Download Oil Circuit Breaker Diagnostics

Oil Circuit Breaker Diagnostics
Thomas A. Prevost
Weidmann Diagnostic Solutions Inc.
Abstract:
Although considered old technology there is still a significant number of oil circuit breakers in operation.
Because their operation is critical to the power system, maintenance of these components is crucial.
Diagnostic evaluation of oil samples gives valuable information to the asset owner so that maintenance can
be based on the condition of the equipment. This paper will review the methodology used for OCB
diagnostic analysis and present some case histories that demonstrate the effectiveness of this approach.
Introduction:
Circuit Breakers are a critical component of an electrical system. They are used to connect and disconnect
transmission lines under normal operating conditions. They are also used to clear sections of a transmission
grid should a short circuit occur in the system, isolating the fault. The technology of circuit breakers has
evolved based primarily on the media in which the circuit breaker contacts are located. Early circuit
breakers used air as the dielectric media. When system voltages and current levels increased oil circuit
breakers were introduced. Later compressed air circuit breakers were developed followed by SF6 and
vacuum breakers, which is today the preferred technology for High Voltage Circuit breakers.
Figure 1
Development of Circuit Breaker Technology (ref. ABB)
Even though the technology for circuit breakers has evolved so that oil circuit breakers are rarely being
purchased and installed there is a significant number of oil circuit breakers installed in today’s electrical
power grid. In a presentation on using DGA as part of a circuit breaker maintenance program one utility
reported that they had approximately 5,800 oil circuit breakers on their system alone.i Many of these
breakers are exceeding forty years of service life.
The cost of replacing oil circuit breakers is prohibitive. Most of these breakers meet the system
requirements for voltage and normal and fault current interruption capability. While these breakers
continue to meet the needs of the industry they do so because of maintenance procedures. Because breakers
utilize moving contacts they will wear out, over time, requiring periodic maintenance. The use of a DGA
and oil quality diagnostic program will permit the asset owner to schedule maintenance based on the
condition of the breaker rather than based on time or operation counts. This can save the utility
significantly by applying maintenance dollars to those breakers which require service.
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Seventh Annual Weidmann Technical Conference, New Orleans, September 15-17, 2008
Oil Circuit Breaker Design
An oil circuit breaker consists of a steel tank partly filled with oil, through the cover of which is mounted
porcelain or composite insulating bushings. Contacts at the bottom of the bushings are bridged by a
conducting cross head carried by a wooden or composite lift rod, which in common designs drops by
gravity following contact separation by spring action, thus opening the breaker. An air cushion above the
oil level serves as an expansion volume to prevent pressure from building up inside the chamber after the
interruption of the short circuit current. Figure 2
Figure 2
Bulk Oil Circuit Breakerii
In an oil circuit breaker, mineral oil serves as the dielectric medium. The mineral oil serves two functions.
First it provides electrical insulation. The oil must have sufficient dielectric strength to insulate the high
voltage components from ground. When the breaker is open the oil must insulate the contacts themselves.
Second, the oil acts to extinguish the arc when the breaker is opened. Under the influence of the extreme
arc heat part of the oil decomposes into gases composed of 70% Hydrogen and 20% Acetylene, and also
produces carbon particles. The hydrogen gas has the effect of de-ionizing and extinguishing arcs at a rapid
rate by cooling the arc.
In order to reduce the arc extinguishing time interrupter chambers are uses. These chambers incorporate a
number of specially designed insulating plates that are stacked together to form a passage for the arc that is
alternately restricted and then laterally vented. This allows venting of the pressure inside the chamber and
aids in extinguishing the arc by creating a cross flow of gas. Figure 3
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Seventh Annual Weidmann Technical Conference, New Orleans, September 15-17, 2008
Figure 3
Example of Arc Suppression Gridiii
Good Condition
Deteriorated
Figure 4
Example of Good and Deteriorated Arc Suppression Grid
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Seventh Annual Weidmann Technical Conference, New Orleans, September 15-17, 2008
Oil Circuit Breaker Fault Types
IEEE C37.10 “Guide for Diagnostics and failure Investigation of Power Circuit Breakers” lists specific
failure types for bulk oil circuit breakers.iv
Dielectric types of failures:
a. Internal bushing deterioration by oil leakage, moisture/tracking
b. Water leakage into main tank
c. Tracking or related deterioration of operating rod
d. Loose and splitting joints
e. Carbonization of the oil
Interruption types of failures
a. Deteriorated arcing contacts or baffle chambers
b. Evolving fault
c. Binding mechanism
d. Inoperative tank heaters
e. Control malfunction including interlocks
f. Operating without a full close cycle
g. Pumping or related pilot valve failures
Specific fault types which can be diagnosed through an oil sample involve the dielectric integrity of the
insulating oil as well as contact and grid condition.
