Abstract—The Polarization and Depolarization Current

(PDC) analysis is a nondestructive dielectric testing method for determining the conductivity and moisture content of insulation materials in a transformer. The predominant influence of oil conductivity on the initial amplitudes of polarization/depolarization currents can be used to estimate the oil conductivity of a transformer even without performing direct conductivity measurements on its oil sample. In the same way, an estimation of the conductivity of the pressboard can be obtained from the long term values of the polarization and depolarization currents.Moisture and ageing strongly influence the dielectric properties of oil-pressboard insulation system of power transformer. A prototype model is developed that allows us to analyse the dielectric response of the composite oil pressboard insulation system by varying moisture content in a controlled manner. Polarization and Depolarization current measurement technique is used to analyse the effect of moisture in the aforesaid model.

Index Terms—Power transformer, polarization current.

I. INTRODUCTION Demand for reliable electricity supply has significantly

increased during the last few decades. Therefore fault free operation of power system has become very important. However due to high cost of power system components, especially transformers, it is not economical to replace them in order to increase reliability, by considering their age. Moisture increases the risk of dielectric failures and has a double function: it accelerates ageing and also ageing generates it. Additional moisture can penetrate from the atmosphere into the tank. Independent of its origin moisture is absorbed into the oil-pressboard insulation system. Hence, measurement of moisture in different stages of transformers life becomes a challenge for transformers experts. A relatively large number of power transformers that are still working in fairly good condition although they have been used longer than their designed life .Therefore correct condition assessment of power transformer is needed before making any conclusion about replacement and refurbishment. Failures of power transformer mainly occur due to degradation of power transformer insulation, which mainly consist of oil and pressboard. Chemical and electrical measurements are used to monitor the condition of

power transformer insulation. Among these, chemical analysis provide direct information on parameters, such as water content, degree of polymerization of paper ,sludge

Manuscript received September 1, 2011; revised October 8, 2011. The authors are with the National Institute of Technology, Hamirpur,

H.P, India.

content in oil and quantity of different gasses dissolved in oil. However most chemical analysis must be performed under laboratory conditions. On the other hand, electrical measurements are simpler and it is possible to perform them on site. Due to this simplycity, Electrical tests, such as Insulation resistance (IR), Polarization index and loss factor (tan δ) provide very little information about the transformer insulation since they are limited to a single value measurement. To overcome this disadvantage, dielectric response measurements, namely return voltage measurements (RVM), polarisation and depolarisation current measurements (PDC) and frequency domain spectroscopy measurements (FDS), have been introduced for condition monitoring of transformer insulation, especially for the evaluation of water content in transformer pressboard. In the early stages, RVM was introduced because voltage measurements were simpler than measurements of low currents. The other two methods, requiring current measurements, were introduced recently due to improvements provided by the use of sophisticated electronic devices.

The focus of this paper is kept on PDC technique which is a non destructive testing method for determining the conductivity and moisture content in the insulation materials like oil and pressboard separately. This paper describes the effect of moisture on a prototype model which is similar to that of power transformer insulation.

II. DIELECTRIC RESPONSE The dielectric response analysis method, which is based

on wide range measurement in time and frequency domain, is a useful tool to evaluate the condition of the electrical insulation systems. Dielectric methods applied on power transformers measure a superposition of conductivity and polarisation phenomena. Moisture in oil-pressboard is obtained by a comparison of measurement data to measurements on oil-pressboard insulation with known moisture content.

If an electric field E(t) is applied to a homogeneous linear dielectric material the current density in the material [1]can be written as:5 -10

J()J J(t) = σ E(t)+ (1) The current density J(t) is the sum of the conduction

current and the displacement current, where σ is the DC conductivity and D(t) is the electric displacement given below.

D(t ) = E(t ) + ΔP(t) (2)

Investigation of Polarization and Depolarization Current for Evaluation of Moisture in Oil-Pressboard Insulation

Amit Kumar, J. Rattan, R.N Sharma, and Sushil Chauhan

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and are respectively, the permittivity of vacuum and of the dielectric material. ΔP(t) is the slow part of the dielectric polarisation. The polarisation is related to the dielectric response function of the material by the relationship:

ΔP(t)= (3)

The response function describes the fundamental memory property of any dielectric system. The dielectric susceptibility is the Fourier transform of the response function.

