Transformers: Basics, Maintenance, and Diagnostics

1. Introduction

2. Introduction to Transformers

2.1 Principle of Operation

2.2 Transformer Action

2.3 Transformer Voltage and Current

2.4 The Magnetic Circuit

2.5 Core Losses

2.6 Copper Losses

2.7 Transformer Rating

2.8 Percent Impedance

2.9 Internal Forces

2.10 Autotransformers

2.11 Instrument Transformers

2.12 Potential Transformers

2.13 Current Transformers

2.14 Transformer Taps

2.15 Transformer Bushings

2.16 Transformer Polarity

2.17 Single-Phase Transformer Connections for Typical Service to Buildings

2.18 Parallel Operation of Single-Phase Transformers for Additional Capacity

2.19 Three-Phase Transformer Connections

2.20 Wye and Delta Connections

2.21 Three-Phase Connections Using Single-Phase Transformers

2.22 Paralleling Three-Phase Transformers

2.23 Methods of Cooling

2.24 Oil-Filled – Self-Cooled Transformers

2.25 Forced-Air and Forced-Oil-Cooled Transformers

2.26 Transformer Oil

2.27 Conservator System

2.28 Oil-Filled, Inert-Gas System

2.29 Indoor Transformers

3. Routine Maintenance

3.1 Introduction to Reclamation Transformers

3.2 Transformer Cooling Methods Introduction

3.3 Dry-Type Transformers

3.3.1 Potential Problems and Remedial Actions for Dry-Type Transformer Cooling Systems

3.4 Liquid-Immersed Transformers

3.4.1. Liquid-Immersed, Air-Cooled

3.4.2 Liquid-Immersed, Air-Cooled/Forced Liquid-Cooled

3.4.3 Liquid-Immersed, Water-Cooled

3.4.4 Liquid-Immersed, Forced Liquid-Cooled

3.4.5 Potential Problems and Remedial Actions for Liquid-Filled Transformer Cooling Systems

3.4.5.1 Leaks

3.4.5.2 Cleaning Radiators

3.4.5.3 Plugged Radiators

3.4.5.4 Sludge Formation

3.4.5.5 Valve Problems

3.4.5.6 Mineral Deposits

3.4.5.7 Low Oil Level

3.4.6 Cooling System Inspections

4. Oil-Filled Transformer Inspections

4.1 Transformer Tank

4.2 Top Oil Thermometers

4. Oil-Filled Transformer Inspections (continued)

4.3 Winding Temperature Thermometers

4.3.1 Temperature Indicators Online

4.3.2 Temperature Indicators Offline

4.4 Oil Level Indicators

4.5 Pressure Relief Devices

4.5.1 Newer Pressure Relief Devices

4.5.2 Older Pressure Relief Devices

4.6 Sudden Pressure Relay

4.6.1 Testing Suggestion

4.7 Buchholz Relay (Found Only on Transformers with Conservators)

4.8 Transformer Bushings: Testing and Maintenance of High-Voltage Bushings

4.9 Oil Preservation Sealing Systems

4.9.1 Sealing Systems Types

4.9.1.1 Free Breathing

4.9.1.2 Sealed or Pressurized Breathing

4.9.1.3 Pressurized Inert Gas Sealed System

4.9.2 Gas Pressure Control Components

4.9.2.1 High-Pressure Gauge

4.9.2.2 High-Pressure Regulator

4.9.2.3 Low-Pressure Regulator

4.9.2.4 Bypass Valve Assembly

4.9.2.5 Oil Sump

4.9.2.6 Shutoff Valves

4.9.2.7 Sampling and Purge Valve

4.9.2.8 Free Breathing Conservator

4.9.2.9 Conservator with Bladder or Diaphragm Design

4.10 Auxiliary Tank Sealing System

5. Gaskets

5.1 Sealing (Mating) Surface Preparation

5.2 Cork-Nitrile

5.3 Cork-Neoprene

5.4 Nitrile “NBR”

5.4.1 Viton

5.5 Gasket Sizing for Standard Groove Depths

5.6 Rectangular Nitrile Gaskets

5.7 Bolting Sequences to Avoid Sealing Problems

6. Transformer Oils

6.1 Transformer Oil Functions

