CONDITION BASED MAINTENANCE OF UNDERGROUND CABLE SYSTEMS
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De-regulation of power market brings changes by creating an increase in competition among utilities. Minimizing the cost of maintenance and losses due to cable failures is a key to successful operation. Simulations demonstrate the possibility of applying condition-based maintenance for the entire service period of a cable system if maintenance cost could be lowered to a certain level. The aging of power cables begins long before the cable actually fails. Preventing incipient failures developing into failures can greatly reduce loses. There are several external phenomena indicating undergoing aging problems, including partial discharges, hot spots, mechanical cracks and changes of insulation dielectric properties. Most sensors currently used are cumbersome to move, complicated to use, or destructive to cables. In the presented project and implimentation, non destructive miniature sensors capable of determining the status of power cable systems are developed and integrated into a monitoring system, including a video sensor for visual inspection, an infrared thermal sensor for detection of hot spots, an acoustic sensor for identifying partial discharge activities, and a fringing electric sensor for determining the aging status of insulating material. Mobile monitoring can greatly reduce the maintenance cost and supply more accurate status of local cables over traditional monitoring techniques. The application range of condition-based maintenance can be expanded greatly with the aid of mobile monitoring. A novel autonomous robot is developed A graphical user interface on the host computer is developed to enable motion control, sensor control and signal processing. Presented work demonstrates the use of mobile monitoring system for underground power cable systems to be viable and economically efficient.
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Deregulation of the power market brings change by creating an increase in
competition amongst utilities. Utilities realize that economical efficiency is becoming
more crucial in order to stay competitive. Minimizing the cost of maintenance and losses
due to cable failures is a key to successful operation.
An economical cost-driven model for maintenance assess is derived to evaluate the
economical performance of traditional maintenance methods. Further, the model is
employed to generate a hybrid maintenance strategy, which is a function of specific cable
system characteristics. The hybrid strategy is used to deploy the optimum methods
among traditional methods to a specific power network according to its status, i.e., it
could combine advantages of different maintenance methods. Simulations demonstrate
the possibility of applying condition-based maintenance for the entire service period of a
cable system if maintenance cost could be lowered to a certain level.
The aging of power cables begins long before the cable actually fails. Preventing
incipient failures developing into failures can greatly reduce losses. There are several
external phenomena indicating undergoing aging problems, including partial discharges,
hot spots, mechanical cracks, and changes of insulation dielectric properties. Most
sensors currently used are cumbersome to move, complicated to use, or destructive to
cables. In the presented project and implimentation, non-destructive miniature sensors capable of determining
the status of power cable systems are developed and integrated into a monitoring system,
including a video sensor for visual inspection, an infrared thermal sensor for detection of Page 5
hot spots, an acoustic sensor for identifying partial discharge (PD) activities, and a
fringing electric sensor for determining aging status of insulation material and detection
presence of water trees. The working principles and experimental setups with these
sensors are discussed.
Mobile monitoring can greatly reduce the maintenance cost and supply more accurate
status of local cables over traditional monitoring techniques. The application range of
condition-based maintenance can be expanded greatly with the aid of mobile monitoring.
A novel autonomous robot is developed to perform a monitoring routine. The modular
robot can move freely and negotiate obstacles. The robotâ„¢s distributed control system
consists of a control board built from four micro-controllers, and a signal-processing
system based on a DSP board and an ADC board. The bilateral communication between
the master micro-controller and slave micro-controllers is achieved by combination of
SPI bus and interrupting routines, while the communication between the control board
and DSP board is implemented through an unsynchronized serial bus. Corresponding
communication protocols are developed to ensure the correct transmission. These
protocols can perform error checking and direction checking for communications.
Wireless communication enables remote control of the robot from a host computer. A
graphical user interface on the host computer is developed to enable motion control,
sensor control, and signal processing. When robot teams are employed to monitor the
status of cable systems, communication and cooperation will be very complicated. A
control algorithm for robot teams is put forward to solve this problem.
Presented work demonstrates the use of mobile monitoring system for underground
power cables systems to be viable and economically efficient
Deregulation of the power market brings changes by creating an increase in competition
amongst utilities. In order to stay competitive, utilities must be economically efficient.
