Voltage Source Converter-based Multi-terminal High-voltage Direct Current Research Articles

Cooperative control of the DC-link voltage in VSC-MTDC grid via virtual synchronous generators

Introduction: Virtual Synchronous Generators (VSGs) are used in Voltage Source Converter-based Multi-Terminal High-Voltage Direct Current (VSC-MTDC) systems to enhance power system stability. However, the MTDC framework can lead to instability due to reduced inertia in certain grid areas, especially during load switching at VSC stations. This instability is exacerbated by untimely adjustments of the VSG's power setpoint, leading to voltage and frequency oscillations.Methods: This study introduces a cooperative control approach for the DC voltage of the VSG, employing a consensus algorithm and Model Predictive Control (MPC). This method aims to achieve incremental power for the VSG and provide interactive power commands for both the grid side and the wind farm side. The consensus algorithm ensures coherent system adjustments, while the MPC algorithm tracks DC-side voltage changes in real time.Results: The application of this cooperative control approach significantly enhances DC voltage regulation performance. It effectively reduces the extent of frequency drops and mitigates secondary frequency drop (SFD) issues, particularly those arising from the use of wind farms for frequency regulation and the associated speed recovery in wind turbine units.Discussion: The increase in supplemental power effectively utilizes the energy stored in DC-side capacitors for power balance regulation and introduces additional inertial power into the system. Electromagnetic transient simulations have confirmed the effectiveness of the Consensus MPC-VSG method, demonstrating its ability to optimize the dynamic performance of VSC-MTDC systems and promote stability in DC voltage and frequency.Conclusion: The findings suggest that employing the Consensus MPC-VSG method offers a promising solution for enhancing the stability and operational efficiency of VSC-MTDC systems, addressing the challenges posed by the inherent segmentation of the grid and the integration of renewable energy sources like wind farms.

Distributed Optimal Power Flow for VSC-MTDC Meshed AC/DC Grids Using ALADIN

The increasing application of voltage source converter (VSC) high voltage direct current (VSC-HVDC) technology in power grids has raised the importance of incorporating DC grids and converters into the existing transmission network. This poses significant challenges in dealing with the resulting optimal power flow (OPF) problem. In this paper, a recently proposed nonconvex distributed optimization algorithm -- Augmented Lagrangian based Alternating Direction Inexact Newton method (ALADIN), is tailored to solve the nonconvex AC/DC OPF problem for emerging voltage source converter (VSC) based multiterminal high voltage direct current (VSC-MTDC) meshed AC/DC hybrid systems. The proposed scheme decomposes this AC/DC hybrid OPF problem and handles it in a fully distributed way. Compared to the existing state-of-art Alternating Direction Method of Multipliers(ADMM), which is in general, not applicable for nonconvex problems, ALADIN has a theoretical convergence guarantee. Applying these two approaches to (VSC-MTDC) coupled with an IEEE benchmark AC power system illustrates that the tailored ALADIN outperforms ADMM in convergence speed and numerical robustness.

Two-Level Combined Control Scheme of VSC-MTDC Integrated Offshore Wind Farms for Onshore System Frequency Support

This paper proposes a two-level combined control (TLCC) scheme of voltage source converter-based multi-terminal high-voltage direct current (VSC-MTDC) integrated offshore wind farms to provide frequency support for onshore system. The proposed TLCC scheme consists of two levels, which are the step start-up and adaptive inertial droop control of the offshore wind turbine level, and the communication-free allocation control of the onshore VSC station level. On the first level, each wind turbine adopts the inertial and droop control with adaptive coefficients, and all wind turbines (WTs) work at the maximum power point tracking (MPPT) mode without energy reserve. To reduce the second frequency drop (SFD), the WTs are divided into different clusters according to their rotor speed, and a step start-up control scheme is adopted for the WT clusters to provide frequency support sequentially. On the system level, the communication-free allocation control strategy is proposed using local frequency signal of onshore VSC stations to share the active power among onshore VSC stations reasonably. The proposed TLCC scheme can provide onshore system with frequency support and reduce the SFD simultaneously, while all WTs work at MPPT mode. Case studies are carried out on a 3-area 4-terminal VSC-MTDC based offshore wind farms (OWFs). Simulation results demonstrate the effectiveness and universality of the proposed TLCC scheme under different scenarios.

