Power system stability enhancement with high-share of power electronic converter interfaced renewable energy sources
Creators: name: Munkhchuluun, Enkhtsetseg
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400803
subject: Electrical energy generation (incl. renewables, excl. photovoltaics)
classificationCode: 400805
subject: Electrical energy transmission, networks and systems
classificationCode: 400808
subject: Photovoltaic power systems

description: Over the last two decades, the penetration level of power electronic converter (PEC) interfaced renewable energy sources (e.g., doubly-fed induction generators (DFIGs), solar photovoltaic (PV) systems) has rapidly increased to address the global warming and climate change concerns. This has significantly affected the frequency and voltage control of the power grid, as several important power system parameters, such as inertia/stored kinetic energy, short-circuit strength, reactive power reserve, and active power reserve, are severely affected by the PEC-interfaced renewable power integration. Therefore, it is necessary to characterise the frequency, voltage, and rotor angle stability of the power system with high penetration of the PEC-interfaced renewable energy sources, and also it is imperative to design appropriate controllers that fully utilise the active and reactive power capabilities of the PEC-interfaced renewable energy sources to improve the power grid stability.
The first phase of this research has been devoted to investigating the possibility of using the reactive power response capability of the PEC-interfaced renewable energy sources to improve the frequency stability of the power grid. Frequency stability is the main problem of modern, low-inertia power systems with high penetration of PEC-interfaced renewable energy sources. The reactive power capability of the PEC interfaced renewable energy sources can be used to support the grid frequency stability via indirect methods. Therefore, the fast-reactive power response capability of PEC interfaced renewable energy sources can indirectly support the grid frequency stability. In this thesis, a new frequency regulation approach based on reactive power control (FRQC) is proposed for solar-PV systems to improve the frequency stability of the power system. The effectiveness of the proposed FRQC scheme is verified using a realistic large-scale test power system (e.g., RVS test system) considering frequency and voltage thresholds, different penetration levels of PV systems, load voltage sensitivities, and the PV loading conditions. The results indicate that the proposed FRQC scheme improves the frequency stability, as it decreases the initial active power imbalance following a large disturbance.

The second phase of this research proposes a new methodology to size the battery energy storage system inverter to mitigate the adverse effects on frequency stability from solar-PV generation. Extensive research efforts have been made to develop methodologies to size a BESS to mitigate the issues associated with renewable energy sources. However, sizing of the BESS inverter to mitigate the negative impact of the solar-PV systems on frequency stability has not been studied thoroughly, as many jurisdictions do not require solar-PV systems to participate in the system frequency regulation. In this thesis, a new methodology to size a BESS inverter is developed based on the compensation of the frequency nadir point. The effectiveness of the proposed methodology is validated by using a realistic large-scale test power system (e.g., RVS test system). The results show that the proposed methodology successfully mitigates the frequency drops due to the solar PV system.
The third phase of this research is focused on improving the reactive power control strategy used in the PEC interfaced renewable energy sources to support long-term voltage stability (LTVS). The existing reactive power control schemes of the PEC-interfaced renewable energy sources are mainly designed to support the short-term voltage stability to support the low-voltage ride-through capability. In this approach, reactive power is prioritised over active power during voltage disturbances, and hence the active power is decreased until the voltage is recovered to a certain value. However, this control scheme does not support the LTVS, as it requires the power system to keep the active power constant and use the remaining capability of the PEC-interfaced renewable energy sources to increase the reactive power. In this thesis, a new active power control strategy with an active power prioritisation logic is used for solar-PV systems to improve the long-term voltage stability while supporting the short-term voltage stability. The effectiveness of the proposed active power priority based control strategy is validated by using a realistic large-scale test power system (e.g., Nordic test system) considering the loading condition of the PVs, inverter rating of PVs, and different control parameters. The results show that the proposed active power priority based control strategy of the PEC-interfaced renewable energy sources improves both the long-term and short-term voltage stability.

The final phase of this research has focused on improving the active power response of PEC-interfaced renewable energy sources to improve the first-swing transient rotor-angle stability of the power grid. Power systems with decreased inertia (due to high penetration of PEC-interfaced renewable energy sources) are subjected to large initial rotor angle deviation or swings following short-circuit faults. This initial deviation is important as it further defines whether the power system is able to maintain rotor-angle stability. Currently, reactive power response capability is mainly used to support the rotor angle stability, and different types of power system stabilisers and damping controllers are proposed to damp the oscillations. However, the appropriate strategies to control the active power response of PEC-interfaced renewable energy sources during and after the short-circuit event can be used as an effective solution for managing the acceleration and deceleration torques of synchronous generators. Hence, with the fast response of the PEC interfaced renewable energy sources, they could enhance the first-swing transient rotor angle stability. In this thesis, a new active power logic (APL) controller is proposed for DFIGs to improve the first-swing rotor angle stability. The effectiveness of the proposed APL controller is verified using a realistic large-scale test power system (e.g. RVS test system) for local and remote faults and high DFIG penetration levels. Results indicate that the proposed APL controller can significantly enhance the first-swing rotor angle stability of the power system, specifically when the DFIGs are located near synchronous generators. 

The key findings of this research have contributed to three prominent areas in the power system stability with PEC interfaced renewable energy sources; first swing rotor angle stability, frequency stability, and long-term voltage stability. Therefore, the outcomes of this thesis would assist in making secure, stable, and reliable power grids with high penetration of PEC-interfaced renewable energy sources in the future.