Qualitrol is happy to announce the introduction of our on-line Bushing Monitoring system for power transformers. Bushings regularly fail and can cause catastrophic damage. You can learn more about Bushing Monitoring from Qualitrol by going here, https://www.qualitrolcorp.com/products/bushing-monitoring/

1. On-line Bushing Monitoring

2. Enhanced Bushing Monitoring
Presented By: Emilio Morales Qualitrol LLC
April, 2018
Emilio is a Technical Application Specialist in Transformer applications at Qualitrol
Company LLC. His main focus is to support solutions in comprehensive monitoring for
Transformer applications. Emilio attended Nuevo Leon State University in Mexico,
receiving his Bachelor degree in Electro Mechanical Engineering in 1980. He has over
30 years of experience in power transformer design which includes transformers up to
500 MVA and 500 kV, furnace and rectifier transformers and reactors. He is member
of the IEEE/PES Transformer Committee, IEC and CIGRE. Emilio joined Qualitrol in
June 2012 and previously worked with GE-Prolec , Ohio Transformer, Sunbelt
Transformer and Efacec.

3. 1. Statistical overview
2. Technical basics about Bushing design
3. Main parameter (Tan δ and Capacitance) and its measurement
4. Monitoring principles
5. Qualitrol solution
Agenda

4. Agenda
1. Statistical overview
2. Technical basics about Bushing design
3. Main parameter (Tan δ and Capacitance) and its measurement
4. Monitoring principles
5. Qualitrol solution

5. 5
• Bushings are a critical component (bottle
necks) in electricity transportation. They
transfer load currents in and out of metal
(grounded) enclosures at system voltages,
and are often exposed to high permanent
stresses
• They are used on transformers/reactors,
instrument transformers and switchgear.
• Capacitive bushings > 25kV have highest
failure rate
• Average Price of Bushing = USD 10-20k
Bushings

6. Transformer Failure Statistics
Source: WG A2.37, Transformer Reliability Survey: Interim Report, No. 261 - April 2012 ELECTRA
HIGHEST SYSTEM VOLTAGE [kV]
FAILURES & POPULATION
INFORMATION
69 kV < 100 100 kV < 200 200 kV < 300 300 kV < 500 kV 700 All
Failures 145 212 163 154 11 685
Transformer -Years 15220 48994 47473 41569 959 156186
FAILURE RATE/ YEAR 0.95% 0.43% 0.34% 0.37% 1.15% 0.44%
• The newest transformer statistic shows, that in case of substations bushings contribute
with 17% to the total failure rate of transformers.
• In case of generator step- up transformers, bushings contributing with 9%
• More than 50% of bushing failures are catastrophic

7. 7
• Impact of a catastrophic failure can be tank rupture, violent explosions and fires
• Result: Higher probability that a new transformer is needed versus failures of other
transformer components
• Violent explosions / fires have high risk of collateral damage and personnel injury/death
(depending on whether anyone is in the substation at time of catastrophic failure)
• Result: Risk of injury claims, collateral damage with other S/S assets, insurance
premium increases etc.
• Other economic impacts can include – environmental fines, contractual penalties, loss
of revenue
The value/benefit of bushing monitoring is not related to the asset value (due to low
replacement cost) but in ALL costs that could be incurred as a result of bushing failure
Bushing Failures

8. Agenda
1. Statistical overview
2. Technical basics about Bushing design
3. Main parameter (Tan δ and Capacitance) and its measurement
4. Monitoring principles
5. Qualitrol solution

9. 9
Condenser bushing construction and key components
C1
C2
Foil/ conductive ink
(capacitive layer)
Oil or epoxy
impregnated paper
conductor
Flange
Bushing tap
Silicone/ Porcelain
insulator
High Voltage Layer
Measuring Layer
Head Cover
The C1 capacitance is the capacitance between the center conductor and the tap.
The C2 capacitance is the capacitance from the tap to ground.

