Design, Function and Operation of On-Load Tap-Changers
Uwe Seltsam
Maschinenfabrik Reinhausen GmbH
TRANSFORM Partner
About the Author
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
Uwe Seltsam, Maschinenfabrik Reinhausen GmbH
07.06.2013
Page 2
Agenda
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
Uwe Seltsam, Maschinenfabrik Reinhausen GmbH
07.06.2013
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1. On-load tap-changer in regulating transformer
On-load tap-changers (OLTCs) are used for changing the transmission ratio under load
110 kV
U1
Mwg = Main winding
Cwg = Coarse winding
Rwg = Regulating winding
LU = Diverter switch insert
Mwg
555 wdgs
Cwg
120 wdgs
Transformer:
OLTC:
Step voltage:
%
40 MVA, 110 kV 16
/ 21 kV
17.6 kV
M III 350 Y – 123/C – 10 19 1G
17.6 kV : 9 steps ≙ 1.955 kV/step ≙ 1.78 %
U2
21 kV
129 wdgs
+
k
Rwg
108 wdgs
12 wdgs/step
Nominal:
Example 1:
Example 2:
9
LU
2
1
U1 = N1
U2 N2
U2 = 21 kV = constant
U1 = 110 kV
N1 = 110 kV • 129 wdgs / 21 kV = 675 wdgs
U2 = 110 kV • 129 wdgs / 675 wdgs = 21.02 kV
U1 = 116 kV
N1 = 116 kV • 129 wdgs / 21 kV = 711 wdgs
U2 = 116 kV • 129 wdgs / 711 wdgs = 21.05 kV
Uwe Seltsam, Maschinenfabrik Reinhausen GmbH
07.06.2013
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2. On-load tap-changer – Design types
Tap-change operation
Vectorial diagram
Contact movement
A,B
Main contacts
(only for V III 350 Y/Δ,
V III 500 Y / Δ,
V I 350, V I 700)
OILTAP®
Type M since 1972
a,b
Main switching
contacts
e. g. M III 600 9 steps
3-pole
Star connection
Insulation to ground
Um = 123 kV
a1, b1
Transition contacts
Ra, Rb
Transition resistors
Ua, Ub
Voltages of tappings
Ust
Step voltage
U, I
Voltage, current of tapchanger terminals
Uwe Seltsam, Maschinenfabrik Reinhausen GmbH
07.06.2013
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2. On-load tap-changer – Design types
Operation
Vectorial diagram
Contact movement
A,B
Main contacts
(only V III 350 Y/Δ,
V III 500 Y/ Δ,
V I 350, V I 700)
OILTAP®
Type V since 1979
a,b
Main switching
conttacts
e. g. V III 200 10 steps
3-pole
Star connection
Insulation to ground
Um = 76 kV
a1, b1
Transition contacts
R
Transition resistor
Ua, Ub
Voltages of tappings
Ust
Step voltage
U, I
Voltage, current of
tap-changer terminals
Uwe Seltsam, Maschinenfabrik Reinhausen GmbH
07.06.2013
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2. On-load tap-changer – Design types
VACUTAP®
Type VR® since 2004
e. g. VRF III 1300 10 steps
3-pole
Star connection
Insulation to ground
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
Uwe Seltsam, Maschinenfabrik Reinhausen GmbH
Transition resistor
07.06.2013
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2. On-load tap-changer – Design types
VACUTAP®
Type VV since 2000
e. g. VV III 400
10 steps
MTS
MSV
TTS
TTV
STC
R
IC
Uwe Seltsam, Maschinenfabrik Reinhausen GmbH
Tap-selector contact, main path
Main switching contact (vacuum interrupter), main path
Tap-selector contact, transition path
Transition contact (vacuum interrupter), transition path
Sliding take-off contact
Transition resistor
Circulating current
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3. Switching sequence of tap selector and diverter switch
IB
Tap pre-selection without current
Diverter switch insert operation type M
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
Uwe Seltsam, Maschinenfabrik Reinhausen GmbH
IB = Operating through-current
IK = Circulating current
R = Transition resistor
07.06.2013
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4.1 Maintenance – Intervals for OILTAP® and
VACUTAP®
OILTAP® - Regular maintenance, depending on
time of service
or
no. of operations
(whichever comes first)
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
Diverter switch insert type M
Service life 800,000 operations
For OLTC-specific maintenance intervals, refer to the respective
operating instructions or the label on the motor-drive unit.
Uwe Seltsam, Maschinenfabrik Reinhausen GmbH
07.06.2013
Page 10
4.1 Maintenance – Intervals for OILTAP® and
VACUTAP®
VACUTAP® - Regular maintenance, only depending on
the number of 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
For network applications, the VACUTAP® is more or less
maintenance-free.
Example:
Switching frequency 5,000 operations per year
 maintenance after 60 years.
