2014 IEEE International Conference on Liquid Dielectrics, Bled, Slovenia, June 30 - July 3, 2014

Straygassing of Insulating Liquids - Manifestation

and Influence on Diagnostics

Ivanka Atanasova-H6hlein Siemens TR , Material Laboratory

Nuremberg, Germany ivanka.hoehlein-atanasova@siemens.com

Abstract- The phenomenon of fault gas generation out of

insulating fluids at moderate temperatures, based solely on their

constituent and not related to an equipment failure is described

with the term "straygassing". Straygassing is not a new

phenomenon, but it seems to be stronger manifisted with modern

mineral oils. Especially the use of some additives, mainly

passivators may accelerate strongly the straygassing

production.This paper is an overview on:

the possible mechanism of straygassing

the typical stray gas formation of mineral oil, as well as

of further insulating liquids like synthetic or natural

esters under various temperature conditions

examples of possible interference of the straygassing

pattern with diagnostics

how to distinguish straygassing from fault cases in the

DGA evaluation

Keywords-straygassing; mineral oil; synthetic and natural esters; influence of oxidation and additives

I. INTRODUCTION

The gassing of insulating liquids is an important diagnostic criterium, since electrical or thermal faults can be detected by certain gassing patterns. This has given the birth to the OGA (dissolved gas analysis) about 50 years ago. Today OGA is described in various evaluation schemes and standards. The phenomenon of fault gas generation out of insulating fluids at moderate temperatures, based solely on their constituent and not related to an equipment failure is described with the term "stray gassing". Straygassing is not a new phenomenon, but it seems to be stronger manifisted with

modern mineral oils. Especially the use of some additives, mainly passivators may accelerate strongly the straygassing production [1- 3]. The raffination techniques of mineral oils change with time and technique or environmental requirements. New insulating liquids enter the market. Intrinsically they can differ in their stability and therefore gassing pattern. The interaction of the fluid with the environment or transformer materials may also have a big impact on the gassing.

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II. MAIN PARAMETER INFLUENCING THE STRAY GASSING

A. Influence of Oxygen

Oxygen out of the air can be solved in mineral oil up to 3,5%. Even in transformer with a rubber bag there is a significant amount of oxygen solved in oil and paper. The reactivity of oxygen increases with the temperature. Since oxygen is paramagnetic, it is to be expected that there is a certain enrichment of it, following the magnetic pattern. In the presence of electrical field or temperature oxygen builds easily peroxides with reactive organic molecules, thus giving start to chain oxidation reactions.These peroxides can react with further compounds or rearange with formation of more stable compounds. There are some methods allowing the estimation of formation of stray gassing components under the influence of oxygen -these are e.g. ASTM 07150 or Cigre brochure 296 (2006) [4]. Typical stray gassing products are hydrogen, carbon monoxide, ethane and methane. Usually the concentrations are strongly increasing at the beginning of operation and later on reaching an equilibrium or even decreasing (Fig. 1 and Fig. 2).

18000 __ CO2 400

16000 -2 350 ........ CO P 14000 300 e �Temper

_12000 ature :::J

E 250 '§ c. 0) e 10000 c.

E N 200 � 0 0 8000 (5 N 150 0 6000 � c.

4000 100 e 0

50 0 2000

0 0 02.05.2010 10.08.2010 18.11.2010 26.02.2011 06.06.2011 14.09.2011

Date

Fig. I. 800 MV A, 400 k V spare transformer, taken in service with aged uninhibited oil. Even after a short time a drastic reduction in dissolved oxygen concentration and increase in carbon monoxide is noticeable.

2014 IEEE International Conference on Liquid Dielectrics, Bled, Slovenia, June 30 - July 3, 2014

30000 4000

3500 25000

3000 20000

2500

15000 2000

1500 10000

1000 5000

500

S!art 200e 80°C

Fig. 2. Straygassing at 80°C for 164h of an aged mineral oil without cellulose - laboratory experiment

In such cases straygassing is strongly related to oxygen depression.

B. Influence of Additives

-02 -CO2

I -C2H6 -CH4 -co -H2

Especially in case of mitigation of corrosive sulfur additives like passivators with aminic groups have been used. Aminic compounds like Irgamet 39 or its degradation products can act as promotors for oxidation, thus causing straygassing (Fig. 3).

5000.0 ,----------------------. 50.0

4500.0 r=:;;����::-::::�L=J 4000,0 50,0 E

3500,0 +-------[ 3000,0 a 8 2500,0 U 8' 2000,0

I!! � 40,0 CD c. E t!!

30,0 1 ���-�--� �

1500,0 *"'-� ... "--------------__j 20,0 � !:l

1000,0 ... I 10,0 U

Carbon Monoxide

---t-Carbon D ioxide

--Hydrogen

Methane

---Ethane

Ambient

with a possible accelerated degradation of solid insulation has been observed - Furanic compounds low (0,050 mg/kg) - Original oil has been partially degassed and stored at 100°C to follow the development of ethylene, carbon monoxide and carbon dioxide. - The passivator has been removed acc. to the procedure described in IEC 60296 and again stored at lOO°C to follow the development of ethylene, carbon monoxide and carbon dioxide.

4000 180

3500 160

3000 140

I 2500 120 �

E .s: 100 ! � 2000 �co

80 <t U l: 61500 N �C02

60 u

u 1000 -+-C2H4

40

500 20

0 0

0 2 3 4 5 6

Days

Fig. 4. DGA production in the partially degassed original oil (with passivator) stored at 100°C

1000

gOO

800

E 700

... 600 .s: N 500 0 U

400 6 U 300

"'" � �/ /"-.

