Maintenance Power
Resume:
*th International Conference on Probabilistic Methods Applied to Power Systems, Iowa State University, Ames, Iowa,
September 12-16, 2004
Circuit Breaker and Transformer Inspection and
Maintenance: Probabilistic Models
Satish Natti, Panida Jirutitijaroen, Mladen Kezunovic, Fellow, IEEE, and Chanan Singh, Fellow, IEEE
out. Finally, a circuit breaker maintenance model is proposed.
Oil filled and air blast circuit breakers are considered in this
Abstract Circuit Breakers and Transformers are two of the
most common components in the power system. Catastrophic paper since most of the circuit breakers that are already in
failure of these components will result in high cost associated service are of the above type.
with the loss of load and component replacement. Preventive
maintenance may reduce these costs by extending the
II. DETERIORATION PROCESSES OF CIRCUIT BREAKER
components lifetime and increasing availability. However, too
much maintenance may be costly while too little maintenance
A. Deterioration of Operating Mechanism
may result in catastrophic failure. Probabilistic models that will
include the effects of maintenance on reliability are needed to This includes the deterioration of interrupter chamber,
perform cost benefit analysis and arrive at an optimal valves and various moving components. Moisture and
maintenance strategy.
corrosion of metal parts are some of the causes that are
A probabilistic maintenance model introduced earlier for
responsible for deterioration process of operating mechanism.
power transformer is proposed for circuit breaker uses in this
As a result, the breaker may fail to operate.
paper. The model is based on data coming from a study of
deterioration process, maintenance tasks, and inspection tests for
B. Deterioration of Contacts
circuit breaker.
Oxidation of contacts results in formation of a thin oxide
Index Terms Circuit Breakers, transformers, inspection,
film over the contact surfaces. At higher temperatures these
maintenance, probabilistic model.
oxide materials will begin to soften and might result in a
plastic deformation. Finally, contact erosion takes place due to
I. INTRODUCTION
the vaporization of electrodes during the current interruption
F AILURE of circuit breakers and power transformers can process [4]. These conditions may result in binding of
greatly affects the power delivery. The remaining life of contacts.
power apparatus and maintenance cost are two most important
C. Deterioration of Oil
aspects, which affects the maintenance policies. Various
maintenance strategies are reported in literature so far [1]. It Arc byproducts combine with moisture and oxygen in the
was concluded that power apparatus service availability and oil and reduce the dielectric strength of the oil. Accumulation
replacement cost should be balanced in order to get an optimal of these products contributes to the deterioration of oil [5]. If
maintenance strategy. Incipient failures have along term- prolonged, this condition causes arcing in the insulation
accumulated effect, which may cause major failures if no gradually developing into an internal fault.
related maintenance action is taken. In reference [2], failure,
D. Deterioration Failure
repair and maintenance sequences are described as Markov
Deterioration process results in deterioration failure, which
processes and optimal maintenance intervals are discussed in
is a long term-accumulated fault. It can happen mostly due to
detail. Based on this concept, a maintenance model for
deterioration of contacts and oil, and break down of insulating
transformer is presented in [3].
materials such as bushings etc. [5, 6].
In this paper, a similar idea is applied to circuit breakers. In
order to build the probabilistic model for estimating circuit
III. MAINTENANCE ACTIONS
breaker failure rate, the deterioration process, inspection tests,
and maintenance actions are discussed. Then, a comparison Various maintenance actions for power circuit breakers are
between circuit breakers and transformers regarding operating summarized in reference [7]. In the proposed model, two
conditions, inspection tests and maintenance actions is carried different levels of maintenance actions are considered.
A. Basic Maintenance
This work was supported by Pserc project, Automated Integration of
1) Operating Mechanism
Condition Monitoring with an Optimized Maintenance Scheduler for Circuit
Clean all insulating parts from dust and smoke
Breakers and Power Transformers, and in part by Texas A&M University.
Satish Natti, Panida Jirutitijaroen, Mladen Kezunovic and Chanan Singh Clean and lubricate operating mechanism and apply
are with the Department of Electrical Engineering, Texas A&M University,
suitable grease for the wearing surfaces of cams,
College Station, TX 77843-3128 USA (e-mails: ******@**.****.***,
rollers, bearings etc.
****@****.***, *******@**.****.***, *****@**.****.***).
Copyright Iowa State University, 2004
processing techniques. The time-axes of reference frequency
Adjust breaker-operating mechanism as described in
pattern and test frequency pattern are aligned to indicate any
the manufacturer s instruction book.
changes in the condition of the operating mechanism. The
Make sure all bolts, nuts, washers, cotter pins etc. are
presence of an abnormal event in the test signature will change
properly tightened.
the frequency, and the time at which this event occurs.
