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, 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 (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.
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, 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.
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 4
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 “Condition-based Maintenance of Electric Power Systems” focuses on the prototype development of mobile monitoring. The long-term goals of this project 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, 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 5
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 of failures.
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.
The electric conductors in high-voltage power cables and in telephone cables are insulated with paper tapes. In the former the paper tapes are impregnated with an insulating oil while in the telephone cables they are dry. The continued operation of both types of cable depends on the integrity of the sheath. One of the greatest causes of sheath failure has been the imperfect die welds along the sheath. The sheath is formed by a ram or plunger acting in a press cylinder to force the hot lead through a die to form the cylindrical sheath. The imperfect die welds are caused by the oxides and impurities which form on the surface of the molten lead in the press cylinder and remain when a new charge is added. Hence the die weld formed between the old and new charges is weakened by the presence of oxides and impurities in the weld. The General Electric Co. has eliminated this difficulty by the following means: The new molten charge is kept under an atmosphere of inert air to prevent oxidization; it is introduced into the press cylinder as molten lead flowing through a spout that goes well into the bottom of the cylinder, and at the end of the spout there is a nozzle that imparts a swirling motion to the molten lead that frees the oxide from the surface of the old charge. The oxides being lighter than the lead rise to the surface and the cylinder is allowed to overflow so that the oxides go out into a flood ring at the top of the cylinder. A hydrogen flame playing into the top of the cylinder prevents oxide forming when the ram is withdrawn from the cylinder. Examinations of the die welds made with this new process show no imperfections.
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.
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 7 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.
Sensing of partial discharges :
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.
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 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. In order to detect inductive pulses, the sensor is placed around the outer sheath of the cable. This sensor doesn’t make any changes to the cable under test, but it can only be applied to cables with a sheath of helically wound wires
Capacitive impulses are detected along with capacitive injected pulses for calibration, making the capacitive sensor useful for broad applications. 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.
Directional coupler belongs here, and it uses superposition of inductive and capacitive coupling. This method has the same limitation like capacitive sensors.
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. 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 parameters.
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 can’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 9 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. 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 chemical sensing method applied to solid insulated cables, although it has been investigated on gas/oil-insulated cables.
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 total failures.
There are different methods available to directly detect the properties of insulation material. 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. 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. 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 test.
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, 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 analysis.
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.
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.
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. 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. There are already successful projects for power transformers, but little work has been done with power cables.
Due to the deregulation policy introduced into power market, utilities are market players now, no longer as monopolies. From the perspective of utilities, the ideal power network should work continuously and without failures. In reality, it is not possible, but the ideal can be approached through minimizing maintenance cost and maximizing the service life and reliability of existing power cable networks, both goals cannot be achieved simultaneously. Timely preventive maintenance can dramatically reduce system failures. Investments must be made to prevent unexpected cable failures, including the addition of hardware/software and possibly more technicians. On the other hand, maintenance does not always mean cost efficiency. For instance, it is meaningless to maintain a power network with certain types of minor cable failures endured within normal system operation. Maintenance or replacement of cables with these problems would result in additional and unneeded financial losses. Therefore a cost-efficient maintenance strategy is necessary to reach a minimum operational cost for utilities.
Since a low operational cost is a necessary condition for a maximum profit, an economic model can be simplified to include only the controllable operational cost, without the fluctuating price of energy. In order to achieve minimum operational cost, the following cost driven rule should be calculated when any maintenance strategies are performed to power systems:
General maintenance methods can be divided into two distinct categories: unplanned maintenance which is event driven and planned maintenance which is carried out with forethought, control, and the use of records to a predetermined plan. These strategies also hold true to the power industry. Three basic levels of maintenance strategies are described and identified by their flexibility , although other classifications are also available:
Corrective/emergency maintenance (CM):
Corrective maintenance (CM) is passive. In other word, there is no action until a failure occurs. CM is economically efficient when failures cause only minimum inconvenience to the customers and minimum financial loss. For many years, the maintenance strategy for power cable systems has been the CM type. Examples of power networks for which CM is a reasonable choice include a recently perfectly installed non-critical distribution network. Another example is an underwater cable network. In the former, the pay from prophylactic maintenance is too low. In the latter, the costs of routine maintenance are very high, at least, with current technology. If unnecessary monitoring is imposed in these situations, the total operational cost C may have the characteristics shown in Figure The loss is introduced, although the profit is expected.
For example, an unexpected power shutdown to some customers will generate costly compensation and emergency maintenance. The cost of this type of outage can be too large to afford for both customers and utilities.
Scheduled maintenance (SM):
Scheduled maintenance (SM), as the name suggests, is one of planned maintenances carried out in predetermined time intervals to prevent possible failures. SM includes routine monitoring, repair of a network, or part replacement when incipient faults are detected by routine monitoring or predicted by estimation.
