an attempt to identify temperature dependent non- linear ... · linear insulation characteristics...

An Attempt to Identify Temperature Dependent Non-

Linear Insulation Characteristics from Dielectric Response Measurements

S. K. Ojha, P. Purkait Department of Electrical Engineering

Haldia Institute of Technology Haldia, India

Email: [email protected], [email protected]

B. Chatterjee, D. Dey, S. Chakravorti Department of Electrical Engineering

Jadavpur University Kolkata, India

Email: [email protected], [email protected], [email protected]

Abstract— Dielectric testing techniques, such as polarization and depolarization current (PDC) and recovery voltage (RV) measurements are gaining importance these days for assessment of transformers’ insulation condition. Many researchers have analyzed the results of these tests quantitatively with the help of a linear dielectric model. It is now an established observation that polarization and depolarization currents are extremely sensitive to temperature. It has been always a big issue to assess insulation condition based on PDC and RV measurement results, especially under varying temperature conditions. The current contribution reports experimental investigations performed on oil-paper insulation system under various conditions of operating temperature. Focus has been concentrated on identifying non-linear dielectric characteristics from PDC and RV measurement results.

Keywords- Transformer, oil-paper, insulation, PDC, RVM, temperature, non-linear, dissipation factor

I. INTRODUCTION Reliable and efficient operation of power transformer is of

essential importance for power plant and transmission network companies. A large number of power transformers are approaching to the end of their design life [1]. Transformer life is significantly influenced by degradation of the insulating material, which is caused largely by thermal stress on insulating oil and paper [2]. Insulation materials in transformers degrade at higher operating temperature in presence of oxygen and moisture. When temperature increases water migrates from paper to oil and vice versa. Hence a small change in temperature modifies the relative water content of oil and paper [1-2]. The moisture in the solid and liquid insulation of transformer in variation forms can deteriorate the electrical and mechanical strength. Experimental results and observation reveal the fact that the mechanical life of insulation is reduced by half of each doubling in water content [3].

All these above facts tell that ageing process is caused by dielectric, chemical, thermal and electrodynamic stresses. So, condition based maintenance and online monitoring are gaining importance now. In recent times, dielectric diagnostic techniques such as polarization and depolarization current (PDC) and recovery voltage (RV) measurements have become popular due to their non-destructive nature.

It has been reported that PDC and RVM results are highly influenced by operating temperature [2]. Polarization process at the oil-paper interfaces can be understood by making measurement at different operating temperature. The responses at higher temperature differ from the measurements at room temperature [2-3].

Due to variations in load as well as ambient condition there is a complex temperature dependent moisture dynamics in oil paper insulation system. It has been observed by several researchers [4,5] that dielectric responses do not follow a linear relationship with temperature due to complex structure of the insulation, its local chemical composition and local physical properties of the solid insulation such as specific density, fibre orientation or surface roughness, and ageing contamination in both oil and paper [6]. Such non-linear temperature dependent dielectric response is also aggregated by unstable moisture distribution between oil, paper and pressboard insulation.

This article reports experimental investigation of traditional insulation testing methods such as capacitance, insulation resistance, and dissipation factor measurements as well as dielectric response measurements (PDC and RVM) performed on oil-paper insulation system with special emphasis on non-linear temperature dependence.

This work has been sponsored by DST FAST TRACK project SR/FTP/ETA-39/2008

16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, 2010 530

Department of Electrical Engineering, Univ. College of Engg., Osmania University, Hyderabad, A.P, INDIA.

II. BRIEF THEORIES OF PDC AND RVM

A. Polarization and depolarization current(PDC) The insulating materials generally used in between live

parts as well as between live and earthed parts of a transformer are oil, paper, pressboard, presspahn etc. If the two terminals of the high voltage windings and the two terminals of low voltage winding are individually shorted, then it can be assumed as a capacitor. The shorted high voltage side and low voltage side act as two plates of the capacitor and the insulation system act as the medium in between the two plates. PDC measurement is done by application of a high DC voltage in between the capacitance thus formed for a long duration. Thus a charging current of the transformer capacitance, i.e. insulation system, the so called polarization current flows. It is a pulse like current during the polarization duration, the final value of which is given by the conductivity of the insulation system [7]. Then the voltage is removed and the test object is short circuited. It now gives rise to depolarization current in the opposite direction, where no contribution of the conductivity is present. In addition to moisture content [8], conductivity and ageing condition [5], contaminations present in the insulation [8], polarization and depolarization current also depend on operating temperature [2- 4].

