Steady-state analysis of directly connected synchronous machines and HVdc converters
Thesis DisciplineElectrical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
In this thesis the results from a conventional ac-dc load flow program for modelling the steady state behaviour of a unit connect salient-pole generator-converter are compared with those obtained from degree by degree dynamic simulation. The Transient Converter Simulation (TCS) program is used to give benchmark results. The unacceptable level of error from the load flow program indicates that the conventional ac-dc formulation is not directly applicable to the unit connected generator-converter. A salient-pole generator converter model has been developed to reduce the errors, which allows for a voltage behind a varying sub-transient reactance to be used as the commutating voltage. The characteristics obtained from this model are better than for the conventional model but are still significantly different from the TCS results. A dynamic simulation of the operation of the unit connected generator converter is necessary, before steady state results can be obtained. Unit and Group-Connections are also compared under varying steady state loading conditions and by reducing the number of generators in service in both schemes at nominal frequency. An accurate algorithm of general applicability, called the Equivalent Inverter, is proposed which used unit-connection characteristics derived from a time domain simulation. Finally the steady state model for an alternative unit connected generator converter scheme is presented. This is based on the use of dc ripple re-injection, which permits using a single bridge (instead of two) to obtain a twelve-pulse converter operation. The proposed scheme considerably reduces the number of transformers and converter bridges, without increasing the harmonic levels with respect to the double bridge configuration.