CEEX 2008 CONFERENCE

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LIFETIME ESTIMATION OF ELECTRICAL MACHINES’

INSULATION SYSTEMS BY ABSORPTION CURRENTS

P.V. NOTINGHER1, L.M. DUMITRAN1, S. BUSOI2, E. BALESCU2, G.

TANASESCU3

The ageing state evaluation and lifetime duration estimation for insulation

systems of medium and high power machines represent important present-day

research objects for manufacturers and users. The aim is to obtain a method based

on nondestructive, in-situ insulation tests. In this paper, a new method for the

analysis of insulation systems degradation based on the measurement of

absorption/resorption currents is presented. Based on the accelerated ageing

measurements and on time variation curves of measured absorption currents and

considering their maximal values and mass loss as diagnostic factors, insulation

lifetime is estimated.

Keywords: Absorption/resorption currents, insulation systems, thermal ageing,

lifetime.

1. Introduction

During electrical machines functioning, the insulation systems are exposed

to normal permanent (thermal, electrical, mechanical, environmental, etc.) and

accidental (overvoltage, overcurrent) stresses. These stresses initiate and maintain

insulation degradation processes (oxidation, molecule fracture, micro-cracks,

insulation loosening, etc.), which lead to the deterioration of their mechanical,

and, more often, of their electrical insulation characteristics (with the increase of

charge carriers and polar radicals concentration, etc.), leading to the insulation

systems breakdown and electrical machines premature take out [1].

Therefore, the knowledge regarding the states of the insulation systems

and their lifetime estimation has become a present-day problem. The existing

methods are destructive (based on the mass loss determination, on the

chemiluminescence’s intensity, etc.) or global (based on the measurement of the

insulation resistance, loss factor, partial discharge level, etc.) [2-8]. Recently,

there is an attempt to analyze the ageing state of electrical equipment insulation

systems based on the values of absorption/resorption currents [6].

These currents are obtained when a step voltage U is applied to condensers

with dielectrics whose ageing state is studied and have the following form:

1 Prof., Electrical Engineering Faculty, POLITEHNICA University of Bucharest, Bucharest,

Romania 2

GENA ELECTRIC S.R.L., Bucharest, Romania 3 SIMTECH INTERNATIONAL S.R.L., Bucharest, Romania

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)()()()()( tititititi csspia , (1)

where ia(t) is the absorption current, ii(t) – the charging current of the condenser

with vacuum dielectric, ip(t) – the polarization current, iss(t) – the space charge

current and ic(t) – the conduction current [2].

The polarization current corresponds to the exterior orbit deformation of

ions and/or atoms inside the dielectric and to the spinning of the trapped charge of

electrical (permanent or temporary) dipoles, respectively to a small movement of

a large number of bound charges [2]. Its values increase for a movement process

(chemical, mechanical, thermal, radiated), which leads to the macromolecules

fracture and polar species generation.

The iss(t) component corresponds to the movement of the existing space

charge (electrons, ions) fixed on the defects of the insulators volume, particularly

on the interfaces between their homogenous areas. During the ageing of

insulations still found in use, there is a molecule fracture process which leads to

the increase of the concentration of charge carriers fixed on their defects.

Therefore, iss(t) values increase with electrical machines’ service time.

The permanent component of the absorption current, respectively the

conduction current ic(t) = Ic is given by the convection of electrons, ions and

molecular ions and allows the determination of the electrical resistance and

resistivity of the dielectric. Ic values increase during machine functioning, because

of the insulation degradation, respectively as a result of the increase of their

electrical conductivity.

In the previous papers [5] and [7], different methods of evaluation of the

insulations’ degradation state based on the conductivity factors and polarization

index were presented. In the presented paper, there is an analysis of the influence

of the ageing state of high-power machine insulation on the maximal values of the

absorption currents Imax. Based on these values, a new non-destructive method for

lifetime estimation of electrical machines insulation is presented.