The successful operation of utilities is determined by many factors. One of the most
important elements is the service quality that utilities can deliver to customers. Service
quality of utilities can be expressed as the dependability and stability of power delivery to
customers. Power service quality has been a serious problem for a long time. There are
about 12 to 20 cable failures per month within Puget Sound region, Washington (Puget
Sound Energy). In the summer of 1996, power outages left large portions of the Western
United States without electricity for periods of up to 14 hours on one of the hottest days
of the summer (CNN). The high-tech industry is especially sensitive to power supply
outages. For instance, a chip fabrication plant can be offline for up to a week from just a
several minute power outage. Millions of dollars worth of e-commerce business can
disappear in one second. According to CEIDS, outage in electric power systems
contributes $104 billion to $164 billion loss annually in the USA . One of the most
significant tasks of today's power industry is to provide high quality power to customers
while keeping the cost of power systems maintenance as low as possible.
The United States has millions of miles of underground cables within power systems.
Many of these cables, which installed dozens of years ago, are on the edge of
replacement. It is well recognized that utilities can save significantly if proper
maintenance is performed to underground cable systems. Currently, there are three
maintenance methods employed by utilities: corrective maintenance, scheduled
maintenance, and condition-based maintenance. Corrective maintenance dominates in
todayâ„¢s power industry. Being able to predict the incipient problems, condition-based Page 12
maintenance emerges to be the most potential method in future applications. One case
study showed that the performance of a network was increased 40% and the maintenance
cost was lowered to 1/7 when cables were repaired instead of replaced .
The insulation material of underground cables keeps aging (degrading) during the
whole service period until a failure or replacement happens. Once aging status reaches a
certain critical level, external aging phenomena can be observed, such as hot spots, partial
discharges, and mechanical cracks. For example, a void introduced by aging can initiate
partial discharges (PDs), and most systems will eventually fail during service time after a
PD initiates, possibly years later . These phenomena can be used to locate the position
of deteriorating cables and estimate the remaining lifetime of these cables. If incipient
failures can be detected, or the aging progress can be predicted accurately, possible
outages and following economical loss will be avoided.
In order to estimate accurately the status of cable insulation, suitable sensor
technologies have to be applied to power systems. Aging estimation of electrical
insulation has remained as a visible and actively investigated field of electric power
industry for many decades. Different types of sensors are being used, and new sensors are
being developed. Resonance type partial discharge (REDI) sensors and ultrasonic sensors
 are used to locate voids of insulation material by detecting electrical pulses introduced
by partial discharges . Ultrasonic sensors  and nuclear magnetic resonance (NMR)
sensors  are used to detect electrical trees. Among failure phenomena, the most
important one is the partial discharge activity. PDs can be regarded as a forerunner for an
insulation failure. PD measurement is an important diagnostic tool, especially
predominant for medium and high voltage cables, where local intensity of electric stress
can reach breakdown values. Figure 1.1 and Figure 1.2 show two types of sensors used to
detect PD defects. Page 13
Figure 1.1. Capacitive measurement of partial discharges (Lemke Probe LDP-5).
Figure 1.2. Acoustic detection of partial discharges (Acoustic Emission Consulting, Inc).
Because of the high price of monitoring devices, there is no way to install them in
every portion of a cable system. Either a wide-angle global monitoring system or a
distributed sensor network is used in power systems, where only important equipment,
such as generators and transformers, can be monitored. Neither of monitoring techniques
is sufficient for adequate diagnosis of aging status and incipient faults, therefore they
should be supplemented with local sensing devices. A local scanning device has
inherently higher resolution and accuracy than a global/distributed system. In order to
implement cost-efficient localized sensing, technicians have to be employed to scan the Page 14
network with handheld instruments. Only trained professionals with limited access to the
underground tunnel can do this dangerous job.
Mobile sensing emerges to be able to solve the local sensing problem and is playing
an increasingly important role in the monitoring of power systems with the advance of
sensor, robotics, MEMS, and IT technologies. This method suggests a robotic platform
carrying a sensor array is used to patrol the power cable network, locate incipient
failures, and estimate the aging status of electrical insulation. This National Science
Foundation and Advanced Power Technologies (APT) Center at the University of
Washington funded project and implimentation Condition-based Maintenance of Electric Power Systems
focuses on the prototype development of mobile monitoring. The long-term goals of this
project and implimentation are to design a powerful robotic platform, develop a sensor array, put forward a
signal-processing algorithm to process multiple sensing signals, and estimate accurately
the aging status of electrical insulation. In this project and implimentation, we are trying to generate evidence
that mobile monitoring can be a viable of the maintenance method in the future.
