Design, Function and Operation of On-Load Tap-Changers

TRANSFORM Partner

Uwe Seltsam

Maschinenfabrik Reinhausen GmbH

Design, Function and Operation of On-Load Tap-Changers

Uwe Seltsam

Maschinenfabrik Reinhausen GmbH, Regensburg / Germany

– Manager Technical Specialists –

Scientific Background

Dipl.-Ing. (FH) Electrical Engineering

Professional Background

10 years at Maschinenfabrik Reinhausen GmbH

Technical Service / Troubleshooting

07.06.2013 Uwe Seltsam, Maschinenfabrik Reinhausen GmbH

About the Author

1. On-load tap-changer in regulating transformer

2. On-load tap-changer – Design types

3. Switching sequence of tap selector and diverter switch insert

4. Maintenance

Intervals for OILTAP® and VACUTAP®

Parameter contact wear

Parameter replacement of parts

Parameter oil carbonization

Checks after maintenance

Checks between maintenances

5. Oil change criteria

6. Influence of humidity on dielectric strength

7. Avoiding carbon deposits or their effects

8. On-load tap-changer – Protective devices

9. Diagnostics


Agenda


1. On-load tap-changer in regulating transformer

On-load tap-changers (OLTCs) are used for changing the transmission ratio under load

U1 =

N1

U2 N2

Transformer: 40 MVA, / 21 kV

OLTC: M III 350 Y – 123/C – 10 19 1G

Step voltage: 17.6 kV : 9 steps ≙ 1.955 kV/step ≙ 1.78 %

Nominal: U2 = 21 kV = constant

Example 1: U1 = 110 kV

N1 = 110 kV • 129 wdgs / 21 kV = 675 wdgs

U2 = 110 kV • 129 wdgs / 675 wdgs = 21.02 kV

Example 2: U1 = 116 kV

N1 = 116 kV • 129 wdgs / 21 kV = 711 wdgs

U2 = 116 kV • 129 wdgs / 711 wdgs = 21.05 kV

21 kV

129 wdgs

U1

Mwg = Main winding

Cwg = Coarse winding

Rwg = Regulating winding

LU = Diverter switch insert

U2

1

110 kV

Rwg

108 wdgs

12 wdgs/step

Cwg

120 wdgs

Mwg

555 wdgs

2

9

k

LU

-

+

%16

6.17110 kVkV

Uwe Seltsam, Maschinenfabrik Reinhausen GmbH


OILTAP® Type M since 1972

e. g. M III 600 9 steps

3-pole

Star connection

Insulation to ground

Um = 123 kV

A,B

Main contacts

(only for V III 350 Y/Δ,

V III 500 Y / Δ,

V I 350, V I 700)

a,b

Main switching

contacts

a1, b1 Transition contacts

Ra, Rb Transition resistors

Ua, Ub Voltages of tappings

Ust Step voltage

U, I

Voltage, current of tap-

changer terminals

Tap-change operation Vectorial diagram Contact movement

07.06.2013



OILTAP® Type V since 1979

e. g. V III 200 10 steps

3-pole

Star connection


Um = 76 kV

A,B

Main contacts

(only V III 350 Y/Δ,

V III 500 Y/ Δ,

V I 350, V I 700)

a,b

Main switching

conttacts

a1, b1 Transition contacts

R

Transition resistor

Ua, Ub Voltages of tappings

Ust Step voltage

U, I

Voltage, current of

tap-changer terminals

Operation Vectorial diagram Contact movement



VACUTAP® Type VR® since 2004

e. g. VRF III 1300 10 steps

3-pole

Star connection


Um = 76 kV

Switching sequence

Switching direction n n+1 MSV Main switching contact

(vacuum interrupter), main path

MTF Transfer contact, main path

TTV Main switching contact (vacuum

interrupter), transition path

TTF Transfer contact, transition path

MCA Main contact side A

MCB Main contact side B

ZnO ZNO varistor

R Transition resistor



VACUTAP® Type VV since 2000

e. g. VV III 400 10 steps

MTS Tap-selector contact, main path

MSV Main switching contact (vacuum interrupter), main path

TTS Tap-selector contact, transition path

TTV Transition contact (vacuum interrupter), transition path

STC Sliding take-off contact

R Transition resistor

IC Circulating current


3. Switching sequence of tap selector and diverter switch

Tap pre-selection without current Diverter switch insert operation type M

IB

IB = Operating through-current

IK = Circulating current

R = Transition resistor

Switching principle of diverter switch insert: Flag switching cycle

Operation switching time approx. 40 – 50 ms

Load period of transition resistors approx. 30 ms

Link to movie

http://www.youtube.com/watch?v=Xwt3nYsVEbQ

Neutral-point application

after every 5 – 7 years

after every 50,000 – 150,000 operations

Delta application

after every 2 – 4 years

after every 20,000 – 100,000 operations

Maintenance of tap selector

after every 1 to 1.2 million operations


4.1 Maintenance – Intervals for OILTAP® and VACUTAP®

OILTAP® - Regular maintenance, depending on

time of service or no. of operations

(whichever comes first)

For OLTC-specific maintenance intervals, refer to the respective

operating instructions or the label on the motor-drive unit.

