This article provides a background on dielectric response measurements in the DFR and the advantages of using this testing technique in liquid-filled power transformers
——–Benefits of dielectric response measurements——–
A healthy insulation condition of an electrical apparatus is essential for the operational reliability of the entire electrical power network. Transformers are without a doubt one of the most critical components in the system and, for this reason, a great amount of research has been performed to better comprehend the values as power factor or dissipation factor (tan delta). Power factor is widely used among operators and manufacturers to set a reference value for the losses encountered in the insulation material.
Typically, Dissipation Factor (DF) or Power Factor (PF) tests are carried out at power frequency of 50/60 Hz. A voltage is applied to one electrode of the capacitive system and the total resulting current is measured. From there, the angle between the total current vector and the applied voltage vector is obtained and the cosine function given in percentage values is the power factor value of the tested capacitance. In this test, the power factor value is a function of frequency. The capacitive reactance of the object is directly related to the excitation signal frequency and therefore a power factor test should be comparable if performed at the same frequencies. But frequency is not the only factor affecting the power factor value.
When repeating the power factor test on the same specimen, if moisture, oil condition and aging have not been altered, but the temperature of the system changes, the power factor will change as well, and the values at two different temperatures will not be comparable. In order to be comparable, the PF values must be normalised to a 20 C reference. Therefore, temperature has a significant effect on the resulting value of the power factor and this fact should be taken into account and improve the existing methods used to compensate power factor measurements for temperature variation.
This article provides a background on dielectric response measurements in the Frequency Domain (DFR), also known as Frequency Domain Spectroscopy (FDS), and the advantages of using this testing technique in liquid-filled power transformers to assess their insulation system condition, as well as to obtain the “unique” individual thermal response of the capacitive system analysed.
The routine dissipation factor or power factor test at power frequenciesThe PF and capacitance test is one of the most effective methods of assessing the overall condition of a transformer. An AC signal is applied to the insulation system at a voltage high enough to allow easy measurement under substation interference conditions, but not too high as to stress the system. Test voltages in the field test instrument range from below 100 V to as high as 12 kV. Field tests are usually performed at rated voltage or a maximum of 10 kV. The AC signal is typically applied at two different frequencies which are very close to the power frequency. The use of two different frequencies is called frequency variation suppression mode, and instead of running one single test at power frequency (50 or 60 Hz), the test is carried out at two frequency values close to the reference line frequency. The AC capacitance test is part of the PF test because the capacitance value and its associated charging current are required to calculate the PF value of that specific capacitive (insulating) system later.PF testing of transformers is carried out to assess the level of contamination of the insulation system and reference limits have been set in international standards [1] [2] to determine the possibility of dielectric degradation/contamination or mechanical damage of the insulation material. Following the limits set by the different international references [3] [4], PF is a trigger to announce potential accelerated aging or degradation of the insulation system.
It is necessary for this test to record the insulation system temperature to later normalise measured values to the 20 C reference. The dissipation factor of insulation can be more or less sensitive to the effect of temperature depending on the condition of the bulk insulation system. So far, the method used to normalise power factor values obtained at temperatures different from 20 C has been to apply correction factors. Correction factors may be available from equipment manufacturers and test equipment manufacturers and are only based on nameplate data. Generic correction factors were available in IEEE standard C57.12.90-2006, section 10.10.5, but were removed in C57.12.90-2010 [5] with the following note:“NOTE 3.b) Experience has shown that the variation in power factor with temperature is substantial and erratic so that no single correction curve will fit all cases.”
A PF test at power frequency by itself is capable of detecting moisture and contamination in a transformer; however, it cannot differentiate whether the source of power factor values beyond recommended limits or in an unexpected accelerated growth from historical values correspond to moisture in the solid insulation or contamination on the liquid insulation. Further analysis is performed to investigate the cause of values beyond the established limits and field users should perform other tests including physicochemical analysis of oil, DGA and of course DFR.
The recommended limits for new and service aged power transformer insulation power factor at 20 C
“The numbers shown here for natural esters are only provisional as there are no correction curves established by the industry yet.”
Temperature dependenceTo be able to determine the correct temperature correction factor, the temperature dependence of the insulation system must be investigated. The susceptibility of the insulation material can be expressed as a function of frequency and temperature [6]:
Where A(T) is a temperature dependent amplitude factor, F(x) a spectral function and ?c(T) a characteristic frequency. A(T) is constant for cellulose. It means that the shape of the spectrum remains unchanged at different temperatures. The dielectric response moves to higher frequency with temperature increase, or conversely, to higher temperature as frequency increases. One can obtain the same effect by increasing the frequency or increasing the temperature. However, the shape is usually not changed. In the special case of an ideal Debye function, the complex permittivity can be written as:

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 This article provides a background on dielectric response measurements in the DFR and the advantages of using this testing technique in liquid-filled power transformers
——–Benefits of dielectric response measurements——–
A healthy insulation condition of an electrical apparatus is essential for the operational reliability of the entire electrical power network. Transformers are without a doubt one of the most critical components in the system and, for this reason, a great amount of research has been performed to better comprehend the values as power factor or dissipation factor (tan delta). Power factor is widely used among operators and manufacturers to set a reference value for the losses encountered in the insulation material.
