The existence of harmonics and oscillations represent major problems for reliable operation of power system components. Therefore, investigating their response requires finding an appropriate model which reflects their response including these variations. The mathematical derivation of the state space models and impedance models of some of voltage source converters in flexible ac transmission systems (VSC-FACTS) systems is presented using synchronous dq and dq-dynamic phasor approach.Two types of the VSC-FACTS devices are studied in this thesis; the static synchronous compensator (STATCOM) due to its popularity in the power system network and static synchronous series compensator (SSSC) due to its effective on damping system oscillations. The effect of mechanical section of the synchronous machine and turbine sections on the machine impedance is analysed. A generalised state space and impedance modelling is proposed by converting the synchronous dq models to dq-dynamic phasor modelsA development of harmonic stability criteria for the proposed modelling is presented. The proposed modelling is employed to present the harmonics effect on the STATCOM and SSSC response and to identify their unbalanced operation in frequency domain. The main features of the proposed modelling technique are compared comprehensively with the conventional modelling techniques for stability studies assessment. It shows the advantages of proposed method and the importance of including the harmonics in the stability studies.A comparison between different control modes of the SSSC is discussed in the frequency domain. The effectiveness of these control modes on damping system oscillations is investigated using the impedance concept. It presented the effectiveness of impedance control mode on damping system oscillations over the other control modes. A fast impedance measurement unit (IMU) is proposed to monitor the small signal stability.The proposed IMU can measure accurately the system impedance within a very short time without any filtering requirements. The effect of changing the STATCOM control gains on the impedance norm is investigated. Also, the effect of shunt and series virtual impedances on the infinite norm of the STATCOM impedance which can be used by network operators to retain the stability is discussed.
|Date of Award||22 Feb 2019|
- University Of Strathclyde
|Supervisor||Derrick Holliday (Supervisor) & Lie Xu (Supervisor)|