Aspects of the Influence of Electromagnetic Field on the Oxidation of Insulating Oil

Tiberiu-Octavian Cujbă, Dorel Cernomazu Faculty of Electrical Engineering and Computer Science

”Ştefan cel Mare” University of Suceava 13, Universitatii Street Suceava – 720229 ROMANIA

E-Mail: tiberiucujba@yahoo.com , dorelc@eed.usv.ro

Abstract—The insulated oil is used in electricity as a medium for cooling and dielectric in transformers, for cooling and extinguishing the arc in switches, etc. Operating conditions of electrical equipment affect properties of insulated oil causing their aging. Some aspects of electromagnetic field influence on the oxidation of insulating oil are listed below. Keywords - electrical field, oxidation, catalysts, height voltage.

I. INTRODUCTION In practice the following methods are known for checking the stability to oxidation: experimental transformer method, the method Baader, Fig.1, the method by heating and ultraviolet method [1].

Figure 1. Method Baader (photo) and Ultraviolet method (scheme)

The methods listed only method Baader and transformer method experimental model in greater operating conditions, although they have some disadvantages, leading to an uncomfortable implementing them: Baader method has high accuracy, does not mean electric field and is quiet and transformer experimental method is very long and requires a large labor consumption (standard transformer should be cleaned after each sample). By using ultraviolet radiation, after the scheme of Fig.1, we obtain better results [2]. But this method does not consider the influence of the electric field that is inside the transformer insulating oil.

Researches led on reasons which drove to the acceleration of the phenomenon of the oxidation of insulating oil shows that this process sets up in maximum intensity in conservatories of the electrical transformers. The products of oxidation enter in vat of the transformer where attacks irremediably insulation (especially the insulation of shortening makes in the low of cellulose).

II. MECHANISM AGING INSULATED OILS Oxidation is in fact a chemical reaction between oxygen

and transformer oil. This reaction is influenced by: temperature (especially about 70-100°C), humidity, catalytic action of metals (especially copper), sunlight (particularly UV radiation), impregnating products polymerizates incomplete. The oxidation of transformer oil in two cycles of reaction:

- first cycle of reaction leads to the formation of soluble oxidation products as acids;

- the second cycle of reaction is characterized by the fact that soluble products oxidation becomes insoluble compounds (waxes, resins, etc.) and forming mud (schlamm).

Insoluble deposits on the transformer windings are troublesome in that it prevents the normal discharge of heat released [3]. Another high-risk is the acids that attack the cellulose-based insulation and as a result of the aging process loses its elasticity and became vulnerable to mechanical forces (example electrodynamics forces of short-circuit) [4].

Figure 2. Damages caused in cellulose-based insulation

As a result the conductor insulation short circuit can occur causing serious damage of the transformer. The practice has established the following classification of transformers insulation: better insulation class I, class II - good insulation,

Class III - insulation brittle type or poor quality, Class IV - poor insulation, which characterize the poor insulation, the cartoon is broken when it bent pressboard at a right angle, and the insulated conductor, when pressed by hand, presents significant deformation or damage [5]. In Fig.2 are presented damage caused in the cellulose-based insulation aging [6].

III. THEORETICAL METHODS OF INVESTIGATION OF ABNORMAL REGIMES ENERGY SYSTEM

A. Method Leonowicz

Spectrum estimation of discretely sampled deterministic and stochastic processes is usually based on procedures employing the Fast Fourier Transform (FFT) [7]. To overcome the limitations the Wigner-Ville Distribution and the Min-Norm subspace method have been applied for spectrum estimation of non-stationary signals caused by switching on capacitor banks and by a short circuit. Investigation results in a distribution system (Generator at 50 Hz, Transformer DY 110/15 Kv, 25 MVA) when two capacitor banks (CB) were installed along the feeder. Fig. 3 shows the current waveform at the beginning of the feeder for the case that the first CB (900 kVAr) was switched on at 0.03s and the second CB (1200 kVAr) at 0.09s.

Figure 3. Waveform of the A–phase current during switching of the

capacitor banks

Because capacitors voltages involved in mounting the operation of transformers, the performance of switching are not generally dangerous even for the main insulation, no insulation inside. However, designing and implementing the entire system so that the transformer insulation withstand surge of atmospheric origin is an important design issue because the dangerous surge of failure.

