Learning Forum

[João Pranke]

I'm simulating a 3D model on transient solver of a trhee-phase power transformer, split winding, with two LV and one HV winding. The HV is Delta, one LV is Delta and other LV is Y.

The model works perfect when the excitation is inserted directly on the windings (voltage tipe, the used the cosine funcition). The flux density is exactly the one on the real prototype, so as the excitation currents.

But, when the same excitation is made using the external circuit, the transformer enters on saturation (almost double the expected density flux), even the voltage on windings being the same, with or without the external circuit.

So, the model works just fine. But, when utilizyng the external circuit, with the same voltage feeding, everithing goes wrong.

Has someone already sttrugled with this problem? How may i correct this? For my research, i do need use the external circuit.

The excitation circuit is show on the figure, with AT = HV; abc the phases; AT1 and AT2 are in parallel (reminding that it's a split winding transformer).

[DELI]

I will need more information, could you plot the voltage report of reach winding for both voltage type and external excitations?

[João Pranke]

Of course!
The line voltage is 36.225 kV; 
With the external circuit:

(This last graph is the max amplitude of the flux density B on sections of core legs)

Without the external circuit:

The excitation on the widings is the following (AT1 and AT2 are in parallel):

AT1_a=36225*sqrt(2)*cos(2*pi*60*time)

AT1_b=36225*sqrt(2)*cos(2*pi*60*time+120deg)

AT1_c=36225*sqrt(2)*cos(2*pi*60*time+240deg)

AT2_a=36225*sqrt(2)*cos(2*pi*60*time)

AT2_b=36225*sqrt(2)*cos(2*pi*60*time+120deg)

AT2_c=36225*sqrt(2)*cos(2*pi*60*time+240deg)

Here the flux density is almost exactly of the project (1,73 T)

As i can see, the excitation is exactly the same with or without the external circuit, but the simulation does't work with the circuit.

[DELI]

Looks like the excitation orders are different: 

1. VA_T: B->C->A (external circuit)          A->C->B (without external circuit)
2. VB_T: B->C->A (external circuilt)         A->C->B (without external circuit)

Could you give it a try by changing the phase shift in the simulation with external circuit to match the one without external circuit?

[João Pranke]

That was one of the tests i already have done, with diferents phase sequences, all simulations result the same :/

Other test i have done was to add a 90° phase on the external circuit (to make it like a cosine), but also doesn't work.