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substation solutions… to power your future 1


Welcome

to the

Theory of Operation

of

On-Load Tap Changers


You can find more than one type of tap changer on a transformer.

There is a:

DETC ~ De-energized Tap Changer. Often referred to as a

NLTC ~ No Load Tap Changer

LTC ~ Load Tap Changer. Often referred to as a

OLTC ~ On-Load Tap Changer

They both allow you to change the winding connections of the

transformer. The main difference is the way the change must take

Place. The DETC must be changed only with the transformer de-

energized and with out load. (NLTC - No Load Tap Changer). The

LTC is designed to be changed with the transformer energized

and under load. (OLTC – On Load Tap Changer). Please refer to the OEM information when hand cranking the LTC under load.

Tap Changers


Tap Changers

DETC ~ De-Energized Tap Changer

The DETC may be used to do the following:

•To match Transformer Primary to Transmission Line Voltage.

•Adjust the travel of the LTC (Load Tap Changer).

A Typical specification of a DETC is:

•Five Positions ~ two above nominal voltage and two below

nominal voltage and one at nominal voltage.

DETC’s are found in the main tank of the transformer. The taps

are changed by rotating the connecting shaft on the outside of the

transformer main tank designated by a hand wheel or crank

handle. Positions are listed as 1-2-3-4-5 or A-B-C-D-E. Never

change the DETC without engineering approval.


Tap Changers

DETC ~ De-Energized Tap Changer


What is the Purpose of a Load Tap Changer

A Load Tap Changer is a devise that is used to change the

taps of a power transformer with the transformer energized

and under load.

A Load Tap Changer (when working correctly) allows the

power output of the transformer to be changed with-out

interruption.

The purpose of the Load Tap Changer is to allow the voltage

out-put to be regulated up or down without interruption to

keep a set voltage available for distribution to customers.

General Information


How does the Load Tap Changer Work

The tap or regulating windings of a load tap changing

transformer are used to adjust the number of transformer turns

usually in the secondary or low voltage windings. The

regulating windings are divided in sections (taps) that can be

added in series to the low voltage windings. Voltage change

must be provided smoothly and efficiently without

interruption to the secondary current flow. In other words,

when changing tap positions, the LTC mechanism must

MAKE-BEFORE-BREAK to avoid opening the secondary

circuit thus dropping voltage to your customers.

General Information


The LTC oil compartment and the main transformer tank are

separated for many reasons. One reason is that you don’t want

Acetylene gas (combustibles) in the main transformer tank.

Remember, Acetylene is a by product of an arc. If the main tank

were to have Acetylene gas in the oil, you couldn’t determine if

you had a problem with the windings or connections in the main

tank or if the gas was a normal function of the LTC. Acetylene

gas in the oil is a normal state for some LTC’s.

Typical specification include:

•33 steps (1 step being nominal voltage)

•+/- 10% range (5/8% per step)

•Why do we change 5/8% per tap change?

•Rating at full capacity above nominal voltage and Rated current

(reduced capacity) below nominal voltage



+

-

+

-

+

-

Coils connected in series

and wound in the same

direction on a common

flux path add voltage

You can “BOOST”

voltage by adding turns

that are wound in the same

direction.

VoltageN1

N2VT

VT LV

RV

V = (VT) (N1) + (VT) (N2) “Boost Voltage”

LV RV

Simple Load Tap Changer Theory 101

Note:

N = Number of Turns

LV = Low Volt Windings

RV = Regulating Volt Windings

VT = Volts per turn


+

-

+

-

-

VoltageN1

N2VT

VT LV

RV

+

V = (VT) (N1) - (VT) (N2) “Buck Voltage”

LV RV

Coils connected in series

and wound in the

opposing direction on a

common flux path

subtract voltage

You can “BUCK” voltage

by adding turns that are

wound in the opposing

direction.

