Optimization of Transformer Drying in the Field

2/25/2014
Optimization of Transformer Drying in the Field
Thomas Prevost
OMICRON USA
March 3, 2014
Denver, Colorado
Agenda
•
•
•
•
The effects of moisture in transformers
Sources of moisture
Sources of moisture
Moisture measurement techniques
Processes used to remove moisture from transformers in the field
• Methods to monitor the insulation moisture content during field drying
• Utilization of DFR to monitor moisture during drying
• Summary and conclusion
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Breakdo
own voltage / kV
Effect #1: Lowered Dielectric Strength
Effect of higher moisture content:
• PD Inception Voltage Decreases
PD Inception Voltage Decreases
• Dielectric Strength of Oil Decreases
• Creep Strength of Insulation Decreases
Krause,et al, “Moisture 100
HOSO
FR3
Midel 7131
Midel eN
NN3000X
80
60
40
20
0
0
20
40
60
80 100
Moisture saturation / %
Effects on the Electric Strength of oil/Pressboard Insulation used in power Transformers” IEEE ICDL, 2005
Risks: • Failure of Transformer Insulation
• Moisture level over 3% increases risk significantly
Berget al,”Discharge signatures from a wedge geometry in transformer insulation paper”, ISHV 2003
Effect #2: Lowers the Bubbling Temperature
Risks: • Inception of Partial Discharge
• Lower Dielectric Strength
• Major Insulation Failure
• Limits Emergency Loading Limits Emergency Loading
Capability
190
Kobayashi rapid heating
Kobayashi slow heating
170
150
Temperature
Effect:
Bubbling inception temperature will decrease p
with higher moisture content
Davydov
130
Oommen gas free
110
Oommen gas saturated
90
70
50
0
2
4
6
8
10
WCP % w/w
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Effect #3: Accelerated Aging
1000
Life expectancce / a
Effect:
High temperature and moisture g
p
content will dramatically lower the mechanical strength of paper insulation
Dr
y
1%
100
10
2%
3%
1
Risks: • Lower the expected life of transformer
• Increase of moisture to 2% will decrease insulation life from 20 years to 2 years
4%
0,1
50
70
90
110
130
Temperature / °C
L. E. Lundgaard, “Aging of oil‐impregnated paper in power transformers”, IEEE Transactions on Power Delivery, Jan. 2004 Agenda
•
•
•
•
The effects of moisture in transformers
Sources of moisture
Sources of moisture
Moisture measurement techniques
Processes used to remove moisture from transformers in the field
• Methods to monitor the insulation moisture content during field drying
• Utilization of DFR to monitor moisture during drying
• Summary and conclusion
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Sources of Water
Leaky seals Installation, repair Water from aging Breathing Residual moisture
Water content in the paper/Pressboard:
 New: 0.4 to 0.8 %
 Aged:
3 to 5 %
Increase of water
0.1‐ 0.2% per year
[Cigré WG12.18 Life Management of Transformers, 1999]
Moisture Distribution
Distribution example:
150 MVA, 7 t cellulose, 70 t Mineral oil, Temperature 30°C
Temperature 30
C
Total amount of
Total amount of water = 211.1 kg
cellulose Cw = 3 %  210 kg water
T+
Oil 15 ppm  1,1 kg H2O
T–
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Moisture Distribution
Distribution example:
150 MVA, 7 t cellulose, 70 t Mineral oil, Temperature increase from 30 increase from 30 °C
to 80 °C
Temperature C to 80 C Total amount of water = 211.1 kg
cellulose Cw = 3 % 210 kg water  200 kg
cellulose Cw = 2.86%
T+
Oil 15 ppm  150 ppm
T–
Water in Oil 1.1 kg ppm  11 kg
Agenda
•
•
•
•
The effects of moisture in transformers
Sources of moisture
Sources of moisture
Moisture measurement techniques
Processes used to remove moisture from transformers in the field
• Methods to monitor the insulation moisture content during field drying
• Utilization of DFR to monitor moisture during drying
• Summary and conclusion
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Moisture Measurement (estimation) Methods
• Direct measurement of paper • Requires sample
q
p
• Requires Karl Fisher Titration because oil soaked
• Equilibrium Methods
• Dew Point
