The service life of a distribution transformer is governed by the condition of the insulating material. Deterioration of the transformer insulating material reduces the dielectric strength and also reduces the ability of the transformer to withstand short-circuit events. Korea's domestic transformer manufacturers use varnish to improve winding mechanical strength. However, the solidified varnish deteriorates over time, which adversely affects the characteristics of the transformers. By improving insulation performance and the short-circuit withstand strength of distribution transformers, the cost of maintaining and replacing these transformers can be reduced.

Many pole-type transformer failures could be avoided by using a hybrid insulation, where layers of aramid papers and cellulose papers are used. Additional design modifications include reducing the number of cooling ducts between layers and reinforcing the frame of the transformer to improve short-circuit withstand strength. Then, varnish impregnating process is not required when this hybrid insulation is used. The higher reliability and longer life anticipated of hybrid-insulation manufactured transformers should result in cost savings to the utility.

Pole-Type Transformers in Korea — Test Result Analysis

As of December 2002, some 1.6-million pole-type transformers were in operation in Korea. The Korea Electrical Power Research Institute (KEPRI) performs pre-commissioning testing of new transformers and disqualifies many of the transformers tested. Figure 1 shows the test results that indicate the major cause of disqualification was due to short-circuit failure of the transformers.

Basic Design Concept

The permissible temperature limit depends on the characteristics of the insulating paper in direct contact with the conductor in the windings. For cellulose insulating paper used in conventional pole-type transformers, the maximum limit is 95°C (203°F), which corresponds to a 55°C (131°F) average winding rise. For thermally enhanced insulating paper, the maximum limit is 110°C (230°F), which corresponds to a 65°C (149°F) average winding rise. Aramid insulation can operate continuously at a maximum temperature of at least 190°C (374°F).

The transformers can be compacted by the increase in permissible temperature limit. For example, a 10% increase in the average temperature of windings results in a 20% reduction in mass of the core and coil. In this application, the hybrid-insulation design allows the maximum winding temperature to exceed the 110°C conventional limit without loss of insulation life. However, the windings are still designed for a conventional average winding temperature rise of 65°C.

Figure 2 illustrates the hybrid-insulation technique, where the aramid paper is placed directly in contact with the primary conductor windings, which reduces the thermal aging of the hybrid insulation. Otherwise, conventional cellulose insulating paper is used.

Performance Test of Model Transformer

Two types of model transformers (shell-type and core-type) with the same voltage ratio, kVA capacity and percentage impedance were manufactured for comparison with a conventional low-loss pole-type transformer. Two additional taps (one above and one below) the nominal primary voltage of 13.2 kV were added to actively cope with the voltage variation. The new design also improved the short-circuit withstand characteristics by enhancing the structural reinforcement without the need for varnish impregnation. Four transformers with hybrid insulation and four with conventional insulation were made. Figure 3 shows the internal structure of the shell-type model transformer.

The unit was designed to 65°C average winding rise, which allowed the conventional eight-panel radiator to be replaced by a six-panel radiator. The L-H-L type structure, separating the secondary winding into two sections, was employed. The clamp assembly was reinforced after testing, identifying that the short-circuit withstand strength of the transformer with conventional insulation was inadequate. The internal view of the core-type model transformer is shown in Fig. 4.

The higher permissible temperature limit of the hybrid insulation allowed the oil cooling ducts to be reduced considerably. The two-layer duct pattern used in the secondary windings of the conventional transformer was replaced by a single-layer duct. The four end-duct patterns required in the primary windings were entirely removed, resulting in a more compact design with lower losses that also was mechanically more robust.

Accelerated Aging Characteristics

Temperature and time are the key factors that affect the aging of winding insulation in pole-type transformers that, in service, operate with a variable load/time characteristic. Therefore, the transformers were evaluated using heat cycle test equipment. Transformer designers and utility operators regard the temperature rise limit of the transformers with cellulose insulation important because when the transformer windings exceed the specified maximum temperature, the insulation is subjected to accelerated aging.

The overload conditions applied are shown in Fig. 5, where the load cycle was defined as 150% of full load applied for nine hours and 90% of full load applied for 15 hours. This accelerated aging test was carried out for about 40 days. The winding reached a maximum temperature of 155~160°C during the 150% loading condition. According to the loading guide ANSI/IEEE C57.92, the expected life at this temperature is about 500 hours for the transformer, whose maximum average winding temperature is 55°C. When the transformer is subjected to the 40-day accelerated aging test, the expected life will be 72% of the normal service life. This aging method gives more than a 14-year accelerative effect.

Figures 6, 7 and 8 show the sensor for determining the aging degree, the placement of the sensor in the insulating oil and the schematics of the aging measurement system of the insulating oil. The amount of the leakage current measured by the sensor depends on the temperature. Therefore, the system indicates the oil temperature measured and compensated in real time. The temperature was measured by four sensors positioned around the transformer: at the lowest oil level, top of the radiator and a sample taken from the bottom of the radiator.

Additionally, the leakage current was measured by the Toid sensor, installed in the model transformer and the conventional transformer, and compared for the duration of the test period. Examination of the increase in the leakage current during the aging period confirmed that the hybrid-insulation-type transformer is less affected by overload than the conventional low-loss-type transformer.

Insulating Paper Aging

The tanδ was measured on the insulating paper between layers for the shell-type transformers with hybrid and conventional insulation after accelerated aging treatment under the same conditions.

In Fig. 9, the test results indicate the tand of the hybrid-insulated transformer was less than that of the conventional low-loss-type transformer. This means that the electrical characteristics after aging can be improved by using the hybrid-insulating method.

Leakage Current of Insulating Oil

Figure 10 shows the leakage currents measured by the aging diagnosis sensors installed in the hybrid-insulated transformer and the conventional low-loss-type transformer. It is seen that transformer loading affects the leakage current. Although the measuring system was calibrated for the automatic temperature measurement, the leakage current increased with the internal temperature of the transformer in proportion to the load. However, the average of the leakage current gradually increased with time, which indicates that the aging progressed in the transformer windings. Considering the trend of the leakage current increase rate during the aging period, it also was shown that the hybrid-insulation method was affected less by the overload than the conventional low-loss-type transformer.


The advantages of the hybrid-insulated pole-type transformer (manufactured with hybrid insulation comprising conventional cellulose and aramid insulating paper) include:

  • Reduced aging of the cellulose insulating paper during overload conditions.

  • Reduction in secondary failure rate due to poor mechanical strength under short-circuit conditions.

  • Improved long-term reliability when compared with a conventional low-loss-type transformer.

  • The cost of manufacturing a transformer with hybrid insulation is about 1.1 times more that than of a conventional transformer. However, due to the predicted superior long-term reliability, the hybrid insulated transformer offers substantial overall economic benefits.


The authors wish to acknowledge the technical support and advice received from Kacey C. Lee and Richard P. Marek, DuPont Ltd., in preparing this article for publication.

I.K. Song received his BE, MS and PhD from Soongsil University in 1984, 1986 and 1996, respectively, in electrical engineering. Presently, he is a principal researcher and team leader in KEPRI, KEPCO. His special fields of interest are the lifetime estimation of power distribution facilities-cable, distribution transformer, insulator and lightning arresters.

Byung-Sung Lee received his MS degree from Chungnam National University in 1995 in high-voltage engineering. He has been with KEPRI, KEPCO as a member of the technical staff since 1995. His special fields of interest are the electrical characteristics of insulators and lightning arresters, the accelerated aging characteristics of polymer insulators and the lifetime estimation of distribution transformers.