Simple Load Tap Changer Theory 101

Note:

N = Number of Turns

LV = Low Volt Windings

RV = Regulating Volt Windings

VT = Volts per turn



Reactance ~ A reactance LTC utilizes a reactive impedance to

limit the circulating current while in the bridging position. A

bridging position is a normal operating position of a reactance

LTC. Another name for this reactive impedance is a Reactor,

Preventive Autotransformer, or simply a “PA”. This type of LTC

was developed in the United States. They were designed for

higher currents and lower insulation levels due their use in low

voltage regulation. Vacuum interrupter’s were introduced to

reactance LTC’s in 1965.

Resistance ~ Resistive LTC’s are typically designed to arc

either at the selector switch or the transfer (diverter) and operate

at higher speeds. The resistive LTC utilizes a resistor to limit the

circulating current during a tap change. The bridging position is

NOT a normal stopping tap position for a resistive LTC.

There are two basic operating designs of LTC’s.


How does the Load Tap Changer WorkReactor, Preventive Autotransformer, or simply a “PA”

Reactance type Load Tap Changer

These are

the leads

that will

connect

to the

Load Tap

Changer


How does the Load Tap Changer WorkResistor, Resistive or Resistance type Load Tap Changer

These are the

resistors of

the Waukesha

UZD Load

Tap Changer.

They are

contained in

the LTC oil

filled

switching

compartment.


How does the Load Tap Changer WorkResistor, Resistive or Resistance type Load Tap Changer

This is a side picture

of the Waukesha

UZD Load Tap

Changer. During

maintenance, the

resistors should be

ohm’ed and all

hardware should be

checked for integrity


What are the Functions of the LTC Switches

The On-Load Tap Changer Operation can be divided in up to

three separate functions:

1. Selection of “raise” or “lower” (Boost or Buck) of the

winding taps by use of the reversing switch (if equipped).

2. Selection of voltage magnitude by use of the selector

switch

3. The arc interruption by use of the transfer switch (if

equipped).

The functions are limited to the devices (switches) in the On

Load Tap Changer design.


There are several different types of reactance load tap changers.

They can be divided into categories by the components or

switches included in their design.

Arcing on the selector switch

Arcing on the transfer / diverter switch

Arcing in a vacuum bottle.

Types of Load Tap Changers

Arcing Reactance


Reactance Load Tap Changers

Arcing on the Selector Switch

Load Tap Changers that arc on the Selector Switch must have special

contacts that have arcing tips where the moveable contacts make

contact with them. These tips are made of a material called Elkonite

or Copper Tungsten. This material is not a low resistant material.

Examples of these types of On-Load Tap Changers are listed below:

McGraw Edison 550 General Electric LR-38

McGraw Edison 550B General Electric LR-48

McGraw Edison 550C General Electric LR-68

Siemens/Allis TLH-10/20/21 General Electric LR-72

Siemens/Allis TLG Siemens/Allis TLF-20/30

Siemens Allis TLH-10 Siemens/Allis TLS


Arcing on the Selector Switch

When the Elkonite/Copper Tungsten Tip of the contact is worn out or

falls off, they must be replaced. If the contacts are not replaced, they

will not last long at all. The copper or silver plated copper contact is

not designed to sustain the arc during a tap change. Below are two

pictures that display what will happen if you don’t change the

contacts when you should:



Arcing on the Selector Switch Sequence of Operation


On Position

(non-bridging)

Selector Switch

Opens

Selector Switch

Closes

On Position

(bridging)

Load Current

Circulating Current


Arcing on the Transfer / Diverter switch

Some LTC’s are equipped with a transfer or diverter switch. These

switches are designed to sustain the arc of the make and break

operation while changing tap positions. The operation is as follows:

The Transfer/Diverter switch opens and breaks the current; the

selector switch opens with no arc; the selector switch moves to the

next tap position; the selector switch closes with no arc; and the

Transfer/Diverter switch closes and makes the arc. No arc should

occur on the selector switch. Examples of these types of On-Load

Tap Changers are listed below:

Westinghouse UTT Series Westinghouse UTS

General Electric LR-65 & 83 Wagner KRL 14

Federal Pacific TC-546 & 525 Federal Pioneer TC-15

Federal Pioneer TC-23 & 23-2 McGraw Edison 396



Arcing on the Transfer / Diverter switch

With the arc occurring on the diverter/transfer switch, the selector

switch contacts could now be made of a low resistance material to

reduce heat on the contact. Below are examples of Diverter/Transfer

switches.


