Download evaluating dielectric condition in sf6 circuit breakers

EVALUATING DIELECTRIC CONDITION IN SF6 CIRCUIT BREAKERS
Linda Nowak
Doble Engineering Company
85 Walnut Street
Watertown, MA 02472
617-393-3003
[email protected]
ABSTRACT
The purpose of the Doble test is to detect the presence of contamination and/or deterioration of the
breaker’s insulating system, this will allow corrective actions to be taken to ensure the integrity of the
breaker. The objective of this paper is to introduce the test techniques that have been developed for SF6
circuit breakers including double pressure and single pressure types. Also included is an explanation of
how to analyze test results and several test case studies illustrating various problems detected by Doble
testing.
INTRODUCTION
The use of SF6 gas as an insulation medium has been used throughout the world for more than 30 years.
The high dielectric strength and thermal conductivity of the gas is why SF6 breakers are currently the
principal breaker type being purchased by electric utilities. These breakers are available in dead-tank
designs ranging from 15 kV up to 800 kV and live-tank designs ranging from 72.5 kV up to 1200 kV. The
insulation system of these breakers is critical to ensuring the safe operation of the device. This paper will
highlight how power factor testing can determine the condition of this insulation.
SIGNIFICANCE OF TESTS
As mentioned above, the purpose of the Doble tests is to detect the presence of contamination and/or
deterioration of the breaker’s insulating system. This is done by measuring the insulations dielectric-loss
and capacitance and calculating the power-factor. The increase of the dielectric-loss, and consequently
the power factor, is representative of an increase in contamination and/or deterioration of the insulating
system and can detect a number of problems including:
• Moisture contamination resulting from leaks or incomplete cleaning and drying
•
Deterioration of line-to-ground and contact-grading capacitors
•
Surface contamination of weathersheds
•
Deterioration of insulating components such as operating rods, interrupters, interrupter supports
caused by corrosive arc by-products.
•
Internal corona damage of the same components listed above as a result of voids within the
insulation system.
•
Impurities, contamination and/or particles within the SF6 gas
1-18
PREPARATION FOR TESTS
The insulation systems of many types of SF6 circuit breakers are of relatively low capacitance (charging
current); accordingly, special attention must be given to the preparation of the test starting with the
isolation of the breaker. Because of the low capacitance involved, it is extremely important to fully
disconnect and ground all of the circuit breaker leads.
The inclusion of leads and their associated standoff insulators is detrimental for two reasons. First, the
leads tend to act as antennae, which amplify the effects of electrostatic interference. When high
interference levels are present, it may not be possible to use the most sensitive Current and Watt Meter
ranges, resulting in a less accurate reading. Disconnecting and grounding these leads not only reduces
the interference transmission, but the grounds tend to act as interference shields, thus further reducing the
effects of the interference. With the interference rejection capabilities of the M4000 the problems listed
above are less prominent then when using an M2H or MEU instrument.
Secondly, the presence of bus and support insulators contribute to the current and dielectric losses that are
being measured which can potentially reduce the sensitivity of the test. A short section of bus with one
or two standoff insulators can increase the measured charging current by 50%. If a moderate amount of
surface contamination is also present on the bus and support insulation, the resulting watts measurement
can be more than twice the amount of watts loss which would have been seen without the added bus
work.
The next step in test preparation is to minimize the effects of surface contamination and humidity which
can cause higher-than-normal losses. If contamination is visually observed, it may be more efficient to
clean the bushing surface prior to the start of testing. Guard collars may also be used to further reduce
surface leakage effects. For additional information, see the “Test Procedures, General” Section of the
Doble Test Set Instruction Manual.
TEST PROCEDURE
The test procedure for SF6 circuit breakers is dependent on the breaker design; this paper will discuss both
live- and grounded-tank breakers. For grounded-tank breakers, the test procedure depends upon whether
the breaker is equipped with single or multiple contacts. Live-tank breakers are divided into two
categories; the first will include T or Y styles, and the second, candlestick or I style. The procedure for
each depends upon the number of contacts per phase.
