HVDC interaction studies using small signal stability assessment

Small signal stability analysis is typically concerned with electromechanical oscillations in a power system. For this purpose, it is adequate to model the transmission system using a constant admittance matrix. For torsional oscillations and HVDC interactions, the frequency of interest is much higher and the constant admittance representation is not sufficient. The modeling details necessary to adequately represent the dynamics of the HVDC converters and the AC network are investigated and the models are validated against an electromagnetic transient simulation program. The thesis shows that AC network dynamics must be modeled in order to obtain meaningful results from the small signal stability study of the high frequency interactions (up to 200Hz). The dynamic phasor representation of the AC network is used to model the network and the dynamic devices are combined with the network model using current injection models. However, it is impractical to model the dynamics of the entire AC network of a large power system because of high computational burden.;The thesis presents an analysis of multi-in-feed HVDC interactions using small signal analysis techniques. A small test system with two HVDC in-feeds and the IEEE New England 39 bus system with two HVDC in-feeds are used to demonstrate the presence of interactions in those systems. The case studies presented in the thesis indicate that it is possible to have interactions between the HVDC terminals in an AC system. The thesis recommends that a small signal interaction study similar to what is presented in the thesis should be performed to identify these interactions. For large power systems, the proposed hybrid model can be used to accurately and efficiently analyze these interactions.;Furthermore, the thesis demonstrates that the small signal stability assessment techniques described in the thesis can be used to identify the HVDC-generator-turbine torsional interactions in power systems. Two case studies are performed using the small signal model including the dynamics of the entire AC network and the proposed hybrid small signal model. The case studies indicate that it is possible to have subsynchronous frequency torsional interactions between the HVDC systems and generator-turbine units and these interactions may cause instabilities under certain conditions. Further, the electromagnetic transient simulations validate these findings.;The thesis further investigates the inclusion of the auxiliary controllers at the HVDC terminal to control the electromechanical and torsional oscillations of the nearby generator-turbine units. The design procedures are briefly described using small signal stability assessment and the performances of them are evaluated using time domain simulations.;This thesis proposes a hybrid model, which allows the parts of the transmission network in the vicinity of HVDC converters or any other dynamic devices to be modeled with their dynamics and the remaining parts to be modeled as constant admittances. This model can be efficiently used for large power systems. The proposed hybrid methodology is validated against an electromechanical transient program using time responses.;In general, the analytical techniques proposed in this thesis would be useful to analyze the high frequency interactions of HVDC systems, generator-turbine units and FACTS devices. The modeling techniques can be used for very large power systems.