After servicing the circuit breaker, verify whether the
3) Control Circuit Monitoring
contacts can move to the fully opened and fully closed
Portable test sets are generally used to monitor the control
positions or not.
circuit. The circuit breaker is forced into operation and the
2) Contacts
control circuit signals are recorded [11]. The following are the
Check the alignment and condition of the contacts and
typical control circuit signals that can be monitored in practice
make adjustments according to the manufacturer s
[12].
instruction book
Trip coil current
Check if the contact wear and travel time meet
Close coil current
specifications
DC Supply voltage
3) Insulating Medium and Arc Extinction
A, B auxiliary contacts
Check for leaks and remove any water content. Check
X & Y Coils
governor and compressor for required pressure
Trip initiation
Recondition oil by filtering
Close initiation
B. Replacement
B. Contacts
This includes the replacement of various components.
Inspections related to circuit breaker contacts are mentioned
Arc chute and nozzle parts if damaged
in this section.
Governors and compressors if worn or malfunctioning
1) Contact Resistance Test
Contacts if badly worn or burned
The resistance of the main contact can be measured with a
Oil if dielectric strength drops below an allowable limit
portable double bridge (Kelvin) or a Ducter [7]. A DC current
and if any arc products are found in the oil.
is injected in one phase of the breaker, and the breaker is forced
into operation. The current and voltage over the contact are
IV. INSPECTION TESTS
measured and the dynamic resistance curve is calculated. The
This section gives an idea about how various inspections can condition of the contacts can be analyzed by comparing the
be done, and what is the information that can be obtained from measured resistance curve with previous measurements [9].
those tests. Some of the possible inspection tests used in practice 2) Contact Temperature Monitoring
for a circuit breaker are mentioned below [8]. The inspection Large changes in contact temperature may be due to broken
tests are grouped according to the order of components that are contact fingers, excessive burning of the main contacts, material
discussed in section II. degradation, oxide formation, weak contact springs, improperly
or not fully closed contacts etc. Optical sensors are used to
A. Operating Mechanism
measure the temperature of the contacts [4].
Inspection tests, which give the performance of operating
C. Inspection of oil
mechanism either directly or indirectly, are presented in this
section. Oil sample can be taken and tested for its dielectric strength.
1) Contact Travel Time Measurement The following are the inspections that can be done in practice
The motion of the breaker contacts can be determined with [5].
contact travel time measurement. It is a plot of position Color and visual inspection
(distance) of contacts with respect to contact travel time, and Interfacial tension (soluble contaminants measurement)
can be obtained by a resistive transducer [9]. The transducer is Dissipation factor (measure of power lost as heat)
usually mounted on a moving part of the breaker. The contact
D. Partial Discharge
travel time measurements provide information about the
operating components of the circuit breaker, which include Insulation failures of circuit breakers can be detected by
mechanical links and interrupter contacts. Partial discharge monitoring [13]. The test procedure and
2) Vibration Analysis equipment for the partial discharge monitoring are discussed in
Mechanical malfunctions, excessive contact wears, detail in reference [14]. Various methods are reported in
maladjustments, other irregularities and failures can be detected literature so far but the cost varies according to the test
through vibration patterns [10]. Accelerometers mounted procedures and accuracy of results.
usually on the arcing chamber and operating mechanism, are
used to record the vibrations. The recorded vibration patterns
are converted into time/frequency patterns using signal-
V. COMPARISON BETWEEN A CIRCUIT BREAKER AND D2
D1 D3 Fail
TRANSFORMER
Circuit breaker is an electrical device that operates on Inspection test Inspection test Inspection test
command. Once the operating mechanism receives trip or close
signal from a control circuit, it starts working and opens or Investigation Investigation Investigation
process process process
closes the main contacts respectively. The overall performance
of the breaker depends on the operating mechanism, which C1 C2 C3 C1 C2 C3 C1 C2 C3
consists of various moving parts. Transformer is a device, D1 D1 D1
Basic Maint. Basic Maint. Basic Maint.
which while in service, is always in an energized state. The D2 D2 D2
D1 D1 D1
insulating oil properties used in breaker and transformer are D3 D3 D3
D2 D2 D2
different. Suggested limits for service-aged insulating oils for
D3 D3 D3
both breaker and transformer are given in table I [5]. Having an
idea about the similarities and differences between the two Replacement
Replacement Replacement
devices, and knowing the maintenance model of the transformer D1 D1 D1
Basic Maint. Basic Maint. Basic Maint.
will help in developing the maintenance model for the circuit D2 D2 D2
D1 D1 D1
breaker. Table II provides a comparison between the breaker D3 D3 D3
D2 D2 D2
and transformer characteristics.
