Designing a robust controller to damp sub-synchronous oscillations in power systems

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.