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Load Tap Changers

for Power Transformers

Dr. Dieter Dohnal / Maschinenfabrik Reinhausen GmbH

Dipl. Ing. Bernhard Kurth / Reinhausen Manufacturing Inc.


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Load Tap Changers for Power Transformers

Content

Basic Principle of the Load Tap Changer (LTC) / General LTC Arrangements

Commonly Used Winding Connections

Basic Arrangements of Regulating (Tap) Windings

Basic Switching Principles of LTCs

Design Principles of Oil Type LTCs

Design Principles of Vacuum Type LTCs

Todays Expectations from LTCs

The Vacuum Switching Technology of Today

Summary of Advantages of the Vacuum Switching Technology

Life-Cycle Cost Considerations


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Basic Principle of LTCs

LTCs allow to change the ratio of a transformer by adding turns to or

subtracting turns from either the primary or the secondary winding

LTC

For this purpose, the transformer is equipped with a regulating or tap

winding , which is connected to the LTC terminals

Principle winding arrangement of a

regulating transformer in Y-connection


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General LTC Arrangements

LTCs divided in 2 groups:


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LTC Application at the Neutral End of Y-Connected

Transformer Windings

Three-phase star point LTC

(LTC operating voltage in generalonly 10% of the line voltage)


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LTC Application in Delta-Connected Transformer Windings


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Examples for basic arrangements of the tap winding for regulation of U2


LTC Application in Autotransformers


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Examples for basic arrangements of the tap winding for regulation of U1


LTC Application in Autotransformers


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LTC Application in Autotransformers with a Series Transformer

Examples for basic arrangements of the tap winding for regulation of the U2


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LTC Application in Phase-Shifting Transformers

Example for s ingle core design / direct circuit arrangement


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LTC Application in Phase-Shifting Transformers

connection diagram phasor diagram

series unit

exciting unit

Example for dual core design / symmetrical circuit


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Basic Arrangements of Regulating (Tap) Windings


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Single Contact Switching

Not applicable due to the loss of the

system load / short circui t

I: through current

arcingI

Basic Switching Principle of LTCs

Transfer of Load Current

Make Before Break Switching

Applicable but requires an impedance to

limit the circulating current that appears

in the bridging or transition positions

reactorprinciple

impedance = reactor (pa)(pa=preventive auto transformer)

resistorprinciple

impedance = resistor

IC

IC

I

I


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Design Principles of Oil Type LTCs

Resistor Oil Type LTCs

Arcing Switch (Diverter Switch) and Tap Selector

(Reinhausen: M, MS, R)

Arcing Tap Switch (Selector Switch)

WES/ABB: UZD, UZE, Reinhausen: V; combine selecting and switching

functions into one device

Reactor Oil Type LTCs

Arcing Switch (Transfer Switch) and Tap Selector (*)

Arcing Tap Switch (Selector Switch) (*)

(Combines selecting and switching functions into one device)

(*) discont inued


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Design Principles of Resistor Oil Type LTCs

Diverter Switch (Arcing Switch) with Tap Selector Design

tap selector

diverter switch

switching principle design


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switching principle example for in-tank design

Design Principles of Resistor Oil Type LTCs

Selector Switch (Arcing Tap Switch) Design


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Design Principles of Vacuum Type LTCs

Resistor Vacuum Type LTCs

Vacuum Diverter Switch and Tap Selector

Reinhausen plans start of series production for 1st

Quarter 2004

Tap Selector with integrated Vacuum Interrupters

Type V V introduced by Reinhausen in 2000

Reactor Vacuum Type LTCs

Vacuum Interrupter Mechanism and Tap Selector

(LRT200)

By-pass Switch, Vacuum Interrupter Mechanism and Tap Selector

Reinhausen RMV, Westinghouse UVT


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Design Principles of Reactor Vacuum Type LTCs

Tap selector, By-pass and Vacuum Interrupter (VI) Design

Switching principle RMV-II

V I

tap selector

by-pass

___reactor___


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Today's Performance Expectations from LTCs

Low failure rate

Long-term uninterrupted availabili ty of the regulatingtransformer

Extension of the maintenance intervals

Reduction of the maintenance work

Main target

Reduced operating costs

The Available Tool

The Vacuum Interrupter


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The Vacuum Switching Technology in LTCs

One Solution for the next Generation of LTCs

Historically:

Rapidly wearing tungsten-copper

arcing contacts in mineral

transformer oil

Today and Tomorrow:

Primarily Vacuum

Interrupters

replaced by


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Highlights

arc has no interaction with the surrounding medium

The vacuum interrupter is a hermetically sealed system

low energy consumption (~ I x Uarc x t)

The arc (drop) voltage in vacuum (5-10V) is considerably lower than in oil

(25-30V) or SF6

the switching characteristics do not depend on the surrounding medium

reduced contact wear


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Highlights

elimination of by-products e.g. carbon when using transformer oil

Elimination of the insulating medium as the arc quenching agent

easy disposal of used insulating medium

Eliminates need of an on-line filtration system


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Highlights

constant switching characteristics throughout the entire lifespan of thevacuum interrupters

