Distribution transformer dimensioning 4

[hhhansen]

Hi there.
Electric power utilities often design their distribution transformer for the peak loading amps even though this conditions only appear few times a year for a very short period (few hours). In fact Christmas evening is sometimes used as reference for maximum transformer loading.

However is this really the right way to do it because the loading of the transformer varies considerably within a day and the average loading could be even less than 50%.

Anyone who knows how to find any litterature about this topic ?

B.R.
HHHH

 

[jghrist]

A thermal loading guide for residential distribution transformers based on time-variant current load models

Heunis, S.W.; Herman, R.;
Heunis Solutions, Pretoria, South Africa

This paper appears in: Power Systems, IEEE Transactions on
Issue Date: Aug. 2004
Volume: 19 Issue:3
On page(s): 1294 - 1298
ISSN: 0885-8950
INSPEC Accession Number: 8052504
Digital Object Identifier: 10.1109/TPWRS.2004.831293
Date of Current Version: 02 August 2004
Sponsored by: IEEE Power & Energy Society


Abstract
This paper presents a practical thermal loading guide for distribution transformers feeding residential customers. Extensive load research data were used to estimate the thermal response of a "standard IEC distribution transformer." A strong relationship between the parameters of a load modeled by load current as a function of time and the hotspot temperature was found. Using this type of load model, a method was developed for estimating the hotspot temperature of a transformer for customers from different load classes. A method was derived to translate the thermal response of a standard IEC distribution transformer to any practical transformer.

http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=1318663

Loading Guide for 65 C Rise Distribution Transformers

Acker, C. R.;
Line Material Industries, Division of the McGraw-Edison Company, Zanesville, Ohio.

This paper appears in: Power Apparatus and Systems, IEEE Transactions on
Issue Date: Sept. 1964
Volume: 83 Issue:9
On page(s): 893 - 901
ISSN: 0018-9510
Digital Object Identifier: 10.1109/TPAS.1964.4766088
Date of Current Version: 27 January 2009


Abstract
Transformer loss of life calculations are made based on a normal transformer life expectancy of 30 years at a continuously sustained hot-spot temperature of 120 C (degrees centigrade). Computer-derived equations are applied to a generalized hypothetical 65 C rise distribution transformer. Hot-spot temperature curves, including the heating and cooling portions of the curve, are calculated for loads from 90% to 280% in ambient temperatures from 0 C to 50 C. The change in transformer time constant with changing loads is considered. The hot-spot temperature curves are converted to aging factor curves. The area under the aging factor curve is compared with a standard area representing a hot-spot temperature of 120 C continuously sustained for 24 hours. A transformer load capability table is presented.

http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=4766088

 

[prc]

The following transformer loading guides will be helpful:
IEC 60076-7 /IEEE C57.91-1995

 

[jtester]

I'm not sure that your premise is true. I worked for electric utilities for 20 years before moving into consulting, and we NEVER sized transformers for peak loading. In fact we tolerated slight overloads as long as voltages were OK.

Jim T

 

[burnt2x]

I'm not sure that your premise is true. I worked for electric utilities for 20 years before moving into consulting, and we NEVER sized transformers for peak loading. In fact we tolerated slight overloads as long as voltages were OK.

Jim T

I agree with you on this one. With regards to transformer sizing, we used a daily demand graph in my other life years ago to fit a transformer size taking into consideration the thermal loading guide as mentioned by jghrist's referenced articles. Tolerances in overloading are also allowed as long as the transformer temp calculations don't get into the red zone.

 

[PHovnanian]

Christmas evening? Maybe in colder climates, but peak loading often occurs in mid summer in hot climates where air conditioning is common.

The distribution transformer loading criteria needs to be adjusted for load types and ambient conditions for each region. The 'Christmas' generalization isn't very useful.