Operation and control of a current source converter series tapping of an LCC-HVDC link for integration of offshore wind power plants

This work presents a series tapping station for integrating Offshore Wind Power Plants (OWPP) into a (Line Commutated Converter High Voltage Direct Current) LCC-HVDC transmission system. The tapping station allows to integrate wind power resources without building a new HVDC link and it is based on a Current Source Converter (CSC). However, the CSC requires a minimum DC current to extract the power coming from the OWPP which may not be guaranteed depending on the power conditions of the HVDC corridor. For this reason, this paper proposes a coordinated operation and control of the CSC and the OWPP. A steady-state analysis is performed to determine the appropriate AC voltage level of the CSC. A power reduction algorithm is presented to limit power extraction during a reduction in the current of the HVDC transmission system and under loss of communications between the CSC and the OWPP. The proposed algorithm and the performance of the system are validated through simulation results.Peer ReviewedPostprint (author's final draft

Modelling and control of an interline Current Flow Controller for meshed HVDC grids

This article is focused on the modelling and control of an interline Current Flow Controller (CFC) for meshed HVDC grids. The operation states of the CFC are presented and an average model is derived. The average model is used to perform steady-state analysis on a 3-terminal meshed grid, showing the current change capabilities and the benefits on the operation area. The converter control is designed using a linearised model. The system performance of the CFC is tested by means of simulation in a 3-terminal grid and in a 5-terminal grid.Postprint (author's final draft

Fault mode operation strategies for dual H-bridge current flow controller in meshed HVDC grid

Peer ReviewedPreprin

AC fault ride through in MMC-based HVDC systems

© 2022 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting /republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other worksVSC-HVDC systems are being increasingly employed in the power systems. The recently installed HVDC systems have a power capacity similar to traditional power plants. Hence, they are expected to have a similar behaviour as traditional synchronous generators during faults in AC grid, within their limits of course. Recent grid codes require HVDC converter stations to incorporate fault ride-through (FRT) capabilities in order to avoid HVDC converter station disconnection from AC grid for certain fault characteristics. In this paper, two FRT mechanisms are suggested for the two converter stations of an HVDC system. One FRT mechanism is added to the DC voltage control loop of the master converter station, while the other FRT mechanism is added to the active power control loop of the slave converter station. The objective is to ensure the stable operation of the HVDC system during faults that may occur in AC grids located on both sides of the HVDC system. The performance and stability of the suggested FRT mechanisms are tested considering the pre-fault power flow direction and all possible types of balanced and unbalanced faults. Simulation results confirm the effectiveness of the FRT mechanisms and revealed the critical modes during FRT operation.Peer ReviewedPostprint (author's final draft

Design methodology of the primary droop voltage control for DC microgrids

© 2018 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.n this article, a complete methodology to design the primary voltage droop control for a generic DC microgrid is proposed. First, a procedure to obtain a linear model of the complete system including the different converters inner and outer loops is detailed. Then, this linear model is analyzed using frequency domain techniques in order to ensure that the system is able to operate in a stable and secure manner. Based on the frequency analysis performed, the system droop gains are selected and tested in simulation to validate that the control design specifications are met.Postprint (author's final draft

Steady-state impedance mapping in grids with power electronics: What is grid strength in modern power systems?

The paper analyzes grid strength and grid impedance for systems with significant penetration of power electronics. The work shows that the traditionally employed Thévenin equivalent cannot capture the different saturation states of power converters and therefore some classical definitions of grid strength based on short-circuit power at the point of connection fail to describe the system behavior. The paper proposes to map the impedance for all the different possible voltages at the point of connection and analyzes how the impedance depend on the voltage and angle of derivation for different possible simplified systems (grid-following converter in parallel with a Thévenin equivalent and grid-forming converter) considering the different saturation states of the converter. Numerical results and case studies are included as examples of the conducted analysis.Peer ReviewedPostprint (author's final draft

Optimal sizing and location of reactive power compensation in offshore HVAC transmission systems for loss minimization

Postprint (published version

Handling of unbalanced faults in HVDC-connected wind power plants

High-voltage DC (HVDC) connections enable integration of wind power plants located very far from shore. The decoupled AC offshore grid comprises multiple WT converters, and the voltage magnitude and frequency is primarily controlled by the offshore high-voltage DC voltage-source converter (VSC-HVDC). Faults in the offshore grid challenge the connected converters to provide an adequate response improving the overall fault behavior. Of special interest are asymmetrical faults due to the resulting unbalanced voltage conditions. This article addresses such conditions in the offshore grid and analyzes the impact on the offshore grid behavior for different converter contributions. Four fault ride-through strategies are studied for the WT converters. The effect of over-modulation of the converter voltages during such voltage conditions is highlighted. A test system is defined to analyze the fault and post-fault behavior. It is found that voltage support from the WT converters in both positive and negative sequence shows the best performance compared to controlled negative sequence current suppression. This scheme helps additionally the VSC-HVDC AC voltage control to return quickly to normal operation. To validate this statement simulations are performed for line-to-line (LL) and single line-to-ground (SLG) faults in immediate vicinity of the VSC-HVDC.Peer ReviewedPostprint (author's final draft

How Many Grid-Forming Converters do We Need? A Perspective From Small Signal Stability and Power Grid Strength

Grid-forming (GFM) control has been considered as a promising solution for accommodating large-scale power electronics converters into modern power grids thanks to its grid-friendly dynamics, in particular, voltage source behavior on the AC side. The voltage source behavior of GFM converters can provide voltage support for the power grid, and therefore enhance the power grid (voltage) strength. However, grid-following (GFL) converters can also perform constant AC voltage magnitude control by properly regulating its reactive current, which may also behave like a voltage source. Currently, it still remains unclear what are the essential difference between the voltage source behaviors of GFL and GFM converters, and which type of voltage source behavior can enhance the power grid strength. In this paper, we will demonstrate that only GFM converters can provide effective voltage source behavior and enhance the power grid strength in terms of small signal dynamics. Based on our analysis, we further study the problem of how to configure GFM converters in the grid and how many GFM converters we will need. We investigate how the capacity ratio between GFM and GFL converters affects the equivalent power grid strength and thus the small signal stability of the system. We give guidelines on how to choose this ratio to achieve a desired stability margin. We validate our analysis using high-fidelity simulations

Dynamic Ancillary Services: From Grid Codes to Transfer Function-Based Converter Control

Conventional grid-code specifications for dynamic ancillary services provision such as fast frequency and voltage regulation are typically defined by means of piece-wise linear step-response capability curves in the time domain. However, although the specification of such time-domain curves is straightforward, their practical implementation in a converter-based generation system is not immediate, and no customary methods have been developed yet. In this paper, we thus propose a systematic approach for the practical implementation of piece-wise linear time-domain curves to provide dynamic ancillary services by converter-based generation systems, while ensuring grid-code and device-level requirements to be reliably satisfied. Namely, we translate the piece-wise linear time-domain curves for active and reactive power provision in response to a frequency and voltage step change into a desired rational parametric transfer function in the frequency domain, which defines a dynamic response behavior to be realized by the converter. The obtained transfer function can be easily implemented e.g. via a PI-based matching control in the power loop of standard converter control architectures. We demonstrate the performance of our method in numerical grid-code compliance tests, and reveal its superiority over classical droop and virtual inertia schemes which may not satisfy the grid codes due to their structural limitations.Comment: 7 pages, 9 figure