EVALUATING DIELECTRIC CONDITION IN SF CIRCUIT BREAKERS

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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

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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.

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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





7 OPEN UST 1 – 2 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.

CIE

ROR CGC

CSI

CB CB

CSI

Terminal No. 2, 4, 6 Terminal No. 1, 3, 5

Insulation Components

CB – Bushing CSI – Support Insulator

CGC – Grading Capacitor CIE – Interrupter Envelope

ROR – Operating Rod

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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.

Dielectric Circuit for Multi-Contact Grounded-Tank Circuit Breakers

Figure 2


CB – Bushing CSI – Support Insulator



CIE1

CGC1

CB

CSI

Terminal No. 2, 4, 6 Terminal No. 1, 3, 5

CIE2

CGC2

CB

CSI

ROR CSI

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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 GST-

GROUND

1 2 – CB + CSI

2 OPEN GST-

GROUND 2 1 – CB + CSI

3 OPEN GST-

GROUND

3 4 – CB + CSI

4 OPEN GST-

GROUND

4 3 – CB + CSI

5 OPEN GST-

GROUND 5 6 – CB + CSI

6 OPEN GST-

GROUND

6 5 – CB + CSI

7 OPEN UST 1 – 2 CIE1&2 + CGC1&2



10 CLOSED GST-

GROUND 1 or 2 – – (2)CB + (3)CSI + ROR

11 CLOSED GST-

GROUND

3 or 4 – – (2)CB + (3)CSI + ROR

12 CLOSED GST-

GROUND

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.

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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.

Terminal

D Terminal

B

Terminal

A

ROR

CRE1

CGC1

CIE1

CSI

CRE2

CGC2

CIE2



CRE – Resistor Envelope CSI – Support Insulator


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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

ROR CSI





CRE CGC CIE

Terminal B

Terminal A

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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 multi-

section 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.

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

No.

Breaker

Position

Test

Mode

Terminal

Energized

Terminal

Grounded

Terminal

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

ROR1

CSI1

CRE1 CGC1 CIE1

Terminal D

Terminal A

Terminal C

CSI12

CC

ROR2

CSI2

CRE2 CGC2 CIE2

Terminal B

Terminal E

CSI2

CC





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Figure 6

Table 7

Test Procedure for Circuit Breaker Shown in Figure 6

Test

No.

Breaker

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 – B CIE2 + CGC2

+

CRE2

3 OPEN GST-

GROUND A – B – CSI1&2 + ROR1&2

4 OPEN GST-

GROUND

B – A – CSI1&2 + ROR1&2

5 OPEN GST-

GROUND

C A – CSI1 + ROR1

6 OPEN GST-

GROUND E B – CSI2

+ ROR2

ROR1

CSI1

CRE1 CGC1 CIE1

Terminal A

Terminal D

Terminal C

CSI1

CC

ROR2

CSI2

CRE2 CGC2 CIE2

Terminal B

Terminal E

CSI2

CC





10-18


Figure 7

Table 8

Test Procedure for Circuit Breaker Shown in Figure 7

Test

No.

Breaker

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 GST-

GROUND A – B – CSI1&2&3 + ROR1&2&3

5 OPEN GST-

GROUND B – A & F – CSI1&2&3 + ROR1&2&3

6 OPEN GST-

GROUND F – B – CSI1&2&3 + ROR1&2&3

7 OPEN GST-

GROUND C A – – CSI1

+ ROR1

8 OPEN GST-

GROUND E B – – CSI2

+ ROR2

9 OPEN GST-

GROUND G F – – CSI3

+ ROR3





Terminal

C ROR1

CSI1

CRE1 CGC1 CIE1

Terminal A

Terminal D

CSI1

CC

ROR2

CSI2

CRE2 CGC2 CIE2

Terminal B

Terminal

E

CSI2

CC

ROR3

CSI3

CRE3 CGC3 CIE3

Terminal F

Terminal

G

CSI3

CC

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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

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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 Ground-

Tank 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

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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 Test Mode Current(uA) Watts-Loss % Power-Factor