• Absolute Moisture in Oil (Karl Fisher)
• Relative Saturation of Oil
• Dielectric Methods
Di l t i M th d
• Dielectric Frequency Response (DFR)
• Polarization / Depolarization Current (PDC)
Direct Method
• Take paper sample from transformer and test for moisture content using KFT
content using KFT
• Limited use since possible only during repair or tear‐down
Sample
injection
Water Vapour
Electrolysis
electrodes
l t d
Detection
electrode
Sample
Heating
Electrolysis cell for coulometric Karl Fischer titration with furnace for
water evaporation
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Dew Point Measurement
• Introduce dry gas and stand idle for 12 to 24 hours
• Measure dew point of gas
• Record tank pressure
• Record Insulation temperature
•Utilize the Piper curve (found in
•Utilize the Piper curve (found in IEEE C57.93) to estimate the % moisture content
Dew Point Measurement
Dew Point ‐30 °C
Tank Pressure
3 psi
Insulation temp.
30 °C
VC = VP (14.7 + TP)/ 14.7
VC = 300 (14.7 + 3)/14.7
VC = 361.2
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Dew Point Measurement
Vapor Pressure (corrected to atmosphere) = 361.2 um
atmosphere) 361.2 um
Insulation Temperature = 30 °C
Moisture Content = 0.9%
Absolute Moisture‐in‐Oil Method
Karl Fischer Titration on oil sample Titration – a chemical reaction where oil is injected into a reaction vessel. The water inside the oil chemically reacts and this is measured. Use Equilibrium Curves to correlate moisture in oil to U
E ilib i
C
l
i
i il
moisture content in paper
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Moisture‐in‐Oil Method
PRO: Easy to Perform
CON: Need to wait for equilibrium (days ~ weeks)
CON: Uncertainty in Estimates
CON: Errors introduced during handling (oil samples)
l )
CON: Curves are not accurate for aged oil
CON: Tendency to over‐estimate
Moisture‐in‐Oil Method
Moisture in Paper Determination:
1.. Onsite
Onsite oil sampling, transportation to laboratory
oil sampling, transportation to laboratory
•
Must record oil temperature
2. Moisture content determination (ppm)
3. Application of an equilibrium diagram
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Moisture Equilibrium Curve (ref TV Oommen)
Inaccuracies of Equilibrium Diagrams
Aging






Sampling
p g
Uncertainty of KFT
Equilibrium conditions
Literature sources
Absorption capacity Aging Aging and other restrictions lead to inferior accuracy
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Assumptions and Errors
Source: [B. Ward: Moisture Estimation in Transformer Insulation, Panel Session IEEE TC, 2004]
Moisture Saturation Method
• Use R.H. sensor to measure moisture saturation level i t d f
instead of ppm as in the lab
i th l b
• Moisture probes inside the transformer measures changes in capacitance of water molecules
• Moisture diffused into the probe and changes its capacitance, from this the moisture saturation is determined
Use equilibrium curve to estimate moisture content of
• Use equilibrium curve to estimate moisture content of cellulose insulation
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Moisture Saturation Method
PRO: More accurate than oil sampling method since no handling is involved
handling is involved
CON: Intrusive
CON: Requires Equilibrium
Moisture in pap
per [%] Equilibrium Diagrams based on Water Saturation
6
5
4
3
21°C
2
40°C
60°C
1
80°C
0
0
10
20
30
40
Moisture relative to saturation [%]
Onsite and online application possible
M. Koch, “Advanced Online Moisture Measurements in Power Transformers” CMD 2006
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Dielectric Frequency Response Methods
Frequency Based Method
Dissipation Factor (Power Factor) vs. Frequency
Time Based Method
Polarization Spectrum
Both methods looks at the electrical response of the dielectric and compares to laboratory data and models
Accuracy of the estimation depends on the accuracy of the model
Accuracy of the estimation depends on the accuracy of the model (e.g. takes into consideration geometry of insulation, conductivity of oil, etc.)
