Mitigating SSO in an Actual DFIG-MTDC System: Field Implementation and Tests

Recently, a new type of subsynchronous oscillation (SSO) event was observed in the doubly fed induction generator (DFIG)-based wind farm connected to a multi-terminal dc (MTDC) grid. It has been established that converter-based devices can interact with each other and other grid components. Generally, it is difficult to replicate an SSO event and devise a mitigation scheme due to the complex and black-box nature of converter-based devices. This paper demonstrates the field implementation of a model-independent method to mitigate the SSO using additional band-rejection filters in the DFIG's converter controller. The proposed method is unique in the sense that the complete design and implementation were carried out without detailed models or prior EMT studies. Field results illustrate that the risk of SSO can be significantly reduced by supplementing the DFIGs with properly tuned band-rejection filters, which reshape the DFIGs' impedance characteristics at the desired frequency. The test results obtained under various operating conditions further confirm the effectiveness of the mitigation scheme. The implementation and field tests of the filter-based mitigation scheme in an actual system can provide a practical reference for solving similar SSO issues in other large-scale systems without going through detailed EMT studies.