Wind power has become one of the major sources of electricity generation in Finland. Fingrid (the Finnish transmission system operator) utilizes series compensation on Finland’s 400 kV north-south power transmission lines and on cross-border AC interconnections with Sweden. Type-3 wind power plants (WPPs) in a series compensated electricity network may cause undamped wind related subsynchronous control interaction (W-SSCI). Undamped subsynchronous oscillations can cause significant problems in the electricity network, such as equipment failures and the disconnection of WPPs.

This study investigates whether the W-SSCI risk in a highly meshed series compensated electricity network can be reliably analyzed using single-input single-output (SISO) impedance scanning based on positive-sequence impedance, or whether W-SSCI should instead be analyzed using multiple-input multiple-output (MIMO) impedance scanning. The differences between SISO and MIMO impedance scans are investigated in the single machine infinite bus (SMIB) model for a Type-3 WPP.

This study also investigates the effect of electricity network topology and other close-by Type-3 WPPs on the possibility of W-SSCI in a series compensated electricity network. The impact of network topology and other close-by WPPs on a series-compensated electricity network is investigated in a semi-wide area network model in which the electricity network is series compensated.

Based on the results of this study, it seems that SISO impedance scanning based on positive-sequence impedance is reliable in Type-3 WPPs up to frequencies of 30 Hz. At frequencies above 30 Hz, the negative-sequence impedance and the cross-coupling between the positive- and negative-sequence impedances have an effect on the results of impedance scans. Since majority of the resonance frequencies, present due to series compensated network, land in frequencies below 30 Hz, the SISO type impedance serves as a feasible tool for assessing the W-SSCI.

Based on research in a semi-wide area network model, it was found that the electricity network topology and other close-by Type-3 WPPs have a significant impact on the possibility of subsynchronous oscillations in a series-compensated electricity network. The results also showed that Type-3 WPPs connected in close-by to the studied WPP often caused a parallel resonance peak in the frequency-domain impedance curve of the background network. The frequency of the oscillation observed in the time-domain analyses almost always corresponded to the frequency of the parallel resonance peak and indicated high risk for undamped W-SSCI.