S.W.H. de Haan

Ridethrough of wind turbines with doubly-fed induction generator during a voltage dip

In this paper, a solution is described that makes it possible for wind turbines using doubly-fed induction generators to stay connected to the grid during grid faults. The key of the solution is to limit the high current in the rotor in order to protect the converter and to provide a bypass for this current via a set of resistors that are connected to the rotor windings. With these resistors, it is possible to ride through grid faults without disconnecting the turbine from the grid. Because the generator and converter stay connected, the synchronism of operation remains established during and after the fault and normal operation can be continued immediately after the fault has been cleared. An additional feature is that reactive power can be supplied to the grid during long dips in order to facilitate voltage restoration. A control strategy has been developed that takes care of the transition back to normal operation. Without special control action, large transients would occur.

General model for representing variable speed wind turbines in power system dynamics simulations

A tendency to erect ever more wind turbines can be observed in order to reduce the environmental consequences of electric power generation. As a result of this, in the near future wind turbines may start to influence the behavior of electric power systems by interacting with conventional generation and loads. Therefore, wind turbine models that can be integrated into power system simulation software are needed. A model that can be used to represent all types of variable-speed wind turbines in power system dynamics simulations is presented. The modeling approach is commented upon, and models of the subsystems of which a variable speed wind turbine consists are discussed. Some results obtained after incorporation of the model in PSS/E, a widely used power system dynamics simulation software package, are presented and compared with measurements.

Wind turbines emulating inertia and supporting primary frequency control

The increasing penetration of variable-speed wind turbines in the electricity grid will result in a reduction of the number of connected conventional power plants. This will require changes in the way the grid frequency is controlled. In this letter, a method is proposed to let variable-speed wind turbines emulate inertia and support primary frequency control. The required power is obtained from the kinetic energy stored in the rotating mass of the turbine blades.

Short-Circuit Current of Wind Turbines With Doubly Fed Induction Generator

The short-circuit current contribution of wind turbines has not received much attention so far. This paper considers the short-circuit behavior, especially the short-circuit current of wind turbines with a doubly fed induction generator. Mostly, these wind turbines have a crowbar to protect the power electronic converter that is connected to the rotor windings of the induction generator. First, the maximum value of the short-circuit current of a conventional induction machine is determined. The differences between a crowbar-protected doubly fed induction generator and a conventional induction generator are highlighted and approximate equations for the maximum short-circuit current of a doubly fed induction generator are determined. The values obtained in this way are compared to the values obtained from time domain simulations. The differences are less then 15%

Grid tied converter with virtual kinetic storage

The increase of relatively small scale dispersed power generation (DG) is likely to impact the structure and operation of power generation and distribution systems. The current power system comprises multiple generators. Their intrinsic kinetic energy buffer (rotor inertia) plays an important role in short term system stability. Generators that are connected via power electronic power converters lack these kinetic buffers. Optimal power transfer from source to grid is often used as a control objective to optimize the energy yield. At power system level this approach may compromise stability. In this paper it is demonstrated that by emulation of rotor inertia, stability problems on a system level can be mitigated. For that purpose a short-term energy buffer is added to the system that can be controlled at a fast rate, thereby forming a so-called Virtual Synchronous Generator (VSG). Power system stability support then becomes an additional control objective of the DG. Maximum rotor angular speed deviation and maximum critical clearing time are used as performance indicators for the evaluation. Sizing of the short term buffer constitutes a second topic of this paper.

Contribution of DG units to primary frequency control

The increasing penetration of distributed generation (DG) in the electricity grid will result in a reduction of the number of connected conventional power plants, which are nowadays responsible for control of the electricity network frequency. Currently DG units do not contribute to frequency control. With increasing penetration of DG it will become necessary however that they also contribute to frequency control. A significant part of the DG units are connected to the grid by a power electronic converter. It is possible to implement additional control in this converter to let the DG unit contribute to frequency control. In this paper it is investigated how these controllers can be implemented and it is analyzed how large the contribution of several types of DG units can be

Simulation of crystal growth with a special purpose computer

A special purpose computer has been constructed which makes it possible to carry out Monte Carlo calculations for the simulation of the crystal growth process, two hundred times faster than an IBM 360/65 computer. Using this special purpose computer, many more and more precise simulations could be carried out which enabled us to check the validity of the two-dimensional nucleation theories in more detail.

Optimal operating ranges of three modulation methods in dual active bridge converters

The dual active bridge (DAB) converter attracts more and more attentions thanks to its advantages over the other isolated dc-dc converters, such as soft-switching, immunity to parasitic inductance, less circuit components and less component stress. Rectangular (also named as phase-shifted) modulation method on DAB has been widely studied and but other two proposed modulations (trapezoidal and triangular methods) have much less applications. Due to the characteristics of the modulation principles, one of these three methods outperforms the other two in certain operating ranges from system efficiency point of view. This paper summarizes the properties of three modulation methods and optimizes the combined use of three modulations on a DAB converter (12V/360V, 1 kW and 25 kHz) in order to obtain the best system efficiencies over full converter operating range. A validated loss model is utilized as the analyzing platform, based on which the optimal operating ranges of three modulations are determined.

Reaching High Power Density in Multikilowatt DC–DC Converters With Galvanic Isolation

In this paper, the possibility of reaching high power densities in multikilowatt dc-dc converters with galvanic isolation is demonstrated and the main design issues are discussed. The issues related to converter topology, transformer design, and thermal management are addressed, and new conceptual solutions are proposed. Implementing zero-voltage-switching quasi-zero-current-switching topology, optimized transformer design with leakage layer, and thermal management based on conduction enhanced by heat pipes at critical places resulted in very high power density and efficiency. The power density reached by the converter prototype is 11.13 kW/L with water cooling and 6.6 kW/L with air cooling. In the same time, the measured efficiency exceeded 97% in a broad load range. The new design concepts are demonstrated on a 50-kW converter prototype that was successfully tested at full-load conditions.

Primary power/frequency control with wind turbines and fuel cells

The increasing penetration of variable speed wind turbines in the electricity grid results in a reduction of the number of connected conventional power plants. This requires changes in the way the grid frequency is controlled. Together with the wind turbines devices like fuel cells can be installed, which can be used to supply the primary frequency control contribution of the wind turbines. In this paper it is analyzed how large the installed capacity of fuel cells should be to enable a correct response to frequency deviations. When fuel cells have stored hydrogen available, their response to changes in the power setpoint is fast. When the hydrogen has to be obtained from natural gas, or other fuels, their response is much slower. In that case the kinetic energy stored in the rotating mass of wind turbines can be used to support primary frequency control for a limited time period. The proposed control schemes are shown with the simulation of two case studies