D3 D3 D3
VI. PROPOSED MAINTENANCE MODEL AND DESCRIPTION Replacement Replacement Replacement
D1 D1 D1
A probabilistic model, based on the concept of representing
the deterioration process by various stages [3, 15] is shown in D2 D2 D2
Figure 1. Three deterioration stages i.e., the initial stage (D1), D3 D3 D3
minor (D2) and, major (D3) deterioration stages, followed by a
Fig 1. Proposed circuit breaker probabilistic maintenance model.
failure stage are considered. Inspection test is implemented at
each stage and the collected data is investigated to determine the
The model takes results from various inspection and
condition of the breaker. In this model, three different levels of
maintenance tasks and the frequency of performing the tasks as
breaker condition are defined: C1- satisfactory and no
inputs and gives the failure rates as output. The changes in the
maintenance is needed, C2- indication of abnormality or caution
mean time to failure indicator can be observed by considering
stage, needs further investigation or related maintenance and
different inspection and maintenance actions. This model can
C3- Failure stage or poor condition, needs replacement. Further,
help in obtaining optimum maintenance intervals such that both
the maintenance process is divided into three levels; (1) Do
the component availability and the total cost are balanced.
nothing, (2) Basic maintenance, and (3) Replacement. Once the
Various inspection tests and maintenance actions considered in
suggested maintenance action is taken, the subsequent condition
the model are discussed next.
of the breaker is determined.
A. Inspection Tests
The following inspection tests are considered in developing
TABLE I
SUGGESTED LIMITS FOR SERVICE-AGED OILS FOR TRANSFORMERS AND CIRCUIT
the proposed model. Air blast and oil circuit breakers are
BREAKERS [5]
considered in this study.
Test and method Transformer Circuit
1) Contact Travel Time Measurement
(Value for voltage class) Breaker
Condition of the circuit breaker can be obtained by
69 kV 69 230 230 kV Suggested
and kV and limit comparing the test curve with the reference curve. Figure 2
below above
shows various contact travel curves during the opening of a
Dielectric strengtha
circuit breaker [4]. The solid lines indicate the reference curve
KV minimum
and the dotted lines indicate the current observation. The
1 mm gap* 23 28 30 20
2 mm gap* 40 47 50 27 following are the possible observations from the Figure 2.
Contact separation occurred sooner than before:
Dissipation factora
contact wear
(power factor),
25 oC, % maximum 0.5 0.5 0.5 1.0 Faster circuit breaker stroke: kinetic energy of the
100 oC, % maximum 5.0 5.0 5.0 -
mechanism is above its upper limit
No damping at the end of the operation: shock
Interfacial tension, 25 30 32 25
mN/m minimum absorber failure
Reduction in total travel distance: binding or
a
Older transformers with inadequate oil preservation systems or
stalling of the mechanism or insufficient stored
maintenance, may have lower values
* *Alternative measurements of 0.04 in and 0.08 in respectively for gaps driving energy
The proposed criterion for assessment of the condition of Condition 3: excessive wear and need complete overhaul
operating mechanism is or replacement
Condition 1: satisfactory, test results follow the reference curve 2) Control Circuit Monitoring
Condition 2: caution stage, test results deviate slightly and need The recorded control signals are analyzed to find any
more attention abnormalities in the breaker operation. A detailed analysis of
circuit breaker operations based on automated monitoring of
control circuit is discussed in reference [12]. Table III shows
some of the possible causes for the behavior of the measured
signals [16].
Figures 3, 4, 5 and 6 show the sluggish trip latch, defective
close coil, defective auxiliary switch and defective battery
respectively.
The proposed criterion for the condition of control circuit is
Fig. 2 Comparison of circuit breaker contact travel curves [4]
Condition 1: within specification and will not require
maintenance
TABLE II
COMPARISON BETWEEN CIRCUIT BREAKER AND TRANSFORMER
Comparison Aspect Circuit breakers Transformers
Main components Contacts, interrupter, insulating medium, Winding, Cooling agent (for example, oil,
control circuit, mechanism which includes gas, or air), Bushing, Tap changer
cam, latches, springs, bearings, coils,
compressors, charging motors etc.