Typically, Dissipation Factor (DF) or Power Factor (PF) tests are carried out at power frequency of 50/60 Hz. A voltage is applied to one electrode of the capacitive system and the total resulting current is measured. From there, the angle between the total current vector and the applied voltage vector is obtained and the cosine function given in percentage values is the power factor value of the tested capacitance. In this test, the power factor value is a function of frequency. The capacitive reactance of the object is directly related to the excitation signal frequency and therefore a power factor test should be comparable if performed at the same frequencies. But frequency is not the only factor affecting the power factor value.
When repeating the power factor test on the same specimen, if moisture, oil condition and aging have not been altered, but the temperature of the system changes, the power factor will change as well, and the values at two different temperatures will not be comparable. In order to be comparable, the PF values must be normalised to a 20 C reference. Therefore, temperature has a significant effect on the resulting value of the power factor and this fact should be taken into account and improve the existing methods used to compensate power factor measurements for temperature variation.
This article provides a background on dielectric response measurements in the Frequency Domain (DFR), also known as Frequency Domain Spectroscopy (FDS), and the advantages of using this testing technique in liquid-filled power transformers to assess their insulation system condition, as well as to obtain the “unique” individual thermal response of the capacitive system analysed.
The routine dissipation factor or power factor test at power frequenciesThe PF and capacitance test is one of the most effective methods of assessing the overall condition of a transformer. An AC signal is applied to the insulation system at a voltage high enough to allow easy measurement under substation interference conditions, but not too high as to stress the system. Test voltages in the field test instrument range from below 100 V to as high as 12 kV. Field tests are usually performed at rated voltage or a maximum of 10 kV. The AC signal is typically applied at two different frequencies which are very close to the power frequency. The use of two different frequencies is called frequency variation suppression mode, and instead of running one single test at power frequency (50 or 60 Hz), the test is carried out at two frequency values close to the reference line frequency. The AC capacitance test is part of the PF test because the capacitance value and its associated charging current are required to calculate the PF value of that specific capacitive (insulating) system later.PF testing of transformers is carried out to assess the level of contamination of the insulation system and reference limits have been set in international standards [1] [2] to determine the possibility of dielectric degradation/contamination or mechanical damage of the insulation material. Following the limits set by the different international references [3] [4], PF is a trigger to announce potential accelerated aging or degradation of the insulation system.
It is necessary for this test to record the insulation system temperature to later normalise measured values to the 20 C reference. The dissipation factor of insulation can be more or less sensitive to the effect of temperature depending on the condition of the bulk insulation system. So far, the method used to normalise power factor values obtained at temperatures different from 20 C has been to apply correction factors. Correction factors may be available from equipment manufacturers and test equipment manufacturers and are only based on nameplate data. Generic correction factors were available in IEEE standard C57.12.90-2006, section 10.10.5, but were removed in C57.12.90-2010 [5] with the following note:“NOTE 3.b) Experience has shown that the variation in power factor with temperature is substantial and erratic so that no single correction curve will fit all cases.”
A PF test at power frequency by itself is capable of detecting moisture and contamination in a transformer; however, it cannot differentiate whether the source of power factor values beyond recommended limits or in an unexpected accelerated growth from historical values correspond to moisture in the solid insulation or contamination on the liquid insulation. Further analysis is performed to investigate the cause of values beyond the established limits and field users should perform other tests including physicochemical analysis of oil, DGA and of course DFR.
The recommended limits for new and service aged power transformer insulation power factor at 20 C
“The numbers shown here for natural esters are only provisional as there are no correction curves established by the industry yet.”
Temperature dependenceTo be able to determine the correct temperature correction factor, the temperature dependence of the insulation system must be investigated. The susceptibility of the insulation material can be expressed as a function of frequency and temperature [6]:
Where A(T) is a temperature dependent amplitude factor, F(x) a spectral function and ?c(T) a characteristic frequency. A(T) is constant for cellulose. It means that the shape of the spectrum remains unchanged at different temperatures. The dielectric response moves to higher frequency with temperature increase, or conversely, to higher temperature as frequency increases. One can obtain the same effect by increasing the frequency or increasing the temperature. However, the shape is usually not changed. In the special case of an ideal Debye function, the complex permittivity can be written as:

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