Diverter switch insert type M

Service life 800,000 operations

Neutral-point or phase application

after every 300,000 operations

Tap selector maintenance

after every 1.2 million operations

Reason for time-independent maintenance:

No oil carbonization since arcs do not occur in insulating oil



VACUTAP® - Regular maintenance, only depending on

the number of operations

Diverter switch insert type VR®

Service life 1.2 million operations

For network applications, the VACUTAP® is more or less

maintenance-free.

Example:

Switching frequency 5,000 operations per year

maintenance after 60 years.



Parts subject to wear

Oil change and cleaning

Contact wear

Visual presentation

Present condition 0 ... 100 %

Prediction for 100 % moment

Semaphore function green/yellow/red

Maintenance time

Maintenance criterion

Monitoring – TM100, Tapguard® 260

Maintenance depending on condition

Calculation

No. of ops. specification for parts change

Calculation algorithm for oil carbonization

Calculation algorithm for single and difference wear

Red signal prevents additional operations

by the motor-drive unit, but it is not

integrated into the tripping circuit for the

circuit breaker.

Front panel Tapguard® 260

Criteria for OILTAP®:

Single wear and

Difference in wear between switching

and transition contact

Parameters for contact wear:

Number of operations: variable value

Arcing current: variable value

Medium arc duration: approx. 6 ms

Arc voltage: 25 – 30 V

Contact material: Cu or CuW sintered material

Wear on main switching contact: proportional to operating through-current

Wear on resistor contact: independent of operating current, but proportional to

circulating current Ic (Ic = Ust / Rres)


4.2 Maintenance – Parameter contact wear

Contact system of diverter switch type M

MSCA

TCA TCB

MSCB

MC = Main contact

MSC = Main switching contact

TC = Transition contact

R = Transition resistor

SG = Spark gap

Max. permissible single wear = 4 mm

Max. permissible difference in wear = 2.5 mm

MCA MSCA TCA TCB SCB MCB

SG

Step voltage

MCA TCA TCB MSCB

MCA MCB

Exceeding the wear differences influences the switching

sequence – a short circuit between taps may occur.

Ust

Depending on no. of ops.

Springs, leads, etc.

Depending on wear

Arc contacts, etc.

Preventively

Cover gasket, etc.

Up-dating

Measure for increasing operating reliability

Adjustment to current state of manufacture

Diverter switch insert:

Replacement after 800,000 operations (OILTAP®)

Replacement after 1.2 million ops. (VACUTAP®)

Tap selector:

Maintenance after every 1.0 to 1.2 million operations,

depending on OLTC type


4.3 Maintenance – Parameter replacement of parts

E.g. Energy accumulator spring for type

M: 400,000 – 500,000 ops.

E.g. Braided lead for type M:

250,000 operations

MR original parts are the basis for safe operation.

Criterion:

Heat removal from the transition resistors must

not be restricted by oil carbonization.

Determining factors for degree of carbonization:

Number of operations

Arc energy

Type of oil: Little difference between naphthenic-base

or paraffin-base oil


4.4 Maintenance – Parameters oil carbonization

Diverter switch type insert T

before cleaning Diverter switch type insert T

after cleaning

Drying of a sooted tap-changer leads to baking of

carbon deposits on the surfaces and to a reduction of

the dielectric strength.

Use only transformer oil for cleaning. Electrical cleaners

evaporate on the surface of the insulating material and

lead to humidity.

The following checks have to be performed:

Identical indication of positions between on-load tap-changer and motor-drive unit

Functional check of final positions

If protective relay is in position “OFF“, it must not be possible to put the circuit breaker in

the circuit

Transformer ratio test during major modification work on the on-load tap-changer (e. g.,

replacement of motor-drive unit, or after connecting or disconnecting tap-selector leads)


4.5 Maintenance – Checks after maintenance

1) Diverter switch with oil compartment

2) Motor-drive unit

3) Protective relay

4) Drive shafts


4.6 Maintenance – Checks between maintenances

Motor-drive unit Visual check

Heater in operation

Door closed

Protective relay Functional check

together with transformer

protection test

Drying agent OLTC

oil conservator

Reliability (effectiveness)

together with transformer test

Oil monitoring Oil exchange criteria, see section 5

OLTC maintenance before re-commissioning of transformer if

operating time and downtime exceed maintenance interval of

on-load tap-changer (OILTAP®)

Disconnected

transformer

*Dielectric strength, measured in accordance with IEC 60156

The temperature of the oil sample must be measured at the time when the oil was taken.

The values specified for dielectric strength apply to oil temperatures of 20 ± 5 °C at the time of

evaluating the oil sample.

Recommendation:

Dielectric strength of > 50 kV / 2.5 mm for new insulating oil that must be filled in after maintenance.