III. POLARIZATION AND DEPOLARIZATION CURRENTS The working principle of the PDC method is based on the

following effect: when a step-like DC charging voltage Uo is suddenly switched to the sample, previously uncharged, there is a motion of charges, due to the interaction of the electrical field with the free and the different kinds of bound charge within the dielectric. This so-called polarization current ipol(t) is a pulse-like current at time=0, then decreases during the polarization time Tc down to a certain value related to the insulation intrinsic conductivity. The polarization current can be stopped when it becomes stable or very low. If the sample is then suddenly short circuited, a discharging current idepol(t) (depolarization current) jumps to a negative value that gradually decreases down to zero. If Tc is large idepol(t) becomes directly proportional to the dielectric response function. The method investigated [2, 3] used a so-called “two active electrodes” technique to measure the polarization and depolarization currents.

From the response function and DC conductivity the polarisation (charging) and depolarisation (discharging) currents can be derived. If an insulation system with geometrical capacitance C0 is exposed to a step voltage, Uo , the polarisation current is given by: ipol = C0U0 + f(t) (4)

and the depolarisation current is given by:

idepol = - C0Uo f (t )- f (t + tpol ) (5) where tpol is the length of the polarisation period. If the polarization time is sufficiently long, then, f(t+tpol) =0, the response function is proportional to the depolarisation current.

Although the dielectric response function reflects the condition of insulation, it is rather difficult to identify the actual state of the insulation (e.g. the exact amount of moisture in the insulation) only by observing the relevant curves, especially when the insulation is a complex combination of different materials.[4] This problem can be solved, to some extent, by comparing the dielectric response of insulation under study with the dielectric response of well-defined similar types of insulation samples.[5] The dielectric response of insulating materials can be expressed as an analytical function of time or frequency, which offers

possibilities for fast and easy mathematical manipulations in such comparisons.[6] In addition, when the insulation is a complex combination of two or more materials, modelling the geometrical arrangement of insulation structure provides better comparisons between the responses of the insulation and the models.

IV. X-Y MODEL The typical winding and insulation arrangement of a

power transformer shown in the figure 1, the LV winding is usually surrounded by the HV winding and these windings are separated by the main insulation duct that consists of pressboard layers and oil channels. Therefore, response measurements between the HV and LV windings are affected by this composite insulation system. In core type transformers, the main insulation consists of cylindrical pressboard barriers in series with oil ducts and spacers as shown in Figure 2. The complex geometrical arrangement shown in this figure can be simplified by combining all oil ducts, barriers and spacers separately, which simplifies the modelling. Then the main insulation is simplified to the so called X-Y model, as shown in Figure.3

Fig. 1. Typical winding configuration of a power transformer

Fig. 2. Cross section of main insulation of a core type transformer

Fig. 3. Simplified insulation structure of a core type power transformer

X = T W

Y = T P

International Journal of Computer and Electrical Engineering, Vol. 3, No. 6, December 2011

854

In real power transformers, X and Y often vary between

0.2 - 0.5 and 0.15 - 0.25, respectively. The dielectric response across the X-Y system can be

calculated when the individual dielectric response of oil, spacers and barriers are known. The response of the oil is described using the constant values of dc conductivity σ and permittivity (= 2.2). Since both barriers and spacers are formed by Pressboard, they can be treated as a single material. The dielectric response of pressboard has significant dispersion in the time and frequency range of interest. Hence, the response of pressboard is described by its dielectric response function or susceptibility in time and frequency domains, respectively. Dielectric measurements of well-defined pressboard samples are used for forming a database, where information of relevant response functions with different moisture content is stored [7]. These responses can be well described by the general response function.

The temperature dependence of oil and paper is also considered. This is characterised by the activation energies of 0.9 eV and 0.7 eV for pressboard and oil, respectively.

V. PDC MEASUREMENT The principle of measurement of polarization and

depolarization current is based on application of a dc voltage across a test object for a long time (e.g., 5000sec.). During this time, the current, arising from the activation of the polarization process with different time constants corresponding to different insulation materials and due to the conductivity of the object is measured. Then the voltage is removed and the object is short-circuited. The previously activated polarization process now gives rise to the discharging current in the opposite direction, where no contribution of the conductivity is present. The schematic diagram of the PDC measuring set-up is shown in Figure 4. Figure 5 shows the typical nature of these currents due to a step charging voltageU0.Charging and discharging currents (i.e. polarization and depolarization currents) are influenced by the properties of the insulating materials as well as by the geometric structure of the insulating system [7]. For current measurement an electrometer (Keithley 6517B) was used which has the capability to generate 1 kV of voltage and can read up to 425 readings per seconds, it also offers full auto ranging over the full span of current, resistance, voltage and charge measurements[8].

