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Просмотр полной версии : Influence of peroxides, acids and moisture on aging of solid insulation of transformers



Batura
16.07.2012, 13:26
In the absence of oxygen (see table) isopropylbenzol hydroperoxide (hyperis) has no significant impact on mechanical strength of insulation.

The table below illustrates the influence of some transformer oil oxidation products on cellulose insulation strength in the absence of oxygen (at the temperature of 95oC, test duration 720 hours)



Test subject
Oil parameters
Water content by weight, %
Relative breaking tension reduction, %
Relative reduction of paper double kinks, %



Acid number
Water soluble acid content

Cable paper
Herring-bone tape




mg KOH per 1 g oil













Sealed vessels filled with oil with nitrogen above oil


Oil with no additives
0.014
None
Below 0.0010
6
0
3


Oil with additive:








Hyperis
0.015
None
Below 0.0020
10
0
38


Acetic acid (acid number 0.1mg KOH/g)
0.013
None
0.001
6
0
55


Acetic acid
0.08
0.034
0.0025
12
7
86


Naphthenic acids
1
0.005
Below 0.0010
0
0
9


Stearic aicd
1
None
Below 0.0010
0
0
10


Water (100% humidity above oil at +20oС)
0.017
0.003
0.005
7
0
0


Water (100% humidity above oil at +95oС)
0.013
None
0.027
12
20
74


Sealed nitrogen filled vessels, no oil


Clean insulation



1
2
49


additives:








Alcoholic acids № 1*



29
49
100


Asphaltenes № 1*



26
53
100


Alcoholic acids № 2**



53
55
100


Asphaltenes № 2**



65
56
100



* Extracted from sediment obtained by oxidizing oil at 95 oС with copper.

** Extracted from sediment obtained by oxidizing the same oil at 150 oС.

Using one of the most active low-molecular acids, acetic, it was determined that concentration corresponding to the acid number of 0.1 mg KOH / 1 g of oil, the acid had almost no effect on insulation destruction. If concentration is increased to the equivalent of 0.1 mg KOH / 1 g of oil, a superficial aging process becomes noticeable.

The presence of high-molecular acids, e.g. stearic and a mix of naphthenic acids (with acid number of 1 mg KOH per 1 g of oil) has no influence of mechanic characteristics of the insulation. Oil-saturated paper insulation of power transformer contains 0.8 – 4.7% moisture; the lower limit refers to new transformers; the upper one refers to transformers currently in operation. Regarding insulation of measurement transformers, which operate with oil temperature slightly higher than the ambient temperature, water content is significantly higher and constitutes 7.4% on average.

Moisturizing of transformer insulation occurs due to a number of natural reasons: thermal paper decomposition (see figure below), oil oxidation, transformer “breathing”, which is the result of shifting thermal balance between the oil and the environment, imperfect gaskets etc. A new well dried transformer should indicate no more that 0.8% moisture in paper insulation.

The figure below illustrates formation of moisture during thermal degradation of kraft-paper

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An approximate rule was derived, which holds true to concentrations of water in paper insulation in the range of 0.3 – 7%, which states that the rate of paper decomposition is proportional to the amount of water therein (see figure below). Tests were run in sealed vessels.

This graph shows influence of moisture content in paper (h) on its thermal aging for various degrees of aging attained (x - relative reduction of cellulose polymerization)

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t(0,3)/t(h) — is the ratio of time required to reach a certain degree of aging paper with 0.3% moisture content, to the time, required to reach the same degradation of paper with h moisture content in per cent.

At 0.027% solved water in the oil and at 95oC temperature, the mechanical characteristics of insulation degrade significantly. Such excessive moisture content is rare (high temperature and 100% relative humidity). Normal operating conditions of transformers warrant moisture content in the range of 0.003 – 0.010%.

At atmospheric pressure, after a passage of time period, duration of which is determined by the temperature, a dynamic balance is established between moisture in the air above the oil, moisture solved in the oil and moisture in paper insulation.

For qualitative assessment of water content in different parts of the air – paper – water system, the curves below may be used. Data obtained is close to practical.

These curves show the equillibrium dispersion of water in air - oil - paper system as a function of ambient air humidity: Θ1 — oil temperature; Θ — air temperature; τ — air relative humidity

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Estimating danger of moisture content in the oil, keep in mind that when the transformer is constantly loaded and the temperature of oil (and copper) is significantly higher than the ambient temperature, the possibility of significant oil and solid insulation moisturizing is all but eliminated.

Moreover, insulation in unsealed transformers can actually be somewhat dehydrated. Such natural process is difficult in sealed transformer, therefore, when manufacturing sealed transformers, moisture must be removed from solid and liquid insulation to the fullest extent possible.

When oil temperature drops below the ambient temperature (which often happens in practice when transformer load is sharply decreased, or the transformer is switched off), moisture may condense on the surface of the oil in unsealed transformers; this moisture may dissolve in the oil and enter solid insulation.

It should be emphasized, that moisturizing of transformer paper insulation is not only tied to the loss of its mechanical strength, but also a significant drop of dielectric strength (see table below). This is due to emissions of gas from oil saturated paper under high voltage.

This table shows breakdown voltages for dry insulation paper in moist transformer oil



Duration of paper-oil contact
Breakdown voltage, kV


Dry oil
Moist oil
Very moist oil


Oil
Paper
Oil
Paper
Oil
Paper


0 hours
116

48

20



1 hour
116
27
50
27
54
27


1 day
80
26
62
26
72
24


3 days
84
25
48
25




11 days
92
25
66
22,5
68
25


30 days
70
25
50
22
96
18


60 days
72
24
60
18
74
10




Note. The breakdown voltage was determined for: a) oil in a cell with spherical electrodes at 5 mm distance; b) paper – same electrodes in oil environment at 0.25 mm between electrodes..

A quadratic dependence of tg δ of oil saturated paper from percentage of water in the paper m has been established: 100 tg δ = p + bm2, where p=0.21 at 80oC; b=0.83 at 80 oC (figure below).

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Correlation between tg δ of oil saturated cable paper and temperature at various moisture content levels.

Relative dielectric permittivity of oil saturated paper εr is a linear function of moisture content: εr = 3.8 + 0.41 m.

Well dehydrated oil can absorb excessive water from cellulose insulation. This principle allows drying of transformers and is referred to as sequential oil processing. Transformer is filled with dry oil with high dielectric strength. After a certain type, the exchange of water between the oil and the solid insulation of the transformer causes a drop of the oil’s dielectric strength. The oil is again dried until initial dielectric strength is obtained. The cycle is repeated until the oil’s electric strength stops dropping. This is the moment of dynamic balance between the quantity of water in the oil and in the paper, i.e. the lowest possible moisture content at existing conditions. Therefore, according to the chart above, if the oil and air temperatures are 50 oC and 20 oC respectively, and relative ambient humidity of 50%, the concentration of water in the oil will constitute 0.0010 – 0.0013%, which corresponds to approximately 2% of water content in the paper.