Aurora Power Consulting was contracted by Windcare to undertake a number of power system studies to facilitate a new 4.5MW, 33kV wind farm being constructed at the Keith’s Hill site near Edinburgh. The studies were more onerous than the usual requirements for the DNO, as the 33kV substation was partly operated by National Grid, so Aurora had to also ensure that the studies met with the additional requirements of the Grid Code.¬† Aurora carried out the following studies:

 

Earthing Study

Aurora carried out a full earthing design using the industry standard CDEGS package. This included constructing a full model of the different soil layers using RESAP, then analysing the fault current distribution and ground fault current inject into the earth using FCDIST before finally design the full earthing grid using the SESCAD/MALT packages. Using good engineering practice and judgement, Aurora managed to ensure that the overall site was cold, thus eliminating the need for additional protective surface layers and HV & LV earth separation.

 

Transformer Energisation P28 Study

Aurora carried out a P28 transformer energisation study using the PSCAD package to calculate the maximum network voltage sag that could occur when the transformer was energised at a zero voltage crossing. Using this package Aurora were able to correctly asses the voltage sag limits and calculated how many transformers could be energised simultaneously without exceeding the P28 limits.

 

Harmonics G5/4 Stage 3 Assessment

Aurora modelled the Keith Hill windfarm site as well as the surrounding AuroraN / National Grid 132kV and 275kV Grid networks using the ETAP simulation package. Once this had been done, Aurora then undertook a full harmonic analysis of the site, to identify the harmonic distortion levels on the key busbars as well as performing frequency sweeps to identify any resonance conditions on the upstream and downstream networks.

 

Reactive Power Flow Study

Using the ETAP model generated for the harmonics study, Aurora ran a series of loadflow studies for both leading and lagging power factor behaviour characteristics of the wind turbine inverter units to assess whether the wind farm could meet the reactive power flow requirements for the differing operation modes.

 

Fault Ride Through Study

This study is normally carried out as a transient stability analysis using ETAP (or similar) to demonstrate the wind farms ability to ride through differing types of faults and voltage sags on the nearby AuroraN / National Grid network. However, following discussion with National Grid, Aurora managed to negotiate an agreement that a full power system study was not required, and that presentation of the wind turbine inverter units characteristic response curves would be sufficient – thus saving our client a considerable cost for this study.