10. Oil to Oil Bushings:
Oil to SF6 Bushings:
Oil to Air Bushings:
Wall Bushings:
Will be used for transformer – cable connection
(installed between transformer tank and cable
box)
Will be used for direct GIS connection to
transformer (installed between transformer tank
and GIS)
Will be used for AIS connection to transformer
(installed between transformer tank and AIS)
Will be used for indoor AIS (installed between
indoor AIS and overhead line)
10
Types of bushing regarding the main application
Source of pictures: Micafil RIP bushings, High-voltage Resin-Impregnated Paper bushings; 2012/ ABB

11. 11
Types of bushings regarding technology
Oil impregnated
paper bushing (OIP)
Oil
impregnated
Paper
Oil
impregnated
paper/ foils
Expansion
housing
Oil level
gauge
Flange
Bottom side
porcelain
Resin impregnated
paper bushing (RIP)
Resin
impregnate
Paper
Resin
impregnated
paper/ foils
Oil or Gel
filling
Flange
Resin bounded
paper/ graphite
Flange
Thick Oil filling
Varnished or
painted
surface
Resin bounded paper
bushing (RBP)

12. 12
Types of bushings regarding technology
Resin impregnated
synthetics bushing (RIS)
Resin
impregnated
synthetics
Resin
impregnated
synthetics / foils
Flange
Direct vulcanized
silicon rubber
SF6 filled hollow insulator
bushing (for GIS)
SF6 filled
Conductor
Silicon/ glass
fiber insulator
Flange
Clamping ring
(porcelain)
Porcelain
insulation
DIN Bushings (porcelain or
silicon – up to 36kV)
Ground field
electrode
HV field
electrode
For these two types of bushings, no tan d and
capacitance measurements are possible. There is no
condenser body (no measuring layer) existing.

13. Agenda
1. Statistical overview
2. Technical basics about Bushing design
3. Main parameter (Tan δ and Capacitance) and its measurement
4. Monitoring principles
5. Qualitrol solution

14. Leakage Current
IC
Ir
High Voltage
Ground
Flange
Measuring Tap
Silicone/ Porcelain
insulator
Condenser core
Conductor
IC - capacitive current
Ir - resistive current
A Leakage current
The leakage current is the current flowing from the
measuring tap pin through ground. The resistive part
is defined by the properties of the insulating system,
as the capacitive part is defined by the mechanical
and electrical design. A part of the leakage current is
flowing also along the surface. Due to surface
contamination or strong surface discharges, the
resistive current can be influenced.

15. Power Factor (≈ tan )
Phase angle between the applied voltage across a capacitance and the total
current through the capacitance
If Ir increases  decreases and PF increases
Ir
IC
It


tan  = Dissipation Factor (DF)
DF ≈ Power Factor (PF) – for small 
E IC Ir
Equivalent circuitEquivalent circuit

16. 16
• In bushings there are a several capacitors in series
• When a capacitor layer shorts out, the value of the capacitance will always
increase
• The capacitors in series act as a voltage divider
• If a capacitor shorts out the voltage at the tap will increase in proportion
• Also, as the voltage varies the leakage current will vary.
• Therefore, if the voltage increases, there will be an increase in leakage current
Capacitance
C1 – Main Capacitance C2 – Tap Capacitance
C1 C9 C10 C11 C12 C13 C14C8C2 C4C3 C5 C6 C7
1
𝐶1 𝑡𝑜𝑡𝑎𝑙
=
1
𝐶1
+
1
𝐶2
+
1
𝐶3
+
1
𝐶3
+
1
𝐶4
+
1
𝐶5
+
1
𝐶6
+
1
𝐶7
+
1
𝐶8
+
1
𝐶9
+
1
𝐶10
+
1
𝐶11
+
1
𝐶12
+
1
𝐶13
+
1
𝐶14

17. Typical bushing defects
Insulation ageing
- Oil
- paper
Moisture Voids/
Delamination’s
Surface
discharges (no
real defect)
Contact
problems
Partial breakdowns
(short circuit between
layers)
Surface
contamination

18. 18
Detectability of different bushing phenomena's
Defect Tan d / PF Capacitance Partial Discharge
Insulation aging + - -/+
Detectable if discharges are the cause
of the degradation/ aging
Moisture + - -
Void/ Delamination (+)
After a certain time once the dielectric
material starts to corrode
- +
Surface contamination + - +/-
If the surface contaminations are
creating surface discharges
Surface discharges +/-
Surface discharges with a high
intensity can be seen by a unstable
tan d
- +
Partial breakdowns +/-
If it is combined with erosion of
insulating material
+ +/-
Contact problems +
Shows up as an increased or unstable
tan d
+
Shows up as decreased or unstable
capacitance
+

19. 19
Some additional on moisture in insulation of Bushings
D20tan
D40tan
D60tan
D40tan
Dry bushing
Normal aged bushing
Bushing with moisture
D20tan D60tan<
The difference in tan d at 20°C
for Bushings with moisture is
small. For higher temperatures
the tan d gradient is increasing
and the moisture content is more
clearly to detect. Nevertheless
the difference between a dry and
a wet bushing could be at 40°C
0.2%, which still needs a high
accuracy to detect moisture/
insulation aging early enough.
Valid for wet bushings only!

