^{1}

^{2}

^{1}

The paper is based on the experimental investigation of accelerated stresses on insulation of power transformer. The effects of individual thermal and electrical stresses have been graphically presented. The factors accelerated thermal aging factor (ATAF) and accelerated electrical aging factor (AEAF) have been introduced, it helps to understand the contribution of thermal and electrical stresses and degradation trends of insulating properties. The accelerated aging factors have been mathematically correlated with different properties of insulation such as moisture, breakdown voltage (BDV), tan delta and resistivity. These parameters were determined experimentally for fresh oil samples and for samples subjected to accelerated aging.

The insulation of power transformer degraded under a combination of various stresses. The stresses reduce the dielectric capability of a transformer and increase the probability of failure. In this paper, the impact of accelerated stresses of power transformer insulation is presented. The various parameters used to measure these impacts are moisture, breakdown voltage (BDV), tan δ and resistivity [

To measure the impact of accelerated thermal and electrical stresses on the transformer insulating oil, the special test cell shown in

Description | Material | Dimension |
---|---|---|

Cover plate | Mica sheet | 5 mm |

Sealing ring | Silicone rubber | 5 mm |

Tank | Mild steel (Coated with enamel paint) | 235 mm × 100 mm × 150 mm |

Round stud and nut | Copper | |

Bolt | Mild steel | |

Transformer oil | As per IS: 335 - 1993 (2005) | 3 liter/cell |

Copper strip without paper wrapped | Copper | 205 mm × 12.5 mm × 1.96 mm |

Insulating paper | Electrical grade paper, as per IS 9335-1993 |

The aging of transformer insulation is intimately connected with the magnitude and duration of stresses. To understand the effect of thermal and electrical stresses on transformer insulation, accelerated thermal aging factor (ATAF) and accelerated electrical aging factor (AEAF) have been introduced:

where T is temperature in ˚C, D is the duration of stresses in hours and E is electrical stresses in kV/mm.

Both factors help in generating the mathematical equation which helps in differentiate the degradation trends. The thermal stresses of 190˚C, 200˚C and 210˚C and electrical stresses of 2 kV/mm, 4 kV/mm and 6 kV/mm were used. The electrical stresses have been performed at room temperature. The ATAF and AEAF were calculated using Equations (1) and (2) and results are shown in

Temperature (˚C) | Aging (hours) | ATAF (˚C-hr) |
---|---|---|

190 | 150 | ATAF1 = 28,500 |

200 | 150 | ATAF2 = 30,000 |

210 | 150 | ATAF3 = 31,500 |

190 | 300 | ATAF4 = 57,000 |

200 | 300 | ATAF5 = 60,000 |

210 | 300 | ATAF6 = 63,000 |

190 | 450 | ATAF7 = 85,500 |

200 | 450 | ATAF8 = 90,000 |

210 | 450 | ATAF9 = 94,500 |

190 | 600 | ATAF10 = 114,000 |

200 | 600 | ATAF11 = 120,000 |

210 | 600 | ATAF12 = 126,000 |

190 | 750 | ATAF13 = 142,500 |

200 | 750 | ATAF14 = 150,000 |

210 | 750 | ATAF15 = 157,500 |

Electrical stress (kV/mm) | Aging (hours) | AEAF (kV/mm-hr) |
---|---|---|

2 | 150 | AEAF1 = 300 |

2 | 300 | AEAF2 = 600 |

4 | 150 | |

2 | 450 | AEAF3 = 900 |

6 | 150 | |

2 | 600 | AEAF4 = 1200 |

4 | 300 | |

2 | 750 | AEAF5 = 1500 |

4 | 450 | AEAF6 = 1800 |

6 | 300 | |

4 | 600 | AEAF7 = 2400 |

6 | 450 | AEAF8 = 2700 |

4 | 750 | AEAF9 = 3000 |

6 | 600 | AEAF10 = 3600 |

6 | 750 | AEAF11 = 4500 |

The virgin insulating oil and paper have moisture content of 50 ppm and about 0.5% by weight respectively. Moisture in the transformer reduces the insulation strength. The main reasons for moisture content changes over the life cycle are: moisture interactivity with environment due to leakage, additional moisture generation due to chemical reactions [