/ ,/

....

- -

- -

.....

14

12

10

8 '[ .s: <t

6 I (j

4 500.0 .X·· • Temperature

200 ./

�co �C02 -+-C2H4

0,0 0,0 12.4.10 2.5.10 22.5.10 11.6.10 1.7.10 21.7.10 10.8.10 Dale

Fig. 3. Formation of "stray gases" after addition of pass iva tor as a consequence of oxidation reactions

oil_sampIB_t emperature 100 (0C)

0 / o

/

2 3

Days

4

2

o 5 6

Fig. 5. DGA production in the oil treated with Chromabond (passivator removed) and stored at 100°C

C. Special Cases ofStraygassing with Ethylene

Example I 12 Mvar, 315 kV Reactor, fi lied with an uninhibited

oil, naphthenic type

- 200 I the oil was passivated, however, the oil had already a measurable acidity and slightly elevated lossfactor. - Afterwards a continuous development of ethylene, carbon monoxide and carbon dioxide, which would suggest hotspot

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2014 IEEE International Conference on Liquid Dielectrics, Bled, Slovenia, June 30 - July 3, 2014

D. Straygassing of Synthetic Ester on Pentaerythrite Base

Synthetic esters on pentaerythrite base developed under laboratory conditions significant amounts of ethylene, when aged without further transformer materials. The addition of copper led to disapperance of this type of gassing, possibly by formation of complexes [5, 6].

800 700 600

� 500 .= 400 .. � u 300

200 100

.- � --inhibited mineral oil non...jnhibted mineral oil natural ester

10 only oil. with Cu 0 CU+fl3per I

,---

� synth. Ester

Fig. 6. Ethylene production of synthetic ester without further transfonner materials at 150°C in comparison to mineral oil and natural ester

E. Straygassing of Natural Ester

Natural ester develop higher amounts of hydrogen and ethane than mineral oils. The production of ethane seems to be dependent on the oxidation stability of the fluid [5, 7].

6000,--------------------------------------------,

6000r------------------------r��,_r_----------�

4000 r-------------------------1

.!: 3000 r-------------------------1

2000

1000

inhibited mineral oil non-inhibited mineral oil natural ester synth. Ester

I Donly oil .with Cu oCu+paper I

Fig. 7. Ethane production of natural ester with and without further transfonner materials at 150°C in comparison to mineral oil and synthetic ester

III. INTERFERENCE OF STRAYGASSING WITH

INTERPRETATION. WAYS OUT

DGA

Stray gassing may in some cases interfere with common schemes of DGA interpretation. Nevertheless it can be distinguished by faults by a consequent implementation of certain interpretation rules. Since DGA is not only a single shot, but a complex interpretation of gas values, gas ratios and gas increase rates, it is advisable to:

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Consider the trend development not only of a

single fault gas, but of all gases in the DGA

Build gas ratios, which are responsible for partial

or electrical discharge or thermal problem, e.g.

these described in IEC 60599

Monitor development of absolute values and the

corresponding ratios.

IV. How TO PREVENT STRA YGASSING

A. Use of high grade inhibited mineral oils

In case of mineral oil, the use of inhibited high grade oils will reduce the stray gassing. Generally fluids with higher oxidation stability show lower trend to stray gassing.

B. Oxygen reduction/removal

Since oxidation stability is very closely connected to stray gassing behavior [8], the reduction of oxygen concentration is always beneficial. Oxygen can be reduced through using of rubber bag in the conservator or further novel preservation systems, generating nitrogen from the air.

C. Cooling optimization

The reduction of operating temperature will reduce the rate of oxidation and thus the straygassing tendency.

D. Reclaiming

Oil purification by fuller earth with subsequent inhibition will reduce straygassing and oxygen depression.

REFERENCES

[I] Atanasova-Hohlein I., "Unusual Cases of Gassing of Transformers in Service", IEEE Electrical Insulation Magazine, vol. 22, Nr. 1,22-

25,2006.

[2] Atanasova-Hohlein I.. Frotscher R. "Carbon Oxides in the Interpretation ofOissolved Gas Analysis in Transfonners and Tap Changers ",

IEEE Electrical Insulation Magazine. vol. 26, Nr. 6, 22 - 26, 20 I O. [3] Cigre Brochure 526"Oxidation Stability of insulating fluids", 2013.

[4] Cigre Brochure 296, Recent Developments in DGA interpretation, DI.01/A2. 11 JWG, 2006

[5] Atanasova-Hoehlein I. Schaefer M., Hammer Th. "Diagnostic Markers for Oxidation Condition of Mineral Oil and Ester Insulating Fluids". Cigre 01_203, Cigre Session Paris 2010.

[6] Martins M. A., Gomes, A. R "Comparative Study of the Thermal

Degradation of Synthetic and Natural Esters and Mineral Oil: Effect of Oil Type in the Thennal Degradation of Insulating Kraft Paper", IEEE Electrical Insulation Magazine. Vol. 28, Nr. 2. 23-28, 2012.

[7] Schaefer M., Atanasova-Hoehlein I. . Fritsche R. Schmidt C. "Einsatz natorlicher Ester in Leistungstransformatoren - Betriebserfahrungen und Stand der Technik, Stuttgarter Hochspannungssymposium 2014.

[8] Pahlavanpour B., Sterner M. , Weesmaa J. and Nunes J. " Mineral insulating oil stray gassing and its effect on OGA Laboratory

study", Trafotech 2014.