xDFM vs DFM
Within xDFM topology, THD
is controlled bellow 1% without harmonics filter need.
Within DFM topologies, THD is significant (typically around 4%) and harmonics filters are necessary as part of the power is delivered to the grid through the power converter.
Within DFM topologies, THD is significant (typically around 4%) and harmonics filters are necessary as part of the power is delivered to the grid through the power converter.
Within DFM topology, in case any fault occurs in the grid, the system suffers a high torque peak and any kind of crowbar is usually employed. Said torque peak implies hard mechanic stress for the wind turbine.
Within xDFM topologies, crowbar may not be necessary and torque peak is limited.
Furthermore, within xDFM topologies in case a voltage sag occurs the converter remains active as voltage is kept due to the existence of the exciter, which is a permanent magnets synchronous machine. Hence, in contrary to DFM systems, the converter is able to keep the control of rotor currents duringthe fault.
Within xDFM topologies, crowbar may not be necessary and torque peak is limited.
Furthermore, within xDFM topologies in case a voltage sag occurs the converter remains active as voltage is kept due to the existence of the exciter, which is a permanent magnets synchronous machine. Hence, in contrary to DFM systems, the converter is able to keep the control of rotor currents duringthe fault.
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Within DFM topologies, if the stator power of the Doubly Fed Induction Generator (DFIG) decreases abruptly due to a grid fault or a disconnection from the grid, the machine tends to speed up. In the case of a wind turbine operating at rated power, the machine may suffer an over-speed. Usually, it is not possible to use electric brake in such moment, because the DFIG’s stator power and, furthermore, the DFIG’s rotor power may be too low.
Within xDFM, the exciter machine power can be used to drive an electric brake. In this case, the exciter machine will be used as generator and, hence, power can be transferred from the exciter machine to the direct current Bus. Thus, part of the electric power is drained in the rheostat of the chopper connected to the DC Bus avoiding over-speed of the generator. In such a way, the wind turbine braking does not solely depend on the mechanical brake. The electric brake may be used together with aerodynamic brake, allowing the wind turbine to brake progressively, minimizing mechanical strengths, peak torque loads and undesired accelerations that might cause premature damages in some components of the wind turbine.
Within xDFM, the exciter machine power can be used to drive an electric brake. In this case, the exciter machine will be used as generator and, hence, power can be transferred from the exciter machine to the direct current Bus. Thus, part of the electric power is drained in the rheostat of the chopper connected to the DC Bus avoiding over-speed of the generator. In such a way, the wind turbine braking does not solely depend on the mechanical brake. The electric brake may be used together with aerodynamic brake, allowing the wind turbine to brake progressively, minimizing mechanical strengths, peak torque loads and undesired accelerations that might cause premature damages in some components of the wind turbine.
The xDFM topology is also
suitable for high voltage DC link transmission (HVDC) in variable speed generation
systems. As shown below, the DC output can be produced by using a high voltage
generator with a rectifier or with a low voltage generator and an additional
transformer with one or more taps, wherein each secondary is rectified and
all of such rectifiers are connected in series or parallel.
The output of the AC voltage can be controlled to have constant power and frequency, allowing a am optimum sizing of required rectifiers and transformers and reducing the ripple content of the DC output voltage under low wind conditions, improving the output power quality.
Within DFM topologies, HVDC connection is not feasible at all.
The output of the AC voltage can be controlled to have constant power and frequency, allowing a am optimum sizing of required rectifiers and transformers and reducing the ripple content of the DC output voltage under low wind conditions, improving the output power quality.
Within DFM topologies, HVDC connection is not feasible at all.
Despite the enlargement of DFIG machine and the incorporation of a new electrical machine –the exciter-, the total cost of the wind turbine could be not only maintained, but even reduced.
Relating to the converter, some savings are achieved, removing the crowbar and harmonics filter.
Thanks to the Dynamic Electric Brake (xDEB), mechanical loads are reduced and, consequently, the nacelle may be mechanically (weight, loads…) optimized.
Finally, in case the stator of the Doubly Fed Induction Generator (DFIG) is designed for medium voltage grid connection, the transformer is remarkably reduced in comparison to the DFM topology.
Relating to the converter, some savings are achieved, removing the crowbar and harmonics filter.
Thanks to the Dynamic Electric Brake (xDEB), mechanical loads are reduced and, consequently, the nacelle may be mechanically (weight, loads…) optimized.
Finally, in case the stator of the Doubly Fed Induction Generator (DFIG) is designed for medium voltage grid connection, the transformer is remarkably reduced in comparison to the DFM topology.