Off-shore grid forming wind power plants can operate in different modes: islanding, black-start, or be connected to an HVDC or HVAC grid, which leads to large impedance variations. These impedance variations can be approached by robust controllers. This paper compares different robust controllers tuned under the same performance conditions by frequency weights and adding unstructured uncertainty to the model. The selected robust controllers correspond to H2and H∞control techniques, which allows the optimisation of the controller considering uncertain plants. It allows to consider the impedance variation in the WTG operation range during the controller synthesis. The performance and robustness of each controller are also presented. Then, both robust controllers are validated and compared in theoretical and offline scenarios of a 32-wind turbine generator wind power plant.

Grid forming wind turbines shall be able to work in different modes of operation. The operating modes that the grid forming wind turbine shall be able to operate are islanding operation, different power transmission levels and fault-ride through. This paper proposes synthesising robust grid forming controllers for the wind turbines of an HVDC Diode Rectifier based Wind Power Plant. The considered grid forming controller is based on a cascade control using Proportional Resonant regulators for the voltage and the current control. The cascade loop is tuned using$\mu$-synthesis to optimise closed-loop robust performance. The validation at the system level has been carried out using a realistic simulation of an HVDC Diode Rectifier-connected Wind Power Plant in a real-time simulator.

The Modular Multilevel Converter (MMC) is one of the most practical topologies for integration of renewable energy power plants into the grid, through submarine HVDC cables and HVDC lines. Nonetheless, the MMC requires significant large DC storage capacitors. This work addresses the recently introduced Asymmetric Alternate Arm Converter (A3C), a hybrid MMC topology which uses modular arms only on the lower side of the main converter whereas the upper side uses only standard ON-OFF valves. The A3C offers a significant reduction in DC energy storage requirements and in the number of modules. In addition, it offers a less restricted operation than the Alternate Arm Converter (A2C), in terms of operating over a wide range of AC to DC voltage ratio, and the possibility of achieving both nearly sinusoidal AC current and nearly constant DC current.The principle of operation of the A3C, but in the inverter mode and using thyristor-based director switches, is revisited here and a suitable control strategy to demonstrate its operation is developed. Emphasis is place on switching off the thyristor valves in a soft commutation fashion. The method requires two A3C operating in interleaving fashion on the DC bus. Proposals are verified through simulation studies.

This paper includes a stability study of a HVDC diode rectifier connected off-shore wind power plant. Usually, the internal control strategy of commercial wind turbines are not completely known, so stability studies based on transfer functions or eigenvalue analysis are of limited use. However, the frequency behaviour of both wind turbine grid side converters and HVDC diode rectifier station can be obtained without a detailed knowledge of the control internals of the wind turbines. This paper includes the validation of impedance-based stability analysis for HVDC diode-rectfier connected off-shore wind farms. The frequency dependent impedance of the wind turbine converters and of the diode rectifier HVDC station are developed both analytically from their respective analytical models and also by using signal injection. It is shown that the stability analysis using the identified impedance frequency response and that from analytic calculation lead the same results, thus validating the use of the impedance method for the stability analysis of grid-forming Diode-rectifier connected wind power plants.

This work deals with the concepts to control a Multilevel Current-Source based Distribution Static Synchronous Compensator (MCSC-DSTATCOM) in order to reduce its steady-state losses. For this purpose, the analysis of the operating region is used to define a low-losses operating mode, leading to phase control of the power converter using a fixed modulation index. The mathematical analysis defines the proposal, which is corroborated using simulation in PSIM 9.0. Results show that it is possible to use the proposed control scheme, reducing the losses in the steady-state due to the use of the minimum dc current demanded by the equipment for required power compensation, also achieving high quality in the DSTATCOM at low switching frequency due to the use of the multilevel topology.

System-on-Chip (SoC) devices combine powerful general purpose processors, a Field-Programmable Gate Array (FPGA) and other peripherals which make them very convenient for Hardware-in-the-Loop (HIL) simulation. One of the limitations of these devices is that control engineers are not particularly familiarized with FPGA programming, which need extensive expertise in order to code these highly sophisticated algorithms using Hardware Description Languages (HDL). Notwithstanding, there exist High-Level Synthesis (HLS) tools which allow to program these devices using more generic programming languages such as C, C++ and SystemC. This paper evaluates SoC devices to implement a Modular Multi-Level Converter (MMC) model using HLS tools for being implemented in the FPGA fabric in order to perform HIL verification of control algorithms in a single low-cost device.

Distributed diode rectifier units have been proposed as a robust and cost efficient solution for the connection of large off-shore wind farms to HVDC links. However, since the diode rectifier platforms are connected in series with cables of varying distances, their dc-side voltage might not be the same in all diode rectifier platforms. This paper introduces a diode rectifier platform dc-voltage balancing control to cancel the aforementioned unbalanced. Balanced operation has been shown by means of detailed PSCAD simulations, considering a wide range of power generation unbalance between the wind power plants connected to the HVDC diode rectifier platforms. Moreover, the proposed balancing control has shown a very good dynamic performance.