Grounded Tank Single-Contact Design
The dielectric circuit and test procedure for single-contact grounded-tank circuit breakers is outlined in
Figure 1 and Table 1, respectively. Not all insulation components depicted in the diagram of the dielectric
circuit are necessarily present in all models of circuit breakers. The optional components are indicated by
symbols which are not in bold print.
2-18
Terminal No. 1, 3, 5
Terminal No. 2, 4, 6
CB
CB
ROR
CGC
CSI
CIE
CSI
Insulation Components
CB – Bushing
CSI – Support Insulator
CGC – Grading Capacitor
CIE – Interrupter Envelope
ROR – Operating Rod
Dielectric Circuit for Single-Contact Grounded-Tank Circuit Breakers
Figure 1
Table 1
Test Procedures for Single-Contact Grounded-Tank Circuit Breakers
Test
No.
Breaker
Position
Test
Mode
Terminal
Energized
Terminal
Floating
Terminal
UST
Insulation
Measured
1
OPEN
GST-GROUND
1
2
–
CB + CSI + ROR
2
OPEN
GST-GROUND
2
1
–
CB + CSI
3
OPEN
GST-GROUND
3
4
–
CB + CSI + ROR
4
OPEN
GST-GROUND
4
3
–
CB + CSI
5
OPEN
GST-GROUND
5
6
–
CB + CSI + ROR
6
OPEN
GST-GROUND
6
5
–
CB + CSI
7
OPEN
UST
1
–
2
CIE + CGC
8
OPEN
UST
3
–
4
CIE + CGC
9
OPEN
UST
5
–
6
CIE + CGC
In addition, supplemental Hot-Collar tests should be performed on all bushings. On large bushings, collar
tests should be performed at several locations (top, middle and bottom) or a multiple collar test can also
be performed with several collars located along the length of the bushing.
Under conditions of severe electrostatic interference, it may be helpful to modify the procedure listed in
Table 1 by applying grounds to the bushing of the tanks not under test.
3-18
Grounded-Tank Multi-Contacts Design
The dielectric circuit and test procedure for a multi-contact grounded-tank SF6 breaker is shown in
Figures 2 and Table 2, respectively. These designs contain additional interrupters, support insulators
and/or operating rods which are connected between the interrupters and are therefore not directly stressed
when the breaker is in the open position. Additional closed breaker tests must be performed to evaluate
the integrity of these added assemblies. These closed breaker tests are listed as tests 10, 11 and 12 in
Table 2. Hot-Collar tests as mentioned in the grounded tank single contact section should be performed
on all bushings.
Terminal No. 1, 3, 5
Terminal No. 2, 4, 6
CB
CSI
CB
CGC1
CGC2
CIE1
CIE2
CSI
CSI
ROR
Insulation Components
CB – Bushing
CSI – Support Insulator
CGC – Grading Capacitor
CIE – Interrupter Envelope
ROR – Operating Rod
Dielectric Circuit for Multi-Contact Grounded-Tank Circuit Breakers
Figure 2
4-18
Table 2
Test Procedures for Multi-Contact Grounded-Tank Circuit Breakers
Test
No.
Breaker
Position
Test
Mode
Terminal
Energized
Terminal
Floating
Terminal
UST
Insulation
Measured
1
OPEN
GSTGROUND
1
2
–
CB + CSI
2
OPEN
GSTGROUND
2
1
–
CB + CSI
3
OPEN
GSTGROUND
3
4
–
CB + CSI
4
OPEN
GSTGROUND
4
3
–
CB + CSI
5
OPEN
GSTGROUND
5
6
–
CB + CSI
6
OPEN
GSTGROUND
6
5
–
CB + CSI
7
OPEN
UST
1
–
2
CIE1&2 + CGC1&2
8
OPEN
UST
3
–
4
CIE1&2 + CGC1&2
9
OPEN
UST
5
–
6
CIE1&2 + CGC1&2
10
CLOSED
GSTGROUND
1 or 2
–
–
(2)CB + (3)CSI + ROR
11
CLOSED
GSTGROUND
3 or 4
–
–
(2)CB + (3)CSI + ROR
12
CLOSED
GSTGROUND
5 or 6
–
–
(2)CB + (3)CSI + ROR
Live-Tank T and Y Module Designs
The dielectric circuit and test technique for T and Y module breakers are outlined in Figures 3 and Table
3, respectively.
