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Reliability-Based Preventive Maintenance of Oil Circuit Breaker Subject to Competing Failure Processes

Authors:

Abstract

This paper investigates technical and organizational tools to improve performances of a multi-state degraded system subject to multiple competing failure processes. The competing failure processes treated in this paper are oil insulating aging and electrical contacts wear out of high voltage oil circuit breaker. To degradation processes, is associated random shocks process highlighted by the stresses due to shortcircuit solicitations. To keep a high level of circuit breaker reliability, two policies are developed and cover reliability increasing of the downstream feeder and reliability based preventive maintenance of the item. The first policy is developed using technical and organizational measures, while the second policy is based on improvement factors method optimizing availability under threshold reliability and a maximum benefit. The results obtained using a case study allow the decision maker to reach better information, to target the equipment that reduces the performances of the system, and to practice suitable maintenance actions.
International Journal of Performability Engineering Vol. 9, No. 5, September 2013, p.495- 504.
© RAMS Consultants
Printed in India
____________________________________
*corresponding author email: r.medjoudj66@gmail.com 495
Reliability-Based Preventive Maintenance of Oil Circuit
Breaker subject to Competing Failure Processes
FAIROUZ IBERRAKEN
1
,
RAFIK MEDJOUDJ
1
,
RABAH
MEDJOUDJ
1
*,
DJAMIL AISSANI
2
and
KLAUS DIETER HAIM
3
1
Lamos Laboratory, Electrical Engineering Department, University of Bejaia,
Algeria
2
Lamos Laboratory, Operational Research Department, University of Bejaia,
Algeria
3
Applied Science of Technology, University of Zittau, Germany
(Received on September 23, 2012, revised on May 09 and May 22, 2013)
Abstract: This paper investigates technical and organizational tools to improve
performances of a multi-state degraded system subject to multiple competing failure
processes. The competing failure processes treated in this paper are oil insulating aging
and electrical contacts wear out of high voltage oil circuit breaker. To degradation
processes, is associated random shocks process highlighted by the stresses due to short-
circuit solicitations. To keep a high level of circuit breaker reliability, two policies are
developed and cover reliability increasing of the downstream feeder and reliability based
preventive maintenance of the item. The first policy is developed using technical and
organizational measures, while the second policy is based on improvement factors method
optimizing availability under threshold reliability and a maximum benefit. The results
obtained using a case study allow the decision maker to reach better information, to target
the equipment that reduces the performances of the system, and to practice suitable
maintenance actions.
Keywords: Shocks process, degradation modeling, reliability, preventive
maintenance, electrical components
1. Introduction
The use of degradation measures to assess reliability has seen some important findings in
the literature and the binary assumption used to analyze, to model and to compute system
reliability is relaxed [1]. The oil circuit breaker (OCB) is highly efficient and there is a
significant number installed in today’s electrical power grid, unless the preferred
technology is the one developed following SF6 and vacuum breakers. Regarding its design
and its function, it offers the possibility of implementing both degradation and shocks
processes. The replacement of the OCB is considered unrealistic and prohibitive looking to
the large life duration of the component (about 40 years) and its cost. To ensure a safety
behavior until the end life of this item, energy utilities should perform maintenance.
Therefore, in practice, before recording a maintenance action, it is useful to ensure its
applicability which is the resultant of ease of implementation and effectiveness of its
results. The novelty introduced in this paper is the adaptation of some maintenance policies
often restricted to less complex than electrical systems, in the case of competing failure
processes, such as: maintenance cost optimization based on equipment conditions under
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Fairouz Iberraken
,
Rafik Medjoudj
,
Rabah
Medjoudj
,
Djamil Aissani
and
Klaus Dieter Haim
reliability threshold and, or maximum benefit constraints. The earliest theoretical
investigations on competing failure processes were developed by Li and Pham [2]. The
degradation modelling constitutes an efficient way to estimate full and residual lifetime
distributions [3]. The case of oil circuit breaker reliability assessment considering both
insulating oil aging and contacts wear out is studied in the present paper. It deals with
degradation processes due to the use of the equipment and shocks process when it is
triggered on the fault.
