DIELECTRIC DIAGNOSTICS MEASUREMENTS OF TRANSFORMERS AND THEIR INTERPRETATION Claudia Martorella Spring 2016

INTRODUCTION The Dielectric Diagnostics Method (DDM) and Measurements is a family of methods used to assess the ageing condition of transformers, by mean of a characterization of the dielectric material(s) of the insulation When a dielectric is subject to an electric field, a shift of positive and negative charges take place within it giving rise to polarization [7]. This phenomena adsorb some part of the field energy and then it corresponds to a energy loss, and it is stronger as σ is higher TIME-BASED MEASUREMENTS 1) Return Voltage Measurement (RVM) 2) Polarization and Depolarization Current Method (PDC) FREQUENCY-BASED MEASUREMENTS Frequency Domain Spectroscopy (FDS) The transformer insulation structure consists mostly of mineral oil and paper. The ageing of insulation material is due mainly to oxidative reactions between the oil and the cellulose (both organic substances) and the oxygen, which take to the formation of water. The water contamination causes a higher electric conductivity σ of the oil and the hydrolytic degradation of the cellulose molecules [9]. The moisture content of transformers pressboards at the end of the operation time can reach the 5% [10] Main components of the transformer insulation: -the insulation between the tank and the HV windings; -the insulation between LV and HV windings; -the inter-phase insulation [7]

RETURN (RECOVERY) VOLTAGE MEASUREMENT (RVM) A step DC voltage is applied across the sample. The test object is first charged for a certain time and then discharged for half of the time. This is done creating a short-circuit either during the charging and the discharging [2]: S1 is closed and S2 is open during the charging and vice versa. Afterwards the return voltage is measured under open circuit conditions, that is with both switches open to allow the voltmeter to take the measure. The recovery voltage is due to the remaining polarization of the dielectric material after the charging/discharging cycle. The test is carried forward applying the DC voltage for increasing charging (and discharging) times and plotting the peak values V r (t) of the recovery voltage measured (the so called “polarization spectrum”). Additional informations are shown in the spectrum: -the initial slope of the recovery voltage dV r /dt: it is proportional to the conductivity σ of the insulation material [1]; -the polarization and depolarization currents; -the central time constant (CTC) Figure 1: basic circuit for RVM measurements[1]

The polarization and depolarization currents are the currents flowing through the sample during the charging and discharging times, due to the application of the electric field. -U 0 = step DC voltage applied -σ = average conductivity of the insulation system -C o = geometrical capacitance of the system -f(t) = response function of the dielectric material -t 1 = time when the voltage is disconnected -ε 0 = electric permittivity of vacuum = 8,854* F/m at the beginning the current is only due to the conductivity of the insulation material, then it grows according to the response function It is due to the charges flowing to the initial (neutral) configuration when the step voltage is no more applied

Figure 2: example of voltage and current spectrum [1]. Charging times: 0,5÷1024 s. Figure 3: peak value of the return voltage as a function of the CTC [1] TransformerT4T5T6 operation time [years] The discharging current i d is negative, as it flows in the opposite direction of i c NOTE: THE CENTRAL TIME CONSTANT (CTC) IS THE TIME REQUIRED TO REACH THE PEAK VALUE OF THE RECOVERY VOLTAGE [3]

Transformers with higher operation times show lower values of the central time constant. This can be explained reminding that CTC is the time needed to reach the peak value of the recovery voltage. This recovery voltage is due to the migration of the charges within the dielectric material after the polarization. Such factors as the presence of humidity (one of the symptoms of ageing) can accelerate this phenomenon, reducing the value of CTC REMARKS FOR THE INTERPRETATION OF THE RESULTS Figure 4: example of results for transformer insulations with different moisture content [1]

POLARIZATION AND DEPOLARIZATION CURRENT METHOD (PDC) A step DC voltage is applied through the sample for a long period of time (e.g s) keeping S1 closed and S2 open, and the polarization current (charging current) is measured by the ammeter. The measurement is stopped when the current becomes stable or very low. Then the test object is short- circuited for a long time (opening S1 and closing S2) and the depolarization current is measured by the ammeter as well. Figure 5: basic circuit for PDC measurements [5]

