Questions Related to Power System Design

[kappa_am]

ok! the equal circuit of transformer in SC is almost a series R-L. So you can obtain X simply bu X=sqrt(Z^2-R^2).

VR=IRcos(theta)+IXsin(theta)
divide both side by V ===> VR%=R(p.u)cos(theta)+X(p.u)sin(theta) for lead load replace + with -

 

[FvM]

So as I said short circuit data is Rsc is 1.759 and Zsc is 2.644 S is 3kVa at 400 V line to line and its star connection.

Hard to believe.... A simple text search reveals that the 2.644 number was never mentioned in this thread. (nor any other value that allows to calculate it). My final question is about the meaning of Rsc and Xsc numbers. It's common practice to specify equivalent short circuit parameters that represent all three phases in one. Respectively you have simply (3 kVA/400V)*Rsc as voltage drop and (3 kVA/400V)*Rsc/400V as %R. Or you can specify a "per phase" short circuit impedance which is nearer to an actual measurement.

 

[kappa_am]

Respectively you have simply (3 kVA/400V)*Rsc as voltage drop

this can be count as total voltage drop. the main part of the voltage drop is related to X. I think the second is better solution, even though there are some approximation.

 

[meyra31]

Sorry for the confusion guys. I was without sleep pretty much everyday last week, and this was stressing me a lot.

And so once we get the reactance percentage, how can i work out the expected voltage regulation by using the full load current (8.2A) and per unit equivalent reactance?

 

[kappa_am]

you have to have load power factor.
and use mentioned equation in post 21

 

[meyra31]

It says assume a purely resistive load current. I m not sure how to find the power factor but i nkow its found from Psc/vsc * Isc so i ll try to get it.
But then from the equation in 21st post, VR=IRcos(theta)+IXsin(theta) is X the short circuit one or the percentage reactance one?

 

[KlausST]

Hi,

assume a purely resistive load current

Means a power factor of 1

Klaus

 

[FvM]

I don't understand the discussion about load power factor. It has no meaning for transformer short circuit impedance definition or measurement. It's performed as such:

Calculate rated current from kVA and rated voltage specification. Short the transformer output, apply the rated current to the input. Measure input voltage vectorial, in-phase (real, Vsc,r) and quadrature (reactive, Vsc,x) component. %R = Vsc,r/Vrated, %X = Vsc,x/Vrated.

 

[kappa_am]

Your load P.F=1.
This equation is approx. voltage drop. Second equation is for voltage regulation. and X is p.u one.

VR%=R(p.u)cos(theta)+X(p.u)sin(theta)

 

[meyra31]

If PF is 1, then isnt the angle theta gonig to be arccos 1 =0 angle?? and I know X is the p.u. one but i calculated Xsc to be 53.60 ohms and i doubled check with my lecturer and its correct. Adn the percentage reactance is 3.88% (i calculated it.) how do we obtain p.u X then?? This is really confusing me... hating this already

 

[kappa_am]

I don't understand the discussion about load power factor. It has no meaning for transformer short circuit impedance definition or measurement. It's performed as such:

Calculate rated current from kVA and rated voltage specification. Short the transformer output, apply the rated current to the input. Measure input voltage vectorial, in-phase (real, Vsc,r) and quadrature (reactive, Vsc,x) component. %R = Vsc,r/Vrated, %X = Vsc,x/Vrated.

Who asked question somehow mixed it up. Now, the discussion is about calculating of voltage regulation of transformer and it depends on load power factor.

%R = Vsc,r/Vrated, %X = Vsc,x/Vrated. are you sure about these equations? Respectfully, I think for obtaining X and R in p.u, we have to calculate base Z from voltage and apparent power. and I think there will be different results. if I am wrong, correct me.

- - - Updated - - -
Reply to post #30
the angle between transformer terminal voltage, which is on the load and current is zero. But there is a angle between current and the voltage behind the transformer impedance. X is inductances of the transformer winding transferred to one side of the transformer. look at the equal circuit of the transformer.

 

[FvM]

Voltage regulation? I don't see how the discussion is related to it. I think the discussion is about the definition of basic terms of electrical power engineering and how to measure it. I thought the short circuit impedance measurement would be passed by all electrical engineering students in a second or third years power engineering lab. But may be these days it's only run on a computer...

 

[meyra31]

Voltage regulation? I don't see how the discussion is related to it. I think the discussion is about the definition of basic terms of electrical power engineering and how to measure it. I thought the short circuit impedance measurement would be passed by all electrical engineering students in a second or third years power engineering lab. But may be these days it's only run on a computer...

Yes we have them but our lab sessions are so poor. I dont even see the point of them, and everyone agrees with me in my class. I mean they are good, they could definitely be useful but I think they are not doing it as they should.