Monitoring of insulated electrical transformers using fiber optic sensors
SEDI-ATI has developed built-in fiber optic sensors consisting of a dielectric multi-fiber optic cable assembly including an inline hermetic feedthrough. It is aimed to be placed directly inside the transformer tank to offer on-line and real-time monitoring of the health of insulated electrical transformers.
Very expensive, insulated electrical transformers should be monitored
Insulated electrical transformers are one of the most critical and expensive investments in today’s electric power transmission and distribution systems. Their reliability typically depends on their insulation condition.
Any overlooked critical fault generated inside a power transformer may lead to a transformer catastrophic failure which could not only cause a disruption to the power system but also significant equipment damage. The cost of a failure can easily reach multi-million dollars.
Therefore, monitoring the health state of transformers with accuracy and prompt information is the critical prerequisite to predict fatal failures, optimize the maintenance, and make a vital decision on a transformer with suspicious conditions.
Safe and permanent insulation by sensors to avoid contamination
In high voltage transformers, a decisive significance is attributed to a secure and permanent insulation. The mineral oil and SF6 gas are good insulating materials. However, due to some inevitable factors such as electrical and thermal stresses as well as mechanical aging, the insulating medium inside a high-voltage transformer might be affected. For instance, some unwanted particles like water and gas can contaminate the oil. As a result, insulation strength gets reduced that may result in partial discharges and hot-spots in transformer oil.
Estimate the health of the transformer by measuring temperature and current changes
Partial discharge is not only a precursor of insulation degradation but also a primary factor to accelerate the deterioration of the insulation system in a transformer. Monitoring of partial discharge activities through the measurement of changes in temperature and in current in the transformer insulating medium has been proven to be an effective procedure for transformer health state estimation.
However, because of the strong electromagnetic interference (EMI) inside transformer tanks and short circuit potential, the current commercially available sensors such as piezoelectric acoustic sensors, can only be installed outside of transformers. They thus offer indirect or delayed information.
Advantages and applications of dielectric materials
Fiber optic sensors made from dielectric materials, such as fused-silica glass, are excellent candidates for on-line and real-time monitoring. Indeed, besides their small size and lightweight, they offer unique advantages such as immunity to electromagnetic interferences, high-sensitivity, stability, repeatability, durability against extreme environments, and fast response. And they can be submerged in transformer mineral oil or SF6 gas.
Such fiber-optic sensors can be used to measure various physical parameters like temperature, current, pressure, acceleration, refractive index of the insulating medium, moisture content in the oil, and a host of other applications.
The potential of multiplexing with a single signal conditioner
Moreover, fiber-optic sensors have the great potential of multiplexing which means that distributed electrical and thermal fault detection could be achieved with a single signal conditioner, providing the temperature profile of the winding and enabling the localization of the partial discharge origin.
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SEDI-ATI has developed built-in sensors consisting of optical fiber cables made of multimode silica glass optical fibers. They are placed inside the transformer tank. An inline hermetic fiber optic feedthrough on the transformer tank wall allows to safely pass the fiber optic sensor through the tank wall without affecting the insulation integrity of the transformer.