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.