Shocks occur randomly in time as a stochastic process and cause a certain amount of
damage to a system. This damage accumulates and gradually weakens the system. A
system fails when the total damage has exceeded a failure level. For application, shocks
data, corresponding to OCB operations were collected from 1999 to 2007, corresponding
to 17 years of system a continuous operation at the national company of electricity and gas
of Bejaia city district (Algeria). Reliability modelling and data analysis for the state
probabilities assessment were introduced and discussed by Medjoudj et al. [4]. The authors
have introduced two types of shocks which occur on the OCB when it is triggered from a
defect, such as: cumulative and extreme shocks. The shocks arrive according to a counting
process {N(t), t≥0}, and it is assumed that the random variable
X
, representing the annual
number of shocks, follows either exponential
)(E
λ
or Weibull
),(W
ηβ
distributions. The
values of
βλ
,
and
η
were determined by probability plotting and maximum likelihood
estimation [5].
Following Li and Pham [2] theoretical investigations on the competing failure
processes, we have developed two degradation processes for the OCB, corresponding to
the aging of oil insulation and the wear out of electrical contacts. Both degradation
processes have finite number of states and the transitions between states are governed by
threshold values. For each process, we have defined degrading states where the system has
a decreasing effectiveness and a degraded failure state where the system fails and need
repair. The rest of the paper is organized as follows: A brief overview of failure
mechanisms of electrical components under study is given in section 2. The reliability data
analysis of a degraded system using experience feedback data is presented in section 3.
The section 4 is concerned by reliability improvement aspects based on extrinsic and
intrinsic reliability characteristics of the studied system. A particular interest is given to the
application of a based-conditions maintenance policy to electrical system. Finally, the
discussions and the conclusion of the research are dressed in section 5.
2. A Brief Overview of Failure Mechanisms in Electrical System under Study
A comprehensive failure mechanism development is needed and can allow to a
practitioner of the considered system to understand the notion of competing failure
processes and highlight their correlation with the operational reliability aspects. It is about
the random shocks highlighted by short-circuits appearance (frequency and magnitudes)
and the degradation phenomena (aging, wear out and sudden break) expressed by the
behaviour of the components.
2.1 Bus Bar Insulated Supports Sudden Break
The term bus bar commonly replaces the term bus on heavy current circuits. Bus bars are
used as distribution points for electrical power. There are two problems which are
common to all types of bus bars, thermal and magnetic.
Thermal: As ambient temperature and load current varies, a bus bar will heat up or cool
down. This will cause considerable expansion and contraction. Sufficient stress may be
Reliability-based Preventive Maintenance of Oil Circuit Breaker subject to Competing Failure Processes 497
497
placed on the support insulators to cause them to break. Any joints in a bus bar system,
which become loose, will give rise to localized heating. In extreme cases, this heating can
result in arcing and cause the bus bar to fail.
Magnetic: When current flows in a conductor, a magnetic flux is produced around the
conductor. When current flows in two adjacent conductors the magnetic fluxes interact
and produce attractive or repulsive forces. With normal load currents, these forces are
small but under short-circuit conditions the forces can be very high and reach tones per
meter length. Clearly, forces of this magnitude can distort conductors and break insulators
[6].
2.2 Circuit Breaker Oil Aging
Circuit breakers are used to connect and disconnect transmission lines under normal
conditions. They are also used to clear sections of a transmission grid should a short -
circuit occur in the system, isolating the fault. Circuit breaker failures resulting in a fire or
explosion are rare events, but have occurred frequently enough in the past to warrant
concern. Fires in mineral oil typically occur due to the breakdown of liquid insulation
within the equipment (caused by switching, lightning surges, or by gradual deterioration),
low insulating oil level, moisture intrusion in the insulating oil, or by failure of an
insulating bushing.
