IEEE Power & Energy Magazine - November/December 2016 - 31

CEPRI started the LVRT capability test program for
wind turbines in 2010. With improved LVRT testing methods, CEPRI has developed seven sets of mobile LVRT
testing devices and one set of fixed testing devices in the
Zhangbei wind power test center; the testing devices are
shown in Figure 7.
These devices are connected with the medium-voltage
busbar of the wind farm and the high-voltage side of the
wind turbine's transformers. The principle of the LVRT testing system is illustrated in Figure 8. When the LVRT testing
device is not connected, the electric power of each individual turbine is delivered to the 35-kV power collecting line in
the wind farm through the 690-V/35-kV unit transformer of
each wind turbine. A sequence of closing and opening the
four switches allows a fault to be imposed without unduly
disturbing the host grid. Since the reactance values of the
current-limiting reactor (Xsr) and the short circuit reactor
(Xsc) are adjustable, short-circuit faults with different levels of voltage drop can be realized by different ratios of Xsr
and Xsc. Xsr is selected with the aim of maintaining a stabilized grid voltage when the voltage drop is imposed. The
time between CB2 closing and opening corresponds to the
duration of the short circuit and can be set for different testing scenarios.
By the end of 2015, over 200 types of wind turbines had
been tested for LVRT capability by CEPRI. CEPRI also
developed testing systems for wind turbines and their connections to the grid. CEPRI covers all types of wind turbines
in volume production, and continues performing research
and the development with the goal of improving wind turbine performance and testing.

Retrofitting Wind Turbines
for Fault Ride Through
FRT Capability Retrofit of Wind Turbine

figure 7. Mobile LVRT capability testing devices.

thyristors in a passive crowbar cannot be turned off until
the current passes through zero, presenting a risk that the
crowbar might be unable to be turned off when there is a dc
component in the fault current. Therefore, an active crowbar
circuit was applied to most DFIG-based wind turbines. An
active crowbar circuit consists of a simple three-phase diode
bridge to a dc node. Across the dc poles, there is a transistor in series with a resistor. The transistor can interrupt current, allowing the crowbar to be deactivated even without
a current zero. The crowbar circuit is connected to the ac
generator rotor circuit, independent of the rotor converter.
The crowbar is put into operation when the current on the
generator rotor side or the voltage on the dc side of the transformer exceeds a preset value. The crowbar circuit is relatively compact and normally can be added to the circuitry of
the wind turbine.
Chopper Circuit

A chopper circuit added to the converter loop is another
method for adding FRT capability to a wind turbine. The
position of a chopper circuit in the converter is on the dc
link between the line- and rotor-side converters for a DFIG.
The configuration for a full converter wind turbine-generator (WTG). WTG is the same except that the converter on

To have FRT capability on a turbine built without it, the wind
turbine's automatic control systems need to be adjusted and
additional hardware circuits added. New hardware typically
includes a crowbar circuit and/
or a chopper circuit. A crowbar
circuit or the combination of a
crowbar circuit and a chopper
circuit were adopted by most
double-fed (DFIG)-based wind
turbines. A chopper circuit was
CB Grid
added to the dc link of the conGrid
220 kV/
verter in most full-power conver35 kV
sion wind turbines.

CB1
CB WEC
Xsr

Xsc

Crowbar Circuit

A crowbar circuit can be classified as passive or active according to the type of power electronic switch devices used. The
november/december 2016

CB2

35 kV/
690 V
LVRT Testing Equipment
for Single Wind Turbinne

figure 8. The LVRT testing principle.
ieee power & energy magazine

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