Inertia enhancement for power systems with high penetration of renewable energy
Date of Issue2019-07-26
School of Electrical and Electronic Engineering
To reduce carbon footprints, renewable energy sources (RESs) are widely employed to take the place of fossil fuels at present. With the increasing penetration of renewable energy generation in modern electric power systems, power electronic converters have drawn attentions extensively. Power electronic converters, which feature fast response and flexible control, are typically built with several fully controllable semiconductor devices. The functions of power converters are to convert and control the electricity as required. Power electronic converters have a wide range of application, including high voltage direct current (HVDC) transmission systems, uninterruptible power supplies (UPS), static var compensation (SVC), etc. A paradigm shift occurs in power systems with the fast development of power converters, that is, the conventional power systems are transforming to power electronics-dominated power systems. Hence, innovative and proper control strategies are vital to the future more-electronics power systems for stability maintenance, harmonic elimination and high conversion efficiency. One specific and severe problem which frequently appears in power electronics-dominated power systems refers to the low inertia. In conventional power systems, the power system inertia is provided by the rotor of the synchronous generators, which could release kinetic energy to the power grids or absorb it from the grids by changing the rotor speed to maintain the grid frequency in an acceptable range when there is a frequency disturbance. Large inertia is always desirable for power systems, because large inertia may reduce the power system frequency deviation and slow down the dynamic of frequency change. However, the synchronous generators are gradually replaced by power converters. With the decreasing of the synchronous generators, power system inertia will be significantly reduced. Less damping can be provided to the power system with the reduced inertia. Therefore, the performances of power system frequency regulation will be weakened due to the increasing shares of renewable energy generation, and the grid frequency would be more sensitive to the disturbance caused by the imbalance between generation and demand in the power systems. Low inertia has already challenged the control and frequency stability of small-scale power system. With the continuous integration of renewable energy generation, the large-scale power systems are likely to face the same issues, such as cascading failures, undesirable load shedding, or even massive black-out, in the future. The focus of this thesis is the inertia enhancement for power systems through energy storage systems. The objectives of the research are the improvement of frequency regulation and enhancement of the power system inertia by emulating the inertia through power converters. Some control schemes for inertia emulation and enhancement will be presented and discussed in detail in the thesis. Specifically, the distributed virtual inertia is generated by different energy storage systems (ESSs). With the proposed methods, the frequency nadir and the rate of change of frequency (RoCoF) can be improved, which means the power system inertia is emulated and the frequency stability is improved. The other issues brought by the extension of power converters, such as voltage sag, harmonics, low power factor, etc., should be further discussed and solved in the future. The concept of grid-friendly power converters is the potential research direction. All the control schemes presented in the thesis are verified in simulation using Matlab / Simulink and PLECS. The down-scale experimental test bed has been built for validating the feasibility of the presented methods.
Engineering::Electrical and electronic engineering::Power electronics