Higher ratings are achieved by adding modules that are connected in series. For these designs, the same
procedure is utilized for each module. It is advantageous to ground modules which are not being tested so
as to minimize the effects of the electrostatic interference.
For circuit breaker modules equipped with multi-section support insulators, greater sensitivity can be
obtained by performing tests on parallel sections of individual module supports. The operating rod is also
included in the measurement for this test due to the capacitive coupling (CC) through the insulating gas.
Refer to Test No. 4 in Table 3. Please note the “Observations and Other Considerations” has additional
comments regarding test techniques and recommendations.
5-18
CGC1
CGC2
Terminal
D
CIE1
Terminal
A
CIE2
Terminal
B
CRE1
ROR
CSI
CRE2
Insulation Components
CGC – Grading Capacitor
CIE – Interrupter Envelope
CRE – Resistor Envelope
CSI – Support Insulator
ROR – Operating Rod
Dielectric Circuit for T and Y Module Circuit Breakers
Figure 3
Table 3
Test Procedures for T and Y Module Circuit Breakers
Test
No.
Test
Mode
Terminal
Energized
Terminal
Ground
Terminal
Guard
Terminal
UST
Insulation
Measured
1
UST
D
B
–
A
CIE1 + CGC1 + CRE1
2
UST
D
A
–
B
CIE2 + CGC2 + CRE2
3
GST-GUARD
D
–
A&B
–
CSI + ROR
4
For multi-section support column, energize between each section with other ends grounded
Live-Tank Candlestick or I Type
The dielectric circuit and test technique for a single-contact candlestick-type breaker is outlined in Figures
4 and Table 4, respectively. This design is similar in concept to the T and Y module breaker; however,
one of the distinctions is that it utilizes only one interrupter per module. These breakers should NOT be
tested with bus-work nor current transformers connected for the same considerations discussed in
the “preparation for tests” section.
6-18
Terminal A
CIE
CGC
CRE
Terminal B
ROR
CSI
Insulation Components
CGC – Grading Capacitor
CIE – Interrupter Envelope
CRE – Resistor Envelope
CSI – Support Insulator
ROR – Operating Rod
Dielectric Circuit for Candlestick or I Type Module Circuit Breakers
Figure 4
Table 4
Test Procedures for Candlestick or I Type Module Circuit Breakers
Test
No.
Breaker
Position
Test
Mode
Terminal
Energized
Terminal
Guard
Terminal
UST
Insulation
Measured
1
OPEN
UST
B
–
A
CIE + CGC + CRE
2
OPEN
GST-GUARD
B
A
–
CSI + ROR
There may be instances where the presence of excessive levels of electrostatic interference prevents the
use of the most sensitive Watt Multiplier or possibly prevent obtaining a null balance of the Watts Adjust
Dial (applies to M2H and MEU instruments). In these situations the alternative test procedure outlined in
Table 5 may be employed.
Table 5
Test Procedures for Candlestick or I Type Module Circuit Breakers (Alternative Method)
Test
No.
Breaker
Position
Test
Mode
Terminal
Energized
Terminal
UST
Insulation
Measured
1
OPEN
UST
A
B
CIE + CGC + CRE
2
CLOSED
GST-GROUND
A
–
CSI + ROR
7-18
Higher voltage ratings for Candlestick or I Type module circuit breakers are achieved by combining
additional single-contact modules in series, per phase. The method of connection for the modules may
vary. If it is possible, the preferred approach is to disconnect and test each module on an individual basis.
If this is not practical, the proper test procedure will depend on the specific configuration of the phases of
the circuit breaker. Diagrams and outlines of procedures for some of the possible design options are
included in the following figures.
The higher-voltage multi-contact Candlestick or I Type circuit breakers are usually designed with multisection support columns. A more thorough test can be performed by testing two individual insulator
sections together, in parallel. The operating rod is also included in the measurement for this test due to
the capacitive coupling (CC) through the insulating gas.