Hierarchical and Robust Scheduling Approach for VSC-MTDC Meshed AC/DC Grid With High Share of Wind Power

This paper proposes a novel hierarchical and robust scheduling paradigm for the emerging voltage source converter (VSC) based multiterminal high voltage direct current (VSC-MTDC) meshed AC/DC hybrid system with high share of wind power. Considering the multilevel structure of the VSC-MTDC meshed AC/DC system, the scheduling problem is decomposed into three interactive levels according to the hierarchical analytical target cascading (ATC) technique. The low level is a two-stage adaptive robust security-constrained unit commitment problem for the AC system, which is solved by the column-and-constraint generation (C&CG) algorithm; the middle level is a day-ahead power transmission optimization problem for the VSC stations, where a day-ahead operation model for the VSC station is presented to fully use its flexible and controllable power adjustment capability to promote wind power accommodation; and the high level is a multi-period optimal power flow problem for the DC grid. An integrated ATC and C&CG algorithm is proposed to enable the hierarchical and robust scheduling. The parallel solution is used to accelerate the hierarchical scheduling. Simulations show that this formulation is effective in dealing with wind power uncertainty, promoting wind power accommodation, and improving the calculation efficiency.

VSC-MTDC Operation Adjustments for Damping Inter-Area Oscillations

Application of voltage source converter based multi-terminal high-voltage direct current (VSC-MTDC) grids operation adjustment for damping inter-area oscillations are addressed and analyzed in this paper. The new operation adjustment method is proposed to meet short-term solution requirements and accommodate critical operation conditions where the conventional damping stabilizer by using active power modulation cannot provide sufficient damping. Based on the sensitivities with respect to the active power injections of VSCs and the interior point algorithm, an operation optimization method is presented to obtain a proper VSC-MTDC operation scheme to achieve better damping performance for an ac/VSC-MTDC system. For some critical operation conditions in which an inter-area oscillation has been detected, an operation rescheduling method is proposed as an emergency oscillation control. Time-domain simulations are performed on a New England system with a five-terminal MTDC grid. A comparative study with a conventional damping stabilizer is also included in the case study. Both the theoretical analysis and the time-domain simulations have verified the effectiveness and superiority of the two proposed methods.

Coordinated power control strategy of voltage source converter‐based multiterminal high‐voltage direct current based on the voltage‐current curve

To achieve power distribution quickly and improve the anti‐interference capability of the voltage source converter‐based multiterminal high‐voltage direct current (VSC‐MTDC) system, a coordinated control strategy based on the V‐I curve is proposed in this article. The proposed control strategy includes the conventional droop control, the improved droop control, which considers the voltage dead band and fixed dc voltage control. The conventional droop control is utilized in one of the multiple converter stations, the fixed dc voltage control is adopted in the other one, and the improved droop control is applied to all of the rest converter stations. In this way, the unbalanced power will be distributed rapidly only through the conventional droop control, when the change of dc voltage is within the dead band. Meanwhile, all the droop coefficients of the improved droop control are adjusted automatically following the changes in dc current and dc voltage, and the converter stations regulate the unbalanced power coordinately, when the dc voltage is changed beyond the dead band. A model of a four‐terminal voltage source converter‐based high‐voltage direct current (VSC‐HVDC) transmission system is built in PSCAD/EMTDC, and the simulation results are presented to validate the expected performance of the proposed control strategy. © 2019 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Singular value decomposition‐based dynamic response analysis of VSC‐MTDC/AC systems for renewable energy integration

The dynamic performance of voltage source converter-based multi-terminal high-voltage direct current (VSC-MTDC) grid for large-scale renewable energy integration is becoming a concern. This study proposes a system-level dynamic response model of wind farm (WF)-MTDC-main AC systems, and then analyses the dynamic performance based on the singular value decomposition (SVD) technique. In the modelling, an improved virtual synchronous machine control is developed, and the interaction between AC frequency and DC voltage can be readily described. Using the SVD technique, parameters of the controllers are tuned, thereby making the AC frequency and DC voltage deviations within the limitations. Some oscillation modes of the system are observed and virtual power system stabiliser is proposed to suppress the oscillations. Additionally, the oscillation can be mitigated by emulating capacitance-based inertia response. The efficiency of the proposed model and analysis is verified through the frequency-domain and time-domain results.

