On-site replacement of OLTC, drying of winding insulation, induced voltage test with PD measurement of 250 MVA, 400/110 kV, 40 year old transformer

摘要

The On-Load Tap Changer (OLTC) was damaged beyond repair, but winding insulation has not been significantly aged in 250 MVA, 400 kV network transformer, despite more than 40 years of service. The transformer maker does not manufacture his type of OLTC anymore, and it was replaced by MR equivalent. To determine the winding-insulation condition induced-voltage test was performed up to 110% U_n, by energizing the tertiary winding from mobile motor-generator set operating at a higher frequency. Insulation of 400 kV winding revealed Partial Discharge (PD) level between 1200 and 1800 pC but no specific PD source was identified. Water content in the winding insulation amounted to 1.1% prior to the OLTC replacement. Very compact design of the tank required an assessment of dielectric stress around the leads from tap-winding to the tap-selector. These leads had to be extended, and clamping the joints and wrapping tape insulation in the cramped space required highly skilled craftsmen. The single access porthole has been sealed when no people were inside the tank, to limit ingress of air. An internal polymeric-membrane was installed inside the tank to separate the working area from the windings, where a small overpressure was kept with synthetic dry-air. Despite such precautions some moisture was absorbed by the winding-insulation surface during the OLTC replacement. To dry this insulation the transformer was heated by filling the tank with new oil warmed up by 65°C, then draining the oil and lowering the pressure to less than 1 Tor (133 Pa) for 24 hours. The moisture was extracted from the insulation outer-layers and lower than 1% content was measured using RVM and PDC methods, which are sensitive to the moisture trapped in the insulation surface, and to the moisture averaged over the whole insulation-volume, respectively. The induced-voltage test was repeated after completion of the OLTC replacement, and PD level in 400 kV winding insulation was reduced by an order of magnitude, and DGA indicated content of Hydrogen 3.5 ppm, Methane 0.35 ppm and no Acetylene. The transformer was put in service after 72 hour trial run. The successful on-site OLTC replacement in the large transformer manufactured in Japan 40 years ago confirmed feasibility of such operation. This meticulously prepared on-site operation, highly skilled engineers and technicians, as well as advanced instruments to assess the winding-insulation condition were the essential factors to extend the technical life of this strategically important and costly transformer.