Terminal A
Terminal B
CIE1
CGC1
CRE1
CGC2
CIE2
CRE2
Terminal D
CSI1
CC
Terminal C
CC
CSI2
ROR1
ROR2
Terminal E
CSI2
CSI12
Insulation Components
CGC – Grading Capacitor
CIE – Interrupter Envelope
CRE – Resistor Envelope
CSI – Support Insulator
ROR – Operating Rod
Multi-Contact Circuit Breaker with Multi-Section Support Columns
Figure 5
Table 6
Test Procedure for Candlestick or I Type Module Circuit Breaker Shown in Figure 5
Test Breaker
No. Position
Test
Mode
Terminal Terminal Terminal
Energized Grounded
UST
Insulation
Measured
1
OPEN
UST
D
B
A
CIE1 + CGC1 + CRE1
2
OPEN
UST
D
A
B
CIE2 + CGC2 + CRE2
3
OPEN
GST-GROUND
D
–
–
CSI1&2 + ROR1&2
4
OPEN
GST-GROUND
C
D
–
CSI1 + ROR1
5
OPEN
GST-GROUND
E
D
–
CSI2 + ROR2
NOTE: Repeat Test Nos. 4 and 5 for each pair of Support Insulators
8-18
Terminal D
CIE1
CGC1
CGC2
CRE1
Terminal A
CSI1
CC
CSI2
ROR1
CRE2
Terminal B
CC
Terminal C
CIE2
ROR2
Terminal E
CSI1
CSI2
Insulation Components
CGC – Grading Capacitor
CIE – Interrupter Envelope
CRE – Resistor Envelope
CSI – Support Insulator
ROR – Operating Rod
Multi-Contact Circuit Breaker with Multi-Section Support Columns
Figure 6
Table 7
Test Procedure for Circuit Breaker Shown in Figure 6
Test Breaker
No. Position
Test
Mode
Terminal Terminal Terminal Terminal
Energized Grounded Guarded
UST
Insulation
Measured
1
OPEN
UST
D
B
–
A
CIE1 + CGC1 +
CRE1
2
OPEN
UST
D
A
–
B
CIE2 + CGC2 +
CRE2
3
OPEN
GSTGROUND
A
–
B
–
CSI1&2 + ROR1&2
4
OPEN
GSTGROUND
B
–
A
–
CSI1&2 + ROR1&2
5
OPEN
GSTGROUND
C
A
–
CSI1 + ROR1
6
OPEN
GSTGROUND
E
B
–
CSI2 + ROR2
9-18
Terminal D
CGC1
CGC2
CRE1
CIE1
Terminal A
CSI1
CIE2
Terminal B
CSI2
CC
Terminal
C
CGC3
CRE2
CRE3
Terminal F
CSI3
CC
Terminal
ROR1
E
ROR2 Terminal
G
CSI2
CSI3
CSI1
CIE3
CC
ROR3
Insulation Components
CGC – Grading Capacitor
CIE – Interrupter Envelope
CRE – Resistor Envelope
CSI – Support Insulator
ROR – Operating Rod
Multi-Contact Circuit Breaker with Multi-Section Support Columns
Figure 7
Table 8
Test Procedure for Circuit Breaker Shown in Figure 7
Test Breaker
No. Position
Test
Mode
Terminal
Energized
Terminal
Grounded
Terminal
Guarded
Terminal
UST
Insulation
Measured
1
OPEN
UST
D
B
–
A
CIE1 + CGC1 + CRE1
2
OPEN
UST
D
A&F
–
B
CIE2 + CGC2 + CRE2
3
OPEN
UST
B
D
–
F
CIE3 + CGC3 + CRE3
4
OPEN
GSTGROUND
A
–
B
–
CSI1&2&3 + ROR1&2&3
5
OPEN
GSTGROUND
B
–
A&F
–
CSI1&2&3 + ROR1&2&3
6
OPEN
GSTGROUND
F
–
B
–
CSI1&2&3 + ROR1&2&3
7
OPEN
GSTGROUND
C
A
–
–
CSI1 + ROR1
8
OPEN
GSTGROUND
E
B
–
–
CSI2 + ROR2
9
OPEN
GSTGROUND
G
F
–
–
CSI3 + ROR3
10-18
ANALYSIS OF TEST RESULTS
Grounded-Tank Breakers
Generally, test results are analyzed on the basis of measured current and watts. If the test includes a
measurement of a grading or line-to-ground capacitor, power factor and capacitance should also be
evaluated. Changes in any of these parameters would warrant concern. Usually, insulation problems are
reflected in an increase in the watts or power factor and either an increase or decrease in the measured
capacitance. Results should be compared with data from similar breakers that are tabulated in the Doble
Test-Data Reference Book. Comparison should also be made with initial and previous test data and with
other bushing tests on the same breaker and similar breakers on your system.
