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Although power and distribution transformers have service lives ranging from 25 to 50 years, there are many electrical transformers that were built and installed shortly after World War II that still remain in service. It’s the internal insulation system and the maintenance of that system that dictates the life cycle of the transformer. Industry professionals have long been protecting these large capital investments with reliability-centered maintenance (RCM) programs. Although these RCM programs have achieved a high level of dependability, they require near-continuous monitoring and diagnoses to show individual and aggregate equipment status and trends.
The life of the electrical transformer actually depends on the life of the internal insulation system. It can be shortened by a number of events - exposure to extreme conditions, aging and wear and tear. Many conditional items can be replaced in a timely manner to extend the life of the transformer. However, the oil-cellulose insulation system is one component of the transformer that cannot be replaced. Due to the omnipresence of oxygen and water, insulating oil deterioration is normal. The reaction between unstable hydrocarbons in the oil with oxygen, moisture or other chemicals and contaminants in the atmosphere, along with the assistance of accelerators such as heat, creates decay products in the oil.
Owners and operators of electrical transmission and distribution systems take action to prevent fires and explosions in transformers with deteriorating insulating oils. But these companies need a reliable method for predicting transformer failure before it reaches such extreme conditions. A new instrument designed to continuously monitor hydrogen in insulating oils promises to aid in predicting when a transformer is in the danger zone. Because a high hydrogen level in electrical insulating oils can indicate an imminent explosion, closely monitoring hydrogen can be an effective tool in predicting and preventing transformer failure.
Through nanotechnology, Applied Nanotech Inc., a subsidiary of Nano-Proprietary Inc., created a palladium alloy nanoparticles-based sensor for the detection of hydrogen gas dissolved in power transformer oil. The absorption of hydrogen by the palladium alloy nanoparticles results in changes in the oil’s physical, electrical and optical properties. The sensor, which was tested for different hydrogen concentrations at different temperatures in transformer oil, exhibited suitable sensitivity and short reaction time.
The hydrogen sensor is made of palladium alloy nanoparticles on a dielectric substrate (Figure 1).
|Figure 1. Micrograph of the Sensor’s Sensing Element. The micrograph shows a well-defined pattern consisting of palladium alloy nanoparticles as small as 100 nanometers.|
It is based on a phase transition of these alloys in the presence of hydrogen whereby the nanoparticles expand by as much as 5 percent to 10 percent. This means that the volume increases in the presence of hydrogen. Applied Nanotech developed certain semiconductor processes whereby palladium alloy nanoparticles are isolated from each other when hydrogen is not present. In the presence of hydrogen, because of the volume growth due to the phase transition, the particles touch each other and considerably change the electrical characteristics of the device.
Because the sensor acts like an open circuit, it uses low to no power in the absence of hydrogen. It is well-known that palladium absorbs hydrogen, which results in this phase transition. However, palladium alone cannot be used as a hydrogen sensor due to the fact that it operates only at room temperature and detects only about 2 percent of the hydrogen. As a result of Applied Nanotech’s research and development, the new alloy nanoparticles sensor can operate at up to 180°C. Additionally, the palladium alloy nanoparticles sensor can measure a vast range of hydrogen concentrations (Figure 2).
Figure 2. Sensor Response at Different Hydrogen Concentrations
The sensor is triggered at levels as low as 0.5 percent hydrogen by volume. Experiments demonstrated that at different transformer oil temperatures, the palladium alloy nanoparticles sensor was able to detect hydrogen in transformer oil from 20 parts per million (ppm) to 4,000 ppm. In addition, it typically can sense changes in the hydrogen levels in less than 10 seconds (Figure 3).
Figure 3. Time Response Characteristics
The sensor can be as small as one square millimeter and can be easily incorporated inside the transformer oil for continuous real-time sensing.
Traditionally transformer oil is analyzed using gas chromatography, which is expensive, time-consuming and cannot be performed remotely and continuously. The hydrogen sensor, however, has the ability to continuously, remotely and simultaneously monitor electrical power systems and transformers. Because it is prepared by standard semiconductor techniques, which offer the tremendous advantages of high-volume manufacturing and low cost, it can be a cost-effective alternative for hydrogen-monitoring in transformer oil. Furthermore, it can reduce costs by providing an early warning of insulating oil degradation, which can mitigate catastrophic failures.
Applied Nanotech plans to integrate the hydrogen sensor with recently developed wireless transmission chips so that the status of all transformers in a simple network can be monitored remotely and simultaneously.