No aging of the quenching medium

No interaction/oxidation during switching

high rate of re-condensation of metal-vapor on contacts

getter effect of metal-vapor plasma with absorption of free gas molecules

significantly extends contact life

consistently low contact resistance


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Reinhausen Vacuum Interrupters for LTCs

Key Design Parameters

Mechanical lifespan

in transformer oil

any other given insulating medium

operating temperature range

expected lifespan of the LTC

Switching performance

(current, step voltage)

Contact lifespan

Physical dimension


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Permanent Improvement in Research and Development

Vacuum interrupter for

1320A, introduced in 2000

(VACUTAP LTC type

RMV-A)

Example for Appl ication in Reactor Type LTCs


2500A, introduced in the

early 70s (VACUTAP

LTC type UVT)


2500A, introduced in 1993

(VACUTAP LTC type

RMV-II)

180 mm


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Comparison of Contact Wear

Vacuum interrupter: Low arc drop voltage and re-condensation of metal-

vapor plasma

The contact wear rate for LTC vacuum interrupters is at least 10

times less than for copper-tungsten contacts

Message

Vacuum interrupters

specifically designed for LTCs

easily reach 500,000

switching operations

Example for 1000 A shows only 3% ofwear found in copper-tungsten contacts


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Reinhausen Experience with the Vacuum Switching

Technology in LTCs

Application in reactor type tap-changers (UVT, RMV-I, RMV-II and RMV-A)

Reactor VACUTAP LTC

type RMV-II for 1,500A,

2,000A and 2,500A

Reactor VACUTAP LTC type

RMV-A for 600A and 1,320A

since the mid-eighties

approx. 4,000 LTCs built with 12,000 vacuum interrupters more than 51,000 years of trouble-free field operation


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Technology in LTCs

Application in resistor type tap changers for external mounting

for the regulation of dry type transformers

Resistor VACUTAP LTC

type VT for 500A

Resistor VACUTAP LTC

type AVT for 150A

since 1995 approx. 640 LTCs built with 2,100 vacuuminterrupters


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Technology in LTCs

Application in resistor type tap-changers for in-tank installation

for the regulation of oi l-filled power transformers

Resistor VACUTAP LTC type

VV for 250A, 400A and 600A

since 2000 approx. 800 LTCs built with 4,800 vacuum interrupters


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Resistor type

In-tank design

Max. rated through current

Max. rated step voltage: 4,000 V

Max. BIL level to ground: 1,175 kV

3-phase/2-phase: 1,300 A

1-phase: 3,000 A

Reinhausen Vacuum Switching Technology

Latest Development: Resistor VACUTAP LTC Type VR

Switching principle


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Reinhausen Vacuum Switching Technology

The Next Step

Series product ion and delivery start: beginning of 2004

Introduction of the vacuum switching technology in all resistor type

diverter switches in Reinhausens main product segments

Vacuum type LTCs to replace Oil type LTCs in the range of 600A to3,000 A

Vacuum type LTCs as an alternative to Oil type LTCs in the range up to

600A


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Advantages of the RMV Vacuum Type LTC

Summary

Vacuum Interrupter is a hermetically sealed system

Arc does not interact with the surrounding medium

Carbon by-product eliminated when working in insulating oil

Reduction of mechanical wear of mechanism

Eliminates need for on-line filtration equipment

Allows use of alternative insulating fluids

Vacuum Interrupter has a fast dielectric recovery of the quenching medium

Short arc time

Reduced contact wear (


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Advantages of the RMV Vacuum Type LTC

Summary (Cont.)

Substantial Reduction of Operating Costs

Elimination of on-line filter (acquisition, operating and maintenance costs)

Fewer oil replacements (oil can be re-used several times), easier oil disposal

Contact replacements basically eliminated for network applications

Easier contact wear evaluation and maintenance procedures

Note: UZ type LTCs have only 30,000 operations contact life per stationary contact.

There are 17 stationary contacts per phase (51 total) that need to be evaluated forcontact wear, together with 18 moving contacts. 500,000 operations can only be

achieved if the customer uses entire operating range uniformly,

16LN16RN16LN16R

RMV-II: 1,000,000 operations vacuum interrupter l ife

RMV-A: 750,000 operations vacuum interrupter li fe

Reduction of maintenance costs (contacts, oil, labor)


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V S it hi T h l i R i h LTC


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Vacuum Type

Maintenance

FREE

10 x

maintenance

Deltaapplication

Oil Type

Maintenance requirements during LTC

lifespan for a typical network application

Significant savings in maintenance costs

Vacuum Switching Technology in Reinhausen LTCs

Life-Cycle-Cost Considerations

Maintenance-free through 150,000 operations with type V V and V R LTCs

Maintenance-free through 250,000/500,000 operations with type RMV LTCs

Example:

lifespan of 40 years

< 150,000 operations

Benefits

Oil Type

no time-based maintenance

longer availability of the transformer

simplified maintenance logistics

protection of environment and naturalresources due to reduction of oil

changes

Considerably reduced operating costs

Star-pointapplication

6-7 xmaintenance

V S it hi T h l i LTC


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At the present time and for the

foreseeable future the Vacuum

Switching Technology in LTCs

presents the best formula toachieve the highest standards of

towards a maintenance-free LTC design

Vacuum Switching Technology in LTCs

Conclusions

Quality

Reliability

Economy

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