Stability analysis and enhancement of hybrid AC/DC power networks
Creators: name: GU, Mingchen
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 power transmission network is a fundamental component of the power system, which transmits the electrical energy from power generation plants to load centres. Thus, the stability of the power system is significantly influenced by the power transmission network. With the integration of large-scale renewable energy sources (RESs), such as wind farms and solar photovoltaic farms, the high-voltage direct-current (HVDC) transmission became a viable option for long-distance bulk power transmission due to comparatively low losses and high control flexibility offered by modern HVDC systems. Therefore, a large-scale HVDC network has been built along-side the AC transmission network while transforming power grids to hybrid AC/DC power grids.

The voltage-source converter (VSC) based HVDC system is the most prominent HVDC technology, since VSCs are not prone to commutation issues under transient conditions, and they are capable of performing complex control tasks to achieve multiple objectives. However, VSC-HVDC networks decouple the natural electromechanical dynamics between two interconnected power grids, which could significantly affect the stability of hybrid AC/DC power grids. Since the hybrid AC/DC power network consists of the AC network and the DC network, both the AC and the DC network should be stable to ensure the stability of the entire hybrid AC/DC power network. This research investigates and addresses the stability and control issues of hybrid AC/DC power grids with a special emphasis on VSC-HVDC multi-terminal networks.

The first phase of this research theoretically investigates the influence of the VSC-HVDC networks on the stability of hybrid AC/DC power grids. The components of hybrid AC/DC power grids are more complex than the conventional AC power grids. Therefore, VSCs and HVDC transmission lines must be represented with adequate details in electromechanical dynamic timeframe to accurately evaluate the dynamics and stability of hybrid AC/DC power grids. A detailed theoretical analysis of hybrid AC/DC power grids is of paramount importance for understanding the performance of the power grid. This research develops a comprehensive state-space model of the hybrid AC/DC power grid with a VSC-HVDC network. Based on the state-space model, a sensitivity analysis is conducted to verify the power grid stability.

In the second phase of this research, the rotor angle stability is analysed, and control schemes are developed to improve the rotor-angle stability of hybrid AC/DC power grids. The critical clearing time (CCT) of the hybrid AC/DC power grid is investigated. The damping torque analysis (DTA) is a rotor angle stability analysis technique, where synchronising and damping torque coefficients are calculated to assess the rotor angle stability in AC power grids. However, for hybrid AC/DC power networks with VSCs, the calculation method of synchronising and damping torque coefficients is more complex. Thus, a new calculation method is developed in this research to extract the synchronising and damping torque coefficients. Based on the calculated synchronising and damping torque coefficients, a rotor angle stability control scheme is proposed for hybrid AC/DC power grids with VSC-HVDC networks. The proposed control scheme is validated using both the two-terminal VSC-HVDC and the multi-terminal HVDC network. 

The final phase of this research develops an adaptive controller for VSCs to control the DC voltage, frequency and AC voltage simultaneously. As the stability of the hybrid AC/DC power grid influences both the AC network stability and the DC network stability, it is essential to ensure an overall control strategy for VSCs, which has not been conducted in previous research studies. The proposed adaptive control scheme is based on a multi-dimensional droop control method. The regulator parameters are able to adapt to the severity of the disturbance. Under small disturbances, constant parameters are applied to maintain the stability of the power grid; while under large disturbances, the parameters are adjusted to suppress oscillations and help to achieve enhanced stability performance. Another feature of the proposed control strategy is it can be implemented without a communication network between VSCs, which avoids issues caused by communication links, such as communication latency and data loss. The simulation results indicate that hybrid AC/DC power grids with the proposed control scheme can achieve better dynamic performance compared with the conventional droop control scheme. Furthermore, this research proposes a distributed model predictive control (MPC) scheme. MPC is a control method which can achieve optimised control objective and have the ability to manipulate multiple variables simultaneously. Thus, the proposed distributed MPC-based control scheme is adopted to regulate the DC voltage, frequency and active power. The MPC controllers are applied in a distributed manner, and a communication network is utilised to share sampled current and voltage data between VSCs to decide the operating point of each VSC. With the communication network, the power-sharing between VSCs can be arranged as required, which is more flexible than conventional droop control scheme. The effectiveness of the proposed MPC-based control scheme is verified with a multi-terminal HVDC network.

In conclusion, this thesis has contributed to the characterisation of the stability of hybrid AC/DC power networks and the development of control schemes to improve the stability of hybrid AC/DC power grids under disturbances.