6.1.1 Dissolved Gas Analysis

6.1.2 Key Gas Method

6.1.2.1 Four-Condition DGA Guide (IEEE C57-104)

6.1.3 Sampling Intervals and Recommended Actions

6.1.4 Atmospheric Gases

6.1.5 Dissolved Gas Software

6.1.6 Temperature

6.1.7 Gas Mixing

6.1.8 Gas Solubility

6.1.9 Diagnosing a Transformer Problem Using Dissolved Gas Analysis and the Duval Triangle

6.1.9.1 Origin of the Duval Triangle

6.1.9.2 How to Use the Duval Triangle

6.1.9.3 Expertise Needed

6.1.9.4 Rogers Ratio Method of DGA

6.1.10 Carbon Dioxide/Carbon Monoxide Ratio

6.1.11 Moisture Problems

6.1.11.2 Moisture in Transformer Insulation

7. Transformer Oil Tests that Should Be Completed Annually with the Dissolved Gas Analysis

7.1 Dielectric Strength

7.1.1 Interfacial Tension

7.2 Acid Number

7.3 Test for Oxygen Inhibitor

7.4 Power Factor

7.5 Oxygen

7.6 Furans

7.7 Oil Treatment Specification

7.7.1 Taking Oil Samples for DGA

7.7.1.1 DGA Oil Sample Container

7.7.1.2 Taking the Sample

8. Silicone Oil-Filled Transformers

8.1 Background

8.2 Carbon Monoxide in Silicone Transformers

8.3 Comparison of Silicone Oil and Mineral Oil Transformers

8.4 Gas Limits

8.5 Physical Test Limits

9. Transformer Testing

9.1 DC Winding Resistance Measurement

9.2 Core Insulation Resistance and Inadvertent Core Ground Test (Megger®)

9.3 Doble Tests on Insulation

9.3.1 Insulation Power Factor Test

9.3.2 Capacitance Test

9.3.3 Excitation Current Test

9.3.4 Bushing Tests

9.3.5 Percent Impedance/Leakage Reactance Test

9.3.6 Sweep Frequency Response Analysis Tests

9.4 Visual Inspection

9.4.1 Background

9.4.2 Oil Leaks

9.4.3 Oil Pumps

9.4.4 Fans and Radiators

9.4.5 Age

9.4.6 Infrared Temperature Analysis

9.4.7 IR for Transformer Tanks

9.4.8 IR for Surge Arresters

9.4.9 IR for Bushings

9.4.10 IR for Radiators and Cooling Systems

9.4.11 Corona Scope Scan

9.5 Ultrasonic and Sonic Fault Detection

9.5.1 Background

9.5.2 Process

9.6 Vibration Analysis

9.6.1 Background

9.6.2 Process

9.7 Turns Ratio Test

9.7.1 Background

9.7.2 Process

9.8 Estimate of Paper Deterioration (Online)

9.8.1 CO2 and CO Accumulated Total Gas Values

9.8.2 CO2 /CO Ratio

9.9 Estimate of Paper Deterioration (Offline During Internal Inspection)

9.9.1 Degree of Polymerization (DP)

9.9.1.1 Background

9.9.1.2 Process

9.9.2 Internal Inspection

9.9.2.1 Background

9.9.2.2 Transformer Borescope

9.10 Transformer Operating History

9.11 Transformer Diagnostics/Condition Assessment Summary

Appendix: Hydroplant Risk Assessment – Transformer Condition Assessment

References

Transformers

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testing transformers

1. Introduction - testing transformers

This document was created to provide guidance to Bureau of Reclamation (Reclamation) powerplant personnel in maintenance, diagnostics, and testing transformers and associated equipment. This document applies primarily to the maintenance and diagnostics of oil-filled power transformers (500 kilovoltamperes [kVA] and larger), owned and operated by Reclamation, although routine maintenance of other transformer types is addressed as well. Specific technical details are included in other documents and are referenced in this...


