Toshiyuki Yanari

Degradation of Insulating Materials of Transformers

Insulating papers and pressboards must have high dielectric strength and high tensile strength. However, these materials are gradually degraded due to thermal stress, oxygen, and moisture. Because the tensile strength decreases owing to degradation, the degradation of insulating material may affect the life of transformers. In our laboratory, through accelerated tests using models of oil-impregnated insulating systems, changes in characteristics of insulating cellulose materials have been investigated. These data were compared with data obtained from insulating papers of transformers with long service life. From these investigations, good correlation was found between the amount of gas generated from insulating papers in insulating oil and the retention of tensile strength and of degree of polymerization. Using this correlation, the degree of degradation of insulating papers in transformers may be known from the amount of gas. The average characteristic curves of insulating papers from transformers coincides with the degradation curve of 90 ° C ob tained in experiments. And if the life of insulating papers is considered to be equal to the life of transformers, the life of transformers is between 20 and 40 years.

Breakdown Characteristics of Moving Transformer Oil

As a result of investigating breakdown characteristics of moving oil, it has become clear that the breakdown voltage of ac and dc for moving oil is higher than that for stationary oil by 10-15 % at a moving velocity of 5 cm/s region; the former voltage becomes almost equal to the latter voltage in the region of 25 cm/s, and the former is reduced to lower than 90 % of the latter when the moving velocity exceeds 100 cm/s. However, for impulse voltage, the breakdown voltage of moving oil is the same as that of stationary oil.

PD and BD Probability Distribution and Equi-Probabilistic V-t Characteristics of Oil-Filled Transformer Insulation

Probability distributions of partial discharge (PD) and dielectric breakdown voltage were investigated for the typical components of oil-filled transformer insulation.

Development of large-capacity gas-insulated transformer

Concentrations of population and business activities result in high electricity demand in urban areas. This requires the construction of large-capacity underground substations. Oilless, nonflammable and nonexplosive equipment is recommended for underground substations. Therefore, several types of large-capacity gas-insulated transformer have been developed. Because the gas forced cooling type was considered to be available up to approximately 60 MVA, all of these gas-insulated transformers are liquid cooled. But the liquid cooling type has the disadvantage of a complex structure for liquid cooling. For this reason, the authors have been studying the development of a simple design for a gas forced cooling, large-capacity gas-insulated transformer. This paper discusses research and development of cooling and insulation technology for a large-capacity gas-insulated transformer and the development of a 275 kV, 300 MVA gas-insulated transformer.

Local Voltage Oscillation in Interleaved Transformer Windings

In this paper, local voltage oscillation of each turn within sections of interleaved transformer winding are discussed. Because of interlacing of turns, each turn within a pair of sections of the winding has initial voltage distributions quite different from final voltage distributions. This difference brings about local oscillation when a steep impulse voltage enters the winding.

Problems of Long-Term Reliability for UHV Transformer Insulation

Through long-term (3 years) partial discharge (PD) tests on some component models for transformer insulation, the authors have revealed that transformer insulation has a threshold voltage, Vol below which no partial discharge will occur regardless of how long the voltage may last. The value of VO is more than 62.5 percent of a 50-percent probability PD inception voltage at one-minute ac test.

HVDC Breakdown of Transformer Oil and the Effect of Space Charge on it

Experiments have been conducted on high voltage dc breakdown characteristics with large oil gaps of typical electrode setups, which are plane to plane and cylinder to plane. These have been confirmed in a voltage-range up to 800 kV and in gap length up to 307 mm.