2. Experiments

The experiments were made on plane plates made up of tape

CALMICAGLAS (ISOVOLTA), which contains glass texture (13 %), mica paper

(47 %) and epoxy resins (40 %). Using steel plates of 650x110x2.5 mm3, seven

layers of tape with 0.18 mm in thickness were rolled up (with ½ superposition)

and pressed at p = 3 bar and T = 160 oC for t = 2.5 hours. From these plates were

made samples of 100x100x(1.6...1.8) mm3, which have been processed to obtain a

requested thickness and a fine surface.

All samples were thermally conditioned at 190 oC for 48 h. The samples’

accelerated thermal ageing was made in a Trade Raypa forced air flow oven with

Lifetime Estimation of Electrical Machines’ Insulation Systems by Absorption Currents

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adjustable temperature (between 30 and 250 oC). The temperature values for the

accelerated ageing were chosen accordingly to the IEC 60216-3/2002 standard.

The experimental set-up used to measure the absorption currents is

presented in Figure 1. The mass loss during the ageing process was measured with

a SHIMADZU AW220 electronic balance.

Fig. 1. Experimental set-up for absorption currents measurement:

1 – Electrometer Keithley 6517, 2 – Resistivity Test Fixture Keithley 8009, 3 – PC.

3. Results. Discussions.

The tests were made on groups of 5 samples for each ageing temperature.

The ageing times τ were 1500 h for T = 210 °C, 1092 h for T = 230 °C and

470 h for T = 250 °C. At determined time intervals, the samples were cooled

(inside the oven) at room temperature and their mass loss and absorption currents

were measured and their medium values determined. Part of the obtained results is

presented in Figures 2–7.

In Figure 2, the variations of medium absorption currents determined on 5

samples, unaged (1), aged at 210 °C (176 h), 230 °C (118 h) and 250 °C (107 h)

are presented. It can be noticed that, in the first part of the experiments, absorption

current values decreased compared to the ones for the unaged samples. This is due

to the elimination of polar products (solvents) left inside the samples after the

fabrication process and, therefore, of the reduction of the polarization component

ip from the absorption current ia. Certainly, with the increase of the ageing

temperature, polar product elimination (diffusion) is faster, thus ia values are

smaller (Figure 2, curve 4).

Time variations of the absorption currents in samples thermally stressed

for longer periods of time are presented in Figure 3. It is noticed that, for all

ageing temperatures, current values increased with , which was due to the

degradation of epoxy resin molecules and the generation of polar products (which

led to the increase of the polarization current ip) and charge carriers (electrons and

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ions), part of them fixed on the interfaces between the mica paper and glass

texture layers, leading to the increase of the space charge current iss.

Fig. 2. Time variation of the absorption current

for plane plates at different ageing

temperatures: 1 – unaged, 2 – T = 210 °C at

τ = 176 h, 3 – T = 230 °C at τ = 118 h, 4 – T =

250 °C at τ = 107 h.

Fig. 3. Time variation of the absorption current

for plane plates at different ageing

temperatures: 1 – unaged, 2 – T = 210 °C at τ =

679 h, 3 – T = 230 °C at τ = 626 h, 4 – T =

250 °C at τ = 432 h.

From all measured absorption currents, the first measured values were

gathered after each interspace (1.4 seconds after applied voltage start), marked

Imax(1.4) and, for each ageing temperature, the currents relative maximal values

were determined, corresponding to the ageing time (Imax,r()), defined as:

)4.1(

)()4.1()(

max

maxmax

max,I

III r

. (2)

Reported currents variations with (Imax,r()) are presented in Figure 4.

Choosing the end-of-life criterion the value Imax,r = 1.8 (meaning an 80 %

increase in the maximal current), lifetime duration curve ln τ = f(1/T) has been

determined (Figure 5). Based on the curve presented in Figure 5, lifetime values

DI are obtained for different temperatures. Thus, for T1 = 155 °C, it is obtained

DI1 = 9.64 years, while for T2 = 120 °C, DI2 = 248 years.

In Figure 6, the epoxy resin relative mass loss variation curves Δmr()

defined by:

)0(

)()0()(

m

mmmr

, (3)

where m(0) is the initial mass of the samples’ epoxy resin and m() – the mass of

the epoxy resin at ageing time , are presented.

Lifetime Estimation of Electrical Machines’ Insulation Systems by Absorption Currents

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Fig. 4. Variations of the relative maximal values

of the absorption current Imax,r with ageing time τ.

