design, function and operation of on-load tap-changers
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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
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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
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Agenda
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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
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 Page 5
2. On-load tap-changer – Design types
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
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2. On-load tap-changer – Design types
OILTAP® Type V since 1979
e. g. V III 200 10 steps
3-pole
Star connection
Insulation to ground
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
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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 Transition resistor
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2. On-load tap-changer – Design types
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
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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
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
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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)
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
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4.1 Maintenance – Intervals for OILTAP® and VACUTAP®
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.
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4.1 Maintenance – Intervals for OILTAP® and VACUTAP®
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)
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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
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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
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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)
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4.5 Maintenance – Checks after maintenance
1) Diverter switch with oil compartment
2) Motor-drive unit
3) Protective relay
4) Drive shafts
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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)
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.
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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
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.)
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5. Oil change criteria
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
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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)
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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
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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.)
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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.
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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.
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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.
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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
07.06.2013 Uwe Seltsam, Maschinenfabrik Reinhausen GmbH Page 27
About MR
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