CONDITION BASED MAINTENANCE OF UNDERGROUND CABLE SYSTEMS
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.
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.