This paper is divided into ten sections. Section 5 gives modelling of the wind turbine and the DFIG while Section 6 covers grid connection requirements, control types, levels, and strategies. IEEE standard defines the basic requirements for integrating distributed generation units including wind power to the utility grids. Examples include use of braking chopper with SRC [ 3 ] or with antiparallel thyristors [ 28 ]. Voltage sag results in high stator currents which in turn induce high rotor currents and reduced capability of the WECS to export power as to the square of the voltage drop. The power coefficient is maintained constant at its maximum between points B and C to maximize power output as the wind speed increases.
At point E, the cut-out speed, the machine is stopped by pitching the turbine blades out of the wind and applying the emergency brake to avoid structural damage due to high wind speeds. This limits its accuracy in modelling rotor current transient behaviour; however, the reduced complexity makes it best suited for large-scale wind farm simulations. In the GSC, the -axis current controls the active power and so indirectly the DC-link voltage, whereas the -axis current controls the reactive power. The EMC filter reduces these emissions to a level where other electrical devices can be operated in the vicinity without problems. The power coefficient is maintained constant at its maximum between points B and C to maximize power output as the wind speed increases. Remote connection and disconnection must also be possible. It does not utilize a DC-link capacitor.
Newer technologies such as the tyesis drive gearless permanent magnet synchronous pscsd have come up to challenge its market share, but the large number of installed machines ensures that it remains of interest in the wind industry. In some cases, the WECS must help support grid voltage during the fault. It deals with direct intervention measures on the turbine, the gearbox, the drive shaft, and the generator, collectively referred to as the power drive train, to create adjustments that result in the output meeting the set limits on a point by point basis.
It is seen that the machines are disconnected due to overcurrents which go up to 1.
Starting with an introduction in Section 1Section 2 covers construction while Section 3 gives insight into the operation of the system. This is handled by the generator and the PEC.
It does not utilize a DC-link simlation. Various combinations of the four control strategies above are also utilized with the aim of taking advantage of the strong points of each of the control strategies in the combination while minimizing their shortcomings.
The variable switching frequency makes design of appropriate filters difficult. Stator active and reactive powers are derived as Hence, the active power is controlled by thesks and reactive power by adjustingrespectively.
On occurrence of voltage sag, the WECS must remain connected for a given set time before disconnecting. This tyesis classified into the following: The CCU is a programmable electronic device with various inputs from its own measurements and the WECS controller, and it sends out output signals to the elements to be controlled.
A Condensed Introduction to the Doubly Fed Induction Generator Wind Energy Conversion Systems
Thus, the main aim of FRT requirement is the instantaneous resumption of active power as soon as the fault is cleared [ 3 ]. This entails frequency regulation capability so as to simulattion desired grid simulatkon. A control system that does not rely on preset values such as a look-up table but instead relies on instantaneous values or has a learning ability can self-adjust for these variations giving a more accurate response.
Figure 1 shows the components of a wind turbine.
The GSC is a bidirectional rectifier bridge, utilizing IGBTs as the switching device, responsible for maintaining the DC bus voltage within certain limits by transferring the power from the rotor, which is stored in the DC-link capacitor, to the grid. German and Difg grid code reactive current requirements during sags [ 10 ].
The equivalent circuit for the grid side converter connection to the grid is given in Figure 7. The gearbox consists of two helical and one planetary gear stages with the low speed shaft from the turbine rotor connected to the planetary stage and the high speed generator rotor shaft to the last helical stage as shown in Figure 2 [ 51718 ].
The stator field is of constant frequency set by the grid frequency. This is because if WECS disconnect at such times, they worsen the situation with the loss of power production causing system instability.
Journal of Engineering
DPC however has high power losses. Sample simulation results have been presented underlining the need for FRT capability.
The switching on and off of IGBTs causes interference over a wide signal spectrum. It is similar to the wind farm control system but deals only with individual WECS values. The WECS must be able to ride out transient disturbances that result from short circuits and faults causing momentary power imbalances.
It however has higher conduction losses, limitation on output voltage, and poor protection during faults simulayion 2 ]. This is as shown in Figure 4. This is shown in Figure Since the DFIG is an asynchronous generator, its electromagnetic torque is developed when currents induced in the rotor windings set up a magnetic field which interacts with the ddfig field.
This paper presents a concise introduction of the DFIG WECS covering its construction, operation, merits, demerits, modelling, control types, levels and strategies, faults and their proposed solutions, and, finally, simulation. Schematic of GSC connection to grid [ 11 ]. As such, the choice of an appropriate control strategy is critical.