A Virtual Synchronous Generator Control Strategy for VSC-MTDC Systems

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 $V^2-P-\omega$ 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 $V^2-P-\omega$ 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.

Design and implementation of perturbation observer‐based robust passivity‐based control for VSC‐MTDC systems considering offshore wind power integration

Voltage source converter-based multi-terminal high-voltage direct current (VSC-MTDC) systems are starting to be commissioned. However, concentrated integration of large-scale wind power demands stronger robustness against power fluctuation and system disturbances to increase the reliability of the whole system. This study proposes a perturbation observer-based robust passivity-based control (PORPC) for VSC-MTDC systems connected to an offshore wind farm to meet the demands. The aggregated effect of system nonlinearities, parameter uncertainties, unmodelled dynamics and external disturbances includes grid faults and time-varying wind power output is estimated by a linear perturbation observer and fully compensated by a passive controller, thus no accurate VSC-MTDC system model is required. PORPC attempts to regulate DC voltage and reactive power at the rectifier side, as well as active power and reactive power at the inverters side connected to an offshore wind farm. Besides, a DC-link voltage droop controller is introduced so as to provide immediate response to the grid unbalance situation. Moreover, a noticeable robustness against parameter uncertainties can be achieved as no accurate system model is needed. Case studies are carried out to compare the performance of PORPC to other typical approaches. Finally, a hardware-in-the-loop test is undertaken via dSPACE which validate its implementation feasibility.

Adaptive Droop Control of VSC-MTDC System for Frequency Support and Power Sharing

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.

Virtual Synchronous Generator Strategy for VSC-MTDC and the Probabilistic Small Signal Stability Analysis

In this paper, a new virtual synchronous generator (VSG) strategy based on the V2-P droop control strategy is proposed for the voltage source converter based multi-terminal high voltage direct current (VSC-MTDC) system. The VSG strategy enables the inverter stations to damp the power oscillation and provide frequency support for the AC system. Virtual inertia coefficient is obtained to couple the DC voltage and the angular frequency. To provide the oscillation damping and frequency support, the inverter stations could adjust the output power by the adjustment of the DC reference voltage. Due to the characteristic of the V2-P droop method, there is no requirement for the communication between inverter stations. The Monte Carlo method is used to test the probabilistic small signal stability of the VSG strategy. The performance of the proposed controller is demonstrated by a hybrid AC/DC system which is modeled in DIgSILENT/PowerFactory.

Dynamic Resonance Analysis and Oscillation Damping of Multiterminal DC Grids

Voltage-source converter-based multiterminal high-voltage direct current (VSC-MTDC) systems/grids are prone to system instability. This critical issue is overlooked in the literature. In order to improve system stability, this paper proposes an effective active damping method as a remedy to suppress voltage and power resonances in the VSC-MTDC grids by injecting damping signals into the inner current loops of VSC-MTDC stations. With dynamic regulation of the damping current, resonance is suppressed by power converter controllers without any additional current and voltage measurement. In this paper, modeling and stability analysis of the VSC-MTDC system/grid is presented considering the dc-side energy storage components, and control with a droop control structure. Then, single-frequency and multifrequency resonance mechanisms of dc-bus voltage and power in the event of transients are analyzed. Later, the stability effect of the MTDC system/grid inductance and capacitance values to the resonance amplitude and frequency droop coefficients is investigated. A PSCAD/EMTDC platform is developed to conduct dynamic simulations, and a scaled-down four-terminal 20-kW experimental prototype is used to validate the effectiveness of the proposed control methodology.

Tuning of a Damping Controller for Multiterminal VSC-HVDC Grids Using the Probabilistic Collocation Method

This paper presents a robust probabilistic controller tuning method to improve the damping of critical system modes through the modulation of active power injected by a voltage-source converter-based multiterminal high-voltage direct current (VSC-MTDC) grid. This methodology first establishes the probabilistic locations of the critical modes based on the known variation in power system operating conditions. Following this, the modal linear quadratic Gaussian (MLQG) controller structure is tuned for a set of probabilistic values of critical eigenvalues. The controller's performance following small disturbances in the network for wide-ranging operating conditions is compared with the conventionally tuned MLQG controller designed for the nominal system operating point. The probabilistic collocation method is shown to facilitate robust probabilistic tuning without the need for large numbers of full system linearizations. The test system used incorporates a large wind farm with variable power output connected to the meshed ac network through the VSC-MTDC grid.