Control of high voltage DC and flexible AC transmission systems (1999)

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 oscillatory instabilities at rectifier and at inverter side. The rectifier side of the system is most likely to experience instability at higher frequencies, whereas at inverter side the instabilities can be expected at lower frequencies. Further analysis shows that reduction in the AC system strength i will predominantly affect the eigenvalues at lower frequencies, where the SCR reduction at inverter side much more affects the system stability. The analysis of interactions between AC and DC systems through the influence of inherent feedback loops gives recommendations for the possible control of interaction variables with the aim of system stability improvement. The root locus technique is used for the stability analysis of HVDC control loops, where all conventional and some alternative control methods suggested in the literature are investigated. It is found that DC feedback, at rectifier side, significantly improves the system stability at lower frequencies, however, at frequencies close to the first harmonic this feedback control degrades the system stability, actually accelerating the development of composite resonance. At inverter side, most of the feedback loops improve the system stability in a certain frequency range, whereas at other frequencies they noticeably deteriorate stability. Among all reported inverter control methods, reactive current feedback is found to be the best option. The last Chapter develops a new controller for HVDC system operation with very weak inverter AC systems. The selection of feedback signal for this controller, is based on the analysis of positioning of zeros for candidate feedback signals. It is found that AC current angle is the best inverter feedback signal. This feedback signal can move the unstable complex eigenvalues left, into the stable region, without significantly affecting remaining eigenvalues. For the additional improvement in the system performance, a second order filter, designed using control theory, is placed into the feedback loop. The main design objective is the system robustness with respect to the AC system parameters changes. The controller designed in this Thesis, tolerates very wide changes in system strength, 1.7<SCR<3.5, with the nominal operating point at SCR=2.5. This wide change of operating parameters, although very unlikely in practice, demonstrates large improvement in the system performance with the new controller. The similar controller especially designed for extremely low SCR, shows that HVDC system can satisfactory operate with SCR=1.0 at inverter side and heralds the possible controller use instead of synchronous condensers or SVC elements at inverter AC bus.