For many years, the main maintenance strategy for power cable systems has been
corrective maintenance (CM), i.e., there is no maintenance reaction until an unexpected
failure. Since utilities have to compensate the economical loss of customers within the
deregulated power market, this method is not applicable if the failure rate is high. The
outage loss, especially for large industrial customers, can be too large for both customers
and utilities. It turns out preventive maintenance is the necessary choice. As with any
preventive maintenance technologies, efforts spent on status monitoring are justified by
the reduction of the fault occurrence and elimination of consequent losses due to
disruption of electric power, damage to equipment, and emergency equipment
replacement costs. Condition-based maintenance (CBM) is becoming a superior choice,
since it is based on real time status data. The only drawback of condition-based
maintenance is monitoring cost. Expensive monitoring devices and extra technicians are
needed to implement condition-based maintenance. The application of CBM is limited Page 15
due to its high cost. The mobile monitoring has low cost properties, making condition-
based monitoring possible.
In addition to sensitivity improvement and subsequent system reliability
enhancement, the use of robotic platforms for power system maintenance has other
advantages. Removing human workers from dangerous and highly specialized operations,
such as live maintenance of high voltage transmission lines, has been a long-standing
effort in the power community. Other needs for robotics in power systems include
operation in hazardous environments such as radioactive locations in nuclear plants,
access to confined spaces such as cable viaducts and cooling pipes, and precise position
Mobile monitoring puts more requirements on sensor systems: they have to be
miniature, non-destructive, and energy-efficient. With the advance on IT, MEMS,
robotics, sensor technology, wireless communication, and signal processing, the
monitoring robot can become a reality.
1.3 State of the art
In the past few years, there have been significant developments in monitoring
technologies for distribution power systems.
1.3.1 Mobile robot platforms
Numerous worldwide project and implimentations attacked this challenging application from different angles.
In 1989, two manipulator systems differing in operating method were developed by
Tokyo Electric Power Co. to traverse and monitor fiber-optic overhead ground
transmission wires (OPGW) above 66 kV power transmission lines . It was shown that
systems were fully capable of performing distribution line construction work using
stereoscopic TV camera system. Several other teleoperated robots have been developed
for live-line maintenance in Japan [9;10], Canada , Spain , and other locations.
An autonomous mobile robot was developed to inspect power transmission lines in 1991
. The robot was able to maneuver over obstructions created by subsidiary equipment
on the ground wire. It was equipped with an arc-shaped arm that acted as a guide rail and
allowed it to negotiate transmission towers. At the same time, a basic synthesis concept Page 16
of an inspection robot was described for electric power cables of railways . Since the
feeder cables are extremely long and have many irregular points, a multi-car structure
with joint connections and biological control architecture was adopted. Thus, the robot
could run on the cable smoothly with a sufficient speed and deal with the shape
irregularities. The Electric Power Research Institute also evaluated the feasibility of
remote, teleoperated, non-destructive evaluation (NDE) and repair activities in coal-fired
electric power plants .
Hardly any successful robot applications have been reported for underground
distribution cables. Numerous problems have to be solved for this type of robot, such as
space confinement, size and weight restrictions, wireless design requirements, and
adverse environmental conditions. Miniaturization has been one of the most difficult
Dozens of methods exist for evaluating the aging status of electrical insulation. Main
sensing principles appropriate for monitoring of power cables include acoustics,
dielectrometry, thermal imaging, and visual inspection.
22.214.171.124 Thermal sensing
Polymers commonly used as electrical insulation are thermally sensitive due to the
limited strength of the covalent bonds that make up their structures. When exposed to
sufficiently high temperatures that cause polymer degradation, insulation materials
experience a drop in the glass transition temperature, which effectively lowers the upper
service temperature and significantly reduces the room-temperature mechanical strength
of materials . Lifetime of electrical insulation is reduced when a unit is subject to
continuing overheating, usually due to overload or failures. The impregnated paper used
in underground cables is particularly prone to aging through overheating, but it also holds
true for all types of polymer insulation.
