Negative value of transformer impedance
Negative value of transformer impedance
(OP)
We have a 3-winding transformer with the negative value of secondary winding impedance. What's the physical meaning of the "Negative Impedance"?
Thanks
Thanks






RE: Negative value of transformer impedance
RE: Negative value of transformer impedance
In deriving a 3 terminal equivalent for a three-winding transformer, it is possible that one of the impedances will be negative. I wouldn't try to attach anything of significance to it. It's just a derived quantity.
RE: Negative value of transformer impedance
Secondary voltage-120V
Impedance voltage-3%
????
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Negative value of transformer impedance
Zpt=30%
Zst=6.5%
Under same MVA base
Zp=1/2(Zps+Zpt-Zst)=21.75%
Zs=1/2(Zps+Zst-Zpt)=-1.75%
-1.75% is what I was talking about
RE: Negative value of transformer impedance
Zps and Zpt are no problem since they are on the primary MVA base, however, Zst is normally on the secondary winding MVA base.
RE: Negative value of transformer impedance
In this case, OEM test sheet shown 75MVA for all three windings.
RE: Negative value of transformer impedance
RE: Negative value of transformer impedance
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Negative value of transformer impedance
Your math looks OK. So what if one of the impedances is negative? It's not a big deal. I've seen it before and there's nothing wrong with it.
You are working with a wye equivalent, where you have Zp, Zs and Zt connected in a wye arrangement. Where is the junction of this wye in an actual transformer? It doesn't exist.
Testing won't verify any negative impedances because all you can measure is impedances between pairs of windings, and these are all positive.
There is no physical meaning of the negative impedance value.
RE: Negative value of transformer impedance
RE: Negative value of transformer impedance
Zps=Zp+Zs
Zpt=Zp+Zt
Zts=Zt+Zs
then you have below
Zp=(Zps+Zpt-Zst)/2
Zs=(Zps+Zst-Zpt)/2
Zt=(Zpt+Zst-Zps)/2
Like a triangle, you have the sum of the length of any two sides, then you solve the individual length of each three side. How can you get a negative number of the length for a triangle? Same as the each individual winding leakage impedance, could you please explain why a negative impedance is okay?
RE: Negative value of transformer impedance
If you take your last set of equations:
Zp=(Zps+Zpt-Zst)/2
Zs=(Zps+Zst-Zpt)/2
Zt=(Zpt+Zst-Zps)/2
If Zps is close to Zpt, and Zst is noticably less, you'll consistently get a negative impedance result for one of the legs.
The math is basically doing a delta to wye conversion, where the actual measurements are legs of a triangle. The calculated legs (Zp, Zs & Zt) apply to a wye arrangement.
RE: Negative value of transformer impedance