Why do some use alpha-beta transformation while others use dq0 transformation? Can anyone explain (if its because of my lack of understanding of both transformation applications)?
The choice of transformation, to a large extent, depends on the choice of (current) controller used.
Different types of controller has different control capability depending on the frequency of the control signal. For eg, the classical PI control is good for controlling dc quatity (zero frequency) but its performance deteriorates as the frequency increases. That's why PI is usually used in dq0 transformation where the control variables (Id Iq) are dc quantities. On the other hand, for controlling variable with a particular frequency, a resonant (Res) controller is used. This is the reason why control in alpha-beta frame, where the control variables (Ialpha Ibeta) are ac quantities, usually uses Res controller.
Similarly, for active-filter, it is important to know the characteristics of the control variables under different reference frame transformation, and choose the suitable transformation-controller combinations.
all the best.
Basically both methods work with parks transformation. But there is a big difference. In alpha-beta transformation, we assume that the voltage is balanced and to get the theta we are using a PLL. There fore its performance in Unbalanced voltage conditions will not be good as d-q method. In d-g method we dont care about the voltage is balanced or not. The theta is calculated by trnsformation itself and we don't need a PLL.
You can get more information from this link
http://ethesis.nitrkl.ac.in/3404/1/FINAL_THESIS_(18,_61,83).pdf
I wanted to add another thing with Hangseng Che. Usually, in synchronous motor control, it is wise to utilize the dq0 transformation. In this application, different frequency ranges are tested. So, controlling the DC quantity is much simpler than to control in the alpha-beta frame.
There are 3 reasons that we may prefer d-q transform on alpha-beta :
First is what Hansheng che said. Tracking non-DC quantities with PID controllers is erroneous. However, cross coupling between variables degrades control actions in dq framework.
Second, from the view point of communications, transmission of DC parameters needs lower bandwidth in comparison with ac quantities. So in secondary or tertiary level of hierarchical and centralized control platforms, we prefer to use dq framework.
Third , when we are going to control more than one generator or DG by a single controller, it is better to use a common frame work , which can be settle down easier within dq framework by choosing nominal frequency of grid as synchronous frame for all of them.
The choice of transformation, to a large extent, depends on the choice of (current) controller used.
dq0 transform is easier to control than alpha-beta frame because it has DC components only.
The following points briefly addresses your concerns to perform the abc-to-dqo transformation:
1. The fundamental reason to transform the three-phase instantaneous voltages and currents into the synchronously rotating reference dqo frame is to make computations much easier. Secondly, it allows the system operator to independently control the active (d-axis) and reactive (q-axis) components of the currents. Similarly, in the aspect of the machine, the flux and torque can be independently controlled. This way, the coupling effect can be minimized to a great extent.
2. Additionally, in the dqo-frame, the mutual inductance is constant. Thus, it allows the system operator to achieve the desired output as the inductance-dependent quantities are constant.
3. Furthermore, the various feedback controller including proportional-integral (PI) and proportional-integral-derivative (PID) practically offers an effective and reliable control solution when the quantities are DC (i.e., constant) in nature. On the contrary, if the input quantities applied to these feedback controllers are periodic or sinusoidal then the integrator (i.e., 1/S) term of these controllers typically fails to introduce the infinite gain. As a result, the steady-state error will not be forced to zero.
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