Scanning frequency response analysis (SFRA) is used to identify mechanical or electrical problems in power transformers, caused by short circuits or shocks during transportation. Since the introduction of the IEC 60076-18 standard, SFRA has become a common test. It is recommended to perform the SFRA at the end of the acceptance test at the manufacturer and again after transport and during commissioning.
The method involves injecting a sinusoidal voltage with increasing frequency into one transformer winding and measuring the response signal at the other end. By comparing the input and output signals with a reference, it is possible to detect changes, movements or deformations in the internal components.
Dielectric response (DR) analysis is used to evaluate the moisture content in the cellulose insulation of power transformers, which is crucial in determining its state. Moisture can result from aging of the paper or ingress through faulty seals, reducing breaking strength and accelerating insulation aging.
To measure insulation, the output is connected to the high-voltage winding and the input to the low-voltage winding, with unwanted currents bypassed by the tank. The power factor/dissipation factor is measured at various frequencies, generating a curve that reveals the condition of the insulation. Very low frequencies indicate moisture in solid insulation, while medium frequencies reflect the conductivity of liquid insulation.
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