B. Recovery Voltage Measurement(RVM) A careful attention is needed before performing the RVM;

the test object must be fully discharged first. After the system is completely discharged, a DC voltage is subjected to the test model. The model is then charged for certain duration and then again discharged to ground. The discharging time is set less than the charging time (typically half). The switching operations have to be done automatically so that no time is lost between each change. After this operation the input source is isolated from the test object and a voltmeter connected parallel with the model measures the voltage appearing across the insulation. The charges bounded by the polarization followed by incomplete depolarization turns into free charges and a voltage will build up in between the dielectric; this phenomenon is called the return voltage. The voltmeter parallel with the model measures this return voltage.

A series of such measurements cycles with gradually increasing charging and discharging times are carried out and the return voltage profile is recorded for each cycle. The peak value of RVM for each cycle and its corresponding time of occurrence are noted down. The plot of such RVM peaks for each cycle when plotted with respect to the corresponding times of peaks (sometimes with respect to the corresponding charging times) is called the RVM spectra or polarization spectra. The peak values of RVM spectra and its corresponding time (called the central time constant CTC or the dominant time constant-DTC) have been found to be influenced by condition of the insulation assembly [9] as well as the operating temperature [3, 4].

III. EXPERIMENTAL SETUP

A. Coil and Insulation Structure Double paper covered round copper conductors is used to

construct the low voltage (LV) and High Voltage (HV) coils. Inner LV coils is wound over a presspahn cylinder. The oil ducts between LV and HV coils are maintained with 2mm

pressboard spacers. Coil and insulation structure has been constructed to match capacitance of a real transformer. Schematic diagram and photograph of coil and insulation structure is shown in Fig. 1(a) and Fig. 1(b) respectively.

B. Tank and Accessories A cylindrical metal tank encloses the whole coil &

insulation assembly with connection bushings and breather vent on top. An inner cylindrical tank fitted inside the presspahn cylinder serves the dual purpose of imitating the core and provides space for placing heater. Photograph of the tank and its schematic diagram is shown in Fig. 2(a) and Fig. 2(b) respectively.

Fig. 1(a). Schematic diagram of coil and insulation structure

Fig. 1(b). Photograph of coil and insulation structure



Fig. 2. (a) Photograph of tank

Fig. 2(b). Schematic diagram of coil inside tank

C. Temperature Controlled Furnace A furnace fitted with PID (Proportional + Integral +

Derivative) temperature controller has been used to vary the operating temperature between ambient and 1500C. The entire transformer tank was housed inside the furnace and temperature was set to different values during the dielectric measurements.

D. PDC and RV measurement system A computer (LABVIEW) controlled instrument involving

major components such as Keithley 6517B electrometer, siemens high voltage relay, timer and switching control blocks was developed following basic principles described in details in [9, 10]. In addition to recording PDC & RV, the instrument has also been programmed to record live temperature. The complete experimental set up is shown in Fig. 3.

E. Traditional methods of insulation assessment Traditional insulation testing methods such as capacitance,

insulation resistance, and dissipation factor measurements

were also performed using MEGGER MIT520/2 to test the condition of insulation between HV and LV windings at different temperatures.

IV. RESULTS AND ANALYSES

A. PDC Measurement Results Fig. 4 and Fig. 5 show polarization and depolarization

currents obtained at different steady operating temperatures. In each case, the transformer model was charged (polarized) with 300 volts for 3600 seconds and then discharged (depolarized) for 3600 seconds. It can be seen that variations in temperature cause significant displacements of both the polarization and depolarization currents. It can be also seen from Fig. 4 and Fig. 5 that magnitude of the polarization and depolarization currents tend to shift to higher values with increasing temperature. It is further noted that deviation between polarization current at different temperatures are not really linear. Similar observation can be made from the plots of depolarization currents at different temperatures in Fig. 5.

