Environmental Engineering Reference
In-Depth Information
Voltag e
100 %
80 %
15 %
0 %
0
1
0
3
0
Time (ms)
Figure 6.7
Fault ride - through voltage profi le
In 2001, the German transmission system operator, E.ON Netz, issued new requirements
including fault ride-through. Since then, most of the other major European transmission
system operators have issued similar requirements, within their respective
grid codes
. The
details vary between countries but the general requirement is as follows.
During a short circuit on the network near to the wind farm, the voltage at the connection
point of the wind farm is assumed to follow the profi le illustrated in Figure 6.7. The wind
turbines must remain connected to the network; they should generate reactive power during
the fault and should resume generation of active power as soon as the fault is cleared. Meeting
this requirement was, and still is, a major challenge for manufacturers of wind turbines,
especially with certain generator confi gurations.
Wind turbines with full converters are readily redesigned to meet the fault ride-through
requirement. Turbines with directly connected induction generators are inherently unsuited.
The drop in terminal voltage causes the fl ux to collapse, the electromechanical torque almost
disappears and the wind turbine accelerates rapidly. To prevent overspeed the brake has to
be applied and the turbine cannot immediately resume generation when the fault is cleared.
To address this (and for other reasons), Siemens (formerly Bonus) added full converters to
some of their models.
Doubly fed induction generator (DFIG) manufacturers also faced a considerable challenge.
In order to protect the IGBT converters from excessive current during a fault, DFIGs are
generally fi tted with a crowbar, which is a set of thyristors that short out the rotor and cause
the DFIG to behave like a squirrel-cage machine. All major manufacturers of DFIGs now
claim to have developed their controllers such that they can ride through faults. In a competi-
tive market, they do not publish the details, but it is understood that they generally employ
an active crowbar, which provides a controlled short circuit to the rotor and allows the IGBT
converters to continue operation.
The details of fault ride-through requirements differ from one country to the next, and there
is concern that these details may be somewhat arbitrary. Meeting fault ride-through require-
ments adds costs to wind turbines and the industry is now concerned that these requirements
should not be unnecessarily applied.
