Load Tap Changers for Power Transformers

Dr. Dieter Dohnal / Maschinenfabrik Reinhausen GmbH

Dipl. Ing. Bernhard Kurth / Reinhausen Manufacturing Inc.

11/24/2003 Page 2

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

• Today’s Expectations from LTCs

• The Vacuum Switching Technology of Today

• Summary of Advantages of the Vacuum Switching Technology

• Life-Cycle Cost Considerations

11/24/2003 Page 3

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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Commonly Used Winding ConnectionsLTC 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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Commonly Used Winding Connections LTC Application in Delta-Connected Transformer Windings

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

Commonly Used Winding ConnectionsLTC Application in Autotransformers

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

Commonly Used Winding ConnectionsLTC Application in Autotransformers

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Commonly Used Winding ConnectionsLTC Application in Autotransformers with a Series Transformer

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

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Commonly Used Winding ConnectionsLTC Application in Phase-Shifting Transformers

Example for single core design / direct circuit arrangement

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Commonly Used Winding ConnectionsLTC 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 circuit

I: through current

arcing I

Basic Switching Principle of LTCsTransfer 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

reactor principle impedance = reactor (pa)

(pa=preventive auto transformer)

resistor principle 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)

(*) discontinued

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Design Principles of Resistor Oil Type LTCsDiverter 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 LTCsSelector 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 LTCsTap 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 availability of the regulating transformer

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 LTCsOne 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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The Vacuum Switching Technology in LTCs 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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The Vacuum Switching Technology in LTCs 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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The Vacuum Switching Technology in LTCs Highlights

constant switching characteristics throughout the entire lifespan of the vacuum 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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The Vacuum Switching Technology in LTCs Highlights

Ensures short arcing times (less than one half-cycle)

Extraordinarily fast dielectric recovery of the quenching distance of up to 10 kV/µs

even in the case of large phase angles between current and voltage

high voltage steepness dU/dt after the current zero e.g. converter transformers

or

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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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Reinhausen Vacuum Interrupters for LTCs Permanent Improvement in Research and Development

Vacuum interrupter for 1320A, introduced in 2000 (VACUTAP® LTC type RMV-A)

Example for Application in Reactor Type LTCs

Vacuum interrupter for 2500A, introduced in the early 70’s (VACUTAP®

LTC type UVT)

Vacuum interrupter for 2500A, introduced in 1993 (VACUTAP® LTC type RMV-II)

180 mm

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Reinhausen Vacuum Interrupters for LTCs 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 10times less than for copper-tungsten contacts

Message

Vacuum interrupters specifically designed for LTCseasily reach 500,000 switching operations

Example for 1000 A shows only 3% of wear 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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Reinhausen Experience with the Vacuum Switching Technology in LTCs

Application in resistor type tap changers for external mountingfor 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 vacuum interrupters

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Reinhausen Experience with the Vacuum Switching Technology in LTCs

Application in resistor type tap-changers for in-tank installation for the regulation of oil-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 TechnologyLatest Development: Resistor VACUTAP® LTC Type VR

Switching principle

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Reinhausen Vacuum Switching TechnologyThe Next Step

Series production and delivery start: beginning of 2004

Introduction of the vacuum switching technology in all resistor type diverter switches in Reinhausen‘s 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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Reinhausen Vacuum Switching TechnologyDevelopment of Market Share

Vacuum Type LTCs versus Oil Type LTCsin percentage of units

1987

< 1 % Vacuum Type

Oil Type

2001

17 % Vacuum Type

Oil Type

2005

50 % Vacuum Type

Oil Type

2010

75 % Vacuum Type

Oil Type

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Advantages of the RMV Vacuum Type LTCSummary

Vacuum Interrupter is a hermetically sealed systemArc 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 mediumShort arc time

Reduced contact wear (<10% of wear found in oil)

No aging of the quenching mediumConstant or improving switching characteristics (gas molecules absorption by getter

effect = the more it switches the better the vacuum interrupter becomes!!)

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Advantages of the RMV Vacuum Type LTCSummary (Cont.)

Substantial Reduction of Operating CostsElimination 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 for contact wear, together with 18 moving contacts. 500,000 operations can only be achieved if the customer uses entire operating range uniformly, 16L…N…16R…N…16L…N…16R…

RMV-II: 1,000,000 operations vacuum interrupter life

RMV-A: 750,000 operations vacuum interrupter life

Reduction of maintenance costs (contacts, oil, labor)

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Advantages of the RMV Vacuum Type LTCSummary (Cont.)

Substantial Reduction of Operating Costs (Cont.)

Reduction of transformer down-time for maintenance

Extension of maintenance interval

Reinhausen Vacuum LTCs: the 1st LTCs without time-based maintenance intervals!!!

Type V V: maintenance free through 150,000 operations

Type V R: maintenance free through 150,000 operations

RMVs with with new “ VIM Model BVIM Model BVIM Model B ”:

Type RMV-II: maintenance free through 500,000 operations

Type RMV-A: maintenance free through 250,000 operationsVacuum Interrupter Monitoring System (VIM System) model A supplied standard with

every RMV

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Vacuum TypeMaintenance

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 LTCsLife-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 logisticsprotection of environment and natural resources due to reduction of oil changes

Considerably reduced operating costs

Star-pointapplication

6-7 x maintenance

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At the present time and for the foreseeable future the Vacuum Switching Technology in LTCspresents the best formula to achieve the highest standards of

towards a maintenance-free LTC design

Vacuum Switching Technology in LTCs Conclusions

Quality

Reliability

Economy