Fig. 3. PDC and RV measuring equipment with complete test setup

Fig. 4. Variation of polarization current with temperature



Fig. 5. Variation of depolarization current with temperature

B. Paper and Oil conductivity

Paper/pressboard and oil conductivities have been computed following X-Y insulation structure described in [2]. Fig. 6 shows the structure of insulation constructed in current study with X = 0.19 Y = 0.34. In a real transformer, the values of X and Y are in the ranges of 15% to 45% and 20% to 40% respectively [2].

Fig. 6. X – Y arrangement of structure of oil and paper

The paper conductivity according to [2] is given by

XYXY

rpaper )1( −+

=σσ (1)

Where paperσ is paper conductivity, X is defined as the ratio of the sum of thickness of the entire barrier in the duct, lumped together, and divided by the duct width. The space coverage Y is defined as the total width of all the spacers divided by the total length of the periphery of the duct [2]. The average conductivity of the composite insulation system is given by:

( ) ( )[ ]∞−∞= iiUC

dpolpolo

r0

0εσ (2)

Where, ε0 is the permittivity of free space, U0 is the applied DC voltage (300 V), and C0 is the geometric capacitance = C/εr. C is the measured capacitance at or near power frequency and εr is the effective permittivity of the composite insulation system at power frequency; ipol (∞) and idepol (∞) are the final values of polarization and depolarization currents. Effective permittivity of the composite insulation system εr is give by [2,11]:

εεεε

ε

boilbsp

r XXY

XXY

+−

−+

+−

=1

11

(3)

Where , , are the spacer permittivity, barrier permittivity , oil permittivity respectively.

ε sp ε b ε oil

Oil conductivity, following [2] is given by

( 00 += iUC

poloor

oiloil

εεεσ ) (4)

Where is 8.854x10ε 0-12 (F/m), ( )0+i pol is the

polarization current value at starting.

Values of & thus calculated are represented in Table 1. Both oil and paper conductivities are found to increase with rising temperature.

σ paper σ oil

Earlier experiments have unfolded the fact that this oil and paper conductivity follows an exponential law [2, 12, 13]:

⎟⎟⎠

⎞⎜⎜⎝

⎛−

≈ kTE

eATac

.)(σ (5) Where T is absolute temperature in Kelvin, A is a constant

related to mobility of ions in the insulation, k is the Boltzman constant and Eac is the activation energy.

Thus (5) can be re-written as:

⎟⎟⎠

⎞⎜⎜⎝

⎛−

≈ kTE

eATac

.ln))(ln(σ (6)

or, kTEAT ac−≈ ln))(ln(σ (7)

Equation (7) represents a linear relation between logarithmic of conductivity and inverse of the absolute temperature (1/T). In Fig. 7 and Fig. 8, natural logarithms of oil and paper conductivities have been plotted against inverse of absolute temperature.

TABLE I.

PAPER AND OIL CONDUCTIVITIES AT DIFFERENT TEMPERATURE

Temperature (0C) σ paper ( pS/m) σ oil (pS/m)

40 0.67 4.02 50 1.81 7.18 60 4.19 16.17 70 14.47 47.44 80 44.77 139.84

Unlike the prediction by (7), nature of the relationships in

Fig. 7 and Fig.8 are not perfectly linear. Similar nature of variation of average oil-paper composite conductivity with temperature was reported by [12, 13].

Fig. 7. Oil conductivity versus reciprocal of absolute temperature



Fig. 8. Paper conductivity versus reciprocal of absolute temperature

C. RVM Results Recovery voltage measurements were performed with the

ratio of charging to discharging time being set at 2 for obtaining the RV spectra at different temperatures. Recovery voltages after each of these charging-discharging cycles were measured. Peak of these recovery voltages in each cycle and its corresponding time were recorded. The peak of each RV cycle when plotted against the corresponding peak time produces the RV spectra as shown in Fig. 9.

