Dear all,
When simulating a grid-connected DFIG with a Y-connected stator, I have seen that the sum of the three phase currents are not zero; this means that there is a zero sequence stator current component which should be due to the neutral of the stator left floating. The simulation is of course regarding the faulty conditions and it is clear that under healthy conditions, the question of neutral point being connected or float is not so important. I have therefore the following questions:
1) How are stator windings of practical DFIG's being in use connected: neutral being grounded or left floating?
2) If we are going to write the dynamic voltage equations of this machine assuming that it is connected to an infinite bus, most papers take va, vb and vc of the machine to be equal to the phase voltages of the grid; I think this is just the case when the neutral is grounded. So what is the strategy to model the machine and write the dynamic voltage equations when the neutral is being left floating?
Thanks a lot for your collaboration
Dear mohammad
There is a difference between an autonomus induction generator and that connected to a grid. In this case the grid will impose the 3 voltages va, vb and vc..
If there is a difference between the three voltages, the best way for modelling is to make the transformation to Valfa, Vbeta and Vo. In this case Vo will be taken in consideration if the neutral is connected, other was it will be a floating voltage
Dear Mohammad Naser Hashemnia ,
In a variable speed wind energy conversion system employing DFIG, the stator is directly connected to the grid. However, the rotor is coupled to the grid using a partial rated back to back voltage source converter topology. The speed of the DFIG is adjusted by varying the slip power injection.
Dear Mohammad Naser Hashemnia
DFIG using back-to-back PWM converters for the rotor-side control has been well established in wind power system.
Dear Professor Ahmad K Jammal
Thanks for your reply.
My concern is a grid connected induction generator with its stator connected directly to the grid and the rotor connected via two back-to-back power electronic converters. My question is regarding the status of the center (neutral) of the star connection of the stator windings: Is it in practice left floating, or is connected to the ground?
In addition to the practical point of importance, in computer simulations, there seems to be a problem for the floating neutral point case: We can not simply assume the winding phase voltages to be balanced sinusoidal even if the grid voltages are so; how can we then derive and simulate the winding phase voltages in this case?
Dear Krishnakumar R. Vasudevan
Thanks for your prompt response, but my concern is regarding the status of the neutral point of the stator windings star connection.
Dear Mohammad Naser Hashemnia,
The main criterion for the flow of zero-sequence current is the availability of the ground path. If the neutral is floating then there will be no zero-sequence current to flow. The flow of zero-sequence current also depends on the type of fault induced to the system. So it is a usual practice to leave the neutral point floating. Moreover, the large DFIG based wind turbines are connected at 11 kV where the system itself is a 3-phase, 3 wire system.
Dear Krishnakumar,
Thanks a lot for your response.
The main problem is how to define the abc phase voltages of the machine if the neutral is floating. Also, it has been assumed that the abc voltages of the machine are those of the grid in most of the papers (when referred to the same side).
Dear Mohammad Naser Hashemnia,
In such a case, it is customary to assume the line voltages for calculation. It is the reason for researchers to treat the line and the stator voltage equal. Moreover, line voltages are used for all the computations.
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