GB2355857A

GB2355857A - Fire risk elimination for flammable-liquid-filled transformers

Info

Publication number: GB2355857A
Application number: GB9924667A
Authority: GB
Inventors: Juergen Bastian
Original assignee: Individual
Current assignee: Individual
Priority date: 1999-10-20
Filing date: 1999-10-20
Publication date: 2001-05-02
Anticipated expiration: 2019-10-20
Also published as: GB9924667D0; GB2355857B

Abstract

A transformer arrangement comprises a first tank containing a transformer which is filled with an oil 1. The first tank is itself located in a second tank 15 which is then filled with an inert atmosphere. The said inert atmosphere comprises 95 % nitrogen, which is not used for cooling. The transformer is cooled using heat exchanger arrangements. A cascade of various heat exchanging formations, involving oil, water, alcohol and/or air and vacuum, evaporation or condensation conditions, may be used as remote cooling means. At least part of the heat exchanging system may be located inside or outside the confines of the inert gas filled tank 15. The inert gas surrounding the oil filled transformer prevents oxygen from entering the first tank when the said tank develops a leak, thus reducing the fire risk. The arrangement further facilitates the recuperation of heat lost from the transformer whilst stabilizing the temperature and pressure of the transformer allowing better fault detection.

Description

2355857 Fire risk elimination for flammable-liquid-filled transformers.

Electrical transformers and rectifiers - for the sake of brevity and simplicity we shall often refer to both as transformers - are necessary for the transport and distribution of electricity and for conversion from alternating to direct current.

Step-up transformers step up the voltage from the generator in order to reduce conductor mass and Ohm losses in overhead lines. Step-down transformers transform high-voltage to lower voltage levels, usually in more than one step. Both the conductors and the iron core generate heat, generally referred to as load losses and no-load losses, respectively.

The vast majority of transformers use mineral oil as the cooling and insulating medium. With very few exceptions the "oil" is cooled by ambient air. The International Electrical Committee (IEC) refers to all liquid transformer dielectrics as "oil", abbreviated "0". The different types of flow of the cooling media with reference to dielectric liquids and to air are referred to as natural (code letter N), forced (code letter F) or directed (code letter D).

Thus, the most widely used cooling media and the most widely used types of their movement are referred to as ONAN Qil Natural/Air Natural). Where natural movement of oil and air is replaced by forced circulation, the type of transformer is referred to as OFAF (Oil Forced/Air Forced). Where air is replaced by water, this is denoted by replacing the letter A by the letter W, e.g. OFWF.

IEC nomenclature is based on a perception of the transformer that discourages rearrangement of the functional components of the transformer. It does not even differentiate between liquids with different fire points. It does not differentiate between adjacent and remote installation of radiators, nor does it differentiate between single-stage or multiple-stage or cascade heat exchange. In this way IEC nomenclature discourages transformer manufacturers and users from considering alternatives to current practice.

State of the art Because of its low fire point of approximately 160'C, mineral oil escaping from a transformer ruptured by an internal high-energy arc, is easy to ignite. The difference between bulk liquid temperature and fire point is usually insufficient to cause self-extinction after combustion of decomposition gases.

Transformer fires are not uncommon. While their incidence is diminishing, mainly due to faster fault detection and better protection by current limiting fuses and circuit breakers, the losses incurred by transformer fires every year are enormous. Loss statistics often include estimates of costs totalling millions of dollars or millions of pounds, a recent example being the loss of a transformer manufacturing site in Britain, caused by a transformer fire. The problem is as old as transformers.

It was, therefore, felt necessary to investigate, and use in critical situations, non-flammable and non-combustible fluids. (We use the term fluids to include single-phase liquids, such as mineral oil and esters, condensable vapours such as fluorocarbons, and gases such as SF6.) This resulted in the use of non-flammable chlorinated hydrocarbons such as polychlorinated biphenyls/chlorinated benzenes, condensable halocarbons, and sulphur hexafluoride, I respectively. Most of these fluids have been banned or are ecologically undesirable. The problem has not been solved.

Description of the invention

The present invention is a radical departure from tradition which, without exception, has assumed that the transformer fire risk can only be eliminated by substituting a combustible by a non-flammable or noncombustible dielectric. By contrast, the present invention draws different conclusions ftom the fact that three elements have to concur for ignition to take place:- (We use the word concur to denote occurrence in the same place.) an ignition source (spark, arc, high-temperature contact); a flammable liquid, i.e. combustible insulation or its decomposition products; oxygen.

