Dissipation Factor (or Power Factor) and Specific Resistivity Testing of Insulating Oils

Insulating oils used in electrical power equipment, such as transformers, circuit breakers, pipe-type cables, among other equipment, should be tested on a regular basis to determine and ensure the serviceability of the oil as an insulating medium. Some of the simplest electrical tests to conduct are determining the dielectric breakdown voltage, the dissipation factor/power factor, and specific resistivity of insulating oils. These tests are useful for acceptance testing as a means of quality control or preventative maintenance testing as a means of determining changes in quality as a result of deterioration in service. More detail on breakdown voltage testing is given in a previous blog post titled Which Oil Testing Standard Should You Choose to Determine Dielectric Breakdown Voltage? We will now focus on electrical tests that can be conducted to determine the dielectric dissipation factor (or power factor) and specific resistivity of insulating oils.

Dissipation Factor (Tan Delta) / Power Factor of Insulating Oils

When an alternating voltage is applied to an insulating medium the capacitive current will lead the voltage by 90°. When impurities, such as moisture, contaminants, or oxidation by-products, are present within the insulating medium creating dielectric losses a resistive current is introduced, which causes the current to deviate from the ideal 90° phase shift from the applied voltage. The image below shows the shift from 90° and this is measured as the dissipation factor (tangent of the loss angle δ) or as the power factor (sine of the loss angle δ).

As mentioned above the difference between dissipation factor (or tan delta) and power factor is the trigonometry function used to calculate the loss angle δ. Both values are commonly expressed as decimal values or as a percentage. It is important to note that at very low loss angles (decimal values up to 0.05) dissipation factor and power factor are equal to each other within one part in one thousand. Therefore, for very good insulating oils with low decimal values (below 0.005) dissipation factor and power factor can be considered interchangeable. The relationship between dissipation factor (D) and power factor (PF) is given as:

The conversion is extremely useful when measuring the losses within natural esters, which are becoming increasingly more prevalent as insulating oils in power equipment. Due to the chemical makeup of natural esters these compounds are more polar than the traditional mineral transformer oils, causing higher dissipation factor values.

Dissipation/Power factor is also highly dependent upon the temperature of the insulating oil and will increase with increasing temperature.

The two main standards used throughout the world to determine the dissipation/power factor of insulating oils are:

• ASTM D924 – Standard Test Method for Dissipation Factor (or Power Factor) and Relative Permittivity (Dielectric Constant) of Electrical Insulating Liquids
• IEC 60247 – Insulating liquids – Measurement of relative permittivity, dielectric dissipation factor (tan δ) and d.c. resistivity

Relative permittivity, or dielectric constant, is measured simultaneously during a dissipation/power factor test. The capacitance of the sample under test in a test cell is compared to the capacitance of an empty test cell to compute the relative permittivity of the insulating oil.  

The differences in the two standards are shown in the table below:

* The relatively high temperature is chosen to mimic the temperature of the insulating oil in a transformer at full load. Test temperatures should be carried out at the indicated temperatures for acceptance tests. However, both ASTM D924 and IEC 60247 allow for testing to be conducted at lower temperatures, such as 25°C or room temperature, for routine tests. In research applications, tests may be made over a range of temperatures.

Specific Resistivity of Insulating Oils

Specific resistivity is another important electrical property of insulating oils and is given in units of Ωm (ohm-meters) or Ωcm (ohm-centimeters). An insulating oil that has a very high resistivity indicates a low number of free ions, ion-forming particles, and low concentration of conductive contaminants. The resistivity of an oil will decrease with the increase of temperature. The inverse of specific resistivity is electrical conductivity expressed in S/m (Siemens per meter).

The two main standards used throughout the world to determine the specific resistivity of insulating oils are:

• ASTM D1169 – Specific Resistance (Resistivity) of Electrical Insulating Liquids
• IEC 60247 – Insulating liquids – Measurement of relative permittivity, dielectric dissipation factor (tan δ) and d.c. resistivity

The differences in the two standards are shown in the table below:

* The relatively high temperature is chosen to mimic the temperature of the insulating oil in a transformer at full load. Test temperatures should be carried out at the indicated temperatures for acceptance tests. However, both ASTM D1169 and IEC 60247 allow for testing to be conducted at lower temperatures, such as 25°C or room temperature, for routine tests. In research applications, tests may be made over a range of temperatures.

The procedure for ASTM D1169 is to apply a positive DC voltage for 1 minute and record the resistivity. Afterwards the electrodes of the test cell should be short-circuited for 5 minutes and then a negative DC voltage should be reapplied for 1 minute and the resistivity recorded. The results are then averaged and the measurement is repeated with a second sample.

The procedure for IEC 60247 is to apply a positive or negative DC voltage for 1 minute and record the resistivity. Afterwards the electrodes of the test cell should be short-circuited for 5 minutes and then the same polarity DC voltage should be reapplied for 1 minute and the resistivity recorded. The results are then averaged and the measurement is repeated with a second sample.

Important Precautions:

• The time it takes for the sample to heat and equilibrate with the temperature of the test cell should be kept at a minimum as the characteristics of the insulating oil may change with time and affect the resistivity results.
• The applied electrical stress may influence the resistivity results and for results to be comparable, measurements should be made with the same electrical stress (voltage level). 
• When wanting to conduct both dissipation/power factor and specific resistivity tests consecutively on a sample, the AC voltage shall be applied first and the cell electrodes should be short-circuited for a minimum of 1 minute before applying a DC voltage.

Advantages of the DTL C

The DTL C is an all-in-one device that offers fully automatic measurement of dissipation/power factor, specific resistance, and relative permittivity for insulating oils. The instrument offers maximum quality, reliability, precision, and safety for continuous use in the laboratory. The DTL C is designed for an above average life time of use in this industry and even today our service department is still calibrating instruments that are 20+ years old!

Features:

• Capable of precise tan delta measurement up to 1×10^-6
• Measurement of the specific resistance with both polarities up to 100 TΩm
• Measurement of the relative permittivity
• Suitable for all types of insulating oils – mineral, silicone, and natural/synthetic esters
• Fast and precise induction heating up to 110°C in less than 12 minutes.
• Direct temperature measurement with sensor in the electrodes
• The test cell stays inside the DTL during the complete measurement, thus allowing for an accurate and constant temperature control
• Pre-programmed fully automatic test standards, including ASTM D924, ASTM D1169, IEC 60247
• Ability to program user-defined test routines
• High quality test cell in accordance to IEC 60247
• Automatic calibration of the empty test cell for quick test sequences
• Automatic draining of the test cell with no requirement to remove and disassemble