Burgos Wind Farm Insulation Coordination Study

Project background

DIgSILENT Pacific was engaged by Vestas Australia to carry out an insulation coordination study of the 34.5 kV collector system at Burgos Wind Farm. The purpose of the study is to evaluate the insulation strength of the 34.5 kV equipment with regard to their Switching Impulse Withstand Level (SIW), Lightning Impulse Withstand Level (LIW) and Critical Flashover Voltage (CFO), as well as the effectiveness of the proposed 34.5 kV surge arrestors.

Burgos Wind Farm is located in the province of Ilocos Norte, Philippines. It has a maximum generation capacity of 150 MW produced by 50 units of Vestas V90 3 MW wind turbines. The wind farm's 115/34.5 kV substation is connecting to the Laoag Substation of the National Grid Corporation of the Philippines via a 42 km, 115 kV overhead transmission line.

The study included the assessment of temporary over-voltage due to faults and switching as well as the assessment of lightning strike over-voltage:

Assessment of temporary over-voltage

During fault conditions and network switching operations, the voltage across network components can increase. This over-voltage stresses the insulation of the equipment. Simulated over-voltage conditions has been used to determine the adequacy of the selected surge arrestors. To evaluate the resultant temporary over-voltage, DIgSILENT used PowerFactory to perform steady state and time-domain electromagnetic transient (EMT) simulations for various disturbance events (e.g. single phase to ground faults and switching of reactive plants).

Assessment of lightning strike over-voltage

DIgSILENT Pacific utilised the EMT function of PowerFactory to conduct high frequency lightning studies for the 34.5 kV system. The studies were performed by simulating direct strike and back flashover to the 115 kV overhead transmission line as well as direct strike to wind turbines.

In PowerFactory, the impulse source element is used to represent a lightning strike current waveform. It is a single-phase, single-port element and it can be connected to any AC terminal. The impulse source element supports three different current impulse waveforms:

  • Waveform according to IEC 62305-1 standard
  • Heidler waveform
  • Double-exponential waveform

During simulation of lightning strikes, the overhead lines and collector system feeder cables were modelled in detail using a frequency-dependent distributed parameter model. For both overhead lines and cables, the phase conductor and earth conductor were modelled explicitly to consider travelling voltage between the earth conductor and the phase conductor. The overhead line was modelled using a tower line coupling model where the geometrical characteristic of the tower, and the conductor type, are required. The cables were modelled using pipe type cables, with a bare earth conductor surrounding three phase conductors. The components of the overhead line and wind turbine mast, such as; string insulator model, surge impedance and footing resistance, were also considered during lightning studies.

The resultant overvoltage at the MV busbar was compared against the insulation strength of the equipment to ensure that the proposed MV surge arrestors are sufficient in protecting the equipment. In addition, the peak current discharged through and the energy absorbed by the surge arrestor were both reported, to ensure that the specification of the proposed surge arrestors would meet the minimum requirements.