Download Short circuit behavior of REpower WTG - pes-psrc

Short circuit behavior of REpower WTG
To have a better understanding of the short circuit behavior of REpower WTG this paper will explain
the WTG’s short-circuit characteristics and give a guide to the necessary consequences for modeling
the machines behavior in case of IEC 909 calculations.
1. General short circuit characteristics
1. The REpower MD/MM WTG types are equipped with a double fed induction generator
(DFIG).
2. The stator of the generator is directly coupled to the network
3. The rotor circuit is connected via an IGBT inverter with an internal DC-link
4. In case of a short circuit the DC link would be charged to a very high voltage that could
damage the inverter.
5. To protect the inverter in cases of short circuits, the rotor windings will be short-circuited via
a thyristor bridge (crow-bar).
LV-terminals
Gear Box
Asynchronous Generator
Rotor
AC
DC
Crow-bar
Picture 1:
Circuit breaker
AC
DC
IGBT-Inverter
General layout of a WTG equipped with a DFIG
6. So the electrical behavior of the REpower MD/MM during a fault close to the WTG will be
similar to an asynchronous squirrel cage generator.
7. To minimize the mechanical peak loading during a short circuit, the crow-bar is equipped
with damping resistors.
8. These additional resistors in the short-circuit rotor have a damping characteristic that is
changing the peak current and the waveform in comparison with a system without crow-bar
resistors.
9. The short circuit characteristic will also be influenced by the WTG transformer.
10. Approximately 100 ms after the short-circuit appearance the LV circuit breaker of the WTG
will open.
11. Based on these facts a worst case three-phase short circuit at the LV-terminals of the WTG
will have a characteristic short circuit at 690V as shown in picture 2.
Author: D. Ehlert
Page 1 of 2
Date: 8.12.2005
Short circuit behavior of REpower WTG
16.000
14.000
Ik" = 8,4 kA
12.000
Is = 14,1 kA
Current [A]
10.000
8.000
IDC (t)
6.000
Ik (t)
4.000
2.000
0
0
0,02
0,04
0,06
0,08
0,1
0,12
0,14
Time [s]
Picture 2:
Short circuit characteristic of a REpower MM82 during a three-phase
fault at the LV-terminals of the WTG
2. Guidance for modeling the REpower WTG in short-circuit calculations according to IEC 909
1. Due to the fact that the machine has a different behavior in short-circuit scenarios compared
to normal operation, a software specific approach is necessary to come to results as to
expected in reality.
2. If the machine is regarded as a asynchronous generator with a nominal active power output
of 2,0 MW, the short circuit relevant machine parameters needs to be adjusted to values that
will result in short circuit values in line with the characteristic shown in picture 2.
3. A standard short circuit calculation software might ask for the following values:
Input Data (example)
IEC 909 calculation result
Asynchronous Generator
Un
=
0,69
kV
Pn
=
2,2
MW
Efficiency
=
0,95
Cos ϕ
=
0,99
0,69 kV
R/X ratio
=
0,585
I/In
=
4,5
Ik": 8,40 kA
is: 14,13 kA
R/X: 0,585
Sk": 10,0 MVA
Ri: 25,1 mOhm
Xi: 43,0 mOhm
Author: D. Ehlert
Page 2 of 2
AsG
Date: 8.12.2005