2.3 Electrical Contacts Wear
The electrical contact function plays a main and critical role in the breaker's proper
operation. The high voltage circuit breaker has three major components:
a) Interrupting chamber, where the current conduction and interruption in the power
circuit occurs. It is usually a closed volume containing the make-break contacts and an
interrupting medium (compressed air, oil, SF6, vacuum, etc.) used for insulation and arc
benching.
b) Operating mechanism, where the needed energy to close or to open the contacts and to
quench the arc is initiated.
c) Control, where the orders to operate the breaker are generated and its status is
monitored.
As mentioned earlier, the power current passes through the conducting material in the
interrupting chamber. Various parts that are joined together form the conducting material
and the different junctions form the electrical contacts. For an increasing temperature of
the contacts, the material of the contacts may soften to the point where it will reduce the
contact force, leading to a quick increase of the contact resistance. It has been proved that
oxidation; wear, fretting, force and temperature directly affect the resistance value (in
micro ohms) of the contacts.
In a recent publication [7], two failure types are distinguished and subdivided as
dielectric and interruptions ones. Dielectric type of failures is internal bushing
deterioration by oil leakage, moisture/tracking; water leakage into main tank; tracking or
related deterioration of operating rod; loose and splitting joints and carbonization of the
oil. However, Interruption type of failure is deteriorated arcing contacts or baffles
chambers; evolving fault; binding mechanism; inoperative tank heaters; control
malfunction including interlocks; operating without a full close cycle and pumping or
related pilot valve failures.
It is well known that for every failure, the circuit breaker trips and the fault research is
initiated and done manually. In some cases, the fault is not isolated and the circuit breaker
498
Fairouz Iberraken
,
Rafik Medjoudj
,
Rabah
Medjoudj
,
Djamil Aissani
and
Klaus Dieter Haim
is closed negatively, therefore it is subject to additional electrodynamics efforts. For the
statistical considerations, the number of shocks is defined as the total number of
operations. Failure research procedure is largely developed in reference [8], where the
number of operations in fault tracking procedure is exposed associated with time of each
stage.
3. Reliability Modelling and Data Analysis
Traditionally, electrical system reliability is perceived as a set of objectives to attempt,
fixed a priori and could be expressed as follows: reliability parameters (failure rate, repair
rate and maintenance rate), mean durations (MUT, MDT and MTBF) and mean
frequencies of scheduled or forced outages. To highlight the reliability indices
improvement, it is useful to assess them for the current state of the system and then
propose tools. By exploiting results gathered in table 1 showing the indices cited above, it
is stated that the circuit breaker minimal tripping (shocks) corresponds to the failure
frequency FF (1/year) = 10.995 of the downstream feeder.
Table 1: Components and Sub-system Reliability Indices for the Current State
To improve system performances, technical and organizational measures are considered
during system planning and operation phases, such as:
- Intensification of maintenance operations to reduce the number of failures;
- Addition of remote control switches on outgoing MV lines to restore quickly
power supply and to limit the geographical area affected by failures;
- Automation of failure research using faults detectors;
- Undergrounding overhead circuits and aging equipment replacement.
As shown in Table 1, the great part of failures appears in overhead circuits. At this stage
of the investigation, the results comforts the decisions of undergrounding the overhead
circuits and the removing out of the oldest sections by taking into account the economic
balance between the desired reliability level and its cost [9]. For the case studied, the
implementation of the above actions has reduced the occurrence frequency of shocks for
about 50%. The reliability indices are improved and a special attention is given to the
failure frequency of the feeder which becomes FF=6.1372 (1/year).
4. Oil Circuit Breaker Preventive Maintenance
4.1 Expert Judgments Review
Oil circuit breakers perform the same function in switchgear assemblies as air circuit
breakers; they are quite different in appearance and mechanical construction. The
principal insulating medium is oil rather than air. The oil, in addition to providing
insulation, acts as an arc extinguishing medium in current interrupters. In this process, it
absorbs arc products and experiences some decomposition in the process. Thus,
Reliability-based Preventive Maintenance of Oil Circuit Breaker subject to Competing Failure Processes 499
499
maintenance of the oil is of great importance. Oil maintenance involves detection and
correction of any condition that would lower its quality. The principal contaminants are
moisture, carbon, and sludge. Moisture will appear as droplets on horizontal members,
while free water will accumulate in the bottom of the tank. Sludge caused by oxidation
will appear as a milky translucent substance. Carbon initially appears as a black trace.