The moisture in cellulosic insulation may be determined from moisture in oil samples using oil/paper moisture equilibrium curves such as Fabre-Pichon curves, Oommen curves, Griffin curves, MIT curves [

The breakdown voltage (BDV) is an important parameter to gauge the condition of oil. The BDV of oil is high when it is dry and clean, it goes down slowly as the moisture contents and conducting impurities increase as a result of oxidation of oil [

The moisture of oil sample has been measured by automatic coulometric karl fischer titration equipment. As per IS 335:2005 and IS 13567:1992, moisture content in virgin oil should not be more than 50 ppm. ^{2} = 93.3%. The mathematical correlation between moisture and ATAF is shown in Equation (1):

In

^{2} = 94.5%. The mathematical correlation generated between moisture and AEAF is shown in Equation (2):

According to Fofana [

The BDV of oil sample have been measured by oil breakdown test set [

^{2}=92.9%. The Equation (3) represents the mathematical correlation between BDV and ATAF.

tion (4) represents the mathematical correlation between BDV and AEAF.

The tan δ is very important parameter to measure the quality of the insulation. The variation of tan δ with applied voltage provides useful information about the source of any imperfection in the insulation. It increases with insulation deterioration and serves as an early indicator of failure hazards. A low value of tan δ is generally desirable. The high value of tan δ gives an early indication of the contamination and presence of moisture content, conductive contamination, soluble varnishes, resins etc. [

Resistivity is the most sensitive property of oil, it varies with temperature. It is desirable to have resistivity of oil as high as possible. It reduces considerably due to presence of moisture, acidity and solid contaminants [

The resistivity of oil sample has been measured by Automatic dielectric constant, tan delta and resistivity (ADTR-2K) equipment. As per IS 335:2005 and IS 6103:1971, the resistivity of fresh oil is 35 TOhm-cm (35 × 10^{12} Ohm-cm) at 90˚C, minimum.

so it should be neglected in the graphical representation. After 750 hours of aging, maximum decrease of resistivity was 1.374 TOhm-cm at 210˚C. The results of ^{2}=94.4%. The mathematical correlation generated between resistivity and ATAF is given in Equation (5):

found with R^{2} = 94.9%. The mathematical correlation generated between resistivity and AEAF is given in Equation (6):

The scattered results of ^{2}=95.1%. The mathematical correlation generated between tan δ and ATAF is shown in Equation (7):

^{2} = 94.3%. The mathematical correlation generated between tan δ and AEAF is shown in Equation 8:

The effect of thermal and electrical stresses on the transformer oil has been experimentally investigated in this paper. The term accelerated thermal aging factor (ATAF) and accelerated electrical aging factor (AEAF) have been introduced in order to quantify the thermal and electrical stresses. The graphically representation between moisture, BDV, tan delta and resistivity with aging, ATAF and AEAF has been presented. It is presented that as the moisture increases with ATAF and AEAF, the BDV decreases in same pattern. Similarly as tan delta increases with ATAF and AEAF, resistivity decreases with same pattern. It is due to the fact that these properties are affected by the same contaminants.

This paper contributes that electrical stresses also play an important role in the degradation of the insulation along with thermal stresses but the degradation of insulation by thermal stresses is comparatively more as compared to electrical stresses. Further, all the properties were correlated with ATAF and AEAF and mathematical correlation has been generated.

The authors are thankful to Punjab Technical University, Jalandhar & Technology Information Forecasting and Assessment Council and Centers of Relevance and Excellence (TIFAC-CORE) on Power Transformer Diagnostics and Dr. R. K. Jarial, Associate Professor and Office-in-charge, HV Lab, NIT Hamirpur for providing necessary infrastructural facilities for carrying out the research work.

Singh, J., Sood, Y.R. and Verma, P. (2017) Impact of Accelerated Stresses on Power Transformer Insulation. Energy and Power Engineering, 9, 217-231. https://doi.org/10.4236/epe.2017.94015