Monitoring of insulated electrical transformers using fiber optic assemblies

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.

Fiber optic cables made of dielectric materials, such as fused silica glass, are excellent candidates for online and real-time monitoring. Indeed, besides their small size and light weight, 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 assemblies help 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.

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.

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.

Moreover, such fiber-optic assemblies have great potential for multiplexing, which means that distributed electrical and thermal fault detection can be achieved with a single signal conditioner, hence providing the temperature profile of the winding, as well as enabling the localization of the partial discharge origin.

SEDI-ATI has built optical fiber cables made of multimode silica glass, aimed to be placed inside the transformer tank. An inline hermetic fiber optic feedthrough on the transformer tank wall allows to safely pass the fiber optic cable through the tank wall without affecting the insulation integrity of the transformer.