It eventually will disperse and go into suspension, causing the oil to darken. A
dielectric breakdown test is a positive method of determining the insulating value of the
oil. Samples can be taken and tested as covered in ASTM D 877, Standard Test Method
for Dielectric Breakdown Voltage of Insulating Liquids Using Disk Electrodes. Oil that
tests too low should be immediately reconditioned and retested or replaced with new oil.
Oil should be tested periodically or following a fault interruption. In replacing the oil,
only the oil recommended by the manufacturer should be used and it should have been
stored in sealed containers. In addition, the oil should be given a dielectric breakdown test
immediately prior to use. An oil pump or other means should be used to avoid aeration.
In the event entrapment of air cannot be avoided, the entrapped air should be removed by
application of vacuum or the equipment should be allowed to stand for 8 to 12 hours prior
to being energized.
The main contacts of an oil circuit breaker are not readily accessible for routine
inspection. Contact resistance should be measured. Contact engagement can be measured
by measuring the travel of the lift rod from the start of contact opening to the point where
contacts separate as indicated by an ohmmeter. More extensive maintenance on main
contacts might require removal of the oil and lowering the tank and should therefore be
performed less frequently than routine maintenance. The frequency should be determined
by the severity of the breaker duty such as the number of operations and operating current
levels.
Any time the breaker has interrupted a fault current at or near its maximum rating, this
type of maintenance should be performed. The contacts should be inspected for erosion or
pitting. Contact pressures and alignment should be checked. All bolted connections and
contact springs should be inspected for looseness.
Arc-quenching assemblies should be inspected for carbon deposits or other surface
contamination in the areas of arc interruption. If cleaning of these surfaces is necessary,
manufacturers’ instructions should be followed. The committee on electrical equipment
maintenance suggested the following the plan dressed in Table 2.
Table 2: Initial Guidelines for Maintenance Actions
Maintenance of the operating mechanism, auxiliary devices and other accessories, such as
oil level gauges, sight glasses, valves, gaskets, breathers, oil lines, and tank lifters should
be inspected following the manufacturer recommendations [10].
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Fairouz Iberraken
,
Rafik Medjoudj
,
Rabah
Medjoudj
,
Djamil Aissani
and
Klaus Dieter Haim
4.2 Reliability-based Preventive Maintenance
For the identification of the deterioration stages, the need for preventive maintenance
(PM) is established through periodic or continuous inspection. The improvement of
maintenance to reliability is developed using two factors and the selection of the action to
do for the components on every PM stage is decided by maximizing system benefit in
maintenance. Depending on the percent of the survival parts of the system when it is
maintained, the reliability function giving the probability that the system is always
working on the time interval
],[
1j
j
tt
is
)
(
.)
(,,0 t
R
Rt
Rjjj
ν
=
where
j,0
R
is the initial
reliability of the
th
j
stage and
)(
,tR j
v
is the reliability degradation of surviving parts
on this stage. Considering periodical PM which interval is
m
t
, the reliability of surviving
parts is defined by:
( )
=
mjv
tjt
m
RtR )1(
1
)(
1
,
(1)
with:
R
being the reliability function,
mm jtttj )1(
and
1
m
(
)
10 1< m
the
improvement factor of PM action
)
1
(a
. This PM action
)1
(a
is defined as a mechanical
service which emphasizes on maintaining a system on normal operating condition. It
usually involves less techniques and tools and just improves the extrinsic state. To model
the reliability of systems following PM, the effects of various actions on
j,0
R
and
j
R
must be evaluated using
)(
1,01,,
0mjjf
j
tRRR
R
=
=
, where
1j,0
R
,
1j,f
R
-
are the
initial and the final reliability values of the system on the
th
)1j
(
stage.
