ABSTRACT. For reliable operation of power transformers, the condition of the insulation system is essential. This paper reports on a detailed study of the effect of ...
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A. Setayeshmehr et al.: Dielectric Spectroscopic Measurements on Transformer Oil-paper Insulation
Dielectric Spectroscopic Measurements on Transformer Oil-paper Insulation under Controlled Laboratory Conditions A. Setayeshmehr1, I. Fofana2, C. Eichler1, A. Akbari1, H. Borsi1 and E. Gockenbach1 1
Institute of Electric Power Systems, High Voltage Engineering Section (Schering- Institute) Leibniz Universität Hannover, Callinstr. 25 A, D-30167 Hanover, Germany 2 Canada Research Chair, tier 2, on Insulating Liquids and Mixed Dielectrics for Electrotechnology, University of Quebec at Chicoutimi, 555, Boulevard de l’Université, G7H 2B1, Chicoutimi, Qc, Canada
ABSTRACT For reliable operation of power transformers, the condition of the insulation system is essential. This paper reports on a detailed study of the effect of ageing, temperature and moisture on frequency and time domain spectroscopic measurements carried out on oil-impregnated pressboard samples as well as on a distribution transformer under controlled laboratory conditions. Because field measurements are generally performed after de-energizing the transformer, extreme care is required in interpreting the results due to inherent temperature instabilities. To avoid large thermal variations that may affect the results, a customized adiabatic room was built around the transformer for measurements above the ambient. Capacitance ratio and direct current conductivity deduced from the spectroscopic measurements, helped to interpret the data. Because, low frequency measurements techniques are time consuming, alternative to a transfer of time domain data into frequency domain data was investigated. Index Terms — Dielectric spectroscopy, time domain, frequency domain, conductivity, Aging, moisture, temperature, oil-paper insulation.
1 INTRODUCTION TRANSFORMERS are the “heart” of any electric power distribution and transmission systems and it is essential that they function properly for many years. In most of power transformers, the main insulation is a combination of mineral oil with cellulose materials. When this insulating paper is adequately impregnated with oil, it offers the user a material with insulating and mechanical properties of remarkable suppleness. The good dielectric properties combined with the wide availability and cost benefit of oil paper insulations have, therefore, made these materials, transformer insulation of choice for nearly a century [1]. However, this insulation system deteriorates in service due to service conditions. The reliable performance depends on its basic character which may affect the performance of the transformer. During service, a variety of processes occur, some of them being inter-related, which degrade the insulation. The degradation/aging of transformer insulation is recognized to be one of the major causes of transformer breakdown [2-5]. The cost of premature and unexpected failure of a power transformer can be several times its initial cost. There is not
Manuscript received on 19 October 2007, in final form 14 March 2008.
only the refurbishment or replacement cost but also possible costs associated with clean-up, loss of revenue, and deterioration in quality of power delivery. With increasing age, there are potential risks of extremely high monetary losses due to unexpected failures and outages. Condition monitoring of the insulation of transformers has become an important issue since many transformers in electrical industries around the world are approaching the end of their design life. Indeed, condition monitoring can be utilized to attempt the prediction of the insulation condition and the remaining lifetime of a transformer. In this context, the adequacy of existing and the application of new diagnostic tools and monitoring techniques gain increasing importance. Increasing requirements for appropriate tools to diagnose power systems insulation non-destructively and reliably in the field drive the development of diagnostic tools based on changes of the dielectric properties of the insulation. Some of these modern diagnostic methods include the Recovery Voltage Measurement (RVM), Frequency Domain Spectroscopy (FDS) and Polarization and Depolarization Current Measurements (PDC) [4]. These two later became only recently available as user-friendly methods, and can be used to monitor, diagnose and check new insulating materials, qualification of insulating systems during/after production of power equipments non-destructively [6, 7].
1070-9878/08/$25.00 © 2008 IEEE Authorized licensed use limited to: Issouf Fofana. Downloaded on October 15, 2008 at 10:56 from IEEE Xplore. Restrictions apply.
IEEE Transactions on Dielectrics and Electrical Insulation
Vol. 15, No. 4; August 2008
Field measurements are generally performed just after deenergising the transformer. Under such circumstances, large thermal variations may affect the results, since moisture distribution inside the insulation is not in complete equilibrium condition [8, 9]. Because, spectroscopic measurement results are highly operating conditions dependant, care are required to interpret them. Low frequency measurements being particularly long time consuming process, large temperature instability may affect interpretations. In this contribution, spectroscopic measurements (FDS and PDC) were performed on a distribution transformer, at temperature above the ambient, moisture content in oil acted as parameter. To reduce the influence of large thermal variations, a customized adiabatic room was built around the transformer. The obtained results are discussed in regard to the wetness and temperature influence on the insulating system. In order to analyse moisture content, insulation temperature and ageing, independent on spectroscopic measurements, investigations have been performed on oil-impregnated pressboard samples under controlled laboratory conditions. Measurement duration in time domain being shorter than frequency domain ones, PDC measurement were transformed in frequency domain using Fourier Transformation. The obtained results depict good agreement with measured one and indicate the feasibility of using low frequency data to separate moisture, ageing and temperature effects.
