This paper proposes an adaptive droop control (ADC) strategy for the voltage source converter (VSC) based multiterminal high voltage direct current (VSC-MTDC) system, which enables the VSC-station to provide frequency regulation for onshore ac grids. A V-I-f characteristic is derived to establish the relationship between the frequency deviation and dc voltage. A VSC-station working with the V-I-f characteristic is able to adjust its dc voltage reference autonomously according to the grid frequency deviation. Thus, the power flow of the VSC-MTDC system will be redistributed and more power is going to be injected into the adjacent grid of which the frequency deviation exceeds the threshold value. The ADC strategy inherits the advantage of the traditional droop control (TDC) that there is no need for communication system. An ac/dc hybrid system that includes VSC-MTDC system, onshore grids, and offshore wind farms is built in DIgSILENT/Powerfactory. Modal analysis and nonlinear simulations are performed to obtain the optimal control parameters and demonstrate the performance of the ADC strategy.

With the rapid increase of renewable energy sources and power electronic converters in the power system, the inertia of the power system is declined and the stability problem is more serious than ever before. In this paper, a virtual synchronous generator (VSG) control strategy is proposed to enable the voltage-source converter-based multiterminal high-voltage direct current (VSC-MTDC) system to damp the low-frequency oscillation modes. Both rotational inertia part and governor part are modeled in the VSG controller. The V ² -P-ω characteristic is developed based on the V ² -P droop control to emulate the action of the speed governor when the frequency of ac grid deviates from the nominal value. The virtual inertia part is used to emulate the inertia response of the synchronous generator. The communication between VSC stations is unnecessary with the autonomous power allocation ability of the V ² -P-ω characteristic. Modal analysis is utilized to test the system stability and optimize the controller parameters. Two test systems which include offshore wind farms, VSC-MTDC systems, and onshore power grids are built in the DIgSIENT/PowerFactory. The simulation results verify the effectiveness of the proposed VSG strategy.

The virtual synchronous generator (VSG) has been widely applied for mimicking the dynamic performance of a synchronous generator (SG) by power-electronic interfaced power sources. When the VSG is implemented in VSC-based multi-terminal direct current (VSC-MTDC) systems, the impact of VSG on both ac- and dc-grid dynamics, i.e., the low frequency oscillatory (LFO) modes and dc voltage stability should be taken into consideration. This paper investigates the participation of the VSG controller in low frequency oscillatory (LFO) modes and the impact on dc voltage stability. The influence of VSG parameters on the system dynamics is investigated by the Singular Value Decomposition (SVD) analysis and modal analysis. A parameter alternating VSG (AVSG) controller is proposed to improve the damping performance to LFO modes and mitigating the negative impact of VSG on dc voltage stability. The effectiveness of the proposed AVSG controller is validated according to the non-linear simulations on two test systems.

A distributed cooperative control (DCC) strategy based on the consensus protocol is proposed for providing frequency support through voltage source converter based multi-terminal direct current transmission (VSC-MTDC) systems. The DCC consists of a local P-V droop controller and a cooperative agent network for cooperation control. Each agent controller is composed of a frequency regulation subagent (FRA) for frequency support and a load ratio subagent (LRA) for balancing the power sharing amongst the stations connected to the same ac grid. Moreover, to reduce the impact of the communication link failure, a topology evaluating method is proposed to select the suitable topology of the communication network via graph theory. The small signal stability analysis and the steady-state analysis are performed to investigate the stability and the consensus performance of the VSC-MTDC systems with DCC. Compared with decentralized control, the DCC can simultaneously achieve better frequency support and power sharing without leading to a large dc voltage deviation. The effectiveness of the DCC is verified by simulations on a hybrid ac/dc grid with a 7-terminal VSC-MTDC in DIgSILENT/PowerFactory under the load step change event and communication failure event.

Grid-forming converters (GFC) are promising technology for improving the grid inertia and frequency stability via voltage source converters (VSC). However, GFCs could make the ac-side oscillations more complicated since it mimics the dynamics of synchronous generators (SG), and the ac-side oscillations could spread to the dc systems. This paper investigates the mechanism of the interactions between GFCs and ac grids, and proposes an enhanced GFC to improve the damping performance on oscillations. By separating the system model into GFC-related transfer functions (TF) and the ac grid-related TFs, the interaction and stability margin can be investigated by the phase difference of these TFs at the cross-gain frequency (CGF). A lead compensator is designed to reshape the phase characteristic of GFC-related TF for enhancing the damping performance of the GFC. The effectiveness of the enhanced GFC is tested according to the simulations on DIgSILENT/PowerFactory.

Low-inertia power grids could suffer from large frequency excursions under even small power disturbances. When a voltage source converter-based high voltage direct current (VSC-HVDC) system is used to integrate a low-inertia grid into a main grid, an ancillary frequency support service can be provided to the low-inertia grid by the VSC-HVDC system. This paper proposes a perturbation observer-based fast frequency support controller (POFFS) of VSC-HVDC systems to improve the frequency stability of low-inertia power grids. According to the feedback linearization and the high gain observer technique, the perturbation observer of the inverter station is designed to estimate the comprehensive impact of multiple perturbations, including the power disturbances in low-inertia grids, the uncertainty of grid inertia, and unknown nonlinear dynamics. The estimate of the perturbations are further compensated by the feedback control loop to achieve robust frequency regulation. Compared with the conventional frequency controller, the proposed POFFS can provide better frequency support to low-inertia power grids, without requiring an accurate system model and parameters. Two test systems are used to verify the effectiveness of the proposed POFFS.