Fig .9. RV spectra plotted against peak time at different temperatures

It can be seen that the effect of temperature causes

significant displacements of the RV spectra peaks. The time of occurrence of an individual RV peak is defined as the dominant time constant. Values of these dominant time constants for RV measurement at different temperature and the corresponding temperature are shown in the Table II and graphically plotted in Fig. 10.

Fig. 10. Dominant time constants plotted against steady operating

temperature

At normal operating conditions, the water particles in moisture gets trapped into the small pores in the paper; at higher temperatures the moisture migrates from paper into the oil. At higher temperatures, the spectrum peaks are found to shift to smaller times. This peak-shifting has been attributed to increased water availability in oil at high temperature [14-15]. Once again non-linearity in temperature dependence of dominant time constants is apparent from Fig. 10.

TABLE II. DOMINANT TIME CONSTANT VERSUS TEMPERATURE

Temperature (0C) Dominant Time Constant (s) 40 4.971 50 4.452 60 2.476 70 1.54 80 0.819

D. PDC Measurement under temperature transients & inadequate equilibrium Non-linearity in dielectric response results are further

aggravated under transient temperature and inadequate oil-paper equilibrium conditions.

PDC measurements was performed under transient temperature condition with starting temperature being set at 600C and was brought down to 500C at a uniform rate during one entire PDC cycle of 7200 seconds. Fig. 11 shows polarization and depolarization current under such transient temperature condition along with reference polarization and depolarization curves at 600C and 500C. It can be concluded from Fig. 11; that non-linear dependence of PDC characteristics on temperature can be further enhanced under transient temperature condition, as may likely be encountered during tests on real transformers in open substations.

Fig. 11. PDC under temperature transient, A1-A2: pol and depol current at 600C, B1-B2: pol and depol current at 500C, C1-C2: pol and depol current

during temperature transient of 600C- 500C, D – Temperature (0C)

Next set of experiments were performed on the same test object, being oil- impregnated for varying periods of time. PDC tests were performed at 800C on three different days. Initially the oil filled test sample was kept in a closed furnace at 800C, the first test was carried out after one week, second test was done after one month and the third test was again carried out after two months of oil impregnation. Corresponding plots of polarization and depolarization currents in Fig. 12 show that the duration available for oil paper to reach equilibrium state greatly affects the polarization and depolarization currents. It is also notable that such variations, are once again, not linear.



Fig. 12. PDC under 800C, A1-A2: pol and depol current at 800C after one week, B1-B2: pol and depol current at 800C after one month, C1-C2: pol

and depol current at 800C after two months

E. Results of traditional insulation tests Table III tabulates results of traditional insulation testing

methods, namely capacitance (C), insulation resistance (IR), and dissipation factor measurements to assess the condition of insulation between HV and LV windings at different temperatures.

TABLE III. RESULTS OF TRADITIONAL INSULATION TESTS

Temperature (0C)

C (nF) IR (MΩ)

Dissipation factor

40 1.56 4830 0.000211 50 1.61 2240 0.000442 60 1.8 778 0.001137 70 1.93 225 0.003667 80 2.3 53 0.013063

It can be seen from Table III that with increasing temperature, insulation condition is found to deteriorate. This finding is in agreement with the results obtained by PDC and RV measurements. Changes in the values of capacitance (C), insulation resistance (IR), and dissipation factor with respect to temperature, however, are once-again non-linear.

V. CONCLUSION The present paper reports PDC and RV measurement results on oil-paper insulation structure under varying temperature conditions. A laboratory test setup has been developed to imitate oil-paper insulation structure in a real transformer, and tests have been carried out under controlled environment conditions. It has been demonstrated both PDC and RV measurement results show signs of non-linearity in the response of the dielectric under varying temperature conditions. Attempts for correlating dielectric response measurements results with any linear insulation model thus needs careful refinements to incorporate such non-linear characteristics.

ACKNOWLEDGMENT The authors would like to take this opportunity to thank

Arnab Dasgupta, Manish Kumar, Urnav Bagchi, and Niraj Kumar, final year students of the Department of Electrical Engineering, Haldia Institute of Technology for their contribution towards supporting the experimental works.

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