19-07-2012, 02:10 PM
Transients in doubly-fed induction machines due to supply voltage sags
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Introduction
Increasing the share of electric power generated by renewable energy sources is an important
political goal in Europe and in many other
countries in the world. It reduces the
environmental pollution caused by traditional power plants as well as the dependence on fossil
fuels, which have limited reserves.
Electric energy, generated by wind power plants is the fastest developing and most promising
renewable energy source in Europe. Off-shore wind power plants provide higher yields because
of better wind conditions. The power output of a
wind turbine is proportional to the cube of the
wind speed (see Eq.1). Its theoretical limit is 59.3% of the wind power input (Betz).
Ride-through” capability during voltage sags
Due to increasing contributions of wind farms to the overall power generation, the necessity of
their participation in grid stabilisation arises
[1,2,4,6]. Especially under fault conditions they
must perform similarly to conventional power
plants. In recent years, grid codes have emerged
in many countries, forcing wind farms to stay
connected and maintain operations for a certain period of time during voltage sags. The level of
the voltage sags during which safe operation has
to be guaranteed differs greatly, e.g. down to zero voltage in Australia or 15% in Germany.
Transients of DFIM during voltage sags
Field components are analysed in order to understand the effects taking place in a doubly-
fed induction machine during voltage sags. This
method is similar to the short circuit analysis for
synchronous machines. The time domain simulation program “Simplorer” was used to verify the theoretical predictions. Simplorer
allows any power electronic circuit model to be built up and include its respective control.
Additionally, models for all major machine types
are provided.
Summary
Grid codes have emerged in many countries,
requiring the “ride through” capability for wind
plants. This is very demanding for wind plants
with doubly-fed induction generators because of
transients that occur after voltage sags.
High voltages are induced in the rotor windings. They are dangerous for the rotor side converter.
Thus, a crow bar circuit is used for protection.
The inherent torque pulsations may reach
significant amplitudes and are dangerous for the
gear box. Torque transients were investigated
and explained in a comprehensible way by
decomposing the field into rotating and stationary
components. The presented simulation results
confirm the theoretical predictions.