Frequency Characterization and Control for Future Low Inertia Systems

The rapid penetration of renewable energy sources (RESs) into power systems has introduced many challenges in securing a stable network operation and has forced power systems to operate in low inertia conditions. Low short-circuit power and intrinsic inertial response from converter-interfaced RES may cause poor dynamic performance of systems and render the system frequency more vulnerable than conventional grids. Instability issues from low-frequency oscillations due to low inertia operating conditions and weak interconnections among power systems are more significant. The objective of this work is to investigate the impact of converter-interfaced generation on system frequency characteristics during disturbances. The combination of synchronous condensers (SCs) and synthetic inertia (SI) of wind power plants (WPPs) for the inertial response is examined. Afterwards, effort is given to optimize the design and parameter setting of power oscillation damping (POD), which incorporates SCs to provide enhanced frequency stability during transient and emergency situations. A review of relevant theoretical concepts in power system frequency characteristics and control in renewable-based systems is analyzed. The analyzed system develops from the current Danish power system, which uses a part of fossil fuel sources toward a future scenario with 100% renewable sources in 2035. The system is modeled and validated based on the measurement data provided by the local operator (Energinet) using a real-time digital simulator (RTDS) in PowerlabDK. The benefit of SCs to frequency stability in terms of inertia support during disturbances is examined to indicate the important role of SCs in modern power systems. Hardware-in-the-loop (HiL) testing of an automatic voltage regulator (AVR) and limiters of SCs in steady-state and dynamic conditions for verifying the control algorithms and tuning the parameters are implemented. HiL testing overcomes the high cost and inefficiency of the tests in real systems. Additionally, software-in-the-loop (SiL) simulation, which is employed for parameter optimization of AVR simulation to achieve similar characteristics as the AVR hardware, is performed. The capacity of WPPs for providing an inertial response referred to as SI during disturbances is investigated. A mathematical model of SI for WPPs is developed and verified in the future Danish power system, which shows that WPPs are able to supply valuable inertial response during disturbances. This study also examines how the combination of SC and SI provides better performance, which enhances the frequency deviation and rate of change of frequency while enabling the low inertia systems to become more synchronized during disturbances. POD that incorporates SCs using a local frequency and a tie line power measurement, which adapts to modern system characteristics, is proposed to damp the power oscillation and improve frequency stability during disturbances. An optimal parameter set of the POD is determined using an SiL simulation based on the damping ratio maximization objective of the dominant oscillation mode. The simulation results show that POD based on the optimization algorithm can perform a significant enhancement to the oscillation damping and frequency stability and help control designers save time compared with the empirical tuning method. xv