Overview of the laboratory tests of cellulose-based samples in the NOVUM project.

Materials used as transformer insulation must fulfil number of requirements including resistance to transformer oil, elevated temperature and very good electrical withstand. Therefore, any new material formulation developed in the frame of NOVUM project has to be tested according to user specification (ABB) as well as according to specific standards.

Insulation resistance to transformer oil is very important factor deciding about proper operation of the equipment for many years. Therefore, each material developed in NOVUM project is tested for an oil compatibility. The samples of the material are placed into oil at 90°C for 7 days, and after that the following parameters have to be tested:

  • Appearance: visual assessment
  • Dielectric dissipation factor: according to IEC 60247
  • Interfacial tension against water: according to ASTM D971-99A
  • Increased gas production: according to IEC 60567

Figure 1. Oil compatibility procedure: immersion in oil, interfacial tension against water (ring method) dielectric dissipation factor of oil

Mechanical properties

Mechanical withstand of the insulation is another important parameter that must be evaluated in order to verify material applicability as the power transformer insulation.

The stand used for tensile measurements consists of a testing machine (located at AGH University of Science and Technology)  equipped with sample holders (jaws) adapted for tensile testing of the specimens for electrical purposes in accordance with the IEC 60641-2 and ISO 1924-2 standards. To get proper statistics of the obtained measurements, an appropriate number of samples for testing, at least nine, must be prepared. In reality, more samples have to be used since the samples have to be cut in few directions, having in mind composite structure of the material (with cellulose fibres).

Figure 2. Tensile and compression withstand tests

Figure 3. Samples prepared for compression withstand tests

It should be mentioned that additional set of tests is performed after 1-month ageing of the material in oil at elevated temperature.

Dielectric properties

Dielectric properties of insulation materials include such parameters as: surface and volume resistivity, electrical permittivity, dielectric dissipation factor and electrical breakdown strength.

Determination of the surface and volume resistivity of flat solid dielectric materials made in accordance to IEC 62631-3-1, IEC 62631-3-2 and IEC 61340-2-3. Test stand consist of electrometer KEYSIGHT B2987A and Resistivity Cell (16008B) equipped with normalized electrodes.

Figure 4. Electrometer with Resistivity Cell

Non-standard testing procedures with Frequency Dielectric Spectroscopy (FDS) method are possible for testing of dielectric material samples. The FDS instrument measures the selected dielectric parameters of insulating materials and systems as a function of frequency. The AGH-UST HV laboratory is equipped with a Frequency Response Analyzer Solartron 1260 with a dielectric interface 1296. The system allows measurements of capacitance (electrical permittivity) and dielectric dissipation factor in a wide frequency range, from extremely low (below 0.001 Hz) suitable for oil-cellulose materials characterization in comparative tests.

Figure 5. Frequency Response Analyzer Solartron

Electrical breakdown strength is one of the most important parameters of the insulation used in high voltage applications. Testing of NOVUM materials is carried out in accordance with IEC and ASTM standards (IEC 60243-1, D 149-09). Testing specimen consists of two profiled stainless-steel electrodes (with diameters 25 mm / 75 mm). The slow rate-of-rise method was applied. Test stand consisted of Voltage Test Transformer TP60 with maximal rating voltage up to 60 kV, Phoenix KiloVolt meter and test electrodes. Due to high value of breakdown voltage test samples with electrode system must be placed in insulation oil.

Figure 6. Testing setup for breakdown voltage testing

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