Unbalanced delta load

[simonfr]

Got a three phase 260kVA Dy11 transformer, powered by a 415Vac input (thyristor stack controller), 400Vac output to a delta resistive load (purely resistive). In theory the elements in the delta limbs should be identical, however in practice the elements arent identical, and im getting an unbalanced load.

The problem is i need a constant voltage across the elements, and measure the current through the elements at this voltage. However the reality is that the elements draw as much current as they need (resistance), but cant get the constant voltage across the elements.

Any good ideas?

Thanks

Simon

 

[rbulsara]

In your case I would say, your voltage source is not balanced. Even if the resitors were of uneven value, they will draw unequal current, but it will not change the voltage across the terminals. It follows I=V/R. If R is different, for a constant voltage you will get different current.

Resistors do not pull current as they "need" but they follow I=V/R rule. This assumes your source impedance is equal in all three phases and/or its negeligible compared to the load R.

 

[mc5w]

You did not state if your thyristors are on the primary or secondary side of the transformer. Putting them on the primary side is a very BAD idea because you will get repetitive transformer inrush current among other bad behavior.

If your thyristor controller is built as a 3-phase unit and is designed for a delta connected load, you should not have problems. However, if the phase rotation is backwards with respect to the thyristor firing sequence you will have very unequal current in the thyristors.

 

[simonfr]

The thyristors are on the primary, firing into the transformer. They are in sync, so the gate firing angles will be identical. One setpoint for all three phases.
The secondary of the transformer is a star, with
star point, however this may not be earthed.
Floating voltages due to unearthed star point??

Otherwise the system isn't rocket science and should work....