GB2361112A

GB2361112A - Oil filled transformer with an expansion chamber including low pressure nitrogen

Info

Publication number: GB2361112A
Application number: GB0008358A
Authority: GB
Inventors: Juergen Bastian
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-04-06
Filing date: 2000-04-06
Publication date: 2001-10-10
Anticipated expiration: 2020-04-06
Also published as: DE10116287A1; GB2361112B; GB0008358D0

Abstract

An oil-filled transformer comprising an expansion chamber containing nitrogen gas at a low pressure (ie a partial vacuum, below atmospheric pressure) in which the vacuum effect is compensated by the hydrostatic pressure created by the oil in the chamber and in the pipe connecting the chamber to the transformer tank. This arrangement excludes oxygen whilst preventing super saturation of the oil by the nitrogen during cooling.

Description

1 2361112 NON-BREATHING POWER TRANSFORMER WITHOUT NITROGEN BLANKET

Background

Electrical transformers are principally of two types - hermetic or air breathing. There are several variants to either type. The vast majority of air breathing transformers are equipped with an exterior expansion vessel, often referred to as a conservator, and an air dryer.

The advantage of the air breathing transformer is seen in the absence of pressure variations, while the most uncontroversial disadvantage is the near-saturation of the oil with oxygen which reacts with the oil, especially at elevated service temperatures.

By contrast, the main advantage of the hermetic transformer is seen to be in the exclusion of oxygen. More controversial is the question of air humidity and water as a product of cellulose aging. Theoretically the air breathing transformer could have an advantage over the hermetic transformer in allowing some of the water produced by the cellulose aging process to escape through the conservator, as the oil in the conservator will become more saturated with water than is the air that has been sucked in through the silica gel dryer.

This question will always be controversial, as the answer to it is influenced by a great many parameters such as the state of the solid insulation, service temperature, hygroscopicity and relative humidity of the oil, state of the air dryer, relative humidity of the ambient air. However, the same question is being asked in the context of the hermetic transformer, irrespective of whether it is nitrogen-blanketed, as are most transformers in the United States of America, or of the integrally filled type. (Most distribution transformers in Western Europe are of that type.) It has been shown, however, that the hermetic transformer can be designed so that water as a product of cellulose aging can be trapped. (European Patent Nos. EP 0 746 000 and EP 0 750 322) Where the formation of water by aging cellulose is not considered important, e.g. oil temperatures are kept sufficiently low, or where solid insulation materials are used that do not produce water, the hermetic transformer will always have a great advantage over the airbreathing type, for as long as it remains airtight.

However, hermetic transformers also have their limitations and drawbacks: The integrally filled or cushion-less transformer is limited as to size, while the nitrogen-blanket transformer suffers from the drawback that in the cooling and contraction phase the oil will always become supersaturated with nitrogen, which can lead to insulation inhomogeneities due to micro-bubble formation. Another disadvantage of the nitrogen blanket transformer is to be seen in the constraints on the placing of bushings.

By oversizing the nitrogen blanket it is possible to overcome the disadvantage associated with the formation of negative pressures. The disadvantages that this measure entails are seen as outweighing the advantages. Another serious disadvantage associated with the nitrogen blanket is to be seen in the reduction of the cooling surface and the constraints on the design of the radiators, as in thermosiphoning systems these cannot be any higher than the minimum oil level in the tank.

A hybrid design in which the pressure in the transformer remains unchanged, while the air is not allowed to come into contact with the oil, is the variant in which a membrane, usually in the form of an inflatable bag floating in the oil in the conservator, is open to the ambient air. While the principle of the design has been widely acclaimed as uniting the most desirable features of both the breathing and the hermetic type, problems of susceptibility of the membrane to chemical attack and to physical wear, such as abrasion, have caused many transformer operators to return to the other variants.

Obviously, as with other hermetic types, the membrane transformer does not allow any gases, including decomposition gases and water as a product of cellulose aging, to escape.

Purpose of the invention The main purpose of the invention is seen in overcoming the limitations of the nitrogen blanket design.