The insulation aging factors interact with each other. For example, overheating may
cause loss of adhesion at an interface of cables, thus creating a void where PD will
initiate. The released energy leads to higher temperature. This process repeats itself with Page 17
accelerated speed until the insulation of the cable fails . Thermal analysis plays an
important role in the evaluation of insulation status by supplying rich system information.
Due to its no-contact measurement characteristics, use of an infrared (IR) sensor is one
suitable solution for measuring of temperature distribution along the cable length.
126.96.36.199 Sensing of partial discharges
Referring to IEC Publication 270, a partial discharge, within the terms of this standard,
is an electric discharge, that only partially bridges the insulation between conductors.
Such discharges may, or may not occur adjacent to the conductor. Partial discharges
occurring in any test object under given conditions may be characterized by different
measurable quantities such as charge, repetition rate, etc. Quantitative results of
measurements are expressed in terms of one or more of the specified quantities.
The history of partial discharge recognition could go back to the year 1777 when
Lichenberg reported novel results of experimental studies during a Session of Royal
Society in GÃƒÂ¶ttingen . No theoretical analysis was done until Maxwell published his
electromagnetic theory in 1873 . Partial discharges can be measured electrically,
acoustically, optically, and chemically.
Generally, electrical PD measurement is preferred over others because of its high
sensitivity and availability of complete test systems . There are different types of
electrical sensors for PD measurement of cables, including metal foil electrode, internal
shield electrode, resonance type high-frequency (REDI), and embedded capacitive
sensors. Basically, they can be categorized as:
Inductive sensors: Inductive impulses are detected along with inductive injected
pulses for calibration, based on the fact that the current pulses from partial
discharges traveling along the cable can be observed to follow the spiraling
structure [21;22]. In order to detect inductive pulses, the sensor is placed around
the outer sheath of the cable. This sensor doesnâ„¢t make any changes to the cable
under test, but it can only be applied to cables with a sheath of helically wound
wires . Page 18
Capacitive sensors: Capacitive impulses are detected along with capacitive
injected pulses for calibration, making the capacitive sensor useful for broad
applications [20;23]. The sensor detects the induced voltage, so it can only be
applied to the cable without metal sheath or the cable with the embedded/buried
sensor. For the latter application, the integrity of the cable is damaged.
Capacitive-Inductive sensors: Directional coupler belongs here, and it uses
superposition of inductive and capacitive coupling [24;25]. This method has the
same limitation like capacitive sensors.
Although the electrical PD sensing is the most popular method in applications and
returns the highest sensitivity (up to 0.1 pC), it either has special requirements on cable
types, or is destructive to the cable. Electromagnetic interference is a serious problem for
electrical sensing as well.
A partial discharge results in a localized release of energy creating a small explosion.
Hence acoustic waves are generated and propagate from the source to the outer surface of
the cable [26;27]. Acoustic sensing has great advantages over electrical sensing,
including being free from electrical interference, very easy to apply, no need to power
down, and not requiring additional components, such as high voltage capacitors .
The amplitude and frequency components of acoustic waves are both factors in
detecting the PD. They can be caused by geometrical spreading of the wave, interface
between materials, absorption of the material (higher frequency components are
removed), frequency-dependent propagation, and so on . Interpretation of the
acoustic signal is a very complicated process since it involves many unknown
Cable applications with acoustic sensing are much scarcer, while transformer
applications are more popular. The main reason is that acoustic signals of PDs attenuate
during propagation. Experiments show that 10 pC partial discharges canâ„¢t be detected if
the sensor is located 70 mm away from the site. Sensitivity of acoustic sensor is also
limited (reported with 10 pC ). Once the sensor can be delivered to a reasonable
proximity of the discharge location, acoustic pickup will become possible. Although Page 19
accelerometers and acoustic emission (AE) sensors both are used for detecting acoustic
waves, only AE sensor can be used to detect PDs because of its higher frequency range.
Optical sensing is a potential direction for partial discharge measurement with advances
in optical fiber technology, which enable accurate measurements even in hostile
environments with a high background noise level. The application of this technology in
the field is still extremely limited to date.