Fig. 5. Lifetime curve corresponding to the

Imax diagnostic factor, for the end-of-life

criterion Imax,r = 1.8.

Fig. 6. Mass loss variations with ageing time

for three ageing temperatures.

Fig. 7. Lifetime duration curve corresponding to

mass loss for end-of-life criterion Δmr = 8%.

Taking as end-of-life criterion the value Δmr = 8 %, lifetime duration

curve was drawn (Figure 7).

Using the lifetime duration curve (Fig. 7), insulation lifetime estimation

was determined for two functioning temperatures: for T1 = 155 °C, it is obtained

Dm1 = 9.46 years, while for T2 = 120 °C, Dm2 = 182 years.

It is noticed that the difference between the values of the lifetime

estimation for the two methods at T = 155 °C is relatively small, respectively:

% 87.164.9

46.964.9

1

11

,

I

mI

mID

DDD .

It should be noticed, however, that the calculated lifetime values do not

take into account other stresses which insulation systems suffer during service.

For multiple synergetic stresses, lifetime duration values are smaller [1].

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4. Conclusions

1. Accelerated thermal stresses of CALMICAGLAS tape samples lead to

an increase of the absorption currents.

2. In the first phase of the experiment, there is a decrease of the absorption

current values due to the elimination of the polar products generated during the

manufacturing process.

3. Based on the maximal values of the absorption currents, thermal

lifetime for both samples and insulation systems can be estimated.

4. Lifetime duration values acquired through the absorption current

method – for 155 °C – are close to the ones obtained through mass loss method.

5. The method of absorption currents presented in this paper is a

nondestructive one and can be applied for the lifetime estimation of the high

power electrical machines in service (through off-line measurements).

Acknowledgements

The financial support of the Romanian Ministry of Education, Research

and Youth (CEEX Project 256/2006) is gratefully appreciated.

R E F E R E N C E S

[1] P.V. Notingher, Materials for electrotechnics, Vol. 1, Politehnica Press, Bucharest, 2005.

[2] P.V. Notingher, L.M. Dumitran, C. Stancu, G. Tanasescu, E. Balescu, S. Busoi, The Influence

of Thermal Stress on Absorption/Resorption Currents, published in CONFERENCE

EXCELLENCE RESEARCH – A WAY TO E.R.A. – Brasov, 2007, pp 271-1 – 271-6, 2007.

[3] P. V.Notingher, Cristina Stancu, L.M. Dumitran, P. Notingher jr., Aleksandra Rakowska, K.

Siodla, Influence of the Ageing State of Insulation Systems on Absorption/Resorption Currents,

Revue Roum. Sci. Tech. - Electr. Et Energ., vol. 53, pp. 1-15, 2008.

[4] R.E. Droper, B.J. Moore, R.H. Rehder, Insulation System Evaluation for Rotating Machinery,

IEEE Electrical Insulation Magazine, vol. 11, 4, pp 19-25, 1995.

[5] P.V. Notingher, L.M. Dumitran, S. Busoi, E. Balescu and G. Tanasescu, The Use of

Conductivity Factors for Estimating the Degradation State of Insulation Systems of Medium-

Power Electrical Machines, Proceedings of 2008 International Conference on Condition

Monitoring and Diagnosis CMD, Beijing, China, pp 126–129, 2008.

[6] G. Reza, E. David, Condition Assessment of Rotating Machine Winding Insulation by Analysis

of Charging and Dischargind Currents, Proceedings of 2006 IEEE International Symposium on

Electrical Insulation, Toronto, pp 336-339, 2006.

[7] S. Busoi, P.V. Notingher, L.M. Dumitran, Estimation of the Degradation State of Medium

Power Machines Insulation by the Polarization Index, Proceedings of Joint International

Conference “Materials for Electrical Engineering”, Bucharest, pp. 420-425, 2008.

[8] M. Farahani, H. Borsi, E. Gockenbach, Dielectric Response Studies on Insulating System of

High Voltage Rotating Machines, IEEE Transactions on Dielectrics and Electrical Insulation, vol.

13, 1, pp 383-393, 2006.