By eliminating oxygen from the space where the two other elements or factors could concur, any ignition of the would-be flammable liquid, such as transformer oil, is made impossible, and the fire risk can be completely eliminated.

However, eliminating oxygen from the transformer necessitates an inertgas atmosphere around the transformer. This is only practicable by placing the transformer tank in an inert-gas-filled exterior housing. By inert gas we understand a gas mixture of sufficiently low oxygen content to disable ignition of decomposition gases such as hydrogen, ethane, and other hydrocrbon compounds.

As the inert gas in the exterior housing can no longer fulfil the r6le of the ambient air, i.e. that of receiving the thermal energy transferred by the circulating oil, cooling has to be effected remotely. By remote cooling we mean that either the oil has to be conducted away from the tank and made to transfer the transformer's loss heat to another medium which can also act as a heat receptor or sink, or an intermediate cooling medium has to be conducted into the inert-gas-filled exterior housing and made to transfer the transformer's loss heat from "within" to "without" the exterior housing.

My invention permits a number of variants for the complementary measures made necessary-- by encapsulation of the transformer tank in an exterior housing filled with an inert atmosphere. These variants extend from the simplest, i.e. air cooling of oil in remotely installed heat exchangers, to different configurations using water either as an intermediate heat transfer medium or as a heat recipient. When the intermediate cooling medium is used as a so-called vapour-phase heat transfer medium, the lower alcohols or blends of water with alcohols may be considered if milder vacuum conditions are desirable.

The variants may either use only the excellent heat transfer properties of water as a liquid, or they may put to use the high heat of vaporization of water for heat transfer and medium temperature control purposes.

As the actual realisation of the different variants relies on engineering practice which is state-ofthe-art, while the discovery of the complementarity of the two elements, i.e. encapsulation of the transformer tank and remote cascade oil cooling, is the innovative step in my invention, only the 2 more complex variants will be further explained in greater detail and shown in two illustrations (Fig. 2 and Fig. 3).

A variant of the configuration using the principle of evaporation/recondensation would be the use of one of the lower alcohols, alone or mixed with water, in an oil/alcohol heat exchanger, which would allow a lower evaporation temperature.

Owing to the excellent heat transfer properties of water, transfer can best be achieved by using an oil/water heat exchanger. Depending on site and installation possibilities, loss heat recovery can be economical and worthwhile. While in most situations one will chose a type of heat exchange in which the cooling water is not made to vaporize, a vaporization heat exchanger offers the possibility of simple control of oil temperature:

By the simple means of causing the water to evaporate at a determined pressure maintained by the action of a vacuum pump whose force can be controlled by the transformer's load at any given time, the cooling water will remain at a predetermined temperature.

In a transformer in which the oil is kept at a near-constant temperature and thus near-constant pressure, early fault detection by overpressure sensor is very effective.

Exerting a negative pressure on the cooling water has other benefits: As the water in the cooling circuit can be kept at a pressure well below that in the oil circuit and in the transformer tank, the risk of contamination of the dielectric liquid by cooling water can be eliminated. This arrangement would overcome the widely held prejudice against water as a transformer cooling medium.

In spite of the very poor heat transfer properties of air as a cooling medium, air has remained by far the most widely used medium to cool transformer oil.

As my invention not only furnishes valid arguments to overcome the oil contamination prejudice, but also eliminates the fire risk while facilitating loss heat recuperation and fault detection, it will be seen that the sum of the advantages by far outweighs additional costs that may arise in particular installations.

A simple realisation of this principle, shown in Fig. 2, would be a configuration in which the cooling water circuit includes a vacuum chamber at its highest point. A more complex realisation of the same principle would entail a configuration in which the water is made to evaporate and re-condense, the heat of vaporization /condensation serving to cool the oil and heat a t1iird medium, respectively. This configuration is illustrated in Fig. 3.

All other configuration variants to which the different claims refer are less complex. The variants are simply adaptations to particular situations but all follow the same principle by which the fire hazard is eliminated.

Explanation of the drawings Fig. I shows schematically the principle of the installation of a flammable-liquid-filled electrical transformer installed in a housing filled with an inert atmosphere, and cooling of the dielectric liquid.