The polarization processes have different time constants corresponding to different insulation materials and different ageing conditions. With the values of the currents obtained, the DC conductivity of the sample can be estimated. Higher σ => higher moisture content in the solid part of the insulation Figure 6: example of measured polarization currents for samples with different moisture content [6] REMARKS FOR THE INTERPRETATION OF THE RESULTS

FREQUENCY DOMAIN SPECTROSCOPY (FDS) A sinusoidal voltage of varying frequency is applied to the test sample. The voltages and currents across the sample are measured with an ammeter and a voltmeter. Unlike time-domain measurements, it is not necessary to discharge the sample before changing the frequency. Usually a frequency range of 1 mHz ÷ 1 kHz is used. The frequency is reduced by discrete steps from the highest to the lowest. The voltage may vary from 5 to 200 V (rms). Figure 7: basic circuit for FDS measurements [7]

Figure 8: Phasor diagram of measured current and applied voltage [9]

Figure 10: the effect of oil conductivity on tanδ for four different transformers [7] The oil of transformers T1 and T2 has a higher conductivity than the one of T3 and T4. Figure 9: Real and imaginary part of dielectric permittivity of pressboards having different moisture content [7] Examples of curves (tanδ, f), (ε, f) according to different conditions of the insulation Different moisture content of the solid part of the insulation Different conductivity of the insulation oil

The change in the dielectric properties of the system as the frequency change is due to the fact that different frequencies are associated with different polarization processes [7]. The value of tanδ changes according to different conductivity and moisture content of the insulation. The higher the tanδ, the higher the moisture content and the higher the conductivity, that is, the more advanced the ageing process is. Note that the reason why the diagrams shown are sketched separately for the solid and liquid part of the insulation is that the dielectric response in the low frequencies (f 10 Hz) is mainly influenced by the properties of the solid insulation, while in the middle it is mainly influenced by the properties of the insulating oil (check fig. 11). Therefore all the range must be analyzed to get useful informations about the conditions of both the parts. REMARKS FOR THE INTERPRETATION OF THE RESULTS Figure 11: loss factor for different moisture contents of the cellulose [6]

REFERENCES [1] “Voltage response measurements for power transformer moisture and ageing condition assessment” - Zheng Tong Yao, Tapan Kumar Saha [2] “Return voltage measurements diagnostic interpretation based on the dielectric time constant” - R. Patsch, D. kamenka, J. Menzel [3] “A New Approach for determination of moisture in paper insulation of In-situ Power Transformers by Combining Polarization-Depolarization Current and Return Voltage Measurement Results” - S. Sarkar, T. Sharma, A. Baral, B. Chatterjee, D. Dey and S. Chakravorti [4] “Investigation of Polarization and Depolarization Current for Evaluation of Moisture in Oil-Pressboard Insulation” - Amit Kumar, J. Rattan, R.N Sharma, Sushil Chauhan [5] “Polarization And Depolarization Current (PDC) Tests On Biodegradable And Mineral Transformer Oils At Different Moisture Levels” - N.A. Muhamad, B.T. Phung, T.R. Blackburn, K.X. Lai [6] “Dielectric diagnostics measurements of transformers” – Xiaolei Wang [7] “Recent trends in the condition monitoring of transformers” - Sivaji Chakravorti, Debangshu Dey, Biswendu Chatterjee [8] “Determination of water content in transformer solid insulation by frequency domain spectroscopy” - Belén García, Baudilio Valecillos, Juan Carlos Burgos [9] “Prediction of insulation degradation of distribution power cables based on chemical analysis and electrical measurements” - Petri Hyvönen [10] “Quantitative Analysis of Insulation Condition of Oil-paper Insulation Based on Frequency Domain Spectroscopy” - Ruijin Liao, Jiefeng Liu, Lijun Yang, Ke Wang, Jian Hao, Zhiqin Ma, Jun Gao, Yandong Lv