basic transformer

Contents

Contents

Principles of T

Chapter 1

Chapter 2

Chapter 3

Chapter 4

Chapter 5

Chapter 6

Chapter 7

Chapter 8

Single-Phase

Chapter 9

Chapter 10

Chapter 11

Chapter 12

Three-Phase T

Chapter 13

Chapter 14

Chapter 15

Chapter 16

Chapter 17

Chapter 18

Chapter 19

Chapter 20

Foreword ...................................................................................................................iv

iii

ransformers

Basic Electrical Theories and Principles ..................................................................1

Alternating Current and Induction Principles ......................................................17

Transformers............................................................................................................27

Transformer Construction......................................................................................35

Transformer Information, Characteristics.............................................................51

Transformer Operation...........................................................................................69

Polarity of Transformers .........................................................................................87

Tap Changers and Operation .................................................................................95

Transformers

Install Transformers to Company Standards.......................................................105

Single-Phase Transformer Connections...............................................................119

Conduct Transformer Load Checks .....................................................................133

Energize Transformers Safely................................................................................143

ransformers

Characteristics of Delta and Wye Systems ...........................................................151

Three-Phase Transformer Connections ...............................................................161

Apply Vector Principles for Three-Phase Transformer Connections.................193

Build Three-Phase Banks to Standard .................................................................217

Apply Transformer Connections ..........................................................................225

Calculate Load Checks on Delta and Wye Banks................................................247

Describe Phasing and Paralleling Procedures for Three-Phase Circuits............257

Ferroresonance .......................................................................................................267

Appendices.............................................................................................................279

Transformation for Lineworkers

36

Describe Transformer Construction

Figure 4-1 illustrates a distribution transformer with the tank partly cut away toshow the internal equipment.

Figure 4-1. Distribution transformer construction.

Basically, a transformer consists of the primary and secondary coils, iron core,insulation, and the container or tank. Other features may be added, such as tapchanging mechanisms, internal secondary breakers, internal primary fuses, orlightning arrestors. The larger power transformers have many more features, suchas oil conservator tanks, oil-filled bushings, cooling tubes, cooling fans, and heatindicating equipment.

Primary and Secondary Coils

It is customary to use copper wire in the manufacture of transformer windings.Before the introduction of modern insulation materials, the copper conductorwas wrapped with a thin cotton cover which added significantly to the bulk of theoverall winding. Today’s transformer uses enamel insulated copper wire, andbecause the insulation is of a better grade and is much thinner than the cottoninsulation, improved electrical characteristics as well as compactness are obtained.

Round wire is used in some of the smaller transformers, while square orrectangular wire is normally used in medium-sized and larger-sized transformers.Square and rectangular wires form a more compact and solidly built coil, and alsoprovide better conductivity for the heat to flow out of the windings.

FLUIDIZED-BEDCOATING

ONE-PIECECOVER BAND

TAP CHANGER

SECONDARYLEADS

CORE AND COILASSEMBLY

HIGH-VOLTAGETERMINALS

HIGH-VOLTAGECOVER BUSHINGS

INSPECTIONPLATE

LOW-VOLTAGETERMINALS

LOW-VOLTAGEBUSHING

CORE CLAMPBRACKETS

Chapter 5: Transformer Information, Characteristics

57

Identify Internal and External Leads

For the connection of a distribution transformer to primary and secondary distri-bution circuits, it is important that:

• Each lead that emerges from the winding be correctly identified.

• Correct connection of each lead to the bushings match the externalconnection required.

The primary leads usually present no problem because the manufacturer connectsthe leads to the designated bushings at the time of assembly. Similarly, the highvoltage taps are permanently connected to the tap changing switch or terminalboard.

Connection of the secondary leads is a different matter. While the normalsecondary connection is a series connection of the two secondary coils, the samecoils may be placed in parallel operation in the following instances:

• Where the intention is to establish a 120/208 volt three-phase four-wire wye connected secondary using three transformers.

• For parallel-series operation of two transformers used to temporarilyreplace one larger transformer.

• In specific cases, where 120 volts two-wire single phase is required,using the total capacity of the transformer.