July 14, 2015

2. Introduction to Transformers and Three-Phase Transformer

Introduction to Transformers and Three-Phase Transformer

Generator step-up (GSU) transformers represent the second largest capital investment in Reclamation power production—second only to generators. Reclamation has hundreds, perhaps thousands, of transformers, in addition to hundreds of large GSU transformers. Reclamation has transformers as small as a camera battery charger, about one-half the size of a coffee cup, to huge generator step-up transformers near the size of a small house. The total investment...


July 13, 2015

2.1 Principle of Operation and Transformer Construction

Transformer Construction

Transformer function is based on the principle that electrical energy istransferred efficiently by magnetic induction from one circuit toanother. When one winding of a transformer is energized from an alternating current (AC) source, an alternating magnetic field is established in the transformer core. Alternating magnetic lines of force, called “flux,” circulate through the core. With a second winding around the same core, a voltage is induced by the alternating...


July 12, 2015

2.2 Transformer Action

Transformer action depends upon magnetic lines of force (flux)mentioned above. At the instant a transformer primary is energized with AC, a flow of electrons (current) begins. During the instant of switch closing, buildup of current and magnetic field occurs. As current begins the positive portion of the sine wave, lines of magnetic force (flux) develop outward from the coil and continue to expand until the current is at its positive...


July 11, 2015

2.3 Transformer Voltage and Current

Transformer Voltage and Current

If the small amount of transformer loss is ignored, the back-voltage (back EMF) of the primary must equal the applied voltage. The magnetic field, which induces the back-voltage in the primary, also cuts the secondary coil. If the secondary coil has the same number of turns as the primary, the voltage induced in the secondary will equal the back-voltage induced in the primary (or the applied voltage)....


July 9, 2015

2.4 The Magnetic Circuits

A Magnetic Circuits or core of a transformer is designed to provide a path for the magnetic field, which is necessary for induction of voltages between windings. A path of low reluctance (i.e., resistance to magnetic lines of force), consisting of thin silicon, sheet steel laminations, is used for this purpose. In addition to providing a low reluctance path for the magnetic field, the core is designed to prevent circulating...


July 8, 2015

2.5 Core Losses transformer

Core Losses transformer

Since magnetic lines of force in a transformer are constantly changing in value and direction, heat is developed because of the hysteresis of the magnetic material (friction of the molecules). This heat must be removed; therefore, it represents an energy loss of the transformer. High temperatures in a transformer will drastically shorten the life of insulating materials used in the windings and structures. For every 8 degrees Celsius...


Copper Losses transformer

2.6 Copper Losses transformer

Copper Losses transformer

There is some loss of energy in a transformer due to resistance of the primary winding to the magnetizing current, even when no load is connected to the transformer. This loss appears as heat generated in the winding and must also be removed by the cooling system. When a load is connected to a transformer and electrical currents exist in both primary and secondary windings, further losses of...


2.7 Transformer Rating

Transformer Rating

Capacity (or rating) of a transformer is limited by the temperature that the insulation can tolerate. Ratings can be increased by reducing core and copper losses, by increasing the rate of heat dissipation (better cooling), or by improving transformer insulation so it will withstand higher temperatures. A physically larger transformer can dissipate more heat, due to the increased area and increased volume of oil. A transformer is only as...


July 4, 2015

2.8 Percent Impedance

Percent Impedance

The percent impedance of a transformer is the total opposition offered an alternating current. This may be calculated for each winding. However, a rather simple test provides a practical method of measuring the equivalent impedance of a transformer without separating the impedance of the windings. When referring to impedance of a transformer, it is the equivalent impedance that is meant. In order to determine equivalent impedance, one winding of

the...


July 3, 2015

2.9 Transformer Internal Forces

Transformer Internal Forces

During normal operation, internal structures and windings are subjected to mechanical forces due to the magnetic forces. These forces are illustrated in figure 8. By designing the internal structure
very strong to withstand these forces over a long period of time, service life can be extended.

