ABB ACS800-67, Описание системы и руководство по запуску

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System description 21
Control of torque and reactive power via rotor-side converter
With DTC technology field orientation is achieved by using advanced generator theory to
calculate the accurate generator torque. The performance of DTC controlled wind turbine
converter is most effective and benefits are eg, fast torque response, accurate control also
at low frequencies, torque repeatability and accuracy of dynamic and static speed
operations.
The main difference between DTC and conventional PWM is that the torque is controlled
at the same time level as power switches (25 microseconds). There is no separate voltage
and frequency controlled PWM modulator. All selections of the switches are based on the
electromagnetic state of the generator and torque demand given by turbine control
system.
The controlling variables of the DTC are generator magnetizing flux and generator torque.
In doubly-fed induction generator systems, the voltage in rotor/stator windings forms the
current and magnetic flux by changing the direction of the voltage in rotor windings. The
direction of the flux can also be changed. By changing the voltage direction in 3-phase
generator rotor windings in the correct order, the magnetic flux of the generator rotor will
follow this flux with a certain slip. There are eight different switching positions in the two-
level converter that can affect the flux and torque of the generator. In two positions the
voltage is zero, ie, when all the phases are connected to the same DC link, either negative
or positive. In remaining six switching positions voltage is created in the generator rotor
windings creating magnetic flux. In addition, the DTC principle is used to control the power
factor at the stator terminals of the doubly-fed induction generator to desired level and to
synchronize the stator to the 3-phase electrical power network.
The figure below shows the block diagram of the torque and power factor controller of the
doubly-fed asynchronous generator fed from the rotor-side.