Abstract:
Subsynchronous Resonance (SSR) phenomenon is an important dynamic problem in power
system which can lead to the failure of the power system and destruction to the rotor shaft.
When series capacitive compensators in EHV transmission line are implemented together
with a steam turbine-generator it can cause to the SSR occurrence. Damping of these SSR
oscillations has to be ensured in an effective manner so that the system dynamics remains
stable under a range of operating scenario. Various types of Flexible AC Transmission
System (FACTS) controllers particularly SVC, TCSC, GCSC, STATCOM, SSSC, UPFC,
IPFC have been used for this purpose in the past. Pulse Width Modulated Series
Compensator (PWMSC) is a newly FACTS device, which can modulate the impedance of a
transmission line through the variation of the duty cycle of a train of pulses with fixed
frequency, resulting in improvement of system performance. The thesis starts with the study
of the nonlinear and linear model of IEEE First benchmark Model (FBM) and performs the
small signal analysis by inserting small disturbance in mechanical power input in the
governor. The eigen value analysis shows that the different torsional modes are becoming
unstable for different compensation levels. The critical compensation levels have been
identified. There are four unstable torsional modes and Mode-1 exhibits the most severe
undamping with 68.5% compensation level. The dynamic current injection model of
PWMSC has been developed in the IEEE FBM and small signal analysis has been performed.
A supplementary controller with lead block and reset block has been designed to control the
parameters based on an eigen value based objective function. Genetic Algorithm (GA) is then
used to solve the optimization problem. The results show that the properly tuned PWMSC
can successfully damp out all the shaft torsional torques over a wide range of compensation
levels. The maximum overshoots and settling times for different states are also satisfactory.
The analysis has been performed at critical compensation levels so that all individual
compensation levels are covered. This feature of PWMSC can be used to increase the limit of
line power transfer in a multi-area power system in stable manner. The outcome confirms the
effectiveness of the PWMSC in damping SSR oscillations in power system.