Higher-than-normal power factors and watts readings are usually indicative of moisture contamination
and/or by-products of arced SF6 which have condensed or deposited on insulating surfaces. If elevated
measurements are noted, consideration should be given to the following:
1. High readings may simply reflect the presence of excessive external surface contamination.
Accordingly, the bushing surfaces should be cleaned. The use of guard collars may also be
effective.
2. The moisture content of the gas should be checked. Low moisture content does not guarantee that
the breaker is dry because the water could be condensed on internal surfaces; however, high
moisture content would confirm a general condition of contamination.
3. If the measurement includes contact grading or line-to-ground capacitors, changes from previous
readings could reflect temperature sensitivity of the capacitors. This condition has been
documented for older air-blast circuit breakers6 7. Capacitors on the High Voltage Breaker, Inc.
Type HVB-242-31.5 are temperature sensitive (see HVB Instruction HVK-102) and this
condition may be common in a number of circuit-breaker types. If a change in test data that
might be related to temperature is noted, the circuit breaker’s manufacturer should be asked to
supply both power factor and capacitance versus temperature curves. As a general rule,
capacitance variations in excess of 5% of the previous or original test data will warrant an
investigation.
4. Suspect measurements that cannot be attributed to an innocuous source should be investigated.
An investigation should include an internal inspection followed by vacuum drying.
Hot-collar measurement should be evaluated on the basis of the current and watts measurements. Under
ideal test conditions, readings in the order of 0.01 watt at 10 kV can be expected. Readings higher than
0.1 watt at 10 kV are unacceptable and require an investigation. Consistency between similar readings on
similar bushings is more important than absolute limits. For example, if 0.02 watts is obtained on five out
of six bushings on one circuit breaker, and 0.08 watts is obtained on the sixth bushing, then the bushing
with the high reading should be investigated.
Multiple Hot-Collar tests are effective for evaluating the overall condition in the upper porcelain region.
Analysis of that is based on a comparison with similar bushing on the same or similar circuit breakers.3
Live-Tank Design
Measurements of support insulators and interrupter assemblies that are not in parallel with grading
capacitors are evaluated on the basis of current and watts. If the interrupter assembly includes a contact
grading capacitor, the evaluation is based primarily on the measured capacitance and calculated power
factor. The results should be compared with other phases, previous tests if available, tests on similar
11-18
breakers on the system, and data contained in the Doble Test-Data Reference Book. Higher-than-normal
watts readings may be related to excessive moisture contamination. Under these situations, operating the
breaker several times may improve the results. Higher-than-normal capacitance readings may indicate
short-circuited sections of the grading capacitor assembly. Under these situations separate tests should be
made with the grading capacitors disconnected from the breaker. See Item 3 under analysis of GroundTank Designs for comments regarding change as a result of temperature variations.
OBSERVATIONS AND OTHER CONSIDERATIONS
1. The current transformers associated with these breakers should be tested. For Live-Tank breakers
the CTs are usually of the free-standing design. Older circuit breaker models may incorporate the
CT as part of the interrupter support column of the module. For information on testing the current
transformers, please refer to the Instrument Transformer Section of the Doble Test-Data
Reference Book or to the test set Instruction Manual.
2. After maintenance is performed, it may be advantageous to perform tests prior to re-pressurizing
the breaker. Assuming that the internal condition of the breaker is clean and dry and that dry air
or SF6 is used to fill it, field experience indicates that virtually identical readings will be obtained
both with the gas at atmospheric pressure and at operating pressure.
3. During the installation of a new circuit breaker, it is advantageous to perform tests which would
be included as part of an investigation of questionable readings or when problems are suspected.