Description

The invention overcomes the limitations of the nitrogen blanket design, with the exception of load-induced pressure changes. This is achieved by a combination of design features. In order to combine exclusion of air, exclusion of gas spaces in the tank, exclusion of the possibility of accumulation of decomposition products in the oil, the transformer tank is connected to an enclosed space filled with nitrogen at very low pressure. In order not to allow the vacuum to exert a deleterious effect on the oil in the transformer tank, the connecting vertical pipe is of sufficient height which is determined by the size of the degassed enclosure, also referred to in the following as the vacuum chamber. The connecting pipe has a compensatory function. The pressure in the transformer is determined mainly by the height of the compensating pipe and the dimensions and negative pressure in the expansion vessel.

The vacuum chamber's dimensions are determined by the oil mass in the transformer the expansion coefficient of the oil the oil's expected service temperature range the negative pressure (vacuum) range chosen in relation to expected oil temperature variation the height of the vertical connecting pipe which is assumed to remain liquid filled at any oil service temperature the nitrogen saturation quasi-equilibrium attained at a given pressure and temperature of the oil A more generous dimensioning of the vacuum chamber permits a relatively short height of the vertical connecting pipe; another advantage could be seen in the smaller pressure variations at given oil service temperature variations.

It will be understood that any oil-temperature-driven volume and mass increase in the expansion vessel will only translate into small increases of the hydrostatic pressure increase, as the total height of the column exerting that hydrostatic pressure on the tank changes only slightly.

However, any pressure increase due to an increase in total column height has to be added to the simultaneous pressure increase that results from a decrease in the negative pressure in the vacuum space.

Variations of the oil mass, attributable to temperature changes in the oil in the connecting pipe are negligible.

Within the limits cited above, considerable variations of vacuum chamber dimensions and vertical connecting pipe height are possible. In the example given in the following, a number of assumptions are made:

The oil used in the transformer is an oil having an expansion coefficient no higher than that of normal transformer oil and a partial pressure no higher than that of normal transformer oil.

2. The mean oil service temperature undergoes changes which will not cause a steep increase or decrease of the oil volume. The expected mean oil temperature variations must be accommodated by sufficiently generous dimensions of the expansion vessel. Where large temperature variations are to be expected, the expansion vessel should be approximately three times the oil volume increase caused by a mean oil temperature rise of approximately 4WK.

3.

The commissioning of the transformer, including filling to the minimum oil level in the vacuum chamber, takes place at approximately 20'C mean oil temperature. The minimum mean oil temperature will remain at approximately that temperature at any future time, which ensures that the vertical connecting pipe will remain oil-filled at all times. In all other cases, the possibility of the need for topping up should lower oil temperatures occur, must be considered, if it is to be avoided that a gas space occurs under the tank cover.

By contrast, a machine transformer operating at near-constant load would not require an expansion vessel of a volume more than twice that caused by a mean oil temperature change of 4WK or even 20'K.

It is also assumed that any decrease of pressure (or increase of the difference between atmospheric pressure and vacuum chamber pressure) due to further dissolution of nitrogen gas in the degassed and undersaturated oil will be slight, and a near-equilibrium will be established after a few weeks of service. The dissolution of nitrogen in the oil can cause a slight decrease of oil pressure in the tank; however, a slight negative pressure under the tank cover will not cause inadvertent air breathing of the tank or the radiators.

Explanation to schematic drawing fig. 1 Fig. 1 shows, in schematic form, a variable oil volume of a power transformer. The variations in volume are caused by variations in the mean oil temperature. The minimum volume and the maximum volume liquid levels are indicated as min and max in the vacuum chamber serving as expansion vessel above the transformer.

The oil in the transformer tank communicates with the expansion vessel via a connecting pipe.

h, marks the height of the compensating liquid column when the liquid is cold, h2 marks the increase of that column when the liquid is at its highest temperature.

VO is the volume of the cold liquid, Ve,,p is the volume increase at maximum oil temperature Commensurate to the oil volume increase, the gas volume will decrease, as indicated by the two arrows extending from the top of the expansion vessel downwards.

Claims

CLAIM

What is claimed is a liquid-filled power transformer disposing of a vacuum chamber serving as an expansion vessel filled with nitrogen constituting a partial vacuum, said vacuum undergoing changes due to mean oil-temperature-driven oil volume changes, the effect of the vacuum chamber's negative pressure on the tank pressure being wholly or partially compensated by the hydrostatic pressure exerted by an oil- filled vertical connecting pipe between the transformer tank and the vacuum chamber.