OLTC application of OILTAP® Dielectric strength* Water content

Neutral-point application Minimum 30 kV / 2.5 mm Maximum 40 ppm

Use in auto-transformer or

regulation in delta winding

Minimum 40 kV / 2.5 mm Maximum 30 ppm

OLTC application of VACUTAP®

All Minimum 30 kV / 2.5 mm Maximum 30 ppm

Time of oil sample test:

It is up to the user of the transformer to check the insulating oil, and the test should be

done at the same time when an oil sample of the transformer is taken

Class 1 in accordance with IEC 60214-1: after every 5 years (use in neutral-point)

Class 2 in accordance with IEC 60214-1: after every 2 years (use in auto-transformer,…)

Whenever it is suspected that humidity is absorbed (drying agent ineffective etc.)



Ask the OLTC manufacturer if other insulating liquids (ester liquids, etc.) are used.

Facts:

Diverter switch oil of OILTAP® is a mixture of oil,

carbon deposits and humidity

Water absorption of carbon deposits is several times

as high as water absorption of oil

Carbonized oil with inadmissibly high water content puts

insulation distances at risk which are exposed to voltage

stress

Effects:

Electrical-field distortion with local discharges

Thermal decomposition of surface of insulating material

Creepage paths

Breakdown of insulating distance with flashover


6. Influence of humidity on dielectric strength

Switching shaft

Example of creepage paths

Vacuum technology in the VACUTAP®

No carbon deposits since arcs do not occur in insulating oil

Oil filter unit, filter cartridge

with paper filter: absorbs carbon particles (6 – 9 microns);

in use for annual number of > 15,000 operations / year

with paper filter and drying agent (4 kg granulate):

absorbs carbon particles and water (approx. 420 g)


7. Avoiding carbon deposits or their effects

The effectiveness of the drying agent of the filter cartridge must be

determined by oil sample and water content analysis.

Vacuum interrupter

Oil filter unit – OF100

Protective relay = flow-controlled relay type RS2001

(standard equipment)

Response value, e. g. for M III 600: 1.2 m/sec ± 20% bei 20°C

Pressure relief (burst membrane) in OLTC cover

Static response pressure about 4 – 5 bar

Pressure relief valve type MPreC® (instead of burst membrane)

Response value about 1.3 – 1.8 bar, depending on OLTC type

Monitoring of drive shafts, e. g., for multiple-column OLTC design

End position limiting device in OLTC and/or motor-drive unit


8. On-load tap-changer – Protective devices

The above-mentioned protective devices must be integrated in the

tripping circuit of the circuit breaker of the transformer.

Find out the cause after a protective device has responded!

RS2001

Burst membrane

MPreC®

Increased tap selector contact resistance

if contacts are not silver-plated:

Signs of impurity layer

Resistance measurement in accordance with

CIGRE report 12-205/1994 rounds off

gas-in-oil analysis;

Tap-selector contacts colored to “black-grey“:

Signs of silver-sulfide (impurity layer)

Check for corrosive sulfur (oil, etc.)


9. Diagnostics

Tap selector contact with silver-sulfide

Extract from CIGRE report

Separated silver-sulfide layer may lead to flashovers.

Gas-in-oil analysis for diverter switch insert oil in OILTAP®:

No informative value with respect to arcing on main switching contacts

and transition contacts.

Gas-in-oil analysis for diverter switch insert oil in VACUTAP®:

An evaluation pattern is currently being developed.


9. Diagnostics

No. of ops. Oil

temp. [°C]

H2 CH4 C2H6 C2H4 C2H2 CO CO2 O2 N2

0 16 127 31 3 70 359 10 344 17500 39400

29000 50 17796 12205 3642 36170 102172 330 3032 10800 48100

50000 22 13553 13000 4200 50000 103000 317 7733 4900 25300

Example for OLTC type M

Torque measurement:

Integral part of monitoring TM100 and

TG260®, including assessment (semaphore

function). Online procedure.

Torque measurement directly on OLTC head

with mobile measuring device.

Tap-change operation sequence assessment:

Indirect assessment of tap-change operation

sequence of diverter switch by measuring the

contact wear (OILTAP®).

Measurement of tap changes of diverter switch

insert operation using direct current.


9. Diagnostics

Example of torque progress

Diverter switch insert type G

• On-load tap-changers are available in/with different

switching principles: high-speed resistor and reactor principle

types: non-vacuum and vacuum

• On-load tap-changers are available for different applications

power transformers for transmission and distribution

transformer for industrial use (e.g. arc-furnace or ladle furnace transformers)

HVDC transformers

phase-shifting-transformers

transformers for operation at explosion-prone sites (e.g. offshore platform)

• Maintenance

maintenance intervals of the respective on-load tap-changer shall be observed at any time to

ensure trouble-free operation of the transformer. Original spare parts and trained service staff

have also be seen as mandatory.


Summary

Maschinenfabrik Reinhausen GmbH

Founded in 1868

Family ownership since 1891

Brand name since1901

World market leader in transformer switching

2,850 employees in 7 business areas

30 locations – world-wide

Doing business in 190 countries

Company sales: 630 million EUR

www.reinhausen.com


About MR

http://www.reinhausen.com/

Design, Function and Operation of On-Load Tap-Changers

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