Operating Stored energy in springs or gas pressure is Transforms voltage from one level to another
mechanism used to move operating mechanism which preserving the same voltage frequency.
either opens or closes the main contacts
Deterioration Operating mechanism, oxidation of contacts Insulation paper in the winding, oxidation of
process and oil oil.
Particles produced Oxides, arc byproducts such as carbon, water, Sludge, Water, Fiber, Gases (CO, CO2, etc.),
by aging process partial discharge Furfural, Partial Discharge
Failure Modes Fails to open on command Thermal related faults
Fails to close on command Dielectric related faults
Fails to conduct continuous or momentary Mechanical related faults
current (while already in use) General degradation related faults
Fails to maintain the insulation
Fails to contain insulating medium
Fails to indicate condition or position
Fails to provide for safety in operation
Inspection tests Contact travel time measurement Routine oil sampling test; dielectric
strength, resistivity, acidity, moisture
Vibration Analysis
content.
Control circuit monitoring
Dissolved gas analysis
Contact Resistance Test
Furfural analysis
Contact temperature monitoring
Partial discharge monitoring
Dielectric strength
Partial Discharge
Maintenance (For oil-immersed transformer)
Basic maintenance: lubricating mechanism
Oil filtering (online/offline)
components, check for compressor pressure
and dielectric strength of oil, adjusting all Oil replacement
components and contacts as per
manufacturer s instructions, check for
control circuit connections
Replacement of contacts, interrupters, oil,
damaged nozzles, springs, coils etc.
Condition 2: caution stage; need more attention TABLE IV
SUGGESTED LIMITS FOR CONTINUED USE OF SERVICE-AGED CIRCUIT
Condition 3: final stage; need major replacement
BREAKER INSULATING OIL [5]
Test and method Suggested limit
TABLE III
Dielectric strength 25
ABNORMAL WAVE FORMS [16]
kV minimum
Signal Name Signal Behavior Figure Possible Cause
Trip Coil Current Dip delayed Fig. 3 Binding or friction Dielectric strength, kV minimum
Close Coil Excessive noise Fig. 4 Defective close coil 1 mm gap* 20
current (distortion) 2 mm gap* 27
A&B Contacts Abnormal noise Fig. 5 Defective auxiliary
on contacts switch Dissipation factor (power factor),
DC Voltages DC Voltage Fig. 6 Defective substation 25 oC, % maximum 1.0
unstable battery or high
impedance short Interfacial tension, mN/m minimum 25
Color, ASTM units, maximum 2.0
*Alternative measurements of 0.04 in and 0.08 in respectively for gaps
Condition 1: satisfactory
Condition 2: should be reconditioned for further use
Condition 3: poor condition; dispose
Suggested limits for oil in condition 1 are listed in table IV.
Fig. 3. Trip current dip delayed Criterion for recondition is excessive carbon in oil and
reduced dielectric strength (dielectric strength drops below the
accepted limit).
A detailed analysis of inspection tests and the collected data
is needed for implementation of the model, and will be
included in further work.
B. Investigation Process
Information out of the inspection tests can be used to
Fig. 4. Close current excessive noise
determine the condition of the device followed by the
necessary maintenance action and rate of the next inspection.
C. Maintenance Action
1) Do nothing
The breaker is in satisfactory condition and no maintenance
Fig. 5. Noisy transition when opening is needed. The probability that the system is set back to same
stage is relatively high.
2) Basic Maintenance
This maintenance action increases the probability of going
back to the previous stage.
3) Replacement
Replacement of damaged components brings the system
Fig. 6. DC Voltage unstable
back to its original stage i.e. beginning stage.
3) Contact Resistance Measurement
The possible causes for abnormal increase in contact
VII. CONCLUSION
resistance are deposition of foreign material in contacts, loose
A probabilistic maintenance model for circuit breakers is
contacts and loose bushing connections [7].
introduced. Information collected during inspection tests is
The proposed criterion for the condition of contacts is
analyzed and the condition of the breaker can be defined.
Condition 1: satisfactory
Maintenance action is taken according to the condition of the
Condition 2: caution stage; need more attention
breaker. Implementation of the model using Monte Carlo
Condition 3: excessive wear and need complete overhaul
simulation is in progress. Maintenance cost and time to failure
4) Inspection of Oil
of each transformer and circuit breaker will be incorporated.
Service-aged oils are classified into the following three
conditions [5].
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