Stability issues and mitigation strategies for hybrid AC/DC microgrids
Creators: name: AHMED, Moudud
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

description: The self-governing small regions of power systems, known as 'microgrids', are enabling wide-scale adaptation of small-scale renewable energy sources (RESs) into electricity networks while improving the reliability and the energy efficiency of the system.  With recent developments of power electronic converters (PECs) and due to the environmental factors, the concept of microgrid is gaining significant popularity in the power industry. Microgrids can be primarily classified into three types based on the voltage characteristics and associated architecture; 1) AC microgrids, 2) DC microgrids, and 3) hybrid AC/DC microgrids. Hybrid AC/DC microgrids provide a great opportunity in terms of economy and efficiency compared to the other microgrid architectures. Microgrids are cost-effective solutions due to the integration of multiple RESs, reducing transmission and distribution costs, and higher energy efficiency; however, major drawbacks of renewable based RESs are their variability and intermittency. Furthermore, maintaining a stable and secure microgrid is a significant challenge in the presence of multiple types of RESs. 

The first phase of this research critically investigates oscillatory stability issues and mitigation strategies for hybrid AC/DC microgrids with dynamic loads. The hybrid AC/DC microgrids are susceptible to stability issues in cases with high penetration of dynamic loads (e.g., induction machines). The non-linear dynamics of induction machines, result in sustained voltage/ frequency oscillations following disturbances in the microgrid, which is a major challenge for stable operation of the hybrid AC/DC microgrid. This issue is even more severe when the hybrid AC/DC microgrid is operated at autonomous mode. The PEC based energy storage systems (ESSs) are used as an effective solution for power balancing in the microgrid; hence with the fast response of the PEC, microgrid voltage/ frequency could be stabilised rapidly. In this thesis, a supplementary power oscillation damping controller is proposed for the ESS to damp the low-frequency oscillations (LFOs) in hybrid AC/DC microgrids. The effectiveness of the proposed damping controller is verified using non-linear simulations considering different penetration levels of dynamic loads and disturbances in a hybrid AC/DC microgrid. Results indicate that proposed supplementary power oscillation damping (POD) controller can significantly damp the LFOs in the hybrid AC/DC microgrid.

The second phase of this research critically investigates the dynamic behaviour of various induction machines in microgrids and their stability issues such as LFOs. Majority of the published literature has investigated these stability issues with aggregated models of induction motors (IMs) in hybrid AC/DC microgrids, which do not properly reflect the actual dynamics of parallel operating IMs; hence, the POD controllers must be designed explicitly considering various oscillations induced by parallel operating IMs. Therefore, to tackle LFO issue with multiple IMs, this research proposes an adaptive neuro-fuzzy inference system (ANFIS) based POD controller to damp LFOs induced by IMs in hybrid AC/DC microgrids. The proposed supplementary POD controller was embedded to the ESS controller, which provides additional damping power proportional to the frequency deviation. The following two features namely: 1) ability to adjust the gain based on the frequency deviation, and 2) ability to handle more non-linearity in the system dynamics, make the proposed adaptive ANFIS based POD controller more unique compared to conventional POD controllers. The effectiveness of the proposed ANFIS-POD controller is verified using non-linear dynamic simulations considering a range of disturbances in a hybrid AC/DC microgrid and different combinations of parallel operating IMs.

Third phase of this research critically investigates the effect of feeder (line) X/R ratio on microgrid stability, particularly focusing on oscillatory stability. The existing literature does not comprehensively cover stability of the hybrid AC/DC microgrid considering the X/R ratio of the microgrid feeders. Majority of the RESs and the ESS controllers are designed by assuming that the microgrid feeders are either inductive or resistive, and then the dynamic performance of the hybrid AC/DC microgrid are investigated by varying the X/R ratio of feeders while maintaining the feeder impedance constant. The theoretical analysis indicates that the frequency can be controlled by the reactive power in a microgrid with resistive feeders, while the frequency can be controlled via the active power in a microgrid with inductive feeders. Furthermore, the feeder resistance, inductance, and capacitance parameters highly depend on the rated voltage of the feeder; hence, these parameters affect the active and reactive power sharing of the parallel connected voltage source converters (VSCs). This research proposes a universal VSC control strategy for hybrid AC/DC microgrids with varying X/R ratios and voltage level of the microgrid feeder. The transient stability is analyzed by: 1) developing a hybrid AC/DC microgrid with 6.6 kV and 22 kV distribution voltage levels; 2) applying active and reactive power load switching; and 3) the outage of a large RES.  It is observed that the proposed universal VSC controller ensures improved stability under both the parallel and radial distribution types hybrid AC/DC microgrids. 

Finally, this research concludes by proposing a new hybrid AC/DC microgrid architecture with a central storage system rather than two separate storage systems for each AC and DC sub-grid, which will also eliminate the necessity of multiple ILCs and ensure stability and economic benefits to users. Most of the available hybrid AC/DC microgrid architectures have two separate storage systems for both the AC and the DC sub-grids. The existing hybrid AC/DC microgrid architecture ILC transfers real power to the DC sub-grid and real/reactive power to the AC sub-grid. Therefore, complex coordinated control strategies between multiple ILCs and the two separate ESSs are required. However, in the proposed hybrid AC/DC microgrid architecture the centralized ESS is the main energy balancing element. The centralized ESS supplies real power to the DC microgrid based on the DC point of common coupling (PCC) voltage and supports DC microgrid voltage, whereas, it supplies real power to the AC microgrid based on the AC PCC voltage and supports AC microgrid voltage. In the proposed hybrid AC/DC microgrid architecture, the ILC transfers real power to the DC sub-grid and real/reactive power to the AC sub-grid; hence, the control complexity is reduced.

In summary, this research contributed towards characterising the oscillatory stability issues in hybrid AC/DC microgrids and developed strategies for hybrid AC/DC microgrids with dynamic (IM) loads to mitigate oscillatory stability problems. Finally, this research developed a universal VSC controller (for both the parallel and the radial type hybrid AC/DC microgrids) and proposed a new hybrid AC/DC microgrid structure with a centralized ESS and single ILC.