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Permissible Loading of Oil-Immersed Transformer and Regulators

Bijan Ghayour, P.E.

Course Outline

This 3-hour course is intended to be used as guide in determining the amount of overload that oil-immersed transformers and regulators can carry under various operating conditions and with varying degrees if insulation deterioration.

This course also discusses topics such as transformer life expectancy, overload limitations, supplemental cooling, effects of ambient temperature and altitude, and transformer operation with part or all of the cooling out of service.

The course materials are based entirely on the Unified States Department of the Interior, Facilities Instructions, Standards, and Techniques Volume 1-5.

This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of course materials.

Learning Objective

The purpose of this course is to provide guidance in determining the amount of overload that oil-immersed transformers and regulators can carry under various operating conditions.

At the conclusion of the course the student will be familiar with the:

Intended Audience

This course is intended for design and operating engineers involved in the design and operation of oil-filled transformers and step-voltage or induction voltage regulators.

Benefit to Attendees

The student will become familiar with the concept of operating oil-filled transformers under overload conditions. Topics discussed include overload limitations, aging of insulation, the concept of hottest-spot and top-oil temperatures, and the effects ambient temperature and altitude service conditions on transformer performance..

This course also includes many easy to use graphs in determining the permissible kVA loadings for many different types of transformers under various operating conditions.

Course Introduction

In general loads in excess of normal ratings may be carried under certain conditions when necessary. Some of the overloads such as recurrent short-time overloads result in no significant loss of transformer life, while others involve some degree of aging of transformer insulation and shortening of the transformer life. This course discusses the permissible overloads for different kinds of transformer cooling methods under different operating conditions.

The life expectancy of transformers, regulators, reactors at various operating temperatures is not accurately known, but the information in presented in this course regarding loss of life of is considered to be conservative. "Conservative" is used in the sense that the expected loss of insulation life for a single recommended load will not be greater than the amount indicated by data presented herein.

The data presented here cover transformers with either of the two commercially available insulation systems: (1) those designed to operate continuously up to 55 °C rise above ambient temperature, and (2) those designed to operate continuously up to 65 °C rise above ambient temperature.

Course Content

This course contains specific examples, table and graphs for estimating the amount of transformer overload for different transformers under various operating conditions. .

You are required to study volume 1-5, Permissible Loading of Oil-Immersed Transformers and Regulators, published by the Unites States Department of the Interior, Bureau of Reclamation.

FIST 1-5 Permissible Loading of Oil-Immersed Transformers and Regulators (PDF File 700 KB)

(For an electronic copy of FIST 1-5 go to and then click on volume 1-5)

Please click on the above underlined hypertext to view, download or print the document for your study. Because of the large file size, we recommend that you first save the file to your computer by right clicking the mouse and choosing "Save Target As ...", and then open the file in Adobe Acrobat Reader. If you still experience any difficulty in downloading or opening this file, you may need to close some applications or reboot your computer to free up some memory.

The following contains the outline of the FIST 1-5 of Department of Interior, Bureau of Reclamation:

FIST 1-5 Permissible Loading of Oil-Immersed Transformers and Regulators (PDF File 700 KB)

Table of Contents

1- Purpose
2- Operation with Overload
3- Overload Limitations
4- Transformer Life Expectancy
5- Aging of Insulation
6- Hottest-Spot Temperature
7- Temperature Devices and Thermal Relays
8- Basic Loading for Usual Temperature and Altitude Service Conditions
9- Effects of ambient Temperature and Altitude
10- Transformer Loading for Normal Life Expectancy
11- Top-Oil Temperature as a Guide
12- Recurrent Short-Time Overloads with Normal Life Expectancy
13- Short-Time Overloads with Moderate Sacrifice of Life Expectancy
14- Overloads During System Emergencies
15- Supplemental Cooling for Existing Transformers
16- Forced-Air and Forced-Oil Cooling
17- Ware Cooling
18- Overload Limitations
19- Transformer Operation with Part or All of Cooling Out of Service


Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

Take a Quiz

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.