A large number of nodes is required for the detailed simulation of large off-shore wind farms. Therefore, the corresponding electromagnetic transient simulations require large simulation times. This paper compares a detailed model of an off-shore wind farm with three clusters of 50 wind turbines each one (i.e. a total of 150 wind turbines), with a simplified model consisting of three aggregated wind farms. The paper includes the aggregation criteria so equal power flows and voltages are obtained at the wind farms points of common coupling. The control is carried out using a centralized PI controller to control total active power, and a distributed droop control for active and reactive power control of each wind turbine. The proposed aggregation method leads to significant savings on computational times, with the aggregated models being more than three hundred and fifty times faster than the detailed ones. The results have been validated by means of detailed PSCAD/EMTDC simulations.

This paper presents a control strategy for a grid connected photovoltaic (PV) system aiming to regulate the active and reactive power injected to the electric system during asymmetrical voltage faults. The active power reference is obtained from a Maximum Power Point Tracking (MPPT) algorithm. The proposed control strategy generates the required reference currents to be imposed by the grid-tied inverter from the desired active and reactive power and the measured supply voltage. The scheme is validated for a single stage PV system where the inverter currents are regulated via predictive control. Results showing the performance of the strategy are presented during unbalanced voltage sags and swells.

New System-on-Chip (SoC) devices, which include powerful general purpose processors and FPGA in the same chip are particularly suitable for Real-Time (RT) simulation of electromechanical systems. This paper introduces a systematic analysis of the capabilities of one of these devices for RT electromagnetic simulation. Considering all the capabilities of these platforms, it is not straightforward to use them efficiently and port existing developments due to the SoC relative complexity and variety of programming models. In this paper, authors evaluate some of the benefits of moving to such platforms through the implementation of an example application on the Xilinx Zynq-7000 SoC. Simulation results concerning execution time, resources usage and precision of the calculations are presented, compared and validated using MATLAB/Simulink.

A double-T DC-DC modular multilevel converter (DC-DC-MMC) topology for high-voltage dc (HVdc) applications is presented in this paper. The topology is intended to interconnect large power dc grids with different voltage levels or to control the power flow in meshed dc grids. A two level control hierarchy is used to regulate the dc voltage of each cell. At the top level, dc and circulating ac currents are used to control the total energy in every branch. At the bottom level, the balance of the cell capacitor voltages within a branch is carried out by balancing (N-1) capacitor voltage deviations with respect to the average capacitor voltage. Finally, the operation of the converter is validated by means of PSCAD simulations. Results for the operation of the DC-DC-MMC controlling the power flow between two HVdc grids with different voltage levels are presented. The robustness of the converter has also been proven by simulating the disconnection of one pole of the bipolar dc grid.

This paper includes the development and validation of a dynamic model suitable for control design and stability analysis of a HVDC diode-based rectifier grid connection for offshore wind power plants. The modular design of the model allows for a relatively easy extension to different grid topologies and control strategies. The developed models include the effects of communication delays and a non-stationary off-shore ac-grid frequency. The developed models have been validated with detailed PSCAD/EMTDC simulations (including distributed cable models). The developed model has been used to carry out a detailed study on the effects of parametric uncertainty on the dynamic stability.

A transformer-less DC-DC Modular Multi-level Converter (DC-DC-MMC) topology based on H-bridge cells is proposed in this paper. The suggested DC-DC-MMC can be used to either control the power flow in a HVdc grid or interconnect HVdc lines with different voltage levels. Branch energy and current control loops are presented as well as a cell capacitor voltage balancing strategy. Finally, the operation of the converter is validated by means of PSCAD simulations. Results for the operation of the DC-DC-MMC controlling the power flow between two HVdc grids with different voltage levels are presented.

This paper proposes the use of a transformer-less DC-DC Modular Multilevel Converter (MMC) topology, based on cascaded H-bridge converters, for power flow control in High Voltage Direct Current (HVDC) grids used to connect off-shore wind power plants to on-shore grids. An energy based approach is used to regulate the DC voltage of H-bridge modules. Results for the operation of the DC-DC MMC supplying energy to a DC network and controlling the power flow in a HVDC system are presented.

VSC-HVDC links have been proposed as a means of connecting large offshore wind power plants to the onshore transmission system. This paper includes the study of a control strategy for the islanded operation of the aforementioned wind power plants. The offshore grid voltage is controlled independently of the HVDC link state. Therefore, offshore grid and HVDC link overvoltage due to onshore short circuits are largely reduced. Moreover, the proposed strategy allows for the contribution of the wind power plant to onshore service restoration after a power outage. The proposed control strategy has been validated in a realistic scenario by means of PSCAD/EMTDC simulations.

This paper includes the study of a control strategy for the connection of PMSG-based wind turbines during unbalanced faults in weak grids. The wind turbine front-end converter is controlled in the stationary frame by the direct calculation of current references. In this way, it is possible to keep a reasonably smooth generated power during the fault, in order to reduce the fault impact on the wind turbine mechanical components. The effects on the mechanical parts of the wind turbine have been studied by using the NREL 5 MW reference wind turbine, modeled with the NREL FAST code. The effects of highly distorted network voltages during the fault and the limitations of the current control loop dynamics are thoroughly studied by means of detailed PSCAD simulations.

Diagnosis of intermittent faults is relevant for productive processes with a high level of maintenance caused by such failures. A characterization of diagnosed faults may save unnecessary repairs and enable better maintenance planning. This paper introduces the concept of diagnosis of intermittent fault dynamics. Moreover, a methodology for modelling intermittent faults and generating information for maintenance planning in discrete event systems is thoroughly covered.