Tom Bishop, P.E.
EASA Senior Technical Support Specialist

Although aluminum magnet wire theoretically can be converted to copper magnet wire of about 5/8 of the original wire area, in some cases this is not advisable. In others, it may result in a change in the magnetic strength of a coil or winding. In this article we will address the most common aluminum-to-copper magnet wire conversions as well as how to deal with whether the wire area should be reduced.

Richard Huber, P.E.
Richard Huber Engineering, Ltd.

The intent of this article is to provide an overview of the more common techniques used for dissolved gas analysis (DGA) of mineral oil.

EASA announces the publication of an update to AR200, now titled the Guide For The Repair Of Dry-Type Transformers. This guide outlines best practices for the repair of dry-type transformers.

EASA members adopt a customer service-centered mission encompassing various electrical apparatus repairs such as electric motors, transformers, controls, electrical troubleshooting and construction. EASA’s Technical Services Committee recently reviewed and extensively edited the AR200 guide in support of transformer repair services.

The previous version of AR200 (published in 2011) was titled Guide For The Repair Of Power And Distribution Transformers. Like preceding versions, it embraced a broad scope of transformer repair that included liquid-filled distribution or power transformers up to 10 MVA and 69 kV and dry-type distribution and power transformers up to 5 MVA and 25 kV.

In preparing this update, the Technical Services Committee chose to narrow the scope to the types of transformer repair activities usually found in EASA service centers (i.e., non-liquid-filled distribution dry-type transformers with high voltage windings up to 69 kV and rated less than 15,000 kVA). After all, liquid-filled transformer repair and service is a specialty few members offer, with many standards and best practices differing from those for dry-type transformers. In recognition of EASA’s international membership, the updated procedures in the new version now reference both IEEE and IEC standards.

Besides being helpful to the transformer repair and service efforts in many EASA service centers, it continues to position EASA as the definitive source for best practices in the electrical apparatus repair industry.

While the AR200 guide was an effort of the AR200 Task Group and EASA’s Technical Services Committee, thanks also go to EASA members who offered their comments and expertise. With a view of continual improvement as industry standards evolve, we welcome your feedback.

Table of Contents Summary

• Section 1 General
  • 1.1 Purpose
  • 1.2 Scope
  • 1.3 Identification
    • 1.3.1 Records
    • 1.3.2 Nameplate
    • 1.3.3 Service center labels
  • 1.4 Condition assessment and failure analysis
  • 1.5 Cleaning
  • 1.6 Terminals
    • 1.6.1 Leads
    • 1.6.2 Connections
    • 1.6.3 Enclosures
  • 1.7 Accessories
  • 1.8 Painting
  • 1.9 Packaging and transportation
• Section 2 Testing Transformers
  • 2.1 Safety considerations
  • 2.2 Instrument calibration
  • 2.3 Insulation condition test
    • 2.3.1 Insulation resistance test
    • 2.3.2 Polarization index test
    • 2.3.3 Insulation power factor test
  • 2.4 Other tests
    • 2.4.1 Winding resistance test
    • 2.4.2 Transformer turns ratio (TTR) test
    • 2.4.3 Polarity test
    • 2.4.4 No-load loss test
    • 2.4.5 Load loss test
    • 2.4.6 Single-phase impedance test
  • 2.5 High-potential tests
    • 2.5.1 50/60 Hz high-potential test
    • 2.5.2 DC high-potential test
    • 2.5.3 High frequency induced potential test
    • 2.5.4 Test levels, windings
• Section 3 Rewinding Transformers
  • 3.1 Investigation
  • 3.2 Gathering data
  • 3.3 Winding coils
  • 3.4 Core laminations
    • 3.4.1 Stacked cores, disassembly
    • 3.4.2 Stacked cores, assembly
    • 3.4.3 Wound cores, disassembly
    • 3.4.4 Wound cores, assembly
  • 3.5 Connections
    • 3.5.1 Connections in the winding
    • 3.5.2 External connections
    • 3.5.3 Insulating connections
  • 3.6 Leads
• Section 4 Transformer Repair
  • 4.1 Checking for service suitability
    • 4.1.1 Tests
    • 4.1.2 Equipment checks
    • 4.1.3 Summary of results
    • 4.1.4 Preparation for shipment
  • 4.2 Rewind
    • 4.2.1 Inspection, test and estimate
    • 4.2.2 Dismantle
    • 4.2.3 Winding new coils
    • 4.2.4 Reassembly
    • 4.2.5 Final tests
    • 4.2.6 Preparation for shipment
• Appendix A Electrical Testing Safety Considerations
  • A.1 Personal safety
    • A.1.1 Training
    • A.1.2 Personal protective equipment (PPE)
    • A.1.3 Supervision
    • A.1.4 First aid and CPR
  • A.2 Test area
    • A.2.1 Enclosure
    • A.2.2 Gates
    • A.2.3 Signs
    • A.2.4 Lighting
    • A.2.5 Safety equipment
    • A.2.6 Test unit clearance
  • A.3 Unit under test
    • A.3.1 Suitability for test
    • A.3.2 Exclusive attention
    • A.3.3 Grounding (earthing)
    • A.3.4 Base
  • A.4 Test panels
    • A.4.1 Construction
    • A.4.2 Voltages
    • A.4.3 Warning lights
    • A.4.4 Disconnect
    • A.4.5 Safety switch
    • A.4.6 Leads
  • A.5 High-potential ground (earth) test
• Appendix B Reference Information
  • Table b-1. Insulation resistance test voltages
  • Table b-2. Temperature correction factors for dry-type transformer insulation resistance tests
  • Table b-3. Recommended minimum insulation resistances for dry-type transformers
  • Table b-4. Recommended test levels for dry-type transformer new windings
• Bibliography
• Standards Organizations & Other Resources

REVISED September 2022!

The EASA Technical Manual, containing more than 900 pages of information specific to electric motor service centers, is available FREE to EASA members as downloadable PDFs of the entire manual or individual sections. The printed version is also available for purchase. Each of the 13 sections features a detailed table of contents.

VIEW, DOWNLOAD OR PURCHASE

Richard Huber, P. Eng.
Richard Huber Engineering, Ltd.
North Vancouver, British Columbia
Canada
Technical Services Committee Member

Introduction
There are many transformers in use rated up to 10 MVA (10,000 kVA) that were originally wound using aluminum conductors. When dam­aged or when selected for rewind, the aluminum conductor is often replaced with copper conductor. This is usu­ally fairly routine when the conductor changes are undertaken within the bounds of the original transformer design. It is this type of change that will be reviewed in this article. It is not the intent to provide information here for the complete redesign of the transformer. It is important that all coil dimensions remain as close to the originals as possible.

Richard Huber, P.E.
Richard Huber Engineering, Ltd.

Transformers are fundamental to an industrial or utility distribution or transmission system. This article will present basic transformer information that may help the reader appreciate how they operate and their many features. 

Many people consider a transformer to be one of the more basic of electrical machines. As a result, many of the design and operational characteristics are taken for granted. From time to time, it may be beneficial to review these characteristics and refresh one’s understanding. Some of the basic concepts are discussed in the following sections. 

Most of the information presented here will be limited to transformers with two separate windings.

Topics covered in the article include:

• Transformer design—how a transformer works
• Volts, amps and flux
• Wind polarity
• Excitation current
• Regulation and efficiency
• Winding taps
• Types of transformers
• Wound cores and stacked cores
• Shell type and core type transformers
• Single-phase and three-phase transformers
• Dry type and liquid filled transformers
• Isolating and shield transformers
• Auto transformer