Abstract:
In the power transmission systems, the power transferring capability is limited due to the
inductive reactance of the transmission lines. In order to mitigate the inductive effect, some
compensation techniques are applied to the transmission lines. One such technique is the series
compensation using capacitor banks. Series compensation method is used to improve the
system voltage with capacitor banks are connected in series with the power transmission line
and it expands the power transferring capability of the line. Although the increase of series
compensation improves the power transfer capability and the steady-state and transient
stability limit of the power transmission line, it can lead to the generation of some natural
frequencies due to the combination of inductor and capacitor (L-C). These frequencies are
called as sub-synchronous frequencies which are below the power frequency of the power
systems. They can arise sub-synchronous resonance (SSR).
The SSR can cause physical damages to the power system equipment unless it is detected and
mitigated punctually. Several number of mitigation techniques for different types of power
system oscillations have been proposed in literature. But existing mechanisms are not
completely damp these oscillations or the mechanisms used to damp these oscillations might
be source for any other control situations. Therefore, this is a phenomenon which should
understand well and damped these oscillations properly. The intension of the work presented
in this thesis is to properly mitigate the undamped power system oscillations which are in the
range of sub-synchronous frequencies.
This research proposed a robust controller which can damp dominant sub-synchronous
resonance. Further, the implemented controller performs well in different operating points.
IEEE First Benchmark Model (FBM) is used as the test system and the dynamic phasor
representation of the system is used to model the small signal model. The operating points of
the test system were generated by changing the series capacitor compensation level of the
power transmission line. Finally, this research introduced a robust controller with PID
controlling to damp out dominant sub-synchronous oscillations which can perform well under
different operating points of the selected power system.
