Max Parker

Although the control of modular multi-level converters (MMCs) in high-voltage direct-current (HVDC) networks has become a mature subject these days, the potential for adverse interactions between different converter controls remains an under-researched challenge attracting the attention from both academia and industry. Even for point-to-point HVDC links (i.e., simple HVDC systems), converter control interactions may result in the shifting of system operating voltages, increased power losses, and unintended power imbalances at converter stations. To bridge this research gap, the risk of multiple cross-over of control characteristics of MMCs is assessed in this paper through mathematical analysis, computational simulation, and experimental validation. Specifically, the following point-to-point HVDC link configurations are examined: (1) one MMC station equipped with a current versus voltage droop control and the other station equipped with a constant power control; and (2) one MMC station equipped with a power versus voltage droop control and the other station equipped with a constant current control. Design guidelines for droop coefficients are provided to prevent adverse control interactions. A 60-kW MMC test-rig is used to experimentally verify the impact of multiple crossing of control characteristics of the DC system configurations, with results verified through software simulation in MATLAB/Simulink using an open access toolbox. Results show that in operating conditions of 650 V and 50 A (DC voltage and DC current), drifts of 7.7% in the DC voltage and of 10% in the DC current occur due to adverse control interactions under the current versus voltage droop and power control scheme. Similarly, drifts of 7.7% both in the DC voltage and power occur under the power versus voltage droop and current control scheme.

Large offshore wind farms located far from shore, as are being planned or built in the North Sea, will require high‐voltage DC (HVDC) transmission to shore, and multi‐terminal HVDC could offer further benefits. Currently proposed methods to protect against faults in the DC network are based on extremely fast‐acting DC circuit breakers located on all cable ends, leading to high cost. A method is proposed based around discharging the DC network to isolate the fault, which drastically reduces the circuit breaker requirement, while making use of the inherent current‐limiting behaviour of the wind turbines. The validity of this approach is demonstrated in simulation.

Decarbonisation of the European electricity system can become dauntingly costly due to transmission and distribution network issues arising from the integration of intermittent renewable generation sources. It is expected that wind energy will be the principal renewable source by 2050 and, as such, a number of initiatives in the academia and in the industry are being carried out to propose solutions to best accommodate the wind resource. This paper presents work carried out by DEMO 1 partners within the EU FP7 project BEST PATHS. A MATLAB/Simulink toolbox consisting of the necessary building blocks for the simulation and integration of offshore wind farms using enabling technologies such as multiterminal high-voltage direct-current grids is presented. To illustrate the toolbox capabilities, a number of system topologies is studied. System performance is assessed and measured against a set of key performance indicators. To ensure knowledge dissemination, the toolbox has been made available as open access in the BEST PATHS project website

A comparison has been made of the power electronics lifetime for 5MW horizontal‐ and vertical‐axis wind turbines, based on dynamic models supplied with generated wind speed time series. Both two‐ and three‐bladed stall‐regulated H‐rotor vertical‐axis turbines were modelled, with several different control parameters. Vertical‐axis turbines are likely to lead to a shorter power electronics lifetime as the aerodynamic torque varies with rotor azimuth, leading to a cyclic generator torque, and increased thermal cycling in the power electronics. An electro‐thermal model of a low‐voltage converter was created, and used to calculate the switching device temperatures based on the generator torque and speed time series from the turbine model. An empirical lifetime model and rainflow‐counting algorithm were used to calculate the lifetime, and this was repeated at different average wind speeds to determine the overall lifetime. The vertical‐axis turbine was found to have a lower power electronics lifetime than the horizontal‐axis, or require a larger number of parallel switching devices to achieve the same lifetime, although this was lessened by running the turbine with a more relaxed speed control, allowing the rotor inertia to partially absorb the aerodynamic torque ripple.

This paper presents an evaluation of mechanical contact less magnetic bearing against the conventional industry standard rolling element bearing in direct drive electrical generators. The evaluations are performed based on the energy consideration and their potential contribution in structural mass reduction of rotor. A cylindrical rolling element bearing is chosen to represent the conventional type and an approximate modelling of magnetic bearing is carried out to evaluate them. A finite element analysis in the rotor structure is performed to estimate its mass and to analyse any potential saving in mass by the use of magnetic bearing

A fault-tolerant tandem converter, based on a current-source converter and cascaded multilevel voltage-source active filter, is proposed as the grid interface for a 10MW superconducting direct-drive wind turbine generator. The converter reduces the possibility of a short-circuit fault, an issue with the extremely low generator reactance, while having a significantly lower DC capacitance requirement than an equivalent modular multilevel converter, although this is still high due to an extremely low generator frequency. Active filter DC capacitance is optimised, and a control system designed to prevent large DC voltage peaks during transients, and is verified in simulation. DC voltage balancing between the phases of the filter limits the torque control bandwidth of the converter if the capacitance is to be minimised, due to the slow speed of the balancing controller.

A comparison has been made of the converter lifetime for a 3MW fully rated converter horizontal axis wind turbine located onshore and offshore. Simulated torque and speed of the turbine shaft were used to calculate voltage and current time series, that was used to calculate the junction temperatures of the diode and IGBT in the generator-side converter by a thermal-electrical model. A rainflow counting algorithm was applied to the junction temperature in combination with an empirical model of the lifetime estimation, to calculate the lifetime of the power electronic modules in the turbine. The number of parallel modules for each location to achieve 20 years life time has also been found. Simulations show the lifetime consumption rate of the diode and IGBT is decreased exponentially by increasing number of parallel modules, lowering the average temperature. The offshore wind turbine has a higher lifetime consumption rate, requiring a slightly higher converter rating to achieve a 20-year lifetime, but this difference is small, and both turbines will use the same number of modules

Costs and losses have been calculated for several different network topologies, which centralise the turbine power electronic converters, in order to improve access for maintenance. These are divided into star topologies, where each turbine is connected individually to its own converter on a platform housing many converters, and cluster topologies, where multiple turbines are connected through a single large converter. Both AC and DC topologies were considered, along with standard string topologies for comparison. Star and cluster topologies were both found to have higher costs and losses than the string topology. In the case of the star topology, this is due to the longer cable length and higher component count. In the case of the cluster topology, this is due to the reduced energy capture from controlling turbine speeds in clusters rather than individually. DC topologies were generally found to have a lower cost and loss than AC, but the fact that the converters are not commercially available makes this advantage less certain.