Influence of impedance voltage 2

[zelvica]

Hello!

In our distribution company there is about 5000 10(20)/0.4 kV transformers. For the purpouses of new control center we have to enter (rather import) data about them. We already have a database for characteristical data (S, Uprim, Usec, uk). Many of these transformers are from the same manufacturer and are the same type, but when you compare the measured values of impedance voltage or no-load/load losses there is as many as 30-40% or even more relative difference between the maximal and minimal values. For example:

a group of 70 transformers,
minimal value of uk=3.5%
maximal value of uk=5.9%
average of the group is 4.9%
relative difference between max and min is about 68%

How would affect the results (load flow, short circ. calc.) if we approximate all the transformers of the same type with average or - even bigger approximation - typical values of these transformers? Or should we import data for each transformer individually?

Rgds, zelvica

 

[luis2107]

Hello,

I think that the most recommended is to take into account the "real" values (name plate data) of each one individually.
Only in this way you will get a more accurate representation in digital simulations like shortcircuit and load flow, otherwise, the results may be very different from the reality and not usefull.
For example, if the impedance value (uk) is lower than the actual, the results for load flow may show a lesser voltage drop than real, and for shortcircuit perhaps higher currents thus, requiring new circuit breakers and other protective settings.
As you see, the best is taken the real values for each equipment, in this way there is more certainty about the results.

Regards

Luis

 

[jghrist]

It depends. If you are concerned with MV load flows and faults, it won't make much difference. It will make a big difference in LV load flows and fault studies.

At some point in the future, some customer will ask you what the available fault is at his service. If you want to give him an accurate answer, you will need the actual impedance. If you want to give him a worst case value, to allow for a transformer change later, you will only need the minimum impedance.

 

[CJFlatters]

If you have manufacturers rating plate data available, use it.

If you have the serial numbers, you may be able to get the manufacturers actual test data including both Z% and losses and / or X/R which should ideally be entered in the model.

If you are using IEC60909, there are formulae for calculating both minimum and maximum values, I cannot comment on NFPA or other codes.

_______________________________________
Regards -

Colin J Flatters
Consulting Engineer & Project Manager

 

[jbartos]

Suggestion: zelvica (Electrical) May 11, 2004 marked ///\\a group of 70 transformers,
minimal value of uk=3.5%
maximal value of uk=5.9%
average of the group is 4.9%
relative difference between max and min is about 68%

How would affect the results (load flow, short circ. calc.) if we approximate all the transformers of the same type with average or - even bigger approximation - typical values of these transformers?
///The transformer uk bigger value affects negatively short circuit values (are smaller) and voltage drop values (are higher) and vice versa, the transformer uk smaller value affects negatively short circuit values (are bigger) and voltage drop values (are lower).\\ Or should we import data for each transformer individually?
///This is suggested depending on each transformer importance in the power distribution system, i.e., it is better to input all available values for important transformers in the power distribution system.\\