•
•
•
•
Fault Types Diagnosed by OCB Oil Diagnostic Program
Erosion of Contact Materials
Increased contact resistance
Arc Suppression grid deterioration
Dielectric integrity of insulation system
Diagnostic Methodology
Dissolved Gas Analysis (DGA) has been used successfully for many years to indicate incipient thermal or
dielectric faults in transformers. In analyzing dissolved gases in transformers one looks for so-called “key
gases” that indicate the type of fault based on the temperature at which they are formed. Acetylene, for
example, is a key gas for electrical arc because acetylene is o nly formed under high temperature conditions
which for transformers only occur when an electrical arc is present. For an OCB, arcing occurs during
every operation, so that the key gases will be produced under normal operation.
Gases
Hydrogen
Ethane, Methane
Ethylene
Indication
Partial Discharge/Heating Arcing
Heating Gases
“Hot Metal” Gas
Acetylene
Arcing
Table 1
DGA Key Gases
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Seventh Annual Weidmann Technical Conference, New Orleans, September 15-17, 2008
During normal OCB operation a gas bubble is produced when the contacts separate. The extremely hot arc
being formed between the contacts produces this gas bubble which is composed of approximately 70%
Hydrogen and 20% Acetylene. The remaining gas is developed at the perimeter of the gas bubble where the
oil is relatively cooler. In this region the so-called hot metal gas Ethylene is formed and as the temperature
further decreases Ethane and Methane are formed. Very little heating occurs in a healthy OCB so that the
amount of hot metal gases generated should be small. The ratio of heating to arcing gases should be
consistent. An increase in the ratio would indicate that heating gases are being generated by some other
means than the normal separation of contacts, such as increased contact resistance due to oxidation.
Arc tip and arc shaft erosion can be measured by analysis of the oil for presence of characteristic metals
such as copper, silver and tungsten using an Inductively Coupled Plasma (ICP) spectrometer.
Arc suppression grids deteriorate every time an arc is extinguished, producing many small particles. As the
grid deteriorates the openings become larger which increases the arc time and larger size particles are
produced. By counting the quantity of particles in the oil and comparing the size distribution, the condition
of the suppression grid can be determined. Microscopic examination of the particles for fibrous materials
can supplement the particle size distribution diagnostic for arc suppression grid wear.
Oil quality tests such as moisture content, dielectric strength, color, and interfacial tension can also be an
effective indicator in indicating OCB problems. The mineral oil must maintain its dielectric strength in
order maintain the insulating strength of the circuit breaker. IEEE C57.106 “Guide for Acceptance and
Maintenance of Insulating Oil in Equipment” gives guidelines for the continued use of service-aged circuit
breaker insulating oil.v The guide states that the chief problem in circuit breaker oil maintenance is to keep
the fluid free of water, arc decomposition products, and other contaminants. If the visual examination of the
oil shows the presence of these contaminants and the dielectric strength is below an acceptable value then
the oil should be reconditioned by using blotter papers or paper cartridge filters.
