waveforms during the single-phase instantaneous fault of the full-power converter-based wind turbine
are shown in Figure 10. The output active power of the
wind turbine fell to about 480 kW during the voltage drop,
and the reactive power shifted from generating 43 kVar
before the fault to absorbing 8 kVar after the fault. It took
600 ms for the active power to recover after the voltage
recovered, during which time the reactive power absorption reached 93 kVar at maximum. The reactive power
also fluctuated greatly when the voltage dropped and later
recovered due to the switching of different control patterns of the wind turbines.

DFIG-Based Wind Turbine
The tests of a DFIG-based wind turbine during the singlephase instant fault demonstrated successful ride-through
for the entire plant. The active power fell to zero during
november/december 2016

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - November/December 2016