Westinghouse UTT Series

General Electric LR-83

General Electric LR-65

Federal Pacific TC-546 Diverters

With TC-525 Upgraded

Tap Head, Selector Switch &

Reversing Switch


Arcing on the Transfer / Diverter Switch Sequence of Operation


On Position

(non-bridging)

Transfer Switch Opens Selector Switch Opens

Transfer Switch Closes

On Position (bridging)

Selector Switch Closes

Load Current

Circulating Current


Arcing in a Vacuum Bottle.

Vacuum Load Tap Changers were developed in the United States and

introduced in 1965. There are benefits of having the arc contained in

a vacuum bottle. With the arcing contacts contained in the vacuum

bottle, there is no arc to contaminate the oil. Vacuum LTC’s must

operate in a precisely timed sequence so that the selector switch

moveable contacts change positions (taps) only when no current is

flowing.



Arcing in a Vacuum Bottle.

With cleaner oil, the probability of filming is greatly reduced. With

the oil having no carbon particulates, there would be less mechanical

wear. Examples of these types of On-Load Tap Changers are listed

below:

General Electric LRT-200 Reinhausen RMV-A

General Electric LRT300/400/500/700 Reinhausen RMV-1

Federal Pioneer TCV-23 Reinhausen RMV-II

McGraw Edison V2 Westinghouse UVT

McGraw Edison 397D



Arcing in a Vacuum Bottle ~ Sequence of Operation


On Position (non-bridging) By-pass Switch Opens Vacuum Bottle Opens Selector Switch Opens

Vacuum Bottle Closes By-pass Switch Closes

On Position (bridging)

Selector Switch Closes

Load Current

Circulating Current


The resistance on-load tap

changer was developed in

Europe. A Resistance Load Tap

Changer utilizes a resistor to

limit the circulating current

during a tap changer operation.

These resistors are in the circuit

for a very short time. These

LTC’s must change tap positions

very rapidly. A typical time from

tap to tap is 60 milliseconds with

an arc limit time of 6

milliseconds.

Resistance Type LTC

Waukesha UZD


Resistance Type LTC

You should never see a

resistance load tap

changer stopped in the

position displayed in this

picture. This is a

“transition-bridging”

position. The transition-

bridging position must

occur to provide a

“Make-Before-Break”

operation. This

make-before-break

operation is a

requirement of all

on-load tap changers.


Resistance Type LTCTap Change Sequence of Operation

Transition Bridging

Position

On Position

1 2 3 4

765

On Position

Load Current

Circulating Current


Simple Transformer Theory 101



Typical Layered Windings

Core Steel


1

2

3

4

5

6

7

8

9

K

M

Regulated

Windings are

covered with

the Secondary

(low voltage)

Windings and

the Secondary

(low voltage)

Windings are

covered with

the Primary

(high voltage)

Windings.This is assuming

that there are NO

tertiary Windings.


Core Steel



+

-

Voltage VT = Volts Per Turn

Current

Magnetic Flux

Volts Per Turn = Voltage / Number of turns



Transformer Core

+

+

-

-N1 N2

VTVT V2V1

Note:

N = Number of Turns

I = Current

V = Voltage

VT = Volts per turn

FLUX

V1 = VT * N1

VT = (V1 / N1) = (V2 / N2)

V2 = VT * N2

Ratio = (V1 / V2) = (N1 / N2)


In the example below, you can see that the voltage on the

secondary side was reduced by 50% when the number of turns

around the core steel was reduced by 50%.

If the input voltage is 120 and the number of turns around the

core steel is 1000, what would the voltage be on the secondary

side with 200 turns around the core steel?