PM action
)1
(b
can improve the surviving parts of the system and also recover the
failed parts. Generally, the impact of this action on the failed parts can be measured by an
improvement factor
2
m
, which is also set between 0 and 1 representing the restored level
except the surviving parts. It will be noted that the improvement factors were already
developed by Tsai et al. [11]. According to the definition, the initial reliability on the
action
)1
(b
can be expressed as:
( )
1,021,,0
+=
jfjfj
RRmRR
(2)
where
0
R
denotes the initial reliability of the new system. When both improvement
factors
1
m
and
2
m
are equal to 1, we define the PM action (2P) corresponding to a
replacement. Using the development of the survival function given bellow, the system
reliability is expressed in a developed form of the expression 1, where is associated the
cumulative distribution function of random shocks designated by
)(
)(
SF
j
x
as:
Reliability-based Preventive Maintenance of Oil Circuit Breaker subject to Competing Failure Processes 501
501
( )
)(
!
)))1((
1
(
))1((
exp
)(
0
1
)))1((
1
(
1
,0
1
SF
j
tjt
m
e
mtjt
RtR
j
x
j
j
m
tjt
m
m
jj
m
=
×
=
λ
η
λ
β
(3)
The benefit of the component maintenance on the
th
j
stage is defined as:
k,i
t t j,i1j,i
k,i
C
dt)t(Rdt)t(R
B
j j
∫ ∫
=
∞ ∞
+
(4)
where:
i
,
k
,
k
i
C
,
denote respectively the
th
i
sub-system or component under
consideration, the
th
k
maintenance action been considered and the action cost. The
advantageous maintenance action will correspond to the maximum of the benefit, i.e.,
)B(
MaxB
k,i
*
i
=
.
Once the action of maintenance is defined and retained, the availability of the system at
any stage is processed as:
+
=
=
n
ia,k
,i
n
1i
t
tj
,im
,b
j,
s
tT
dt)
t
(htT
A
j
1j
(5)
where:
n
,
ak
i
t
,,
,
T
denote respectively the number of components or sub-systems, the
time of the
th
k
preventive maintenance action and the cycle time. In the following, we
discuss the methodology allowing the assessment of reliability under preventive
maintenance.
Let’s consider
m
T
the minimal value of preventive maintenance intervals of the
system’s components,
{ }
i,mm tminT =
. At every maintenance stage
j
, verify for the
system, if its reliability
)T)1j((R m
+
for the coming stage
)1j( +
is greater or equal
to the threshold reliability value fixed a priori
th
R
, using equation (3). If the condition is
realized, the decision is do nothing. If no, compute the benefit given by equation (4) for
each action proposed and choose the maximum value. For the case study application, the
maintenance actions are defined as follows:
- Action
)1
(a
corresponds to visual inspection, optical and radiography inspection,
- Action
)1
(b
corresponds to oil replacement,
- However, the action
)2( p
is gathering the following operations: checking components
for wear examination, cleaning, repainting, correcting any identified problems,
calibrating to meet original manufacture’s specifications and certifying the condition of
reconditioned circuit breaker.
502
Fairouz Iberraken
,
Rafik Medjoudj
,
Rabah
Medjoudj
,
Djamil Aissani
and
Klaus Dieter Haim
For reliability, benefit and availability assessments, maintenance characteristics are
dressed in Table 3 and consist on estimated improvement factors (
21
m,m
), preventive
and curative maintenance durations (
b
a
t,
t
) and their costs in US Dollars, respectively.
Table 3: Maintenance Actions Characteristics
The results of the assessments of rreliability, benefit and availability were obtained, using
equations (3), (4) and (5), respectively. They are gathered in table 4, providing a
maintenance plan which illustrates the influence of maintenance actions on failure
processes cited above. It will be noted that the maintenance interval is
years2tm=
. The
case (a) denote the consideration of oil insulation degradation process, however the case
(b), corresponds to the consideration of both oil insulation degradation and contacts wear
out degradation processes.
Table 4: Circuit Breaker Maintenance Actions Plan
The improvement due to technical and organizational mesures taken on the downstream
feeder influences directly the effects of schoks, while, the improvement of circuit breaker
performances is more significant when these mesures are simultaneously taken with
mainteance actions.