Preposition: Contributions of Single Materials as Obtained from Laboratory Tests
Paper / Pressboard
DF
1.000
Polarization dominates for higher frequencies
g
q
Conductivity dominates for lower frequencies
3
0 100
0.100
2
1
0.010
DF
1.000
BNC-connector
lid
oil level
inner electrode
0 100
0.100
0.1p
1p
outer electrode
10p
cavity for thermometer
Mineral oil:
0.010
0.0001
0.001 0.01
0.1
1.0
10
f/Hz 1000
Conductivity dominates
Dissipation factor with slope of ‐
1/dec
DF is very low at high frequencies, but high at low frequencies
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Superposition at Oil‐Barrier System: Modelling by XY‐Model
Simplification of insulation to „XY model“
All barriers are accumulated to one, X
All sticks are accumulated to one Y
(Explanation valid without model too, but model increases clarity)
HV-winding
Y
Barrier
Stick
Oil
Oil
Sticks
Core
LV inding
LV-winding
Barriers
X
Tan Delta ‐ Frequency Dependency: Frequency Domain Spectroscopy (FDS)
Insulation geometry
Dissipation factor
Pressboard: water, acids
Oil: carbon, acids
,
10
Pressboard: Water, Acids
1
Overall response
0.1
1%, 1pS/m,
0.01
0 001
0.001
0.0001
0.0001
0.001
0.01
0.1
1.0
10
100
1000
f/Hz
• Frequency range 0,0001Hz – 1000Hz  Allows differentiation of the Material properties
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How Long to Measure?
low
high
Typical:
moisture,
aging of
cellulose
• Dry transformer or low temperature ‐> 0.1 mHz, 2:50 hours
,
0,1
moisture and
aging of cellulose
low
insulation
geometry
0,01
oil
conductivity
0,1
1
Dissipation factor
Dissipation fa
actor
0,001
0,001 0,01
5
2
1
0.5
• Moderate wetness / temperature ‐> 1 mHz, 22 min high
New
0.2
0.1
0.05
• Wet transformer or hot temperature ‐> 0.1 Hz, 5 min
low
10
100 1000
5
Moderate
q
y
/ Hz
Frequency
2
Dissipation
n factor
Diss
sipation factor
Sufficient data high
1
44°C
1
0.5
0.2
0.1
0.05
0.002
0.0001 0.001 0.01 0.1
1.0
10 Freq/Hz 1000
2
1
0.5
H
Heavily
il aged
d
0.2
0.1
0.05
0.02
0.01
0.005
0.02
0.01
0.005
0.02
0.01
0.005
5
0.002
0.0001 0.001 0.01 0.1
0.002
0.0001 0.001 0.01 0.1
1.0
10
Freq/Hz
1000
1.0
10 Freq/Hz 1000
Dissipation fa
factor
Effects of Conductive Aging By‐products
10
2,1%
1,2%
,
2,0%
0,8%
1
aged
aged
g
new
new
0,1
0,01
0,001
1E-04 0,001 0,01
Conductive aging by‐products
behave similar to water
behave similar to water
‐> Overestimated moisture
content without compensation
Model needs to
compensates for this influence
0,1
1
10
100 1000
Frequency / Hz
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Assessment Settings
Moisture Assessment
Observe fitting left of the hump
Result:
Moisture content
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Agenda
•
•
•
•
The effects of moisture in transformers
Sources of moisture
Sources of moisture
Moisture measurement techniques
Processes used to remove moisture from transformers in the field
• Methods to monitor the insulation moisture content during field drying
• Utilization of DFR to monitor moisture during drying
• Summary and conclusion
Methods to Extract Moisture from Transformers in the Field
•
•
•
•
Circulating hot oil
On‐Line drying with external water absorbent
Hot air
Vacuum
– Without heat
– With heat
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Hot Oil Circulation
• Heat and maintain oil at ~85 ºC
• Circulate
Circulate oil through:
oil through:
• Vacuum Drier
• Oil Filter
• Blotter press
• Water is removed from solid insulation via the oil
• This
This process is VERY INEFFICIENT and should be process is VERY INEFFICIENT and should be
avoided
• Use hot oil circulation to heat up insulation and then Vacuum drying to remove moisture
On‐Line Drying with external absorbent
• Effective but slow process
• Plan on months to remove sufficient moisture
• Relies on the daily temperature cycle to remove moisture from insulation through the oil
• Process efficiency is based on maintaining a dry external absorbent
ref: Siemens
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Hot Air Drying of Transformer in Field
• Typically used for older/smaller transformers with tanks not designed to withstand full vacuum.