2 DIELECTRIC SPECTROSCOPY TECHNIQUES Dielectric spectroscopy in time or frequency domain offers new opportunities for an off-line, insulation condition assessment of HV electric power equipment and its predictive maintenance nondestructively and reliably in the field. These techniques are global methods, i.e. each test object is regarded as a “black box” accessible only by its electric terminals. Therefore, only global changes of the insulation can be identified but not localized defects [6]. Because, results from these tests are highly operating conditions dependant, practical measurements issues should be considered [615]: • Inherent to all dielectric spectroscopy measurements in either time or frequency domains is their “off-line” character, i.e. equipment in operation must be removed from service before performing measurements. • Dielectric measurements require constant insulation temperatures during application for accurateness, as the polarization phenomena are temperature dependant. Adequate experience and extreme care are needed to interpret the results in the presence of temperature variations and thermal instability within the equipment. • The charging time/period for PDC measurements should be long enough to allow all polarization processes to be completed. For good results, a minimum polarization time of 10,000 s should be used on large power transformers. • Rain seems to generate leakage dc currents which superimpose the desired measurement currents. The magnitude depends on
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the rain intensity and sometimes exceeds100 nA, what makes a measurement difficult to interpret. • As measurement instruments used are usually quite sensitive to electromagnetic disturbances, the “electromagnetic compatibility” of such instruments must be guaranteed. Therefore, the test voltage levels of the instruments cannot be too low. • The maximum polarization voltage is determined by the transformer’s geometry and the resulting electrical field strengths in the oil-paper-insulation. If field strength exceeds 10 V/mm, different effects (charge injection from the electrons and field-induced dissociation leading to apparent higher oil conductivity) can lead to non-linear effects. The polarization voltage should be kept as low as possible. A polarization voltage between 100 and 500 V has led to satisfying results on all measurements performed on various distributions and power transformers in power plants and switchyards. The fundamental theories behind dielectric measurements are already well known [6] while dielectric phenomena are discussed in Jonscher’s publications [16, 17]. However, to facilitate interpreting the measurements presented in sections 3 and 4, there is a short review that includes theory behind time and frequency domain measurement techniques. 2.1 TIME DOMAIN SPECTROSCOPY The measurement of polarization and depolarization currents (PDC) following a dc voltage step is one way in the time domain to investigate the slow polarization processes [4, 613]. The dielectric memory of the test object must be cleared before the PDC measurement. The voltage source should be free of any ripple and noise in order to record the small polarization current with sufficient accuracy. The procedure consists in applying a dc charging voltage of magnitude Uc to the test object for a long time (e.g., 10,000 s). During this time, the polarization current Ipol(t) through the test object is measured, arising from the activation of the polarization process with different time constants corresponding to different insulation materials and to the conductivity of the object, which has been previously carefully discharged. Then the polarization (or absorption, or charging) current Ipol(t) through the test object can be expressed by [6-11]: ⎡σ ⎤ I pol (t ) = CoU c ⎢ o + ε ∞δ (t ) + f (t )⎥ (1) ε ⎣ o ⎦ where: Co : geometrical capacitance of the test object, Uc : the step voltage (charging voltage), σo : the dc conductivity of the dielectric material, εo : 8.852 10-12 As/Vm is the vacuum permittivity, ε∞ : the high frequency component of the permittivity, δ(t): the delta function arising from the suddenly applied step voltage at t = t0. f(t) : the response function of the dielectric material. The geometric capacitance of a core type transformer can be estimated by the cylindrical capacitance (equation (2)), where h is the average winding height and ra and rb are, respectively, the inner and outer radius of the insulation between windings [8].
Authorized licensed use limited to: Issouf Fofana. Downloaded on October 15, 2008 at 10:56 from IEEE Xplore. Restrictions apply.
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C0 =
A. Setayeshmehr et al.: Dielectric Spectroscopic Measurements on Transformer Oil-paper Insulation
ε 0 2πh r log⎛⎜ b ⎞⎟ ⎝ ra ⎠
Ipol (t)
If the design of the transformers insulation is available, the geometric capacitance can be calculated. Otherwise, geometric capacitance can be estimated by measuring the capacitance Cm between the two terminals of the insulation system under test (it can be measured with any capacitance measuring ac bridge at/around the power frequency) and dividing by the effective relative permittivity εr of the combination of the composite oil-paper insulation system (Co = Cm/εr) [8]. The effective permittivity of oil impregnated pressboard samples may be written in terms of paper and oil permittivity (εpaper and εoil respectively) as [18]: ε paper .ε oil εr = (3) ε paper (1 − X ) + ε oil X where X is the relative amount of paper in the composite system. The range of X is typically 20% to 50% [18] for a transformer. The voltage is then removed and the object is short-circuited at t = tC, enabling the measurement of the depolarization current (or discharging, or de-sorption) Idpol(t) in the opposite direction, without contribution of the conductivity. The polarization current measurement can usually be stopped if the current becomes either stable or very low. According to the superposition principle the sudden reduction of the voltage UC to zero is regarded as a negative voltage step at time t = tc. Neglecting the second term in (1) we get for t = (t0 + TC) [610]: I depol (t ) = −CoU c [ f (t ) − f (t + Tc )]
(4)
where Tc is the charging time of the test object. Figure 1 shows the schematic diagram of the PDC measuring technique while Figure 2 shows the typical nature of these currents due to a step charging voltage UC [6].
Idepol
Ipol Uc
Uc
(2)
Test object Electrometer
Figure 1. Principle of test arrangement for the “PDC” measuring technique.
The insulation between windings is charged by the dc voltage step Uc. A long charging time is required (10,000 s) in order to assess the interfacial polarization and paper condition. The initial time dependence of the polarization and depolarisation currents (
