Modeling and variable structure control of AC/DC systems
Abstract
The use of simplified model of the bridge converter in conjunction with a dynamic AC network representation has to be implemented carefully. This research reveals a potential modeling pitfall that such a combination has when following what others have been doing so far. Depending on the operating condition, direct implementation of such a combination can yield erroneous results that would suggest instability of the system. To minimize potential errors, a modified approach to the implementation is proposed. The proposed implementation does show significant improvement in the case of the inverter. Its validity has been also cross-checked by comparing the predicted response to that from a full detail simulation. Another primary facet of this research is an investigation of a variable structure control (VSC) approach for controlling the ignition angle of an HVDC converter. There are several potential problems encountered with HVDC terminals interconnected to AC systems and these can be exacerbated if the connecting AC system is considered weak. These problems are: harmonic instability, possibility of repetitive commutation failures, and DC control system sensitivities to network parameter variations. This research will be limited to investigating the latter. Converter station operating conditions and system contingencies determine the shunt impedance and the connecting AC network Thevenin impedance. These network parameters can be varied significantly due to the DC power schedule or from line openings following faults. These network parameter uncertainties will impact the converter controls. Typical linear control techniques, which have been frequently used in the past, tend to be sensitive to parameter variations. Consequently, this yields a compromise between stability, performance and robustness. Therefore the advantages of VSC theory applied to this problem of parameter sensitivity is investigated.
Degree
Ph.D.
Advisors
Ong, Purdue University.
Subject Area
Electrical engineering
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