• Clean dry air is circulated over an external heating element then through an opening at the base of the transformer tank where it circulates around the insulation and exhausts from an opening at the top of the transformer.
• NOT EFFICIENT
• Need to limit the temperature of inlet air to 100 ºC
Hot Air Drying
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Vacuum
• Liquid is drained from the tank to expose surface of solid insulation to vacuum
lid i l i
• Heating the insulation will increase the efficiency
• Cold traps in the vacuum line can increase the efficiency of the vacuum pumps and provide a means to measure the amount of moisture extraction
to measure the amount of moisture extraction
Vacuum Drying
ref: Baron USA
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Vacuum: Methods to Heat Windings
Short Circuiting Winding
• Short LV winding terminals
• Apply a low voltage, low frequency (0.4 –
A l l
lt
l f
(0 4 2 Hz) current to 2H )
tt
HV windings
• Can be combined with oil circulation or hot oil spray
• Winding losses heat up the winding and insulation material.
• Drain oil and apply high vacuum after sufficient temperature is reached
temperature is reached
• Repeat process until reach desired temperature of 110 ºC
in stages . Vacuum: Methods to Heat Windings
Hot Oil Circulation
• Transformer is filled with oil
Transformer is filled with oil
• Circulate oil through external heat exchanger until insulation reaches desired temperature of typically 80 ºC
• Check inlet to outlet temperature differential
• < 5 ºC
• Drain oil and apply High vacuum
• May need to repeat process should temperature decrease too far (60 ºC)
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Vacuum: Methods to Heat Windings
Hot Oil Spray
• Transformer is filled with enough oil to cover the bottom drain valve
• Apply a vacuum to the transformer
• Circulate oil through external heat exchanger until insulation reaches desired temperature of typically 80 ºC
• Check inlet to outlet temperature differential
• < 5 ºC
• Apply High vacuum
Typical Hot Oil Spray System
ref: Baron USA
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Agenda
•
•
•
•
The effects of moisture in transformers
Sources of moisture
Sources of moisture
Moisture measurement techniques
Processes used to remove moisture from transformers in the field
• Methods to monitor the insulation moisture content during field drying
• Utilization of DFR to monitor moisture during drying
• Summary and conclusion
Different Measurements Used to Measure the Moisture Content of the Paper Insulation • Cold Trap – Measures Water Extracted • Humidity Sensor in Vacuum Line
• With pump speed and pressure calculates weight of water extracted
• Vapor Rise Test (Calculates surface moisture)
• Dew Point Measurement
• Dielectric Response Tests
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Cold Trap
Pros:
• Increases efficiency of vacuum pump
• Can directly measure extracted water
• Can estimate final moisture if know mass of insulation
Cons:
• Initial dryness and mass of insulation are difficult to obtain
are difficult to obtain
• Must be sure that you capture all of the water extracted and that it is measured accurately
Vapor Rise Test
• Often done at the same time that the condensed moisture is being removed from the cold trap.