5. Conclusion
In this article the main idea was driven by new theoretical developments on competing
failure process initiated by Pham. This concept seems more robust and realistic than the
one modelling the behaviour of equipment in binary system. The objective attended was
the stochastic modelling with application to a multi-state and multi-degraded system
considering multiple causes of failures. Those treated were: failures due to the use of the
equipment (aging and wear out) and failures due to sudden break (shocks). The study was
based on statistical analysis of real data issued from the experience feedback.
Degradations were modelled using increasing functions based on Weibull, exponential
Reliability-based Preventive Maintenance of Oil Circuit Breaker subject to Competing Failure Processes 503
503
and uniform distributions; however shocks were modelled using non homogeneous
Poisson process. Three types of shocks were illustrated considering state probabilities and
reliability changing. It was observed that the random shock process governs the
behaviour of the reliability function.
Both reliability improvements of the downstream feeder and the effects of intrinsic
preventive maintenance actions were discussed and treated for the improvement of the oil
circuit breaker performances. Investigations conducted by Pham in a theoretical
framework have been applied successfully to complex system such as electrical one. The
models applied on simple numerical examples have been validated by application to a real
case of engineering area. In practical operation, the results analysis of the current state of
the network allows to the decision maker to reach better information and target the
equipment that reduces the performances of the system and practicing suitable
maintenance actions. This work has shown that it is possible to maintain equipment using
other than the traditional methods (systematic maintenance). By following the
methodology explained and applied, this work encourages the introduction of multi-stage
degraded system practice in power system reliability modelling.
References
[1]. Pham, H. Statistical Maintenance Modelling for Complex Systems. In Handbook of
Engineering Statistics. Springer, London, 2006.
[2]. Li, W., and H. Pham. Reliability Modeling of Multi-state Degraded Systems with Multi-
competing Failures and Random Shocks. IEEE Transactions on Reliability, 2005; 54(2):
297-310.
[3]. Kharoufeh, J. P., and S. M. Cox. Stochastic Models for Degradation-based Reliability. IIE
Transactions, 2005; 37(6): 533-542.
[4]. Medjoudj, R., D. Aissani, and K. D. Haim. Competing Risk Model for Oil Circuit Breaker
Dynamic Reliability Assessment. 15th IEEE Mediterranean Electrotechnical Conference,
Malta, April 26-28, 2010.
[5]. Zhao, W., and E. A. Elsayed. An Accelerated Life Testing Model Involving Performance
Degradation. Annual Reliability and Maintainability Symposium, Los Angeles, USA,
January 26-29, 2004.
[6]. Slade, P. G. Electrical Contacts: Principals and Applications. CRC Press, 1999.
[7]. Prevost, T. A. Oil Circuit Breaker Diagnostics. 7th Annual Weedmann Technical
Conference, New-Orleans, USA, September 15-17, 2008.
[8]. Slootweg, J. G., and P. M. Van Oirsouw. Incorporating Reliability Calculations in
Routine Network Planning. 18th Int. Conf. on Electricity Distribution, Turin, Italy, June 6-
9, 2005.
[9]. Medjoudj, R., A. Laifa, and D. Aissani. Decision Making on Power Customer Satisfaction
and Enterprise Profitability Analysis. International Journal of Production Research, 2012;
50(17): 4793-4805.
[10]. Bingham, R. Report of the committee on Electrical Equipment Maintenance, NFPA 70B,
2002.
[11]. Tsai Y. T., K. S. Wang, and L. C. Tsai. A Study of Availability Centered Preventive
Maintenance for Multi-component Systems. Reliability Engineering and System Safety,
2004; 84(3): 261-269.
Fairouz IBERRAKEN is a Ph.D. student in Electrical Engineering at the University of
Bejaia, Algeria. Her areas of interest are smart grids projects development and renewable
energy. She worked on the failure mechanisms and degradation models of photovoltaic
systems. She is also interested in maintenance policies of electrical equipment.
