Abstract
The voltage-source converter (VSC) based multi-terminal direct-current (MTDC) networks are currently being developed in large-scale power grids for efficient and economical transmission of electrical energy generated from remotely sited renewable energy sources (RESs). As the MTDC network is developed alongside the conventional AC grid, it has transformed the power grid to a hybrid AC/MTDC power network. However, since the MTDC network is controlled via the power electronic converters (PECs), it significantly affects the AC power grid damping and synchronising performance as it decouples the natural dynamics between various electro-mechanical systems in the power grid. This paper proposes a supplementary control scheme to improve the damping performance of the entire hybrid AC/MTDC power grid. The damping torque analysis (DTA) technique is used as the primary technique to develop the supplementary control scheme. The preliminary analysis is carried out using a two-terminal hybrid AC/DC power grid and has suggested a supplementary control loop based on the rotor speed deviation for the DC voltage controller of the VSC to improve the damping performance. With the state-space model, the synchronising and damping torque coefficients are calculated and accordingly, the feedback gain is determined to provide optimal synchronising and damping torque components. Subsequently, the fidelity of the supplementary controller is verified using a four-terminal hybrid AC/MTDC grid. Simulation studies prove that the proposed supplementary controller can improve the hybrid AC/MTDC network damping performance, and it performs very effectively with the master-slave control and the conventional droop control scheme.
Highlights
The conventional AC power grid architecture is rapidly evolving with the integration of voltage-source converter (VSC) based high-voltage direct-current (HVDC) transmission systems
In order to verify the effectiveness of the supplementary control loop in combination with the droop control scheme, the supplementary control loop is introduced to VSC2∼VSC4
A calculation procedure was presented to determine the values of the synchronising and damping torque coefficients, since these coefficients must be appropriately calculated for each network to effectively damp out oscillations
Summary
The conventional AC power grid architecture is rapidly evolving with the integration of voltage-source converter (VSC) based high-voltage direct-current (HVDC) transmission systems. Reference [16] proposed a decentralised inter-area oscillation damping method for a hybrid AC/MTDC network based on the analysis conducted with the state-space model. A coordinated control method based on the CLF was proposed in [21] to improve rotor angle stability of the onshore power network during an offshore disturbance Both the modal analysis and the Lyapunov method have some drawbacks in terms of analysing the influence of the control scheme from the synchronising and damping torque perspective for a given power network. The DTA technique is applied with the modal analysis to analyse the damping and synchronising performance of the hybrid AC/MTDC power network. A supplementary control loop is designed based on the rotor speed deviation to improve the damping performance of hybrid AC/MTDC power grids;. The detailed generator state-space matrix can be found in the Appendix-B
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