Cost and losses associated with offshore wind farm collection networks which centralise the turbine power electronic converters

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

Distributed control of a fault tolerant modular multilevel inverter for direct-drive wind turbine grid interfacing

Modular generator and converter topologies are being pursued for large offshore wind turbines to achieve fault tolerance and high reliability. A centralized controller presents a single critical point of failure which has prevented a truly modular and fault tolerant system from being obtained. This study analyses the inverter circuit control requirements during normal operation and grid fault ride-through, and proposes a distributed controller design to allow inverter modules to operate independently of each other. All the modules independently estimate the grid voltage magnitude and position, and the modules are synchronised together over a CAN bus. The CAN bus is also used to interleave the PWM switching of the modules and synchronise the ADC sampling. The controller structure and algorithms are tested by laboratory experiments with respect to normal operation, initial synchronization to the grid, module fault tolerance and grid fault ride-through

Power conversion for a modular lightweight direct-drive wind turbine generator

A power conversion system for a modular lightweight direct-drive wind turbine generator has been proposed, based on a modular cascaded multilevel voltage-source inverter. Each module of the inverter is connected to two generator coils, which eliminates the problem of DC-link voltage balancing found in multilevel inverters with a large number of levels.The slotless design of the generator, and modular inverter, means that a high output voltage can be achieved from the inverter, while using standard components in the modules. Analysis of the high voltage issues shows that isolating the modules to a high voltage is easily possible, but insulating the generator coils could result in a signicant increase in the airgap size, reducing the generator effciency. A boost rectier input to the modules was calculated to have the highest electrical effciency of all the rectier systems tested, as well as the highest annual power extraction, while having a competitive cost. A rectier control system, based on estimating the generator EMF from the coil current and drawing a sinusoidal current in phase with the EMF, was developed. The control system can mitigate the problem of airgap eccentricity, likely to be present in a lightweight generator. A laboratory test rig was developed, based on two 2.5kW generators, with 12 coils each. A single phase of the inverter, with 12 power modules, was implemented, with each module featuring it's own microcontroller. The system is able to produce a good quality AC voltage waveform, and is able to tolerate the fault of a single module during operation. A decentralised inverter control system was developed, based on all modules estimating the grid voltage position and synchronising their estimates. Distributed output current limiting was also implemented, and the system is capable of riding through grid faults

Estimation of the power electronics lifetime for a wind turbine

A comparison has been made of the converter lifetime for a 3MW horizontal axis wind turbine for different wind turbulence levels. Torque and speed of the turbine shaft were used to calculate voltage and current time series that those were used to calculate the junction temperatures of diode and IGBT in the generator-side converter by a thermal-electrical model. A rainflow counting algorithm of the junction temperature in combination with an empirical model of the lifetime estimation has been used to calculate the lifetime of the power electronic module in the turbine. The number of parallel converters for each wind condition to achieve 20 years life time also has been found. it is found greater turbulence levels will lead to less lifetime of the converter in the wind turbine

DC protection of a muti-terminal HVDC network featuring offshore wind farms

A protection scheme for DC faults has been designed for a multi-terminal HVDC network used to transfer energy from three large offshore wind farms to shore. The system uses open access models created in the EU-funded BEST-PATHS project, including a manufacturer-supplied wind farm model. Tripping conditions for the DC circuit breakers are found through simulation, along with current limiting inductor sizes, based on the use of a hybrid circuit breaker. Simulations of faults in the HVDC network show the ability of the protection scheme to isolate the fault, and the converter stations and wind turbines are able to ride-through the fault without tripping based on the 5ms switching time of the circuit breakers Longer switching times will cause significant rises in the offshore grid frequency, which could cause the turbines to trip

DC protection of a muti-terminal HVDC network featuring offshore wind farms

A protection scheme for DC faults has been designed for a multi-terminal HVDC network used to transfer energy from three large offshore wind farms to shore. The system uses open access models created in the EU-funded BEST-PATHS project, including a manufacturer-supplied wind farm model. Tripping conditions for the DC circuit breakers are found through simulation, along with current limiting inductor sizes, based on the use of a hybrid circuit breaker. Simulations of faults in the HVDC network show the ability of the protection scheme to isolate the fault, and the converter stations and wind turbines are able to ride-through the fault without tripping based on the 5ms switching time of the circuit breakers Longer switching times will cause significant rises in the offshore grid frequency, which could cause the turbines to trip

Comparison of power electronics lifetime between vertical- and horizontal-axis wind turbines

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

Status of Birds Newly Recorded in Arkansas Since 1985

In 1994 we published an annotated list of 14 bird species that were newly discovered in Arkansas since the publication in 1986 of the monograph Arkansas Birds, Their Distribution and Abundance. We now add 22 more new species found in Arkansas since the 1994 publication, and update the status of the original 14. Adding these 36 species to the number included in Arkansas Birds totals 402 bird species currently reported in Arkansas