The secondary winding of a distribution transformer normally consists of twoseparate windings, and each of the two windings is designed to produce one-halfof the secondary output voltage. The two windings connected in series wouldproduce the total secondary output voltage.

Figure 5-6 illustrates the internal winding leads for additive and subtractive distri-bution transformers.

Figure 5-6. Internal winding leads for additive and subtractive distribution transformers.

The letters on the secondary leads indicate that A and B designate the leads of onecoil, while C and D designate the leads of the other coil. The center leads B and Ccross over at a point near the core and windings and emerge above the oil in thecrossed position.

A B D

H2

X1

C

H1

ADDITIVE

X2X3 X2 X3X1

D C A

H2

B

H1

SUBTRACTIVE

A B DC D C AB

Chapter 6: Transformer Operation

75

Transformers are tapped up one tap at a time if required, but only after thevoltage conditions have been checked on a 24 hour basis. Recording voltmetersand other instruments are used for this purpose.

On-load tap changers differ in design and purpose from off-load tap switches.These may be found on the large power transformers and on regulators. As thename implies, the on-load tap changer is designed to tap the winding up or down,as voltage conditions require, without interrupting the load circuit.

At some point in the transition from one tap to another, both taps must be joinedtogether; and as the two taps will be at different voltages with respect to the endof the transformer winding, a circulating current will flow in the (bridge) circuit.This circulating current is limited to a safe value by using either a center-tappedreactor or a preventative autotransformer in the bridge circuit.

Describe Transformer Winding Taps

Older double-bushing distribution transformers were usually built with threepercentage taps below full winding. Figure 6-2 illustrates an elementary primarywinding with tap percentages shown.

Figure 6-2. Elementary primary winding.

The taps are provided to permit changes in the voltage ratio. Changing thenumber of primary turns in use will change the ratio of the transformer, and inthis way we can either raise or lower the secondary voltage when the primary(applied) voltage is consistently high or low.

Examination of Figure 6-2 will show that placing the transformer on 4.5%, 9%,or 13.5% tap actually means we are cutting out this percentage of the primarywinding or, full winding minus 4.5%, full winding minus 9%, etc.

The following table (see Figure 6-3) shows what the primary voltage setting willneed to match a corresponding drop in primary line voltage and in order toproduce 120/240 volts at the secondary terminals of the transformer.

FULL WINDING

(F.W.) - 4 1/2%(F.W.) - 9%

(F.W.) - 13 1/2%

Chapter 9: Install Transformers to Company Standards

117

Self Test Answers

Note: Answers to the fill-in-the-blank questions are italicized.

1. All transformer tanks must be grounded during installation and before energization.

2. You would find the information that identifies secondary drop lead sizes in the

company standards manual. See also, Figure 9-1.

3. You would select a 2/0 AWG copper for a 50 kVA 120/240 volt transformer installation.

4. Pre-installation checks that you would make before installing a transformer include

checking

• Primary voltage rating

• Secondary voltage rating

• kVA capacity

• Internal connections to ensure that they are tight

• Loose or cracked bushings and terminals

• The dual voltage switch and placing it in the correct position for the installation

• The case ground to ensure it is intact

5. A CSP transformer means that it is completely self-protected. It is equipped with light-

ning arresters, a secondary breaker and internal primary fuse links.

6. When the red light of the CSP transformer is on, it means that that transformer is

slightly overloaded.

7. You would obtain information regarding correct fuse sizes for transformer installations

in the company standard fuse charts.

8. For a 50 kVA, 14,400 - 120/240 volt transformer installation, you would select a 5H

fuse.


118

9. Following are the differences between a conventional transformer and a CSP:

• A CSP transformer is completely self-protected and is equipped with lightning

arresters, a secondary breaker and internal primary fuse links.

• A conventional transformer is fused externally at a disconnect and is only protected

by lightning arresters when a utility chooses to do so because of a high frequency

lightning area.

10. Secondary leads are sized to at least 100% of currents shown for a given transformer size

and voltage that the drop lead is used for.

If you had any problems with this Self Test, please review the material and, if necessary, see

your sponsor before proceeding.

143

Energize TransformersSafely

Introduction

Before energizing a transformer, checks must be done to ensure that everything isin order. After energizing a transformer more checks must be done to ensure thatit is operating as required.