Current measurements from 1fA to 20 mA. Voltage measurements from 10 µV to 200V. Resistance measurements from 50Ω to10 Ω

The internal voltage source was used to excite the test object. The equipment was used in FVMI mode. Measurements were made on oil and pressboard. The overall geometric capacitance of the system was 1.26 pF and the applied voltage was 500 V DC. The polarization and depolarization currents were measured for 5000 sec. each

Fig. 4. Basic PDC measuring circuit

Fig. 5. Waveforms of polarization and depolarization currents

VI.

TEST

RESULTS

A.

MODEL

A prototype model of transformer insulation has been developed .The model takes care of geometric design of transformer insulation. The model was kept in a temperature controlled environment. An electrometer (keithley 6517B) was used for measurement of currents. The overall geometric capacitance of the model was 50pf and the applied voltage for polarization and depolarization

currents

was 100 volts

DC. Polarization and

depolarization currents

were measured for 5000 s. B.

Result and Discussion The polarization and depolarization currents increased in

magnitude with increase in moisture level .Figure 6 shows polarization current where as depolarization current in figure in 7, taken at moisture levels (0%,1%,2%,3%,4%) by dry weight. The abrupt rise in polarization and depolarization current can be seen clearly.

As one can observe, polarization current at 0% starts at 65 nA and is considered to be ended at 64.65 nA (reading after 10000 s), the curve starts with a gradual slope in the beginning, step in the middle and tends to reach a constant value in the end, on the other hand in the curve representing depolarization current shows steep decrease in the values of depolarization current. The polarization curve at 1% moisture also starts at 65 nA and goes up to 64.6 nA, it is almost a linear fall in the values of current. Depolarization current for the same percentage of moisture starts nearly at 65 nA and then there is a decay in the values of current.

International Journal of Computer and Electrical Engineering, Vol. 3, No. 6, December 2011

855

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[3]

[4]

[5]

[6]

[7]

Institu

T. K Saha andepolarization cuinsulation of ageInsulation, Vol.1U. Gafvert, G.measurements 103,1998Van Deof power transfomeasurements”,CSymposium on E1, pp.28-34, 199T. Leibfried andTransformers U(PDC) Analysis,Symposium on E2002 G. Frimpong, Uof Dielectric respICPADM, KoreaC. Ekanayake,“moisture contenPower EngineeriKeithley instrumresistance meterOhio,USA

ute of Technolog

nd P. Purkit, “urrent measuremed transformers”,11, pp-144-153, 2Frimpong and Jon power tra

er-Houhanessianaormers by means oConference Reco

Electrical Insulati8. d A. J. Kachler,

Using the Polari,” Conference Re

Electrical Insulati

. Gafvert and J. ponse of Compo

a,pp. 86-89, 1997Application of d

nt in power traning Dept.,Chalme

ments operating r, Keithley Instr

Mr. Amit kum1969. He obtainengineering fro1993 and mastetechnical univepursuing his Ph Jivitesh rattanB.Tech in ESrinagar in 2Hamirpur in 2lecturer in ElecHamirpur,H.P.I

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gy, Hamirpur, HP

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“Investigation oment for the asses

, IEEE Trans.Die2004. J. Furh, “Modelansformers,” Cand W.S. Zaenglof polarisation/deord of the 1998 ion, IEEE, NewY

, “Insulation Diaisation and Depecord of the 2002ion, Boston, MA,

Fuhr, “Measuremosite Oil/Paper In.

dielectric spectrosnsformers,” M.Teers University of manual, 6517 Bruments Interma

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College GorakhpuPower System Ene year 1988. He oower Syste

f polarization assment of oil paelectric and Elec

ling of dielecCIGRE session,l, “On-site diagnoepolarisation currIEEE Internatio

York, NY, USA, V

agnostics on Powpolarisation Curr2 IEEE Internatio, USA, pp.170-1

ment and Modellnsulation,” Proc

scopy for estimatech Thesis, ElecTech,2003.

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