B. Method Jizierski

Assuming for simplicity that the transformer windings shall consist of the same number of elements, the same capacity C1 from the ground and being coupled together by the same capacity K1, where a rectangular pulse voltage where the first time results of a hyperbolic distribution tension along the winding [8]. Neutral point of windings can be considered forever tied to the ground, Fig.4. Analytical expression of blood distribution from the ground is where neutral connected to earth:

bobbob xLL

xx

bobx ee

eeULsh

xshUU −

−

== α

αα

αα

00 (1)

For neutral isolated: bob

x LchxchUu

αα

0= (2)

where: KC

Lbob

1=α

C is total capacity from the ground; K - total capacity coupling between winding elements: L - axial length of winding.

Figure 4. Initial voltage distribution along a winding to a rectangular pulse

If the general distribution of blood at a time t > 0 will be every other time until a steady achieved, which generally occurs after a few tens of microseconds. The operating conditions in case of exceptionally unfavorable circumstances competition may occur surge with a high peak and a steep wave front, that the construction of medium and small power transformers, having a dielectric rigidity to give an absolute security against the surge, would require expensive insulation and therefore their implementation would be justified in economic terms. For these transformers to accept a reasonable compromise between absolute safety of the insulation from the pulse voltage and economic consequences

C. Method Frid

Russian literature [9] indicates a relatively easy method of calculating the maximum gradients voltage for voltage pulses of different shapes and windings of transformers having more buckets. After Frid, each of which corresponds to a mirror image, relative to the start winding. Variation field is represented in Fig. 5, e1 and e2 waves travel at a speed v along the winding height and e3 and e4 waves moving in opposite directions at the same speed. Initial distribution of voltage corresponding expression:

bobbob LL

xx

eeeeUu αα

αα

−

−

−−= 0 (3)

in which x is measured from the end windings. This expression can be simplified by neglecting the term, and for values x near

Lbob neglecting the term bobLe α−

. If rectangular amplitude is considered as a unit, we can write: bobLxeu αα −≈ . Substituting

XLx bob −= we have: ( ) XLx eeu bob αα −− =≈ (4)

where X is measured from the start winding.

Figure 5. Distribution voltage gradients after Frid: a - initial distribution

b - component waves after crossing time

IV. INSTALATION FOR CHECKING STABILITY OF INSULATED OILS .

Installation as shown Fig.6, according to the invention [10] is as faithful to the modeling of operating parameters to accelerate the oxidation in laboratory conditions, namely: catalytic effect of various metals (Cu), the effect of radiation heat (100 º C ), ultraviolet radiation effect (mercury vapor lamp 400W) and the effect of electric field (10 kV).

The installation used as catalyst for evidence of oil a spiral of copper (1) connected to the ground and insulated by an insulator (2) to the metal wall of a vessel (3), which is connected to high voltage terminal of a transformer for tests with increased power (4), isolated, in turn, to an oil thermostat bath (5), all being placed under a mercury vapor lamp (6), and signals required for the voltage being applied through a control block and signal (7), which provides adjustment of the oil bath temperature of 100 ° C, at which ultraviolet radiation source is coupled. Irradiation time can be programmed in a range of 3 ... 5 h, after which the scheme is automatically disengaged.

Vat reproduce the status of the power transformers in operation, that the electric field, which is an important component favoring oxidation of oil. Remove oil sample, Fig.7 and determine the characteristics that indicate the degree

of alteration: dielectric losses (tgδ), acid value (ia), kinematic viscosity (σ), flash point (Ti) and breakdown voltage (Ustr().

Figure 6. Installation for checking oxidation stability of insulated oils

Figure 7. Oil – new (left); Oil after 3h (center); Oil after 6h (right)

TABLE I. INSULATED OIL CHARACTERISTICS

tgδ [%]

iA [mgKOH/g]

Ti [°C]

σ [dyne/cm]

Ustr.

[kv/cm]

Oil new 0.15 0.3 145 40 120 Oil after 3h 0.65 0.5 135 26 8 Oil after 6h 14.1 0.75 120 16 4

δ

V CONCLUSIONS - There are several methods for checking the oil stability to

oxidation, not all provide protection solutions, known methods for checking stability to oxidation lasted too long (in some cases 28 days) and therefore was forced to experiment with ways which reduce their length till 10 hours. The results confirm the unfavorable influence of electromagnetic field on the stability of insulating oil. Oxidation is accelerated by the electric field in the first cycle. The magnetic field has a small effect, contributing only a limited acceleration of the second cycle, Fig.8.