Partial discharges can produce ultrasonic pressure waves which can also be detected
with suitable pressure optical transducers [29;30]. The optical fiber sensor system can be
safely immersed inside the transformer. The perturbation caused by pressure wave
induces stress on the fiber core and affects the light beam traversing the fiber. By using
interferometric techniques, the optical phase shift caused by the perturbation can be
detected accurately with a phase-modulated type optical sensor. The use of optical fiber is
being exploited, primarily in acoustic detection of PD , but not limited to it. Optical
sensors can also detect the electrical pulses introduced by partial discharges, which are
measured by using light-emitting diodes (LEDs) and fiber optics under impulse voltage
conditions . The sensor attached to a high-voltage power transmission cable couples
signals with enough intensity from a partial discharge to an electro-optic modulator to
measurably change the amplitude of an optical carrier. One advantage of the optical
sensor is there is no power requirement at its site .
Partial discharge activity also results in changes to the chemical composition of power
systems. These changes have been exploited in the detection of PD activities. A low cost
SOF2 transducer was used with GIS to detect PD activity . The gas generated in the
oil by PDs is exploited to identify PD activities . Hydro-Quebec has developed a
large database and tables of acceptable levels. Paper insulation degradation bi-products
can be detected with liquid chromatography, but there is no future for this method
because of its poor sensitivity and complex data analysis . Basically, there is no Page 20
chemical sensing method applied to solid insulated cables, although it has been
investigated on gas/oil-insulated cables [37;38].
None of above PD sensing methods is perfect. Their characteristics determine the
range of their applications. Based on requirements of mobile monitoring, the acoustic
sensor is the most suitable sensor type, mainly because it is non-destructive to cables,
small in size, and easy to implement. Since the robot is an auto-tracking system, the
acoustic attenuation problem is easily solved.
188.8.131.52 Fringing electric field sensing
Water trees/electrical trees are dangerous incipient failures, which are not detectable by
the previously described thermal or acoustic methods. Water trees or electrical trees may
develop for a long time without any PDs until the insulation is damaged suddenly within
a short period. Generally, these failures are very harmful and contribute a large portion of
There are different methods available to directly detect the properties of insulation
material [6;7]. Since changes in the dielectric properties are usually induced by changes
in various physical, chemical, or structural properties of materials, the dielectrometry
measurements provide effective means for indirect non-destructive evaluation of vital
parameters in industrial and scientific applications [39;40]. One effective method is
fringing electrical field sensing, which relies on direct measurement of dielectric
properties of insulating and semi-insulating materials from one side [41;42]. The basic
idea is to apply a spatially periodic electrical potential to the surface of the material under
test. The combination of signals produced by the variation of the spatial period of
interdigital electrodes, combined with the variation of electrical excitation frequency,
potentially provides extensive information about the spatial profiles of the material under
While interdigital electrode structures have been used since the beginning of the
century, the application of multiple penetration depth electric fields started in the 1960s
. Later, independent dielectrometry studies with single  and multiple 
penetration depths using interdigital electrodes have been continued. Generally speaking, Page 21
the evaluation of material properties with fringing electric fields is a much less developed
area than comparable techniques. This field holds a tremendous potential due to the
inherent accuracy of capacitance and conductance measurements and due to imaging
capabilities combined with noninvasive measurement principles and model-based signal
Another important application of fringing electrical sensors is to detect water uptake.
As a highly polar material, water is easily detectable by low frequency dielectrometry
techniques. The spatial moisture distribution has been measured successfully with a
three-wavelength interdigital sensor for transformers .
184.108.40.206 Video sensing on mechanical damage
As one of the aging factors, mechanical aging contributes to cable failures too.
Mechanical aging could cause cables to rupture mechanically, lose adhesion at interfaces,
lose or absorb liquids and gases . The mechanical aging can initiate and accelerate
the process of PDs and electrical trees/water trees; therefore it is not trivial to locate the
mechanical damages. The active sensors, such as sonar, can be used to investigate the
structure changes of the cable, while a digital video camera is helpful to find the external
abnormal appearance of the cable by vision or through video signal processing.