The topics covered in this chapter are:

• Inspect and troubleshoot

• Energize transformers safely

• Perform post-energization checks

CHAPTER 12


168

As discussed earlier, the line-to-line voltage is equal to the winding voltage of thetransformers. In this case, the secondary winding voltage is 240 volts, therefore,the line-to-line voltage will also be 240 volts. In a four-wire delta secondary, theneutral or mid point of one of the secondary windings is grounded so that single-phase 120/240 volt service can also be supplied from the same bank.

Voltages to ground are established as soon as this ground connection is made toone transformer. By referring to Figure 14-10 and 14-11 it is easy to establish thea - N voltage is 120 volts and the b - N voltage as 120 volts. What then, is thevoltage from c to neutral (c - N)? This phase is what is commonly referred to asthe high or wild leg, or bastard phase and its voltage to neutral will always be 1.73times the phase to neutral voltage of the grounded transformer. In this example,the c - N voltage is 1.73 × 120 = 208 volts.

This can be easily seen in a simple vector diagram of the connections. Figure 14-12 illustrates a vector diagram of the connection shown in Figure 14-10.

Figure 14-12. Vector diagram of the connections.

Note: The c - N is a voltage through a full transformer winding and ahalf. The full winding is 120° displaced from the half winding.

By referring to the vector representation of the secondary in Figure 14-12, itshould be easy to establish the following voltages:

a - b = 240 volts a - N = 120 volts

b - c = 240 volts b - N = 120 volts

a - c = 240 volts c - N = 208 volts

As stated before, the c - N voltage will be 1.73 times the phase to neutral voltageof the grounded transformer. In this case, the voltage c - N = 208 volts. If thesecondary diagram in Figure 14-12 is drawn to scale with each angle 60 degrees, ameasurement from c - N will also indicate 208 volts. Always remember that thephase which is not connected to the grounded transformer is the bastard phase orphase which will measure approximately 208 volts to neutral.

Common delta three phase service voltages are 240 volts and 480 volts.

ca

b

C PHASE IS THE FARTHESTDISPLACEMENT FROM THENEUTRAL POINT

ca

b

120 V

120 V

240 V

240 V

208 V


222

Self Test Answers

1. Standard three phase wye-wye 120/208V connection diagram using 3 x 75 kVA

120/240V transformers.

2. Standard three phase wye-delta 240V connection diagram using 3 x 75 kVA 120/240V

transformers.

H1 H2

X1X3X2

H1 H2

X1X3X2

H1 H2

X1X3X2

H1 H2

X1X4X3 X2

H1 H2

X1X4X3 X2

H1 H2

X1X4X3 X2

Chapter 20: Ferroresonance

269

Figure 20-2. AC circuit response curve.

Now consider the typical case of a three-phase pad or pole mounted floatingneutral wye/delta connected transformer installation, fed via an overhead tap orunderground cable run from pole mounted cutouts, switching kiosk or othersupply point (Figure 20-3).

Figure 20-3. Three-phase pad mounted installation.

Under switching conditions with one (or two) cutouts open and the other two (orone) cutouts closed, Figure 20-3 can be represented by the same circuit configura-tion as Figure 20-1, except that the capacitive reactance (XC) is provided by thoseconductors or cables that are not connected to the source, and the inductivereactance (XL) by the iron core primary winding of the energized transformers.

TO MAIN LINE(SOURCE)

POLE MOUNTED CUTOUTS,SWITCHING KIOSK, ETC.

2 CUTOUTSCLOSED

1 CUTOUTOPEN

2 CONDUCTORS OR CABLESENERGIZED AT LINE VOLTAGE

3 PHASE INSULATED (FLOATING)NEUTRAL

3 PHASE TRANSFORMER BANK(PADMOUNT OR OVERHEAD)

CAPACITIVE REACTANCE (XC) TO GROUND OF OPEN CONDUCTOR(S) OR CABLE(S) IS IN SERIES WITH INDUCTIVE REACTANCE (XI) OF TRANSFORMER(S)

I= ER

I= EX+R

I

XLXC = XLXL = 0

basic transformer

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