Wind Energy

34
Methodical Failure Detection in Grid
Connected Wind Parks
Detlef Schulz, Kaspar Knorr and Rolf Hanitsch
34.1 Problem Description
This contribution deals with the handling of very common failures and the
discussion of outages of wind energy converters (WECs) with doubly-fed
induction generators (DFIGs) located in wind parks. Measurements were conducted
over a period of time of some months in order to find the tripping
source for a high number of WEC-outages. The system overview, a failure
detection approach and measurement results are presented. Almost all failures
were followed by discussions about the causes for errors and the resulting
responsibility due to the high cost of manpower and cost of necessary measurements.
One part of the discussion is the sensitivity of the WECs against
external over- and under-voltage, transients and voltage dips. These tolerances
are given in EN standards and IEC-guidelines [1, 2].
34.2 Doubly-fed Induction Generators
The DFIG’s stator is directly connected to the grid. The rotor is connected to
a pulse-width modulation (pwm) converter which is connected in turn to the
mains. Figure 34.1 shows the system overview of the DFIG. The rotor of a
DFIG has, compared to standard machines, a high winding number in order
to have a high-rotor voltage at nominal speed. Therefore, no transformer is
required on the rotor side. Due to the high-rotor voltage, additional induction
effects at stator voltage sags or at fast supply voltage outages may destroy
the winding insulation. Transients result, which are avoided by short-circuiting
the rotor windings, using the crowbar [3].
As the generator torque change is highly influenced by the control circuit
and the reaction of the electrical side to failures, electrical influences contribute
to the fatigue loads of the drive train. These effects are not considered for
the gear rating, yet. There are different causes for external failures [4, 5].
External distortion sources may be grid-side short-circuits, voltage sags or