1 GST-Ground 690 0.020 0.29

2 GST-Ground 520 0.180 3.46

3 GST-Ground 680 0.020 0.29

4 GST-Ground 500 0.038 0.76

5 GST-Ground 690 0.120 1.74

6 GST-Ground 520 0.070 1.34

1 – 2 UST 9 0.002 N/A

3 – 4 UST 8 0.002 N/A

5 – 6 UST 8 0.003 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


1 GST-Ground 520 0.006 0.12

2 GST-Ground 350 0.005 0.14

3 GST-Ground 520 0.006 0.12

4 GST-Ground 350 0.004 0.11

5 GST-Ground 520 0.006 0.12

6 GST-Ground 350 0.004 0.11

1 – 2 UST 7 0.001 N/A

3 – 4 UST 7 0.001 N/A

5 – 6 UST 7 0.001 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.

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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 Test Mode Ph. Current(uA) Watts-Loss % Power-Factor

1 GST-Ground C 364 0.086 2.36

2 GST-Ground C 533 0.078 1.46

3 GST-Ground B 360 0.003 0.08

4 GST-Ground B 530 0.003 0.06

5 GST-Ground A 359 0.003 0.08

6 GST-Ground A 525 0.003 0.06

1 – 2 UST C 17 0.073 N/A

3 – 4 UST B 10 0.000 N/A

5 – 6 UST A 10 0.000 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 Test Mode Ph. Current(uA) Watts-Loss % Power-Factor

1 GST-Ground C 360 0.002 0.06

2 GST-Ground C 527 0.008 0.15

3 GST-Ground B 359 0.002 0.06

4 GST-Ground B 530 0.002 0.04

5 GST-Ground A 359 0.002 0.06

6 GST-Ground A 525 0.002 0.04

1 – 2 UST C 13 0.000 N/A

3 – 4 UST B 11 0.000 N/A

5 – 6 UST A 11 0.000 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


1 GST-Ground 727 0.018 0.25

2 GST-Ground 531 0.028 0.53

3 GST-Ground 724 0.017 0.23

4 GST-Ground 528 0.011 0.21

5 GST-Ground 732 0.013 0.18

6 GST-Ground 528 0.008 0.15

1 – 2 UST 20 0.008 N/A

3 – 4 UST 16 0.001 N/A

5 – 6 UST 20 0.000 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 # Bushing Test Mode Current(uA) Watts-Loss % Power-Factor

1 1 – 2 UST 20 0.008 N/A

1 3 – 4 UST 16 0.001 N/A

1 5 – 6 UST 20 0.000 N/A

2 1 – 2 UST 21 0.001 N/A

2 3 – 4 UST 18 0.001 N/A

2 5 – 6 UST 20 0.001 N/A

3 1 – 2 UST 21 0.001 N/A

3 3 – 4 UST 17 0.001 N/A

3 5 – 6 UST 20 0.001 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


1 GST-Ground 729 0.006 0.08

2 GST-Ground 531 0.006 0.11

3 GST-Ground 727 0.006 0.08

4 GST-Ground 533 0.010 0.19

5 GST-Ground 734 0.006 0.08

6 GST-Ground 530 0.005 0.09

1 – 2 UST 18 0.000 N/A

3 – 4 UST 15 0.000 N/A

5 – 6 UST 18 0.000 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:

17-18

Table 16

High Power Factor Results on B Phase, C2 Assembly

Test Current (mA) Watts % Power Factor Capacitance (pF)

C1 (A) 9.304 0.428 0.46 2467.9

C2 (A) 9.328 0.477 0.51 2474.4

S1 (A) 0.220 0.020 N/A 58.26

C1 (B) 9.292 0.424 0.46 2464.6

C2 (B) 9.462 2.818 2.98 2508.7

S1 (B) 0.235 0.014 N/A 62.37

C1 (C) 9.218 0.372 0.40 2445.1

C2 (C) 9.141 0.384 0.42 2424.7

S1 (C) 0.219 0.014 N/A 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 Current (mA) Watts % Power Factor Capacitance (pF)

C2 (B) 9.283 0.126 0.14 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. 5-

10.1.

[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.

18-18

[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 Thirty-

Fifth 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.

EVALUATING DIELECTRIC CONDITION IN SF CIRCUIT BREAKERS

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