It is inherently difficult to determine optimum maintenance time intervals. Since the standard to estimate network/components lifetime is generally conservative, losses are generated if network/components are replaced prematurely. On the other hand, failure risk increases as components approach their predicted life expectancy.
Condition-based maintenance (CBM)
Condition-based maintenance (CBM) is defined as planned maintenance based on periodic or continuous monitoring of equipment status. CBM is very attractive since the maintenance action is only taken when required by the power components/systems. By predicting failure, this strategy helps to extend the service lifetime of power systems. CBM has received increasing attention in recent years in the power industry as a viable maintenance method for cable networks. A case study of CBM showed the performance of a network was increased 40% and that the maintenance cost was lowered by a factor 7 when cables were repaired not replaced . Figure shows one possible functional relationship of operational cost when CBM is applied to the power system. The drawback of CBM is possible unavailability and cost of monitoring technologies.
Hybrid maintenance (HM)
No single maintenance method is always the most efficient for power systems at a given time. Every method has its own advantages and disadvantages. Based on the cost driven rule, a hybrid maintenance strategy is presented here. The performance of this approach is maximized when C is calculated by using a combination of CM, SM, and CBM in a predetermined period. System data and records of similar systems can determine the time periods used in this process. The maintenance method with minimum will be selected for this specific period. For example, a newly installed network has a low expected failure occurrence. CM is cost efficient for this period of network life. As time progresses, network components begin to age and the system then requires routine C monitoring and maintenance. Scheduled maintenance works well for this time period. When a cable system becomes prone to fail and approaches its life expectancy, CBM becomes the best maintenance method. Finally, when repair is no longer effective, replacement of the network is required. Upon replacement of the cable system, the described process starts again. Simulated results for different maintenance strategies are shown in Figure. The minimum operational cost is shown as the bottom boundary generated by CM, SM, and CBM. The complexity of the system introduces uncertainty making it difficult to determine the intersection points of the curves in Figure. Therefore, a hybrid method is expected to have discontinuities, as illustrated by the bold curve in Figure.
As with any preventive maintenance technologies, it is crucial to obtain the accurate information about the system. The efforts spent on monitoring of a power system are justified by the reduction of fault occurrence and the elimination of consequent losses due to electric power disruptions. Traditional monitoring methods are expensive and complicated to operate for global monitoring and distributed monitoring methods, making the application of condition-based maintenance very limited. This fact directly affects the operational cost of the system. Mobile monitoring is able to solve this problem and is playing an increasingly important role in the monitoring of power systems with the advance of sensing technologies.
A novel mobile robot platform, which can perform monitoring tasks on underground cable systems, is developed in our lab. The platform measures cable aging status through the use of thermal, acoustic, dielectric and visual sensors to detect hot spots, partial discharges, cable insulation dielectric properties and visual flaws. These particular sensors were chosen based on the following standards:
· Able to report the actual condition of the system
· Small and light-weight
The autonomous robot exercises the following monitoring procedures:
· Execute the operational loop “run – stop – detect”. A local DSP board on robot is used to process raw sensor data.
· Once the incipient failure is detected, the robot will automatically perform detailed scans, i.e., more scans will be executed and the signals will be transmitted back and processed by a host computer.
· When there are no incipient failures detected, processed information can be saved locally.
The information gathered by the robot is used to generate an analysis of the cable aging status used to plan future maintenance procedures.
Information gathered by the robot is used to generate an analysis of the cable aging status used to plan future maintenance procedures. The total hardware cost of this robot is about $2,500, including sensors, a DSP board, and a control board. Assuming the robot runs at an average speed of 5m/min, the traveling distance of the robot can be up to 7.2 km per day. When only using 5 distributed sensing stations, the cost of hardware, installation, and maintenance of the stations would be at least 5 times that of the robot, i.e., more than $12,500. It can be seen that mobile monitoring has lower cost than distributed sensing, not even mentioning higher resolution and sensitivity. Figure 2.5 shows following observations when CBM is implemented with mobile monitoring :
· Mobile monitoring can greatly reduce the cost of condition-based maintenance.
· Operational costs will be less when condition-based maintenance is implemented earlier.
· Once the cost of Condition-Based Maintenance is reduced to a certain level, condition-based maintenance can even replace scheduled maintenance completely.
· If sufficient information is not available to utilize a hybrid maintenance method, condition-based maintenance can be applied for the entire cable service period.
Based on the derived cost driven model, the economical assess is performed for traditional corrective maintenance, scheduled maintenance, and condition-based maintenance. Further, the hybrid maintenance method derived can nearly reach the minimum operational cost by combining the advantages of above strategies. As one of implementation methods of condition-based monitoring, mobile monitoring demonstrates the most honorable property, the lowest cost. Results show that mobile monitoring can be one of future maintenance strategies.