The diagram of DFR measurement for 3-windings transformers

DFR measurement has been conducted by Center Electrical Testing Company Limited (CPCETC) for assessing main insulators of transformers and bushings of transformers, with scanning frequency range from 1 mHz ÷ 1 kHz. This measurement has shown a reliable result for accurate assessment of insulation level of transformers, also offering recommendations for premature treatment to prevent equipment failure. Apart from data of humidity concentration in solid insulators, the measurement provides additional data, such as: (i) inspection of capacitance change and dielectric loss parameter of components in power grid (transformers, insulators, generators...) via frequency-response characteristic; (ii) evaluation of oil conductance; (iii) evaluation of temperature dependency and performance of temperature calibration for each material or actual condition; (iv) evaluation of oil recovery, transformer-drying process and oil impregnation…

In order to clearly show the effectiveness of DFR measurement for transformers, we conducted analyses on a specific case in 2022: 25MVA transformer, type 25000/3P; 115/23 (15,75) kV, manufactured in 2005. Necessary measurements were performed between windings (secondary 1 + secondary 2) and balance winding. The result shows that humidity concentration in solid insulator of the transformer is 3.1%, evaluated as “humid” level (according to the Procedure for repair, maintenance of equipment of 110kV substations and power lines based on working condition_CBM). Such result is consistent with the transformer condition, having operated for over 15 years, its insulation (including paper and oil) was heavily deteriorated with humidity and was in ageing period. In order to keep the safe operation of the transformer, it is recommended that (i) the oil is assessed, (ii) oil filtration for reuse is performed, and (iii) insulators are dried with proper method.

The result for DFR measurement for transformer, having operated since 2005

Drying insulators was impossible due to site condition, operating agency isolated the transformer and conducted oil filtration for reuse. After oil filtration, the transformer was assessed again with the DFR measurement and the result is shown in the picture below. The value of humidity concentration in solid insulation decreased from 3.1% to 2.5% at the “fairly humid” level. Oil quality considerably increased, specifically (i) the value of oil conductance decreased from 10.7 pS/m to 0.424 pS/m, (ii) the value of Tgδ decreased from 0,714% to 0,606%, (iii) the value of capacitance was almost unchanged. This result demonstrates that oil filtration for reuse significantly improved insulation condition of the transformer. However, for transformers, it is normal that water concentration in solid insulation is higher than that of oil, this reflects the fact that oil filtration is not completely effective to help oil return to “good” condition. To eradicate humidity issue in this transformer, it is necessary to perform further steps, including: drying process or change of solid insulation (paper) and periodical DFR measurement for evaluation of deterioration condition of insulators, preventing transformer failure.

The result of DFR measurement for transformer after oil filtration for reuse

In order to further assess DFR measurement, apart from the above-mentioned humidity data in solid insulation, operators should reference the characteristic diagram of parameters, including dielectric loss_tgδ, capacitance… based on frequency. According to the characteristic diagram, operators can categorize separate frequency zones for evaluation; accordingly, each zone has its own demonstration of internal change in insulation, including topology, humidity or ageing process of insulation materials. Besides, there remains one important parameter which can impact on response characteristic, it is temperature. Changes in temperature result in that of the characteristic diagram, the diagram tends to move rightwards when temperature rises.

After time of application of DFR measurement for transformers of EVNCPC, CPCETC has detected many issues, defects of ageing and humidity of transformer insulators which was never detected by periodical testing. DFR measurement is specially important and necessary to transformers which have been operated for a long time, the probability for these transformers to be deteriorated with humidity and ageing in insulation is high, leading to equipment failure without timely detection.

Author: Dương Ngọc Quốc