Figure 8. Insulating oil oxidation stages

- There are several theoretical methods for calculating voltages, all using simplifying assumptions. For example Frid calculated appears first oil channel with a voltage equal to 50% of peak pulse. This voltage decreases very rapidly, being only 30% start from buckets sixth. These values of course refers only to the model created by Frid. Real tensions that occur in transformers oil channels are smaller. Electrotechnical Institute of Poland (1966) found, in a pulse of 630 kV to 110 kV applied to a winding of a transformer of 10 MVA, 126 kV only the first channel of oil, ie 20%. It is easy to understand that these voltages depend on the coefficient valuesα .

- To draw the magnetic flux lines in the middle of a transformer 110/20 kV, 10 MVA presented in Fig.9 we used program FLUX 9 [11].

a b

Figure 9. Magnetic field lines in the core of power transfomer: a – normal regime, b – damage

- For optimal dimensions of the insulation in high voltage transformers 400/121/20 kV, 250 MVA Trafoconsult-Craiova [12] has developed a program based on the study of electric field lines inside the transformer, as show in Fig.10.

- Using a high quality insulating oil with a excellent resistance to aging and also inhibited by an antioxidant, guarantee, together with technical and manufacturing improvements introduced in recent years, an important extension of the useful life of transformers [13].

Figure 10. Electric field lines inside the power transformer

- Finally mentioned that improving the chromatographic analysis method of gas emanating from transformer oil we can states cause of the defect in the transformer [14].

REFERENCES [1] T. O. Cujbă, D. Cernomazu: “Contributions concernant la protection de

l’huile isolante contre l’oxydation”, 7-th International Conference on Electromechanical and Power Systems SIELMEN 2009, Iaşi, 8-10 October 2009, pp.270-273.

[2] A. I. Sabău: “Aparat pentru determinarea rezistenţei la oxidare a uleiurilor”, Patent 66209, OSIM, Bucureşti,1979.

[3] T. Salomon : “Aprecierea uleiurilor electroizolante în exploatare”, Buletin de la Société Française des Electricians 7, nr.46 IV, oct. 1954, pp.570-600.

[4] I. S. Aptov, M. V. Homeakov: “Întreţinerea uleiului electroizolant” Traducere din limba rusă, Editura Tehnică, Bucureşti, 1973.

[5] S. A. Farbman, I. A. Bun: “Repararea şi modernizarea transformatoarelor”, Editura Tehnică, Bucureşti, 1962, pp..131-132.

[6] D. Cernomazu: “Avariile transformatoarelor electrice – Culegere de fotografii realizate la URTAE Roman în perioada 1970-1990”.

[7] Z. Leonowicz: “Analysis of Non-Stationary Signals in Power Systems using Wigner Transform and Min-Norm Method”, 7th EEEIC Conference on Environment and Electrical Engineering, Wroclaw-Cottbus, 5-11 May 2008, pp.43-46.

[8] E. Jizierski, Z. Gogolewski, I. Z. Kopczynski, J. Szmit: “Transformatoare electrice. Construcţie şi proiectare”, Traducere din limba polonă, Editura Tehnică, Bucureşti, 1966, pp.186-205.

[9] E S. Frid: “Osnovnîe empericeskie zakonomernosti impulsnîh gadientov v obmotkah”, Electicestovo nr. 9, 1950.

[10] T. O. Cujbă: ”Instalaţie pentru verificarea stabilităţii la oxidare a uleiurilor electroizolante”, Patent Application A/00622, OSIM, Bucureşti, 07.08.2009.

[11] *** www.cedrat.com [12] Gh. Toma, C. Treschi, V. Iordachi: “New developmenţs in the insulation

of the hign power high voltag”, Conferinţa Internaţională de Transformatoare Electrice, ICPE-ME SA, Bucureşti 9-10 mai 1996, pp. MS48-MS54.

[13] W. Weber, K. Volf: “Mesures pour maintenir en bon état l´huile des transfomateurs de distribution”, Revue Brown Boweri, nr.11/12, 1965, pp.916-921.

[14] F. Mosinski, J. Galosh, Khalaf Y. S. Al-Mualla: “A numerical progam for interpretation of the results of cromatogrphic measurements of gases dissolved in a power transformer oil”, International Conferince of Electrical Transformers ICPE-ME SA, Bucureşti, 9-10 May 1996, pp.TT10-TT13.