Subsynchronous resonance risk assessment in interconnected power systems
Creators: name: Yousif, Nadia
Publisher: name: RMIT University
Year: 2024

description: Series compensation and high voltage direct current (HVDC) links, technically increase the viability of suppling remote areas by long loading transmission lines, or connecting new remote generation hub to the main network. Nowadays such technologies have become part of utilities’ plans to increase power transfer capability between regions. This is usually due to lack of local reserve within a region under system wide contingencies or when there is redundancy of large unscheduled generation (usually in case of extensive wind generation during low demand durations).<br><br>Both series compensation and HVDC converters excite frequencies below the synchronous frequency of the system called subsynchronous frequencies. These frequencies will be seen as voltage and current components superimposed on the 50 Hz voltage and current signals. These components may excite tortional torque at the adjacent generator causing excessive voltages and currents, and damage to the generator–turbine shaft. Subsynchronous currents or voltages can also be generated in power systems as inter-harmonics emission from a distorting load such as HV high-capacity variable speed drives (VSDs). In other cases, a series resonance between the cables’ capacitance and the transformer’s inductance can occur, resulting in resonant frequency components. Under all these circumstances, there is a risk of SSR conditions. Subsynchronous resonance will become an important network condition to investigate when the new technologies are introduced in power systems. <br><br>The purpose of this research is to establish understanding of challenges and issues facing the power networks planning, in particular the SSR conditions. The thesis presents key characteristics of power systems and modelling considerations, which need to be taken into account for SSR assessment. Further, a good understanding of the general requirements and criteria for addressing SSR problems is established. Design requirements for HVDC links and series compensation to reduce the risk of SSR are presented. <br><br>A guide for power system elements representation for simulation studies is established. New modelling and simulation approaches for SSR analysis are proposed to overcome the difficulty of incorporating the discrete nature of the operation of Thyristor Controlled Series Capacitor (TCSC). A computation algorithm for eigenvalue calculation is developed based on Time Frequency Distribution (TFD) concept, by capturing the time variation of a frequency component. <br><br>The disproportion between the cost of mitigation solution and the cost implication of the risk of low probability condition such as SSR conditions led the utilities to underestimate the risk, thus increasing the system vulnerability to failure. Two context are realised in SSR risk, firstly is the system states probability and the correlation with SSR risk, and secondly the relationship between SSR risk and the system’s operating conditions. <br>The probability weighted cost implication of the SSR risk is estimated and compared with the cost of mitigation measure. This comparison will assist in the economical justification of the countermeasure. It is concluded that it is possible to optimise system’s operating condition to achieve acceptable risk of SSR by quantifying a relationship between system’s constraints and the system’s operating conditions.