Uwe Seltsam, Maschinenfabrik Reinhausen GmbH
Diverter switch insert type VR®
Service life 1.2 million operations
07.06.2013
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4.1 Maintenance – Intervals for OILTAP® and
VACUTAP®
Monitoring – TM100, Tapguard® 260
Maintenance depending on condition
Maintenance criterion
Parts subject to wear
Oil change and cleaning
Contact wear
Calculation
No. of ops. specification for parts change
Calculation algorithm for oil carbonization
Calculation algorithm for single and difference wear
Maintenance time
Visual presentation
Present condition 0 ... 100 %
Prediction for 100 % moment
Semaphore function green/yellow/red
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
Uwe Seltsam, Maschinenfabrik Reinhausen GmbH
07.06.2013
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4.2 Maintenance – Parameter contact wear
MSCB
TCB
TCA
MCA
Criteria for OILTAP®:
Single wear and
Difference in wear between switching
and transition contact
Parameters for contact wear:
Number of operations:
Arcing current:
Medium arc duration:
Arc voltage:
Contact material:
MCA
MCB
MSCA
MSCB
TCA
TCB
variable value
Contact system of diverter switch type M
variable value
approx. 6 ms
25 – 30 V
Step voltage
Ust
Cu or CuW sintered material
Wear on main switching contact:
SG
proportional to operating through-current
Wear on resistor contact:
independent of operating current, but proportional to
circulating current Ic (Ic = Ust / Rres)
Exceeding the wear differences influences the switching
sequence – a short circuit between taps may occur.
Uwe Seltsam, Maschinenfabrik Reinhausen GmbH
MCA
MSCA
TCA
TCB
SCB
MCB
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
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4.3 Maintenance – Parameter replacement of parts
Depending on no. of ops.
Springs, leads, etc.
Depending on wear
Arc contacts, etc.
Preventively
Cover gasket, etc.
Up-dating
E.g. Energy accumulator spring for type
M: 400,000 – 500,000 ops.
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
E.g. Braided lead for type M:
250,000 operations
MR original parts are the basis for safe operation.
Uwe Seltsam, Maschinenfabrik Reinhausen GmbH
07.06.2013
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4.4 Maintenance – Parameters oil carbonization
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
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.
Diverter switch type insert T
before cleaning
Uwe Seltsam, Maschinenfabrik Reinhausen GmbH
Diverter switch type insert T
after cleaning
07.06.2013
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4.5 Maintenance – Checks after maintenance
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)
1)
2)
3)
4)
Diverter switch with oil compartment
Motor-drive unit
Protective relay
Drive shafts
Uwe Seltsam, Maschinenfabrik Reinhausen GmbH
07.06.2013
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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)
Oil monitoring
Oil exchange criteria, see section 5
Disconnected
transformer
OLTC maintenance before re-commissioning of transformer if
operating time and downtime exceed maintenance interval of
on-load tap-changer (OILTAP®)
together with transformer test
Uwe Seltsam, Maschinenfabrik Reinhausen GmbH
07.06.2013
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5. Oil change criteria
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
Minimum 30 kV / 2.5 mm
Maximum 30 ppm
OLTC application of VACUTAP®
All
*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.
Uwe Seltsam, Maschinenfabrik Reinhausen GmbH
07.06.2013
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5. Oil change criteria
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.
Uwe Seltsam, Maschinenfabrik Reinhausen GmbH
07.06.2013
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6. Influence of humidity on dielectric strength
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
Switching shaft
Example of creepage paths
Uwe Seltsam, Maschinenfabrik Reinhausen GmbH
07.06.2013
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7. Avoiding carbon deposits or their effects
Vacuum technology in the VACUTAP®
No carbon deposits since arcs do not occur in insulating oil
Vacuum interrupter
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)
The effectiveness of the drying agent of the filter cartridge must be
determined by oil sample and water content analysis.
Oil filter unit – OF100
Uwe Seltsam, Maschinenfabrik Reinhausen GmbH
07.06.2013
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8. On-load tap-changer – Protective devices
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)
RS2001
Response value about 1.3 – 1.8 bar, depending on OLTC type
Monitoring of drive shafts, e. g., for multiple-column OLTC design
The above-mentioned protective devices must be integrated in the
tripping circuit of the circuit breaker of the transformer.
End position limiting device in OLTC and/or motor-drive unit
Burst membrane
Find out the cause after a protective device has responded!
MPreC®
Uwe Seltsam, Maschinenfabrik Reinhausen GmbH
07.06.2013
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9. Diagnostics
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;
Extract from CIGRE report
Tap-selector contacts colored to “black-grey“:
Signs of silver-sulfide (impurity layer)
Check for corrosive sulfur (oil, etc.)
Separated silver-sulfide layer may lead to flashovers.
Tap selector contact with silver-sulfide
Uwe Seltsam, Maschinenfabrik Reinhausen GmbH
07.06.2013
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9. Diagnostics
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.
No. of ops.
Oil
temp.
[°C]
H2
CH4
C2H6
C2H4
C2H2
CO
CO2
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
O2
N2
25300
Example for OLTC type M
Gas-in-oil analysis for diverter switch insert oil in VACUTAP®:
An evaluation pattern is currently being developed.
Uwe Seltsam, Maschinenfabrik Reinhausen GmbH
07.06.2013
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9. Diagnostics
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.
Example of torque progress
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.
Diverter switch insert type G
Uwe Seltsam, Maschinenfabrik Reinhausen GmbH
07.06.2013
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Summary
• On-load tap-changers are available in/with different
switching principles:
types:
high-speed resistor and reactor principle
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.
Uwe Seltsam, Maschinenfabrik Reinhausen GmbH
07.06.2013
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About MR
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
Uwe Seltsam, Maschinenfabrik Reinhausen GmbH
07.06.2013
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