This benchmark data can then be used as a reference when trouble is experienced.
Investigative tests can include:
Ground-Tank Circuit Breakers
a) Hot-Collar tests on each skirt of all bushings
b) Separate measurement of internal capacitors
c) Hot-Collar tests on internal support insulators.
Live-Tank Circuit Breakers
a) Measurements of the grading capacitors with them isolated.
b) Measurements of the resistor envelope with it isolated.
c) Standard test across interrupter assembly with the capacitors and resistors removed; then
repeat the tests with the addition of each assembly.
d) Hot-Collar tests at several locations along the interrupter envelope with the circuit breaker in
the closed position.
e) Hot-Collar tests at several locations along the insulated support column.
FIELD EXPERIENCE – CASE STUDIES
With the intention of helping field-testing personnel and engineers with the analysis of test data for SF6
circuit breakers, we present the following Case Studies obtained as a contribution from several clients.
1. Testing with and without the bus connected.
Due to the low currents and watts-losses expected on most SF6 Circuit Breakers diagnostic tests, leaving
pieces of bus connected will have a significant influence on the test results. Let’s illustrate this point by
12-18
analyzing test data from an ABB 72PM31-12 Dead-Tank circuit breaker. When the circuit breaker was
tested with the bus connected, the following test results were obtained (Table 9):
Table 9
Doble Test with Bus Connected
Bushing
1
2
3
4
5
6
1–2
3–4
5–6
Test Mode
GST-Ground
GST-Ground
GST-Ground
GST-Ground
GST-Ground
GST-Ground
UST
UST
UST
Current(uA)
690
520
680
500
690
520
9
8
8
Watts-Loss
0.020
0.180
0.020
0.038
0.120
0.070
0.002
0.002
0.003
% Power-Factor
0.29
3.46
0.29
0.76
1.74
1.34
N/A
N/A
N/A
At first sight, the current readings show a typical pattern for SF6 circuit breakers, with similar readings for
bushings 1, 3, and 5, where the operating rod is installed. Higher than normal watts-losses readings were
obtained for bushings 2, 5, and 6, and this prompted the testing crew to investigate what could be causing
these readings. The UST tests showed acceptable and comparable test results.
They decided to disconnect all incoming and outgoing bus sections and re-tested the breaker, obtaining
the following test results (Table 10):
Table 10
Doble Test with Bus Disconnected
Bushing
1
2
3
4
5
6
1–2
3–4
5–6
Test Mode
GST-Ground
GST-Ground
GST-Ground
GST-Ground
GST-Ground
GST-Ground
UST
UST
UST
Current(uA)
520
350
520
350
520
350
7
7
7
Watts-Loss
0.006
0.005
0.006
0.004
0.006
0.004
0.001
0.001
0.001
% Power-Factor
0.12
0.14
0.12
0.11
0.12
0.11
N/A
N/A
N/A
This example demonstrates the importance of isolating this type of specimen from all other parts of the
electrical system to obtain suitable test results. It shows that the charging current and watts-losses
associated to the bus sections will affect the ground tests by including the contribution from these sections
and their standoff insulators. Note that the UST tests performed across each pole will not be affected by
leaving the bus connected because all ground currents will not be measured in the UST mode.
Additionally, if the bus sections are left connected, they tend to act as antennae amplifying the effects of
electrostatic interference. Disconnecting and grounding these sections will reduce this interference and
the grounds will also act as interference shields.
13-18
2. Moisture Effects on Test Results
The following example shows the effects that moisture can have on the Doble Test Results. This ABB
Type 72-PM-31-20 Circuit Breaker was manufactured in 2000. In 2004 the client discovered a leak in the
C phase pole of the breaker. The SF6 gas was tested and C phase tested high for moisture. A Doble test
was performed with the following results (Table 11):
Table 11
Doble Test after Leak Discovered
Bushing
1
2
3
4
5
6
1–2
3–4
5–6
Test Mode
GST-Ground
GST-Ground
GST-Ground
GST-Ground
GST-Ground
GST-Ground
UST
UST
UST
Ph.