20. 1. Statistical overview
2. Technical basics about Bushing design
3. Main parameter (Tan δ and Capacitance) and its measurement
4. Monitoring principles
5. Qualitrol solution
Agenda

21. 21
On-line Bushing Monitoring parameters challenges
• Partial Discharge
• Advantage:
• Can be measured with conventional PD techniques
• Gives an early warning before the oil or the solid insulation system is partly
damaged
• Disadvantage:
• Very difficult to determine between external and internal discharges
• Bushing discharges are usually small compared to corona discharges or
discharges out of the transformer tank. Therefore most of the time the
discharges of the bushing itself will not be early enough detected.
Discharges on overhead lines/ bushing surface 
up to some nC
Accepted discharges inside transformer  500pC up to 1 nC
Typical PD level for bushing faults  5pC up to 100 pC
Due to the low level of expected internal PD caused by bushings, the
Bushing PD will be hided behind the discharges coming from
transformer or outside. The determination where the discharges are
coming from is also very difficult for online purpose.

22. 1. Sum of currents (Balanced current) method
• Summation of all currents under consideration of their phases (ideal case = 0)
• Algorithms are considering increase of amplitude
2. Reference signal method
a) From sister bushing
• Bushings from same transformer and different phases will be measured against each
other (bridge circuit or phase shift measurement)
b) External reference signal
• Bushings will be compared to an independent reference source from the same phase
(like VT, CVT or existing coupling capacitances)
Monitoring principles overview

23. 23
On-line Bushing Monitoring balanced current method
Balance current method
• Assumption: all phases will have absolutely the
same voltage amplitude and the phase angles
between the phases are perfectly 120 degree
• Under these circumstances (assuming all bushings
have the same temperature and the same condition)
the sum of all bushing leakage currents will be zero
• A change in the amplitude of a single leakage current
will be counted as capacity increase
• An increase of the current sum without increased
single leakage current is counted as Power Factor
(Dissipation Factor) increase of a certain bushing
• In reality the phase voltages and angles are fluctuating
according to the balance of the load and the network
conditions.
C1
C2
C1
C2
C1
C2
Balancing
Unit
Summation
Unit
L2
L3
L1
Null
Meter
Conclusion: The fluctuation of the measured values prevents to detect changes of the insulation system causes
by e.g. moisture or creeping degradation or aging. Only changes in capacitance (partial breakdowns) or big
changes in Dissipation factor (Power factor) can be detected

24. On-line Bushing Monitoring balanced current principles
• Ideally, the sum of the three bushing currents should be zero
• In reality, not all parameters are equal from each phase
• During commissioning of the system, the leakage currents are adjusted so the sum of the (3)
currents is equal or close to zero
I. All three currents perfectly balanced and the sum equal to zero
II. A change in capacitance results in additional current perpendicular to phase voltage
III. A change in tangent delta results in additional active current, increase in DF (resp. PF)
• The magnitude of the change is an indicator of the problem’s severity
• The vector change indicates which bushing is going bad and
• Whether the power factor or capacitance is changing
I1
I2
I3
IS=0
I1
I2
I3
ISK0
I’2
I1
I2
I3
ISK0
I’2
I. III.II.

25. On-line Bushing Monitoring balanced current
Some thoughts about inaccuracy regarding typical phase and voltage
asymmetries due to unbalanced load:
Considering typical phase and voltage asymmetries (0.2 degree for the phase and 1.0%
for the voltage), the following deviations for the leakage currents of the different phases
can be assumed:
- 1 to 1.5% in amplitude (follows the voltage)
- 0.2 degree in angle
- Comparing the leakage currents of different phases that will result in an error of:
- 1 to 1.5% in the capacitance determination
- 0.0035 in tan d absolute (absolute tan d value for a typical RIP Bushing is 0.00325)
While the capacitance inaccuracy can be compensated by consideration of the real
phase voltage, the phase angle imbalance is difficult to compensate.
As result, changes caused by degradation of insulation material and moisture hardly can
be clearly detected.