Table 2 summarizes the diagnostic tests that Weidmann Diagnostic Solutions (WDS) has incorporated into
its OCB diagnostic program and the type of fault that it can diagnose.vi
OCB Problem
Increased contact
resistance
Contact tip erosion
Test(s)
DGA
Arc suppression grid
degradation
Insulation condition
Particle count, ICP, Oil
dielectric, Color
DGA, Oil dielectric,
Moisture content, IFT, Color
DGA, ICP, Dielectric of oil
Result(s)
Increased hot metal gases, Increased heating to
arcing gas ratios
Hot metal gases, Metals in oil, Metal particles
observed by microscope, Lower oil dielectric
Large particles in oil, Presence of cellulose fibers,
Lower oil dielectric
Tracking on insulation components, Partial
discharge,
Table 2
Diagnostic Tests
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Seventh Annual Weidmann Technical Conference, New Orleans, September 15-17, 2008
Establishment of Diagnostic Thresholds
The establishment of acceptable levels for gases was determined following the same approach
recommended by IEEE in the development of gas threshold levels for transformers. The 90th percentile
value of a large database with approximately 8,000 records was calculated for each gas level. These levels
are given in Table 3.
Fault Gas
Hydrogen
Carbon Monoxide
Methane
Ethane
Ethylene
Acetylene
Total Dissolved Combustible Gas
90th Percentile Concentration (ppm)
46
146
39
17
155
397
800
Table 3
90th Percentile Values
The 90th percentile values were used to establish the normal levels for individual fault gases. A similar
approach was utilized to establish the gas ratios. The WDS OCB diagnostic program utilizes three ratios for
diagnostics:
Ratio 1 – Ethylene/Acetylene
Ratio 2 – (Methane+Ethane+Ethylene)/Acetylene
Ratio 3- Hydrogen/Acetylene
Ratios 1 and 2 which are ratios of heating to arcing gases indicate that abnormal heating is occurring. Ratio
3 is an indication of partial discharge activity.
A similar statistical approach was utilized to establish the normal levels for particles according to size
distribution as well as metals. IEEE C57.106-2006 levels were utilized for the normal thresholds for oil
quality.
The overall condition of the OCB is calculated by utilizing a weighted average of the following tests;
Individual key gases, gas ratios, metals, particles (number and size distribution), and oil quality tests. Each
test result is weighted according to its diagnostic contribution. For example metals and gas ratios are
weighted more heavily because they can indicate contact problems which should be investigated as soon as
possible. Oil quality, individual gas levels and particles are indicators of a developing problem and while
they contribute to the overall condition code they are weighted less. The WDS OCB diagnostic program
establishes four levels of condition codes which in turn give recommendations for sampling intervals.
These are given in table 4.
Condition Code
Normal
Pre-Caution
Caution
Warning
Recommended Sample Interval
Utility Practice
Six Months
Three Months
Internal Inspection/ One month
Table 4
Condition Codes
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Example Report, Normal Condition:
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Seventh Annual Weidmann Technical Conference, New Orleans, September 15-17, 2008
Example Report, Warning Condition:
The customer did an internal inspection based on the OCB Diagnostic recommendation. He found severe
coking and a damaged contact.
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Conclusion:
There will continue to be a significant number of oil circuit breakers in service for the foreseeable future.
This equipment, due to its design, will require maintenance to replace worn contacts, deteriorated arc
suppression grids and other moving parts as well as reconditioning of the oil. By utilizing the WDS OCB
Diagnostic program the equipment owner can ascertain the condition of the OCB and plan sampling and
maintenance intervals based on the results.
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Seventh Annual Weidmann Technical Conference, New Orleans, September 15-17, 2008
References:
i
Salinas, Alex, “Southern California Edison Oil Circuit Breaker Analysis Program”, Presentation at the
Fourth Annual Weidmann Technical Conference, November 16, 2005.
ii
Sampson, Mark, “Power Circuit Breakers”
iii
Sampson, Mark, “Power Circuit Breakers”
iv
IEEE C57.13.10-1995,” Guide for Diagnostics and failure Investigation of Power Circuit Breakers”
v
IEEE C57.106-2006, “Guide for Acceptance and Maintenance of Insulating Oil in Equipment”
vi
Jakob, F., Jakob, K., Jones, S., Youngblood, R., Salinas, A. , “OCB Diagnostics”
Acknowledgement:
I would like to thank Steve Skinner of Idaho Power Company for his help in preparing this paper.
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Seventh Annual Weidmann Technical Conference, New Orleans, September 15-17, 2008