Control of high voltage DC and flexible AC transmission systems

Analytical modeling of HVDC systems has been a difficult task without a to-date reported model convenient for serious analysis of practically reported HVDC stability problems. In order to cover the frequency range f<100Hz, and to cater for different model requirements, three different HVDC-HVAC models are developed in this Thesis: Detailed linear-continuous model, simplified linear continuous model and linear discrete model. Detailed HVDC-HVAC system model is intended for small signal analysis of HVDC-HVAC interactions and resulting stability problems. It demonstrates good response matching against PSCAD/EMTDC simulation, where the CIGRE HVDC Benchmark model is used as the test system. All model variables (states) and parameters have physical meaning, and the model consists of modules, which reflect actual physical subsystems. Simplified HVDC-HVAC system dynamic model is developed as a fourth order dynamic model, which is less accurate but more convenient for the analysis, than the detailed model. The model proves to be reliable for controller design for mitigation of composite resonance and for the study of non-linear effects in HVDC systems. The developed linear discrete model is primarily intended for the system analysis at frequencies close to 100Hz on DC side of HVDC system. A new approach in modeling of TCR/TCSC, based on the same principles for HVDC modeling, is presented in this Thesis. The model development is far less difficult than the similar models presented in literature. PSCAD/EMTDC simulation confirms the model validity. The simplified, linear-continuous model is used for the analysis of dominant non-linear effects in HVDC systems. The analysis of non-linear mode transformation between constant beta and constant gamma operation, shows that limit-cycle oscillations are not expected to develop, for normal operating conditions. The analysis of converter firing angle modulation shows that converter behaves as a non-linear element only for some unlikely operating conditions. In this Thesis, it is attempted to counteract the composite resonance phenomenon by modifying the resonant condition on DC system impedance profile. This is accomplished by designing a supplementary HVDC controller that acts on HVDC firing angle on both line ends. PSCAD/EMTDC simulation results show that significant reduction in DC side first harmonic component (in some cases to 1/4 of the original value) is possible with the newly designed controller. Chapter 5 studies 100Hz oscillations on DC side of HVDC system. A methodology for designing a new controller to counteract negative affects of these oscillations is presented. Linear simulation of the detailed controller design confirms noticeable reduction in second harmonic on DC side. The eigenvalue decomposition and singular value decomposition is used for small-signal analysis of HVDC-HVAC interactions. The analysis of sensitivity of the dominant system eigenvalues with respect to the AC system parameters, shows the frequency range for the possible…