26. 26
Bushing Monitoring Reference signal method
C1
C2
C1
C2
C1
C2
L2
L3
L1
R R R
Reference Method
• The phase difference of the
bushing leakage current and
the reference voltage will be
measured.
• 90° - phase difference
represents the angle Delta
• The phase difference
represents direct the angle
Phi
• By using the RMS value of
the leakage current and the
RMS value of the phase
voltage the capacitance can
be calculated.
• Bushings measured
independently
Bushing Monitor
Conclusion: Applying advanced software algorithms for noise and disturbance
reduction allows achieving a high accuracy in phase difference measurement.
VT
VT
VT VT
Phase shift Phase shift Phase shift
Tap Tap Tap

27. Phase shift measurement
90° - d
On-line Bushing Monitoring phase shift method
Raw Signal
Software algorithm
Signal processing
• Noise and harmonics needs
to be eliminated by advanced
software algorithms
• The accuracy of the phase
measurement is better than
0.1mrad (0.0057 Degree)
• The accuracy enables to detect
changes in tan d form e.g.
0.325% to 0.340%
• That enables to detect
moisture increase, insulation
system aging and
degradation early enough
• Temperature compensation
will be necessary to achieve
this accuracy

28. Calculation of tan Delta/ Power Factor and Capacitance
• Each bushing will be compared separately to its corresponding voltage measured via a
highly reliable voltage transformer as reference signal
• The phase shift between reference signal and leakage current from the bushing will be
measured
• Out of the phase shift, the tan delta can be directly calculated
• The capacitance will be calculated out of the magnitude of the leakage current of the
bushing and the magnitude of the reference voltage.
On-line Bushing Monitoring phase shift method
Note: Sister bushings can be used too as reference source. In that case the
asymmetries will influence the measured results in the same way as for the balanced
current method!

29. On-line Bushing Monitoring methods comparison
Method Advantages Disadvantages
Capacitance • Easy to apply
• Disturbances can be handled by
common hardware filtering
• Costs are significant lower than the
two other methods
• Can detect only capacitive changes
• Voltage changes across the additional Tap
capacity (C3) needs to be related manually
or by additional hardware to voltage
changes in the network
Balanced current • Easy to apply, no separate reference
source is needed
• Bushings are measured against each other
influenced results due to asymmetries
• Accuracy of results due to changing and
unbalanced network load conditions limited
(phase shift between phases and phase
voltage changes according balance of load)
• Assessment results often not reliable
Reference source
(independent)
• Each bushing will be measured and
assessed separately
• Slight changes are already visible
• Separate reference source is needed
• Phase shift calibration procedure needed
during commissioning

30. 1. Statistical overview
2. Technical basics about Bushing design
3. Main parameter (Tan δ and Capacitance) and its measurement
4. Monitoring principles
5. Qualitrol solution
Agenda

31. High voltage lines
Reference
Voltage
signal
Principle
Transformer
Bushing Sensor
Signal
Bushing Monitor
VT’s/CVT’s in the Switchyard of the
substation
Monitoring Principle

32. • USB and serial port
connections; IEC61850, DNP
3.0, Modbus, IEC60870
• Web based interface
• Universal power supply
• Versatile control of cooling systems,
alarms, and trip signals
• 8 relays per module
• Real time monitoring of bushing
health
• Detect early changes in Tan Delta
• Detect early changes in
Capacitance
Bushing
Monitor
BUSHINGS
• Spare slot to accommodate any of the
available modules for expandability of
your asset monitoring
•Hot-Swappable
SPARE MODULE SLOT
Bushing Monitor

33. Analytics
Dashboard
Trending
Notification Center
Web Based
Software
Dashboard
Provides a full view
of the complete
asset in one window
Trending Complete
and combined input
data overtime, great
tool to identify
chronical conditions
Notification Center
Useful tool to indicate
when any threshold was
meet or a ROC
Screen shot (1)

34. Tan δ/Power Factor
• Phase angle between
the applied voltage
across a capacitance
and the total current
through the
capacitance
Capacitance
• Indicates partial
breakdown capacitive
layers & Indicates
contact problems in
measuring tap, in the
connection to
conductor & conductive
tube
Temperature
• Utilized to compensate
the power factor (Tan δ)
to the reference
temperature at the factory
test value
Voltage
• Transformer Phase or Line
Voltage
• User Selectable
Leakage Current
• The current flowing from the
measuring tap pin through
ground
Signal Name
• Customizable signal name for
user identification
Bushing Module
UI
Screen shot (2)

35. On-line Bushing Monitoring Qualitrol solution

36. On-line Bushing Monitoring Qualitrol solution

37. On-line Bushing Monitoring Qualitrol solution

38. WWW.QUALITROLCORP.COM
Q & AFor technical questions or for a quote, please
contact our Applications Engineers at
info@qualitrolcorp.com