For the sake of clarity, Fig. I shows the remote cooling variant in which the secondary cooling medium, i.e. domestic hot water storage tank acting as a heat "sink", is placed outside the 3 confines of the inert-gas-filled exterior housing around the transformer. It would, of course, be possible to effect the oil/water heat exchange inside the inert-gas-filled housing. This would necessitate a slightly different configuration, such as replenishing of the secondary cooling medium or circulation of a tertiary cooling medium which would give off the heat outside the housing. By contrast, if the secondary medium were ambient (outside) air, the oil circuit would have to penetrate the confines of the exterior housing.

Being an abstract schematic representation, Fig. I does not have numbers. The numbering in Fig. 2/Fig. 3A/Fig. 3B is consistent: identical numbers indicate identical or functionally analogous parts.

Fig. 2 shows part of a transformer tank and the secondary cooling circuit using water, in the liquid state only, as a secondary heat transfer medium with water under vacuum, transferring heat to a heat "sink" such as a domestic hot water tank. A mild vacuum serves to ensure lower pressure in the secondary, or water, cooling circuit to eliminate any risk of contamination of the oil.

The flammable transformer dielectric liquid I (also referred to as "oil") is heated as it passes upwards through and along the windings 2, as indicated schematically, and is cooled as it passes down and along a metal dividing plate 3 serving as primary heat transfer surface between the transformer and a cooling mantle or "belt" 4 through which a secondary heat transfer medium such as water 5 flows in an upward direction. The dotted line 15 indicates the inert housing through which pass pipes in which the secondary heat transfer medium such as water 5 circulates, transferring the transformer loss heat from the transformer tank to a recipient 14 containing either a buffer medium or a medium which is continually withdrawn 11 and replenished 12.

Oil thermosiphoning inside the transformer tank is encouraged by a deflecting barrier 6 and by the density and temperature differences on both sides of the deflector 6. (Details such as a deflector plate are not essential and are meant only as illustrations of the variability of a particular configuration.) Hot water is continuously flowing from the top of the cooling belt to a heat recipient 14, where it gives off part of its heat as it heats up air (variant not shown in fig.2) or domestic hot water or any heat "sink", and flows back to the bottom of the cooling mantle or "belt". The vacuum chamber 7 has no other function than to ensure that the water circuit will always be at a lower pressure than the oil in the transformer, thus preventing any leakage of water into the oil. Ambient (outside) air would be ftmctionally equivalent to a domestic hot water storage tank, as would be any tertiary medium heated by the secondary medium 5. Pipe connections 11 and 12 denote flow and return or withdrawal 11 and replenishment 12 of the tertiary cooling medium (refers to claim 3).

Fig. 3:

Part A of Fig. 3 shows part of a transformer tank and the evaporator part of a cooling arrangement in which water is used as a bi-phase heat carrier.

Part B of Fig. 3 shows the condenser and its connections - (Condenser B is downstream of A and therefore called B and not A. For obvious physical reasons (gravity), B has to be above A: and, the cooling mantle and the condenser section being communicating vessels under the same vacuum conditions, the condensate surface and the vaporization interface must be at the same level.) 4 The flammable transformer dielectric liquid 1 (also referred to as "oil") is heated as it passes upwards through and along the windings 2, as indicated schematically, and is cooled as it passes down and along a metal dividing plate 3 serving as primary heat transfer surface between the transformer and a cooling mantle or "belt" 4 through which a secondary heat transfer medium such as water, alternatively one of the lower alcohols, 5 flows in an upward direction. The dotted line 15 indicates an inert-gas-filled housing through which pass the ducting from the vaporization zone of the cooling belt of the transformer (Fig. 3A) to the top of the condensor (Fig. 3B), and also the condensate pipe connection from the condenser (Fig. 3B) to the bottom of the cooling belt (Fig. 3A).

Due to the fact that the condenser part of the arrangement is under vacuum 7, the cooling liquid (water or one of the lower alcohols) boils at a much lower temperature than under normal atmospheric conditions. The vacuum pressure, which is meant to be much lower than that in the arrangement shown in Fig. 2, determines the boiling zone of the liquid.

The heat exchange liquid boiling at the top of the cooling belt generates vapour which is sucked into the condenser B through ducting 8 and condenses along condenser tubes 9, and is collected as condensed water 13 at the bottom of the condenser and gravity-fed via a pipe 10 to the bottom of the cooling belt 4 (Fig 3 A).