However, in a large transformer during a “through fault” (fault current passing through a transformer), forces can reach millions of pounds, pulling the...


July 2, 2015

2.10 Autotransformers

Autotransformers

It is possible to obtain transformer action by means of a single coil, provided that there is a “tap connection” somewhere along the winding. Transformers having only one winding are called autotransformers, shown schematically in figure 9.

An autotransformer has the usual magnetic core but only one winding, which is common to both the primary and secondary circuits. The primary is always the portion of the winding connected to the AC...


July 1, 2015

2.11 Instrument Transformers

Instrument transformers (figure 10) are used for measuring and control purposes. They provide currents and voltages proportional to the primary, but there is less danger to instruments and personnel.

Figure 10 – Connections of Instrument Transformers

Those transformers used to step voltage down are known as potential transformers (PTs) and those used to step current down are known as current transformers (CTs).

The function of...


June 30, 2015

2.12 Potential Transformers

Potential transformers (figure 11) are used with voltmeters, wattmeters, watt-hour meters, power-factor meters, frequency meters, synchroscopes and synchronizing apparatus, protective and regulating relays, and undervoltage and overvoltage trip coils of circuit breakers. One potential transformer can be used for a number of instruments if the total current required by the instruments connected to the secondary winding does not exceed the transformer rating.

Potential transformers are usually rated 50 to 200 volt-amperes...


June 29, 2015

2.13 Current Transformers

Current Transformers

The primary of a current transformer typically has only one turn. This is not really a turn or wrap around the core but just a conductor or bus going through the “window.” The primary never has more than a very few turns, while the secondary may have a great many turns, depending upon how much the current must be stepped down. In most cases, the primary of a current...


2.14 Transformer Taps

Transformer Taps

Most power transformers have taps on either primary or secondary windings to vary the number of turns and, thus, the output voltage. The percentage of voltage change, above or below normal, between different tap positions varies in different transformers. In oil-cooled transformers, tap leads are brought to a tap changer, located beneath the oil inside the tank, or brought to an oil-filled tap changer, externally located. Taps on dry-type...


2.15 Transformer Bushings

Transformer Bushings

The two most common types of bushings used on transformers as main lead entrances are solid porcelain bushings on smaller transformers and oil-filled condenser bushings on larger transformers. Solid porcelain bushings consist of high-grade porcelain cylinders that conductors pass through. Outside surfaces have a series of skirts to increase the leakage path distance to the grounded metal case. Highvoltage bushings are generally oil-filled condenser type. Condenser types have a...


June 26, 2015

2.16 Transformer Polarity

Transformer Polarity

With power or distribution transformers, polarity is important only if the need arises to parallel transformers to gain additional capacity or to hook up three single-phase transformers to make a three-phase bank. The way the connections are made affects angular displacement, phase rotation, and direction of rotation of connected motors. Polarity is also important when hooking up current transformers for relay protection and metering. Transformer polarity depends on which...


June 25, 2015

2.17 Single-Phase Transformer Connections for Typical Service to Buildings

Single-Phase Transformer

Figure 15 shows a typical arrangement of bringing leads out of a single-phase distribution transformer. To provide flexibility for connection, the secondary winding is arranged in two sections. Each section has the same number of turns and, consequently, the same voltage. Two primary leads (H1, H2) are brought out from the top through porcelain bushings. Three secondary leads (X1, X2, X3) are brought out through insulating bushings on the...


June 24, 2015

2.18 Parallel Operation of Single-Phase Transformers for Additional Capacity

Single-Phase Transformers

In perfect parallel operation of two or more transformers, current in each transformer would be directly proportional to the transformer capacity, and the arithmetic sum would equal one-half the total current. In practice, this is seldom achieved because of small variations in transformers. However, there are conditions for operating transformers in parallel. They are:

1. Any combination of positive and negative polarity transformers can be used. However, in all cases,...


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обновлено: September 16, 2016 автором: dannik