C
C
B
B
A
A
C
B
A
Current(uA)
364
533
360
530
359
525
17
10
10
Watts-Loss
0.086
0.078
0.003
0.003
0.003
0.003
0.073
0.000
0.000
% Power-Factor
2.36
1.46
0.08
0.06
0.08
0.06
N/A
N/A
N/A
Technicians opened the C phase pole, cleaned the interior and fixed a faulty seal located at the end of the
tank. The SF6 gas was filtered and placed back in the pole. The following test was performed after
repairs and cleaning (Table 12):
Table 12
Doble Test after Leak Repaired and Moisture Removed
Bushing
1
2
3
4
5
6
1–2
3–4
5–6
Test Mode
GST-Ground
GST-Ground
GST-Ground
GST-Ground
GST-Ground
GST-Ground
UST
UST
UST
Ph.
C
C
B
B
A
A
C
B
A
Current(uA)
360
527
359
530
359
525
13
11
11
Watts-Loss
0.002
0.008
0.002
0.002
0.002
0.002
0.000
0.000
0.000
% Power-Factor
0.06
0.15
0.06
0.04
0.06
0.04
N/A
N/A
N/A
This example shows that moisture inside a SF6 circuit breaker will cause increased losses for the ground
and UST tests.
3. Flash-over inside Dead Tank Circuit Breaker
The following example will show the effects of an internal flash-over on Doble Test Results.
The breaker for this example is an ABB Type 72-PM-31-20, manufactured in 1996. This breaker along
with two others, same vintage and type were being moved from one location to another. Before placing
them in service the three breakers were Doble Tested. One of the UST readings was much higher than the
other phases on all three breakers. The following results are for the problem breaker (Table 13):
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Table 13
Doble Test after Moving Circuit Breaker
Bushing
1
2
3
4
5
6
1–2
3–4
5–6
Test Mode
GST-Ground
GST-Ground
GST-Ground
GST-Ground
GST-Ground
GST-Ground
UST
UST
UST
Current(uA)
727
531
724
528
732
528
20
16
20
Watts-Loss
0.018
0.028
0.017
0.011
0.013
0.008
0.008
0.001
0.000
% Power-Factor
0.25
0.53
0.23
0.21
0.18
0.15
N/A
N/A
N/A
A comparison of all three circuit breakers UST results can be seen in Table 14 below:
Table 14
Comparison of UST Results for Three Similar Circuit Breakers
Breaker #
1
1
1
2
2
2
3
3
3
Bushing
1–2
3–4
5–6
1–2
3–4
5–6
1–2
3–4
5–6
Test Mode
UST
UST
UST
UST
UST
UST
UST
UST
UST
Current(uA)
20
16
20
21
18
20
21
17
20
Watts-Loss
0.008
0.001
0.000
0.001
0.001
0.001
0.001
0.001
0.001
% Power-Factor
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
The client opened the pole for the first breaker and discovered clear signs of a flash-over. Photos of the
damage can be seen in Figure 8 and 9.
Internal Inspection of Suspect Pole
Figure 8
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Close Up of Flash-Over Evidence
Figure 9
After finding the flash-over signs for this pole the client replaced the pole with a new unit. The breaker
was retested after pole replacement. The final results can be seen in Table 15 below:
Table 15
Final Results for Circuit Breaker after Replacing Pole
Bushing
1
2
3
4
5
6
1–2
3–4
5–6
Test Mode
GST-Ground
GST-Ground
GST-Ground
GST-Ground
GST-Ground
GST-Ground
UST
UST
UST
Current(uA)
729
531
727
533
734
530
18
15
18
Watts-Loss
0.006
0.006
0.006
0.010
0.006
0.005
0.000
0.000
0.000
% Power-Factor
0.08
0.11
0.08
0.19
0.08
0.09
N/A
N/A
N/A
This example shows the benefit of comparing data with similar apparatus. In this case, comparing the
results with the same type and vintage breakers showed the high watts results for the UST test and high
power factor on the ground tests for the same phase.
