Development of an analytical model of a simultaneous AC-DC power transmission system

Abstract

In high voltage long transmission line the power flow is much lower than its thermal limit due to the constraints related to stability and corona discharge. However, in case of simultaneous AC-DC transmission system the power flow can be very close to its thermal limit. Simultaneous AC-DC power transmission system generally increases the load carrying capability and stability of an existing long AC transmission line. In this system, conductors transmit usual AC along with DC. It is possible to convert existing AC transmission line into a simultaneous AC-DC power transmission line without the alterations of conductors, insulator strings and towers. Note that, two zigzag transformers and two converter stations must be added to the system for converting pure AC line into simultaneous AC-DC line. Simultaneous AC-DC system provides the benefit of parallel AC-DC power transmission system without constructing separate DC transmission line. An analytical model for the loadability and the transient stability analysis of a simultaneous AC-DC transmission system, transmitting power through an existing AC transmission line, is proposed in this thesis. The model has two independent components; one represents the loadability feature and other represents the stability feature. The loadability model is based on the development of the correlation between the total power flow of simultaneous AC-DC system and that of pure AC system. It incorporates the thermal constraint of a line, the allowable magnitude of DC voltage in an already installed AC line and the operating practice of a long transmission line. Initially the model considers only a simultaneous AC-DC transmission line. However, it is also generalized by considering the transmission line as part of the power system. The stability model is based on the equal area criterion principle. The model includes the overloading feature of the converters for a short period after clearing the fault. Basically, the stability model is developed considering the three phase to ground fault at the sending end bus. However, the power flow scenario at the receiving end is different when a similar type of fault occurs in one of the circuits of the double circuit transmission line, other circuit remaining healthy, compared to that for a fault occurring at the sending end bus. The model is extended for such type of fault location also. The model development process also includes the validation of the models and their application into an existing AC transmission system. The validation is executed through two different approaches. The developed models are applied to a simultaneous AC-DC system considered in the literature and the results are compared with the published ones. The models are also compared through evaluated results obtained using the model and those obtained using standard softwares. In case of loadability model MATLAB simulink and an electronic circuit simulation software are used but in case of stability model only MATLAB simulink software is used as the electronic circuit simulation software does not have stability feature. The validation process also examines some of the inferences of loadability model. Both loadability and stability models are also applied to a realistic system. The benefits of simultaneous AC-DC system are evaluated and the results are critically discussed.