Transformer Voltage Drop

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EW87301

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After viewing several discussions on voltage drop I was unable to find information pertaining to the inclusion of the voltage drop of the transformer. When is the voltage drop of the transformer included? If that value is unknown how can I calculate the voltage drop of the transformer?
 
Voltage does not "drop" in a transformer, it "rises." What I mean is that voltage is created by the transformer. That is the starting point for the system's supply voltage, and voltage is dropped from that point through the conductors and the loads. I am curious as to how this issue came up. Can you tell us?

Welcome to the forum.
 
In referencing company construction standards I found under the section containing transformer data a table that includes a % voltage impeadance (Z,X,R) and next to that a % voltage drop (at 100% and 130% load). I am currently working on a project where we are converting a 2400V pot bank set up to a pad mount for the service to a well. The pump sits at about 2000 ft from the transformer. I wanted to make sure when I do the voltage drop calculation I did not leave anything out. For Example:

150 KVA, 277/480V,
% voltage impeadance: Z=1.55
R=.79,X = 1.33

% voltage drop
100% load = 1.24
130% load = 1.61
 
090112-1315 EST

charlie b:

I would disagree.

There is voltage drop in a transformer. Or you can assume the output of the transformer is ideal, a perfect voltage source, then put a little box in the output lead of the transformer that contains an equivalent circuit of the transformer output impedance. For prectical purposes this is a series inductance and resistance.

The inductance is largely the transformer leakage inductance reflected to the secondary. The resistive component is the primary resistance reflected to the secondary added to the secondary resistance.

.
 
Sorry, I wasn't clear about my intended message. Of course there is a voltage drop internal to a transformer. And yes, there are several ways to model it. But most of the time, when we speak of "voltage drop calculations," we are speaking of the losses in the wires, and the resultant voltage that is available to the loads. We use primary or secondary transformer taps to compensate for excessive VD, and to provide an acceptable voltage level at the transformer secondary. That is why I asked how the subject came up.
 
Would it be ok then to conclude that the internal voltage drop of the transformer is not a major component of the voltage drop and does not need to be included in the overall voltage drop total?
 
I'd suggest that you think in terms of 'voltage regulation' rather than 'voltage drop' for the purpose of this analysis.

Because a transformer has adjustable 'taps' you can compensate to an extent for voltage drop. But once you've set the taps, and designed your wiring, then the changing load will mean changing voltage delivered to the load.

Transformer regulation and impedance may very well be significant components to the change in voltage delivered to the load.

-Jon
 
EW87301 said:
After viewing several discussions on voltage drop I was unable to find information pertaining to the inclusion of the voltage drop of the transformer. When is the voltage drop of the transformer included? If that value is unknown how can I calculate the voltage drop of the transformer?
Click to expand...

Do you do Excel?
 
Vd of transformer

Vd of transformer

With all due respect to all the post . We know voltage drop is in every tap joint inductor conductor in the electrical system, but what i think the question was intended to be was is it really important to include the V D of the transformer ? NO it is not the most important the big voltage drop would be that 2000 foot wire drop . Iam just a plain old electrician but now you have me interested so how do i find voltage drop inside that transformer ? make it simple and on paper ? comments best to ya
 
Voltage regulation - how to compute

Voltage regulation - how to compute

The voltage regulation of the transformer is the percentage change in the output voltage from no-load to full-load. And since power factor is a determining factor in the secondary voltage, power factor influences voltage regulation. This means the voltage regulation of a transformer is a dynamic and a load-dependent number. The numbers you see in the nameplate data are fixed; the number of primary windings won't change; the number of secondary windings won't change, etc. But, the voltage regulation will vary as power factor varies.
Having said those, here it is:
  1. If the resistance (R) and reactance (X) of the transformer is known; VD = IR(Cos q) + IX(Sin q) - (From IEEE Red Book); where I is the load current of the transformer; R is the transformer resistance; X is the transformer inductive reactance; Cos q is the power factor(PF) of the transformer load; and Sin q is the reactive factor = square root of (1- PF^2)
  2. If you have no data on resistances/ reactances, you can calculate the voltage regulation by using the formula:
    V(no-load) - V (full-load)​
    %regulation = ----------------------
    V(no-load)​
In the example presented by EW87301:
150 KVA, 277/480V,
% voltage impeadance: Z=1.55
R=.79,X = 1.33

% voltage drop
100% load = 1.24
130% load = 1.61
Click to expand...
We need to assume the value of the load power factor, say PF = unity (or 1), then:
Resistance in ohms = 0.0079 X (480^2/150 kVA) = 0.0079 X 1.536 = 0.01213 ohms.
Reactance in ohms = 0.0133 X (480^2/150 kVA) = 0.0133 X 1.536 = 0.02042 ohms.
At full load current = 150kVA/480 = 312.5 amperes (assuming 1-phase)
The Voltage Drop will then be:
VD = (312.5)(0.01213)(1) + (312.5)(0.02042)(1) = 10.17 volts (@ PF = 1)
The % regulation will be = (10.17 volts/ 480 volts) X 100 = 2.12 %
The percent regulation will be greater if the power factor is other than unity.
 
Transformer internal voltage drop

Transformer internal voltage drop

Well thanks for the input on voltage drop we understand why now . Does heat or temp of that transformer effect the vd like eddy currents in the core ? And what about harmonics on the neutral or unbalance loads on secondary current should that also be included it might cause a slight percent of voltage drop ? why dont they include this in the nec code as 2 percent is a major percent . comments ? best to ya
 
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