• Vacuum leak rate of transformer needs to be established before drying process begins
established before drying process begins.
• Transformer is isolated from the vacuum pump and cold trap.
• Pressure rise due to moisture evaporation over a given time period is used to estimate remaining moisture level.
Pros:
• Can be done utilizing vacuum gauges during the process
Cons:
• System may not be in equilibrium (thin vs thick components)
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Dew Point
Pros
• Industry accepted practice to estimate moisture content Cons
• Must fill with dry air and wait 12 ‐24 hours for equilibrium
• Often done as final confirmation of dryness
• Typically not used during drying process
• Because based on equilibrium it is usually only influenced by surface moisture
Agenda
•
•
•
•
The effects of moisture in transformers
Sources of moisture
Sources of moisture
Moisture measurement techniques
Processes used to remove moisture from transformers in the field
• Methods to monitor the insulation moisture content during field drying
• Utilization of DFR to monitor moisture during drying
• Summary and conclusion
25
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• Active part in tank
• Normal connection:
oVoltage to HV
oInput current to LV
oGuard to tank
Instru
ument
Setup for Monitoring Insulation Dryness within Tank with DFR
=
Voltage source
A
Current sense 1
A
Guar
d
CL
CHL
LV
CH
HV
How DRYmon Works...
Set up the software,
connect the transformer
Temperature & dielectric
measurement
Moisture assessment
Data output
Visualization
Measurement
Database
Measurement of dielectric response and temperature for highly automated monitoring
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How does the curve look like for a drying process?
‐ empty tank; no oil present → rather flat
→ no interfacial polarisa on
‐ mainly dependent on temperature and water content
i l d
d t
t
t
d t
t t
Tan δ
5.000
2.000
1.000
0.500
water content: 1,8%
0.200
0 100
0.100
0.050
0.020
water content: 0,3%
0.010
0.010
0.100
1.000
10.000
100.000
E1320065A
CHL
CHL (1)
CHL (2)
CHL (3)
CHL (4)
CHL (5)
CHL (6)
CHL (7)
CHL (8)
f/Hz
DRYmon features
monitoring the drying process during transformer dryout in the field
‐ automated cyclic measurements ‐ temperature can be measured
‐ automatic moisture assessment during each cycle
‐ result: moisture vs. time
‐ Indication when final dryness is reached
‐ report functions
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DRYmon software – main window
DRYmon software – settings
Page: 56
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DRYmon software – temperature input
winding
resistance
1. Measurement of Winding Resistance
R2
By knowing one reference point
R1
Rref
all other temperatures
(
can be calculated
ref
1
2
temperature
Cable Setup for Drying of a Transformer
DRYmon
Ground
LV
HV
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Dielectric measurement
DRYmon
~
A
Voltage source
Current sense
Current sense
Ground
CHL
LV
HV
Resistance/Temperature measurement
DRYmon
Current source
Voltage sense
U
Ground
RW
LV
HV
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Connection Diagrams are shown in DRYmon Software
A connection diagram is shown in the software for each type of sample
All 10 cables are connected – no risk of short circuits by disconnected cables
Examples: N on LV or Autotransformer with Tertiary Winding
DRYmon software – connection diagrams
• are shown in software for an easy use
• color coded cables
• examples: N on HV and LV or without N
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Summary & Conclusion
• Excessive moisture in transformers should be avoided
• There are several methods to estimate moisture, care should be taken with methods based on equilibrium
• Dielectric Frequency Response is an accepted method to estimate moisture, however it is an off‐line test
• Vacuum and heat are the most efficient methods to dry insulation in the field
• It is difficult to determine the amount of moisture removed during the drying process
dd
h d
• On‐Line DFR and resistance measurement provides a reliable moisture estimation method for in field drying of transformers
Please Remember to Complete Your Evaluation Form
Thank You!
Thank You!
32

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