504
Fairouz Iberraken
,
Rafik Medjoudj
,
Rabah
Medjoudj
,
Djamil Aissani
and
Klaus Dieter Haim
Rafik MEDJOUDJ is a Ph.D. student in Electrical Engineering at the University of
Bejaia, Algeria. His area of interest is stochastic models applied to electrical equipment. A
particular interest is made for competing failure processes and state probabilities
assessment of sub-station components. He also works on costs evaluation of photovoltaic
systems installations.
Rabah MEDJOUDJ is a Doctor of sciences from the polytechnic high school of Algiers;
he is associate professor in Electrical Engineering at University of Bejaia, Algeria. He
worked for almost 20 years on the reliability aspects, namely: modelling, simulation,
technical and economic calculation. Currently he is working on multi-criteria decision
making methods in the field of electrical systems and sustainable electricity projects. He is
also a consultant to industrial and economic organizations in the areas of networking.
Djamil AISSANI is a professor, director of research at University of Bejaia, Algeria. He
is also president of the association GEHIMAB, and has published hundreds of articles
relating to queues, the reliability of systems, and the history of mathematics in Bejaia in
the middle Ages. He has contributed to the publication of several books. He activated
several times as curated exhibitions and as advisor to the Algerians Ministry of Culture
and Ministry of Higher Education.
Klaus Dieter HAIM is a professor and director of research at University of Zittau,
Germany. His area of interest is MV network design and optimization. His career covered
a diverse range of assignments, from a research project for EDF to serving as a Professor
in Algeria. Between 1994 and 2005, he worked as Head of Production for medium
voltage cable accessories at Cellpack before assuming his current position. He is a Senior
Fellow for electrical power systems and networks.
... Therefore, the utilization of insulating fluids in power transformers has attracted the attention of researchers in recent years [2]. The use of MO in power accessories like transformers [3], high voltage cables [4], high voltage capacitors [5] and highpressure oil circuit breakers [6] is necessary for achieving insulation and cooling. But the adverse effects of using MO during operation followed by reclamation or post-disposal will affect the environment with Particulate Matters ...
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... Also, the oil is preferred as the transformer most suitable liquid insulation because oil can disperse heat evenly from top to bottom (along its column) instantaneously without creating local hot spots [10]. Hence mineral oil is used as insulator and coolant in oil-cooled transformers [11], high-pressure oil circuit breakers [12], high voltage capacitors [13] and underground cables [14]. These oils are stable at higher temperature and hazardous in situations where insulation fails leading to transformer fire [15]. ...
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This paper describes the implementation of reliability calculations into the routine network planning of a distribution network operator. Firstly, actual developments putting this issue on the agenda are described. Then, it is argued why medium voltage (MV) networks form the logical starting point. Hereafter, the basics of reliability calculations and the approach towards reliability calculations, as it is implemented in the MV network analysis program Vision, are elaborated upon. Finally, the practical implementation and applicability of reliability calculations is discussed. It is argued that the results of reliability calculations must be interpreted carefully, but that they can nevertheless be of good use in specific situations.
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In this paper, we develop a generalized multi-state degraded system reliability model subject to multiple competing failure processes, including two degradation processes, and random shocks. The operating condition of the multi-state systems is characterized by a finite number of states. We also present a methodology to generate the system states when there are multi-failure processes. The model can be used not only to determine the reliability of the degraded systems in the context of multi-state functions, but also to obtain the states of the systems by calculating the system state probabilities. Several numerical examples are given to illustrate the concepts.
An Accelerated Life Testing Model Involving Performance Degradation. Annual Reliability and Maintainability Symposium
  • W Zhao
  • E A Elsayed
Zhao, W., and E. A. Elsayed. An Accelerated Life Testing Model Involving Performance Degradation. Annual Reliability and Maintainability Symposium, Los Angeles, USA, January 26-29, 2004.
Oil Circuit Breaker Diagnostics. 7th Annual Weedmann Technical Conference
  • T A Prevost
Prevost, T. A. Oil Circuit Breaker Diagnostics. 7th Annual Weedmann Technical Conference, New-Orleans, USA, September 15-17, 2008.