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Transformer station near the Lillgrund wind turbine park. Operated by swedish Watenfall AB it produces 330 GWh annually
Transformer station near the Lillgrund wind turbine park. Operated by swedish Watenfall AB it produces 330 GWh annually (photo credit: ty-stange.dk)

130 kV system

Lillgrund offshore wind power plant is connected to E.ON’s 130 kV station Bunkeflo, near Malmö. The 130 kV system is illustrated below in figure 1. The 130 kV system consists of a 130 kV bay at the onshore substation Bunkeflo, 2 km land cable and 7 km sea cable.

At the offshore substation the sea cable is connected directly to the main transformer, i.e. there is no 130 kV circuit breaker or any other switchgear on the offshore substation.

Single line layout diagram of the electrical system at Lillgrund
Figure 1 – Single line layout diagram of the electrical system at Lillgrund

The 130 kV cable system generates approximately 10 MVAr reactive power. One requirement from E.ON is that the wind power plant keeps unity power factor at the point of common connection at Bunkeflo, e.g. the reactive power exchange shall be equal to zero.

Lillgrund does not have any reactor to absorb the reactive power. Instead, the reactive power at Lillgrund is controlled through the frequency converters in the wind turbines.

Onshore substation “Bunkeflo”

The main circuit breaker for Lillgrund is located at the onshore substation in Bunkeflo.


130 kV Circuit breaker

Figure 2 shows the new 130 kV bay for Lillgrund at Bunkeflo. Closest to the camera is the surge arresters followed by cable termination, voltage transformer, current transformer, grounding switch and finally the circuit breaker.

The new 130 kV bay for Lillgrund.
Figure 2 – The new 130 kV bay for Lillgrund

Switching transients

During the construction period, a complete systems study (i.e. load-flow calculations, dynamic simulations and transient studies) were performed by Siemens AG in Erlangen, Germany. The switching transient study revealed some large transients occurring at the 130 kV busbar in Bunkeflo when the main circuit breaker for Lillgrund was switched on, see figure 3.

Simulation of direct switch of the 130 kV circuit breaker. The second graph from the top shows the voltage at the busbars at Bunkeflo.
Figure 3 – Simulation of direct switch of the 130 kV circuit breaker. The second graph from the top shows the voltage at the busbars at Bunkeflo.

The switching transient causes an oscillation, which is composed of a high frequency voltage (approximately 650 Hz) overlapping the fundamental voltage (50 Hz). The oscillation emanates from the capacitance in the 130 kV cable and the inductance in the main transformer.

Title:Technical Description Lillgrund Wind Power Plant – Joakim Jeppsson, Poul Erik and Larsen Åke Larsson (The Swedish Energy Agency)
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Technical Description Lillgrund Wind Power Plant
Technical Description Lillgrund Wind Power Plant

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Page edited by E.C. (Google).

One Comment


  1. Mark McCloy
    Apr 07, 2017

    Do the frequency converters at the Wind Turbines require any significant changes, modifications or additions beyond “typical” Frequency Converter performance per the existing equipment/product Standard? Are there any additional specification needed and will those Standards cover or protect against any possible anomolies above and beyond just those inherent switching transients?

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