Supplementary Power Controllers for Modern VSC-HVDC transmission links

Abstract

The growing deployment of geographically close Voltage Source Converter-based High Voltage Direct Current (VSC-HVDC) links enables their interconnection into meshed or multi-terminal HVDC networks. Ensuring the interoperability of such networks, particularly when they evolve from progressively expanded point-to-point links, is a complex task that requires a detailed understanding of control strategies and their influence on power system stability. Simulation models play a critical role in evaluating how expanded VSC-HVDC systems support AC grid stability. However, existing models often require modification to represent multi-terminal operation and to comply with modern grid code requirements. Therefore, upgrading simulation models for analyzing the multi-terminal expansion of VSC-HVDC links remains an important research challenge.

This dissertation addresses this challenge by extending a VSC-HVDC simulation model within a root-mean-square (RMS) simulation framework through the development of several supplementary power controllers. The controllers are implemented in DigSILENT PowerFactory and modify the active and reactive power regulation of a VSC-HVDC link depending on the stability phenomenon being analyzed. Reactive power regulation is adapted to support voltage stability through dynamic power factor control and polynomial-based reactive current injection control. Active power regulation is modified to provide primary frequency support through a power-line communication-based controller, a post-fault active power recovery control, and an open-loop frequency controller.

In addition to these modelling improvements, the dissertation proposes methods for performance assessment and control design. A directional derivative-based method (DDBM) is introduced to evaluate the quasi-stationary voltage support provided by reactive power controllers without requiring time-domain simulations. This method helps identify the most suitable control strategy under different power flow conditions and network strengths. Furthermore, a dynamically adjustable fault impedance (DAFI) concept is proposed to improve the active and reactive power response of VSC-HVDC links during fault ride-through (FRT) and post-fault operation.

The results show that expanding a point-to-point VSC-HVDC link into a multi-terminal configuration affects both active and reactive power responses and their interaction with the AC system in steady-state and dynamic conditions. For example, dynamic power factor regulation can lead to AC voltage deviations of up to 3% during active power reversal events. The DDBM analysis indicates that dynamic power factor control is generally less effective in supporting quasi-stationary voltage stability under the studied operating conditions. The DAFI concept demonstrates that inductive system characteristics can be emulated through first-order dynamic responses, improving controller performance during fault and post-fault periods.

Additional control strategies are proposed to support frequency stability. A power-line communication-based controller using harmonic amplitude modulation enables primary frequency support and reduces the rate-of-change-of-frequency and frequency nadir during network split events. An open-loop frequency controller is also introduced to coordinate frequency responses between asynchronous AC systems under severe power imbalances.

Finally, the study shows that multi-terminal HVDC expansion requires transient DC voltage control to manage post-fault active power recovery. A multi-terminal DC voltage controller based on an exponential function is proposed to regulate DC voltage during recovery periods. Simulation results demonstrate that coordinating this controller with DC choppers can reduce AC/DC power imbalances by up to 80% while restoring active power within 200 ms.

Overall, the proposed modelling and control approaches improve the analysis and operation of multi-terminal VSC-HVDC systems and contribute to the reliable integration of HVDC networks into future power systems.