1.3.3 Signal processing and diagnosis
The major purpose of signal processing and diagnosis is to determine the fault type, fault
extent, and aging status. Then the accurate estimation can be given to aid the decision on
maintenance. The non-destructive measurement methods are often treated in the
framework of the inverse problem theory. In our case, the definition of forward and
inverse problems can be presented as shown in Figure 1.3. For most applications, the
inverse problem is inherently more difficult. It does not necessarily have a unique or any
solutions, since it requires solving for unknown properties given a known subset of
material and geometrical properties as well as the measured partial discharges,
temperature, transcapacitance and transconductance. Furthermore, even if a unique and
exact mathematical solution exists for a given set of input values, it may have no
resemblance to the true physical parameters because of the effects of measurement noise. Page 22
There are already successful project and implimentations for power transformers [46;48], but little work has
been done with power cables.
replace, repair, etc)
Figure 1.3. A conceptual representation of forward and inverse problems in the
framework of dielectrometry.
Some work has been done on the acoustic measurement of partial discharges.
Experiments performed determined the detection limits for acoustic PD signals in a cable
joint made from EPDM rubber. Results show that there were several propagation modes:
high frequency is dominant one when the sensor is close to the PD source, whereas low
frequency is dominant when the sensor is away from the PD source . Acoustic
emission spectral analysis tool was applied to identify specific types of PDs: point-plane
type discharge in oil, surface discharges in oil, gas bubble discharge in oil, or discharges
in indeterminate-potential particles moving in oil . This demonstrated a specific type
of PD can be identified if suitable spectral descriptors are selected. The result was limited
to pattern identification of partial discharges in oil. One back-propagation (BP) artificial
neural network (ANN) with using acoustic emission measurement on PDs with high
voltage cables was investigated. The signals were processed with three-dimensional
patterns and short duration Fourier Transforms (SDFT) . Regular shape and Page 23
arrangement of voids can be effectively identified with this method. It has to be pointed
out that voids in practical condition are generally irregular, there is still no methods
developed to for irregular voids that are common in power systems. Besides these
methods, MLP Neural Network [52;53], fuzzy logic , fracture geometry feature ,
statistic estimation , and wavelet techniques  have been used to analyze PDs.
Current practice still relies heavily on human expertise in the identification and location
of PD sources in electrical apparatus and cables . Future development needs to
include following things: developing more powerful noise rejection procedures;
improving the reliability of monitoring systems; developing sophisticated expert-systems
which can integrate multiple data for quick recognition of dangerous PD faults .
When several sensors are used together for insulation estimation, they can supplement
each other to supply more information than only one sensor system. There are still no
signal-processing methods available for a multi-sensor system that can perform
temperature, mechanical, PD, and dielectrometry measurement. One of the most
important tasks is to develop an algorithm that can integrate multiple signals and access
accurately the status of insulation.
1.4 Thesis Outline
This thesis focuses on developing a mobile platform and a sensor array that could be later
used in the monitoring of underground power cable systems.
In chapter two, a profit-driven model is derived and traditional maintenance methods
are evaluated by their economical performance. The model is also employed to generate a
hybrid maintenance method, which is a function of specific cable systemsâ„¢
characteristics. It demonstrates economic efficiency can be improved greatly if mobile
monitoring technology can be introduced.
In chapter three, a mobile robotic platform is designed to deal with the monitoring
task. Based on the information flow scheme configured, the control board is implemented
with four micro-controllers, and a data acquisition system is developed on a DSP board.
Bilateral communication among micro-controllers is implemented with the serial
peripheral interface bus and external interrupts. Page 24
the development of the digital signal processor (DSP) board 6711 and
the analog to digital converter THS1408 is explained. The direct memory access (DMA)
routine and thresh-hold voltage detection are developed for sensing data transferring on
the background of the DSP. The serial communication between the DSP board and
control board is implemented by adopting a synchronous serial port on the DSP chip into
an asynchronous serial port through software.
In chapter five, applications of the infrared sensor, the acoustic sensor and the
fringing electric field sensor are addressed. The working principles and experimental
setups are discussed, respectively.
In chapter six, some existing problems are addressed in detail, including stability
problem of the robot and experiment problems. Future research directions are put forward
towards to solve these problem
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