Answer__________

Primary

120 volts input

with 10 turns

Secondary

60 volts output

with 5 turns



Reversing Switch

Federal Pacific TC-525

Waukesha UZD

Siemens / Allis TLS Reinhausen RMV-II

McGraw Edison 550B


How does the Reversing Switch Work

A reversing switch (if equipped) located inside the LTC

mechanism, enables the windings to double the number of tap

positions without doubling the number of tap leads from the

tap (regulating) windings. All reversing switches do not

operate the same. There are basically two types of reversing

switches. The most common type is the “Standard Reversing

Switch”. The other type is the “Isolated Reversing Switch”:

also referred to as the “Coarse-Fine Pre-selector”. Both of

these types of reversing switched allow the same function as

noted in bold italic print above. The difference is how they

allow the function to take place. Some manufactures call a

reversing switch a “change-over-selector” switch.

Reversing Switch


The standard reversing switch swaps the polarity of the

regulating windings. This means that the end of the regulating

winding that is connected in series with the secondary

windings is changed when the reversing switch goes through

neutral. See the diagram below:

Standard Reversing Switch


N X1

N X1


The isolated reversing switch does not swaps the polarity of

the regulating windings. The function of the isolated

reversing switch is to add or remove a large section of the

transformer secondary windings. This is considered a “Coarse

Adjustment”. The fine adjustment is made by the selector

switch. This is why the isolated reversing switch is also called

a “Coarse Fine Pre-selector”

Isolated Reversing Switch

Isolated or Coarse/Fine

Coarse Adjustment

9

M

K

01

2

3

4

8

7

5

6

876543210

Preventive Auto

Neutral

Fine AdjustmentIsolated or Coarse/Fine

Coarse Adjustment

9

M

K

01

2

3

4

8

7

5

6

876543210

Preventive Auto

Neutral

Fine Adjustment



Regulated WindingsSecondary Windings

Primary Windings

Isolated Reversing Switch ~ Operation Above Neutral

Watch What

Happens


Isolated Reversing Switch ~ Operation Below Neutral


Primary Windings

Watch What

Happens


“Boosting Power” ~ Standard Reversing Switch

Going Above Neutral


Primary Windings

Watch What

Happens

Watch What

Happens


“Bucking Power” ~ Standard Reversing Switch

Going Below Neutral


Primary Windings

Watch What

Happens

Watch What

Happens


Series Transformer


Primary Windings

Series

Transformer

6:1 Ratio

1200 amps200 amps

Reversing Switch

Secondary WindingsRegulating Windings

Load 1200 amps

LTC Rating 600 amps

Supply

Transition

Resistors

Resistance Type LTC

Series Transformer

N X1


Heating is a problem in LTC’s as heat aids in the filming process.

Filming aids in coking and coking causes failure. Below are pictures

of coking on contacts. If you have coking on contacts, what do you

do? Check for the heat source. It could be from contacts that are

worn and need replacing; unbalanced spring pressure; too little spring

pressure; a weak contact point; a loose connection or other issues that

could cause a hot spot or elevated heating in the LTC oil

compartment.

Coking


0

10

20

30

40

50

60

70

80

90

100

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

1400

1500

Temperature oC

Rela

tive A

mo

un

t

Ethane Ethylene

Methane Acetylene

Hydrogen

PropylenePropane

Decomposition of N-octane

Dissolved Gas Analysis

Different gases are produced at different temperatures


Mild Moderate Severe

400-1,500 1,500-20,000 >20,000

600-2,400 2,400-24,000 >24,000

1,500-7,500 7,500-75,000 >75000

<25 >25 >50

Dessicant Breather

Sealed Tank

Vaccum LTC

Problem Severity*

Free Breathing

Tank Design

Gas Levels Of Combined Readings Of Methane, Ethane, and Ethylene in Parts Per Million

Interpretation Of DGA Readings


Learning the

TERMSfor LTC’s

LTC Terms


LTC TermsReactor

The reactor may also be referred to as a "PA, preventive auto or

preventive autotransformer". The reactor serves as a current limiting

device when the LTC is sitting on a bridging (odd number) tap

position or passing through a bridging (odd number) tap position.

