Eduardo Prieto-Araujo

Methodology for Droop Control Dynamic Analysis of Multiterminal VSC-HVDC Grids for Offshore Wind Farms

This paper addresses the control of multiterminal voltage-source converters at high-voltage direct current in the context of offshore wind farms. Droop control is commonly used to regulate the dc voltage in this kind of grid, and droop parameters are selected on the basis of steady-state analyses. Here, a control design methodology is proposed based on the frequency-response analysis. This methodology provides a criterion to select the droop gains, taking into account the performance specifications [i.e., the desired voltage errors and the maximum control inputs (currents)]. The application of the methodology is illustrated with a four-terminal grid.

DC Voltage Droop Control Design for Multiterminal HVDC Systems Considering AC and DC Grid Dynamics

This paper focuses on the droop-based dc voltage control design for multiterminal VSC-HVDC grid systems, considering the ac and the dc system dynamics. The droop control design relies on detailed linearized models of the complete multiterminal grid, including the different system dynamics, such as the dc grid, the ac grid, the ac connection filters, and the converter inner controllers. Based on the derived linear models, classical and modern control techniques are applied to design the different controllers, including a multivariable frequency analysis to design the grid voltage droop control. In combination with the droop control, a dc oscillation damping scheme is proposed in order to improve system performance. The control design is validated through simulations of a three-terminal system.

Decentralized Control of a Nine-Phase Permanent Magnet Generator for Offshore Wind Turbines

Summary form only given: This paper presents a decentralized current control approach for a nine-phase wind turbine generator. This type of generator has three different three-phase stators sharing the same machine yoke and connected to the grid by means of three different voltage source back-to-back power converters. Due to the machine configuration, magnetic couplings are present between the three stators, complicating the design and implementation of the machine current controllers. Rather than a centralized control approach, this paper proposes a methodology to design a decentralized machine control to regulate the active and reactive power flowing through each stator independently. A complete dynamic analysis is performed in order to design the controller to reduce the coupling effects within the machine, while ensuring a proper dynamic performance. The control strategy is validated through simulation and experimental results.

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

This paper focuses 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 linearized model. The system performance of the CFC is tested by means of simulation in a 3-terminal grid and in a 5-terminal grid.

Control of Modular Multilevel Converters Under Singular Unbalanced Voltage Conditions With Equal Positive and Negative Sequence Components

This paper focuses on the control of Modular Multilevel Converters (MMC) for High Voltage DC (HVDC) applications during unbalanced AC grid voltage sags where positive and negative sequence voltages are equal. The control scheme is based on six arm energy regulators, six independent current controllers, and two reference calculation stages that convert the power references into grid and inner current references. Conventional inner AC currents reference calculation fails if the amplitude of the positive and the negative sequence AC grid voltages are equal, a state which is referred to in this paper as singular voltage condition. This paper discusses the types of network faults that cause this condition and proposes three different solutions to operate the converter in such scenarios. The adequacy of the proposed solutions is validated through simulations considering each of the problematic fault scenarios.

DC voltage droop control design for multi-terminal HVDC systems considering AC and DC grid dynamics

Summary form only given: This article is focused on the droop-based DC voltage control design for multi-terminal VSC-HVDC grid systems, considering the AC and the DC system dynamics. The droop control design relies on detailed linearized models of the complete multi-terminal grid, including the different system dynamics, such as the DC grid, the AC grid, the AC connection filters and the converter inner controllers. Based on the derived linear models, classical and modern control techniques are applied to design the different controllers, including a multi-variable frequency analysis to design the grid voltage droop control. In combination with the droop control, a DC oscillation damping scheme is proposed, in order to improve the system performance. The control design is validated through simulations of a three terminal system.

Impact of the DC cable models on the SVD analysis of a Multi-Terminal HVDC system

High Voltage Direct Current (HVDC) grids are complex Multi-Inputs Multi-Outputs (MIMO) systems whose dynamics are difficult to assess. This paper first describes the modelling of VSC-based Multi-Terminal HVDC systems (MTDC) using different existing cable models. It then recalls the benefits of the Singular Value Decomposition (SVD) approach for the frequency analysis of such systems, and describes how to perform an SVD analysis on a Multi-Terminal HVDC (MTDC) system from its state-space representation. The paper discusses the results of the SVD analysis with regards to the selection of the voltage-droop parameter and the DC voltage constraint of the DC grid. It then emphasises the impact of the choice of the DC cable model on the results of the SVD study and illustrates the cable model influence on the MTDC system through an Electro-Magnetic Transient (EMT) simulation.

Control and experimental validation of a Dual Active Bridge Series Resonant Converter

This article addresses the control and experimental validation of a Dual Active Bridge Series Resonant Converter (DBSRC) for interconnecting two DC systems. The DBSRC provides High Frequency (HF) isolation between the primary and the secondary circuits and allows to transfer power in both directions. Conventionally, it is operated at fixed frequency, applying an angle shift between the square waves generated by both H-bridges, to establish the power transfer. This work proposes a control strategy to operate the converter, based on optimization, that allows to reduce the AC HF current. Also, a real prototype of the converter is built and its operation is shown through experimental results.

Integrated HVDC Circuit Breakers With Current Flow Control Capability

Two key problems in meshed high-voltage direct current (HVDC) transmission grids are managing line power flows and protection against dc faults. Current flow controllers (CFCs) will be required to balance cable currents in meshed dc grids, in order to prevent individual line power capacity limits restricting overall power flow in the grid. Direct current circuit breakers (DCCBs) will be also required to protect HVDC grids from dc faults. This paper demonstrates that the current flow controller functionality can be added into a hybrid CBs design. This paper proposes integrating an interline CFC into the load commutation switch (LCS) of a hybrid DCCB. The integrated design LCS/CFC is analyzed and a state-space model is derived. The control of the CFC is designed and the performance of the LCS/CFC during normal operation is verified by means of MATLAB Simulink and PSCAD simulations. A comparison of the integrated LCS/CFC and the separate design is given. The case studies show a reduction in total power losses, and improved protection operation times can be achieved.

Series Interline DC/DC Current Flow Controller for Meshed HVDC Grids

This paper proposes a DC/DC current flow controller (CFC) for meshed high voltage direct current (HVDC) grids. The CFC has the advantage of a simplified structure that allows us to use the minimum number of switches when unidirectional current flows through the DC lines are expected. An extended CFC topology that is able to operate with all possible current flows is also presented. Then, the operating principle of the CFC is discussed and its structure is compared with a dual H-bridge analyzing advantages and disadvantages. This paper also details the applied modulation strategy and the control methodology. Finally, a five-terminal meshed HVDC grid is used to validate the CFC by means of simulations.