Hybrid AC/DC Power Grids: Stability and Control Aspects

The transmission network is one of the key elements of the power network. This chapter presents an overview of the power transmission system evolution, the high-voltage direct-current (HVDC) transmission system technologies, and the ongoing transition of the AC transmission network to a hybrid AC/DC power grid with the integration of large-scale variable renewable energy sources, such as large-scale onshore and offshore wind farms. This chapter sheds light on the key elements and functionalities of the HVDC transmission systems by providing examples of various real-world HVDC systems. Moreover, this chapter also provides a general overview of operational challenges faced by the emerging hybrid AC/DC power grids.

This chapter introduces different HVDC technologies used in HVDC systems with fundamental theories associated with each HVDC technology. The line commutated converter HVDC technology is introduced with details of different line commutated converter HVDC topologies and the fundamental operation principles of line commutated converter HVDC technology. Similarly, the voltage source converter HVDC technology is discussed in detail, including two-level, multi-level and modular multi-level converter topologies. Moreover, emerging HVDC technologies, such as hybrid LCC-VSC HVDC technology and a comparison between different VSC technologies are also presented in this chapter. Finally, this chapter presents power system stability classification with details on each stability phenomenon and a brief overview of analytical approaches used for assessing each stability issue.

Modelling and control of hybrid AC/DC power grids are of paramount importance for power grid stability and dynamics studies. Therefore, accurate representation of the AC and DC grid equipment is vital to capture the stability boundaries of the hybrid AC/DC power grid. This chapter provides a comprehensive overview on the modelling and control techniques used for various components in hybrid AC/DC power grids. The modelling techniques for LCC and VSC converters are discussed separately describing the pros and cons of each technology. The control techniques are also discussed in detail while providing a detailed classification of different control techniques used in HVDC converter stations. Moreover, modelling of various AC power grid components and the multi-terminal power grids are also discussed in this chapter.

The utilisation of HVDC transmission has brought complex dynamics to the conventional AC power grids dominated by the synchronous generator. Rotor angle stability, as one of the major dynamic concerns, has been considerably affected by the frequent interactions with different HVDC systems. This chapter provides a comprehensive overview on the concepts, and main features and causes of two rotor angle stability issues. Transient stability of hybrid AC/DC networks is investigated by introducing fundamental concepts associated with transient stability evaluation. Subsequently, how the transient stability phenomenon affected by hybrid AC/DC power networks is exemplified with exemplar case studies. The small-signal modelling (i.e., linearised state-space model) of the hybrid AC/DC power grids covering both LCC and VSC based HVDC systems and relevant control schemes is introduced. The general small-disturbance stability analysis concerning the critical oscillation and damping mechanism as affected by HVDC systems is presented by using the eigenvalue analysis and eigenvalue sensitivity analysis with respect to some key system parameters. The influence of LCC and VSC based HVDC systems on the synchronising and damping torque of the synchronous generator is studied. Time domain simulations of hybrid AC/DC power grids are also provided for validation purposes in this chapter.

Voltage stability is considered as one of the key concerns of hybrid AC/DC power grids, the mechanism of which can be quite complicated and involves multiple factors. This chapter provides a comprehensive overview on the concepts, and main causes and analysis methods of two voltage stability issues including the large-disturbance and small-disturbance voltage stability of the hybrid AC/DC power grids. The small-disturbance voltage stability analysis covering both the AC and DC system-oriented issues and relevant control measures are introduced, with the emphasis mainly on the operational equilibrium associated small-disturbance voltage stability issue solved by voltage  sensitivity factor method and maximum power curve method. The influence of various uncertainties resulting from the high penetration of renewable power generation on the voltage stability of the hybrid AC/DC power grids is effectively investigated by using the probabilistic voltage stability analysis and associated variance indices in this chapter.

This chapter delineates the fundamental and advanced concepts associated with frequency stability and control of hybrid AC/DC power grids. As power grids are transforming to hybrid AC/DC power grids with HVDC transmission systems and high penetration of power electronic converter (PEC) interfaced renewable power generation sources, the electrical dynamics are decoupled at a rapid rate resulting in less natural response contribution from the generation sources during frequency disturbances. Therefore, it is imperative to pay a strong attention on improving the frequency stability and control in hybrid AC/DC power grids. This chapter exemplifies the frequency stability issues and frequency control strategies of hybrid AC/DC power grids with case study examples.

In contrast to the conventional synchronous generators, power electronic converter interfaced renewable power generation, such as wind and solar-PV plants is typically sited in remote geographic locations in the power grid. Therefore, inevitably, the generated power from these sources has to be transmitted through long-distance transmission corridors. In many cases, they are being connected to main power grid at remote locations with a low short-circuit strength creating instability conditions. Furthermore, power electronic converter interfaced sources provide low short-circuit current, and hence they further deteriorate the short-circuit strength at the remote locations of the network. As HVDC transmission links are used in these transmission corridors, they may have to operate under a low short-circuit strength triggering stability concerns. In addition, HVDC transmission links connecting two synchronous regions are also sometimes connected to busbars with a low short-circuit strength, and hence it is essential to investigate these stability issues and develop remedial strategies to overcome these challenges. Moreover, the power electronic converters associated with these HVDC systems are also prone to various emerging stability issues ranging from converter control interaction issues to converter control induced resonance conditions. This chapter investigates these stability and operation issues associated with a low short-circuit strength and power electronic converter control associated stability in hybrid AC/DC power grids. Moreover, various strategies to overcome these stability issues are also discussed in detail within this chapter.

This chapter introduces coordinated control schemes used in hybrid AC/DC power networks. A variety of control schemes including control schemes which can control multiple system parameters, realise balanced power sharing, minimise transmission power losses, and damp oscillations are discussed in this chapter. Moreover, the model predictive control is introduced as an exemplar optimisation algorithm that can be implemented in hybrid AC/DC power networks to damp power oscillations. Numerical examples and case studies are presented to illustrate the capability and performance of each control scheme. This chapter also provides examples of complex control objectives that can be implemented in hybrid AC/DC power networks to improve stability.