The vapour generated as shown in Fig. 3 A is sucked in by the vacuum in B which is maintained by continuous condensation. Condensation along the inside of the condensing tubes or "flutes" 9 is effected by cold water 12 coming from a cold medium such as the bottom of a cold water storage tank. Hot water from the condenser top passes via pipe 11 to the medium that is to be heated. A typical example would be the top of a domestic hot water storage tank. Alternatively, heated water could be continuously withdrawn at 11 and replenished at 12 with cold water.

The arrangement described can be used for the heating of any medium in which thermal energy is continuously drawn off, the condition for heat recovery feasibility being useful temperature levels for cooling water, and thermal energy demand being met, at least in part, by the heated medium (refers to claim 4).

Claims

Claims

I. Systemic configuration for the installation of a flammable-liquidfilled electrical transformer having a tank solely dedicated to containing the active part and the dielectric liquid, said tank placed in a housing filled with an inert atmosphere containing at least 95% nitrogen, said inert atmosphere not serving a cooling function, the cooling of the liquid being effected by an oil/water heat exchanger placed inside or outside the inert-gas-filled housing, the water being continuously replenished upstream of the heat exchanger and allowed to run off downstream of the heat exchanger, the water serving as secondary heat transfer liquid and heat recipient.
2. Systemic configuration for the installation of a flammable-liquidfilled electrical transformer having a tank solely dedicated to containing the active part and the dielectric liquid, said tank placed in a housing filled with an inert atmosphere containing at least 95% nitrogen, said inert atmosphere not serving a cooling function, the cooling of the liquid being effected by an oil/air heat exchanger placed outside the inertgas-filled housing, the ambient air serving as a secondary heat transfer medium and final heat recipient.
3. Systemic configuration for the installation of a flammable-liquidfilled electrical transformer having a tank solely dedicated to containing the active part and the dielectric liquid, said tank placed in a housing filled with an inert atmosphere containing at least 95% nitrogen, said inert atmosphere not serving a cooling function, the cooling of the liquid being effected by an oil/water heat exchanger being placed inside or outside the inert-gas-filled housing, the water acting as a secondary heat transfer medium operating at a pressure below the pressure in the oil circuit, said lower pressure being induced by the incorporation of a vacuum chamber at the highest point of the secondary, i.e. water, cooling circuit.
4. Systemic configuration for the installation of a flammable-liquidfilled electrical transformer having a tank solely dedicated to containing the active part and the dielectric liquid, said tank placed in a housing filled with an inert atmosphere containing at least 95% nitrogen, said inert atmosphere not serving a cooling function, the cooling of the liquid being effected by the oil/water heat exchanger being placed inside or outside the inert-gas-filled housing, the water acting as a secondary heat transfer medium operating at a pressure below the pressure in the oil circuit, said lower pressure being induced by the evaporation and re-condensation of water at a pressure well below the boiling point at atmospheric pressure, the heat of condensation serving to heat a tertiary heat transfer medium such as domestic hot water.
5. Systemic configuration for the installation of a flammable-liquidfilled electrical transformer having a tank solely dedicated to containing the active part and the dielectric liquid, said tank placed in a housing filled with an inert atmosphere containing at least 95% nitrogen, said inert atmosphere not serving a cooling function, the cooling of the liquid being effected by an oil/water heat exchanger placed inside or outside the inert-gas-filled housing, in which water is continuously replenished upstream of the heat exchanger and evaporated downstream of the heat exchanger by the action of a vacuum pump.
6. Systemic configuration for the installation of a flammable-liquidfilled electrical transformer having a tank solely dedicated to containing the active part and the dielectric liquid, said 6 tank placed in a housing filled with an inert atmosphere containing at least 95% nitrogen, said inert atmosphere not serving a cooling ftmction, the cooling of the liquid being effected by an oil/alcohol heat exchanger placed inside or outside the inert-gas-filled housing, a lower alcohol such as ethanol or a solution of ethanol in water, acting as a secondary heat transfer medium operating at a pressure below the pressure in the oil circuit, said lower pressure being induced by the evaporation and recondensation of the alcohol at a pressure below the boiling point at atmospheric pressure, the heat of condensation serving to heat a tertiary heat transfer medium such as domestic hot water.

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