4. Defective Capacitor Found on Live Tank Circuit Breaker
During a routine test on a Merlin-Gerin Type FA-2 Circuit Breaker, a client detected higher test results on
one of the entrance bushing and grading capacitor combination. The following table shows the test data
obtained:
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Table 16
High Power Factor Results on B Phase, C2 Assembly
Test
C1 (A)
C2 (A)
S1 (A)
C1 (B)
C2 (B)
S1 (B)
C1 (C)
C2 (C)
S1 (C)
Current (mA)
9.304
9.328
0.220
9.292
9.462
0.235
9.218
9.141
0.219
Watts
0.428
0.477
0.020
0.424
2.818
0.014
0.372
0.384
0.014
% Power Factor
0.46
0.51
N/A
0.46
2.98
N/A
0.40
0.42
N/A
Capacitance (pF)
2467.9
2474.4
58.26
2464.6
2508.7
62.37
2445.1
2424.7
58.13
Higher watts and percent power-factor were obtained on entrance bushing and grading capacitor assembly
C2 on phase B. The capacitance value was comparable with all other assemblies.
The client suspected a problem with the capacitor and decided to replace the capacitor. After replacing the
capacitor the C2 test was repeated with the following results (Table 17):
Table 17
Results after Replacing Capacitor on B Phase, C2 Assembly
Test
C2 (B)
Current (mA)
9.283
Watts
0.126
% Power Factor
0.14
Capacitance (pF)
2462.4
The test in Table 17 verified that the capacitor was defective. This example shows that a defective grading
capacitor can affect the watts and power factor for the associated assembly test.
ACKNOWLEDGEMENTS
I would like to acknowledge Leah Simmons, Doble Engineering Company, for her assistance with the
original version of this paper presented at the 2011 Doble Conference.
REFERENCES
[1] Gryszkiewicz, F.J., Bailey, W.L. and Salmeron, M.A., “Doble Testing SF6 Puffer Circuit Breakers (A
Progress Report),” Minutes of the Sixty-Seventh Annual International Conference of Doble Clients,
2000, Sec. 4-5.
[2] Rivers, M.H., Manifase, S.J. and Leech, J.F., “Doble Testing SF6 Puffer Circuit Breakers (A Progress
Report),” Minutes of the Fifty-Sixth Annual International Conference of Doble Clients, 1989, Sec. 510.1.
[3] Dodds, J. J., “Moisture Content In SF6 Equipment,” Minutes of the Fifty-Second Annual International
Conference of Doble Clients, 1985, Sec. 5-601.
[4] Manifase, S. J. and Osborn, Jr., S. H., “Doble Testing of SF6 Puffer Circuit Breakers,” Minutes of the
Fiftieth Annual International Conference of Doble Clients, 1983, Sec. 5-501.
[5] Stallard, B. K., “Hot Collar Testing,” Minutes of the Forty-Fifth Annual International Conference of
Doble Clients, 1978, Sec. 4-301.
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[6] Rickley, A.L. and Osborn, Jr., S.H., “EHV Circuit-Breaker Test Procedures (A Progress Report),”
Minutes of the Forty-Third Annual International Conference of Doble Clients, 1976, Sec. 5-401.
[7] Connors, Sr., G. J. “Doble Testing and Design Features of the General Electric Company ATB 550-3
Air Blast Circuit Breaker,” Minutes of the Thirty-Eighth Annual International Conference of Doble
Clients, 1971, Sec. 5-601.
[8] Rickley, A. L. and Osborn, Jr., S. H., “EHV Circuit Breaker Test Procedures,” Minutes of the ThirtyFifth Annual International Conference of Doble Clients, 1968, Sec. 5-901.
BIOGRAPHY
Linda A. Nowak is a Principal Engineer for Doble Engineering. She previously has 13 years of
experience working for Northern States Power and Xcel Energy. During her time at these utilities she
spent time in Electric Maintenance, Substation Engineering and Power Plant Engineering and
Maintenance. At Doble and Northern States Power she compiled extensive experience in the area of
diagnostic testing and condition assessment. While at Doble she has published several papers on various
power equipment topics. She is currently the secretary of the Doble Circuit Breaker Committee which
concentrates on issues associated with circuit breakers, disconnect switches and batteries. Ms. Nowak has
a B.S. in Electrical Engineering from the University of Minnesota. She is an IEEE member and Licensed
Professional Engineer in the State of Minnesota.
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