Transformer testing and monitoring
Transformer insulation testing

Transformer insulation testing

Transformer insulation testing

The primary test methods used on site to assess the condition of the active insulation components in transformers and bushings include: 

1) Power factor/tan delta at 50/60 Hz  
2) 1 Hz power factor/ tan delta and variable frequency power factor/tan delta (1 - 500 Hz) 
3) Dielectric frequency response (DFR) [a dielectric response method], and 
4) Moisture in oil measurements (for transformers). 

A traditional power frequency (50/60 Hz) power factor/tan delta test provides some useful information for the assessment of insulation, but the analysis may be inconclusive. Power frequency responds slowly to insulation deterioration and creates doubt when readings are not as expected.  

A more effective and reliable test program utilizes variable frequency power factor / tan delta measurements to accurately identify changes in insulation and dielectric frequency response (DFR) to pinpoint where the issues reside. Variable frequency power factor/tan delta testing complements power factor/ tan delta measurements by providing 1 Hz power factor measurements. Using the same instrument and test connection as the power frequency measurement, the DELTA4000 and TRAX + TDX performs 1 Hz power factor to provide results that are extremely sensitive to insulation deterioration due to moisture or oil conductivity.  Combined with power frequency measurements, 1 Hz provides immediate, conclusive insulation condition assessment without the need for trending or proprietary databases. In addition, frequencies from 1 to 505 Hz are used to provide Individual Temperature Correction (ITC), which provides reliable temperature correction for any frequency, eliminating the need for inaccurate temperature tables. 

Meanwhile, dielectric frequency response (DFR) testing delivers a very reliable and accurate moisture assessment of the insulation and the conductivity of the oil. A thorough review of DFR elucidates why this is a better tool for moisture assessment than traditional moisture in oil measurement, which deduces the moisture content in cellulose from the measured moisture in oil. Megger’s IDAX is a best-in-class dielectric response analyzer and the only instrument that can perform true FDS measurements (which is the preferred DFR method due to its robustness against noise) all the way down to 0.1 mHz in record time. A noteworthy accessory for the IDAX is the VAX, an amplifier that boosts the output signal of the IDAX to 2 kV, which may be important for low capacitive specimens, particularly CTs, to obtain meaningful moisture measurements.