Bypass Switch

The bypass switch is used to shunt current away from the

selector switch to a vacuum interrupter so the selector switch

can switch taps without arcing. Bypass switches are almost always found on vacuum LTC's only.

Preventive Autotransformer (PA or Preventive Auto)

The preventive autotransformer may also be referred to as a

"reactor". The preventive autotransformer serves as a current

limiting device when the LTC is sitting on a bridging (odd

number) tap position or passing through a bridging (odd number) tap position


LTC TermsSpring Drive

The spring drive may be either a single spring or a bank of

springs that are utilized to change from one tap to another. A

hand crank or a motor charges the spring battery. When the

spring drive is fully charged, the mechanism then releases

the energy contained in the spring drive to allow the change of tap positions.Full Cycle Position

A Non-bridging position in an LTC. Both or all moveable

selector contacts of the selector switch are on the same

stationary contact. Half Cycle Position

A Bridging position in an LTC. The moveable selector

contacts of the selector switch are on separate (different)

stationary contacts.


Selector Switch

The selector switch is used to determine the tap voltage

(select the voltage magnitude) the transformer operates at

and supplies to the load.

LTC Terms

Reversing Switch (change over switch)

There are two different types of reversing switches, standard

and isolated. Both types of the reversing switch allow the

number of tap positions to be doubled without doubling the

number of tap leads coming out of the windings. While the

standard reversing switch swaps the polarity of the tap

windings, the isolated reversing switch changes a large

section of the secondary windings in or out of the circuit.


LTC Terms

Connection Diagram of a Typical Load Tap Changer with a

Standard Reversing Switch Used To Swap Polarity (Without a

Series Transformer)

Standard Reversing Switch

The standard reversing switch is used to swap the polarity of the tap

winding in order to raise (boost) or lower (buck) voltage.


LTC TermsIsolated Reversing Switch

The isolated reversing switch is used to swap a section of the

secondary windings in or out of the circuit. This design is always

boosting and never bucking voltage. Polarity of the tap windings is

NOT swapped.

Connection Diagram of Typical Load Tap Changer With Isolated

Reversing Switch Used To “Boost” Output Voltage. “Bucking” is

NOT a characteristic of this tap changer design.


LTC Terms

Reactance (Reactive)

Reactive LTC’s are typically designed to arc either on the selector

switch, the transfer (diverter) switch, or the vacuum interrupter.

They typically operate at relatively slow speeds (1 - 3 seconds). The

reactive LTC utilizes a reactor or preventive autotransformer to limit

the circulating current during a tap change and or when in the

bridging position. The bridging position is a normal stopping tap

position for a reactive LTC.

A vacuum tap changer is a reactive type LTC.

Two Basic Types of LTC’s


LTC Terms

Resistance (Resistor)

Resistive LTC’s are typically designed to arc either at the selector

switch or the transfer (diverter) and operate at higher speeds (.3 - .7

seconds). The resistive LTC utilizes a resistor to limit the

circulating current during a tap change. The bridging position is

NOT a normal stopping tap position for a resistive LTC.

Two Basic Types of LTC’s


Transfer Switch

The transfer switch may also be referred to as the “Diverter

Switch”. The transfer switch is used to interrupt the load so the

selector switch can change position without arcing. The transfer

switch contacts are made of an arc erosion resistant material called

Elkonite (copper-tungsten) and is designed to arc (not heat) under

normal conditions

LTC Terms

Transition Resistor

A resistor that is used to limit the circulating current during

transition only from one tap position to another. The transition

resistor is only utilized in transition. The transition resistor is not

designed to carry full load current for any long period of time. The

Transition resistor is found on a resistance type LTC.


LTC TermsStationary Contact

A contact that is fixed in position. (Stationary Selector)

Moving Contact

A contact that is not fixed in position. A contact that is allowed to

change positions. (Moving Selector)

Reversing Contact

This contact is associated with the reversing switch of an LTC

mechanism. (See Stationary Contact and Moving Contact for definition of term).

Neutral Position

The neutral position is the position where the LTC is neither

bucking nor boosting voltage and / or where the tap windings are

not in the circuit. This is nominal position.


Questions?

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