GB2161989A

GB2161989A - Protection of electric power equipment

Info

Publication number: GB2161989A
Application number: GB08517608A
Authority: GB
Inventors: Donald Francis Binns; Geoffrey Herbert Cooke
Original assignee: Individual
Current assignee: Individual
Priority date: 1984-07-12
Filing date: 1985-07-12
Publication date: 1986-01-22
Also published as: GB8417762D0; GB8517608D0; GB2161989B

Abstract

A power transformer contained within a sealed tank (3) having corrugated sections (4) utilises a vaporisable fire-resistant liquid for insulation and cooling. The tank (3) and corrugated sections (4) are evacuated prior to filling with the liquid in a degassed and dehydrated state at a suitable elevated temperature below its boiling point. The corrugated sections (4) are expansible to allow for expansion of the liquid volume and for the production of vapour during operation of the transformer. When electrical connections (20, 21,) are made from the transformer into gas-filled metal-clad equipment accumulation of vapour and/or gas near the connections may be avoided by making the connections in the side or bottom of the tank. Connections at the top of the tank may be made through a chamber in which any accumulation of vapour is not disadvantageous and from which gas may be vented. <IMAGE>

Description

SPECI FICATION Protection of electric power equipment This invention relates to the protection of electric power equipment and is particularly concerned with the protection against overheating of power transformers by immersion in an electrically insulating liquid which is non-flammable or of low fammability.

It is known to cool the core and windings of a power transformer by immersion in an electrically insulating, fireresistant mineral oil. The oil is contained within a sealed tank which houses the transformer and, in order to accommodate thermal expansion of the oil and any trapped air, the housing is provided with expansible, corrugated sections.

With this known arrangement, the cooling effect is derived from convection currents within the body of liquid and is of a limited nature. Better cooling is possible with a more volatile liquid which is capable of vaporising at the temperatures produced within the transformer when operating under load. However, there are then the problems of accommodating the relatively large change in volume and of avoiding undue accumulation of evolved gases which can adversely affect cooling and/or insulation of the transformer.

An object of the present invention is to overcome or at least appreciably reduce these problems and thereby enable vaporisable liquids to be used with advantage in the cooling of electric power equipment.

According to the invention therefore there is provided a method of protecting electric power equipment housed within an enclosure which incorporates an expansible and contractable chamber, wherein said enclosure including said chamber is evacuated before being filled with a vaporisable electrically insulating liquid which is non-flammable or of low flammability, and said enclosure is then sealed, the liquid used for filling said enclosure being wholly or partially degassed and dehydrated, and the filling and sealing being effected at a temperature above ambient whereby after cooling to ambient the liquid and the said chamber undergo contraction.

With this procedure it will be appreciated that expansion and vaporisation of the liquid, which occurs as a consequence of heat generated during use of the equipment, can be readily accommodated by expansion of the contracted chamber and without requiring any specially provided additional expansion volume. Moreover, due to the use of a wholly or partially degassed and dehydrated liquid it is possible to achieve vaporisation of the liquid without undue accumulation of gas capable of adversely affecting. cooling, electrical insulation and eventual condensation of the evolved vapour.

Most preferably the enclosure is evacuated before being wholly or substantially wholly filled with the vaporisable liquid i.e. so that parts of the electrical equipment to be protected are completely submerged and the sealed enclosure contains only (or substantially only) the liquid and any vapour derived therefrom. Preferably also the liquid is wholly or substantially wholly degassed and dehydrated.

If desired, provision may be made for venting the sealed enclosure at an appropriate position so that any accumulation of unwanted gas which in fact occurs can be removed.

With regard to the vaporisable liquid fluorocarbons may be particularly suitable and a typical example is the substance sold under the Trade Mark FLUORINERT FC75 which has the generalformula C8F,60 and which is a colourless liquid having a boiling point of about 101 "C at normal atmospheric pressure.

It is however to be understood that the invention is not restricted to the use of this fluorocarbon and any suitable liquid or combination of liquids may be used.

The vaporisable liquid may be appropriarely treated prior to filling of the enclosure to achieve the said degassed and dehydrated state thereof, in so far as this is necessary.

Such treatment may involve distillation of the liquid.

Filling of the enclosure may be effected at any suitable temperature which is preferably a constant temperature appreciably above ambient but below the boiling point of the liquid.

Preferably the equipment,enclosure and liquid are heated to the required temperature prior to introduction of the liquid into the previously evacuated enclosure although other procedures are possible. The expansion of the liquid into the evacuated tank will cause cooling and the rate of cooling may have to be controlled to prevent unacceptably low temperatures occuring inside the tank.

The arrangement is preferably such that adequate cooling of the electrical equipment can be achieved under expected conditions of use (including overload as well as normal load conditions) without requiring vaporisation of more than a minor proportion of the liquid. In this respect it will be understood that the nonvaporised body of liquid provides a cooling effect due to convection currents within same.

Thus, the sealed enclosure may contain up to a maximum of 10% vapour at any time, and the internal pressure may be no greater than say 1.5 bar, or preferably 1.3 bar. Other ranges can however be used and, for example, pressures up to say 5 bars may be possible if the tank and other pressurised parts are so designed.

The pressure within the enclosure at ambient temperature is preferably at or close to ambient pressure.

The said enclosure may simply comprise a tank containing the electrical equipment. In this case the said expansible and contractable chamber may be defined by corrugated or convoluted sections applied to any one or more surfaces of the tank. Condensation of evolved vapour may occur by contact with surfaces of the tank and/or the remaining liquid and/or such corrugated or convoluted sections.

Alternatively, the enclosure may comprise a tank containing the electrical equipment in conjunction with a separate heat exchanger of any suitable form connected to the tank by pipework whereby evolved vapour flows from the tank to the heat exchanger to be condensed therein and returned as liquid to the tank. In this case the said expansible and contractable chamber may be defined by corrugated or convoluted sections which may be applied to any one or more surfaces of the tank and/or the heat exchanger.

Other arrangements are also possible and it will be appreciated that the said enclosure may comprise any suitable kind of tank containing the electrical equipment alone or in conjunction with any suitable additional vessels or fluid systems and incorporating as required any suitable kind of heat exchanger.

Moreover, the or each expansible and contractable chamber may take any suitable form and may be arranged in any suitable manner as appropriate.

The electrical power equipment to which the method of the invention is applied preferably comprises an electric power transformer.

With electrical power equipment, such as power transformers, operating at high voltages, it is desirable to avoid or minimise exposure of electrical connections to air and other gases which have poor insulation properties.

According to a second aspect of the present invention therefore there is provided electric power equipment which is housed within a tank filled with vaporisable electrically insulating liquid which is non-flammable or of low flammability, and has at least one electrical connection extending along at least one passage through at least one wall of the tank, the or each said passage being arranged so that vapour produced from said liquid in the tank tends to rise above said connection.

With this arrangement it can be more readily ensured that the connection is maintained immersed in or otherwise protected by said liquid without any undue accumulation therearound of vaporised liquid or gases released from such liquid.

The electric power equipment of the second aspect of the invention may embody any or all of the features of case is particularly suitable for use in conjunction with gas-filled metal clad switchgear (including gas-filled busbar assemblies), the transformer and switchgear (or busbar) being located close to each other and arranged so that the said connection or connections connect directly with the switchgear or busbar without requiring any intermediate connection or conductor which is exposed to the air. The or each said connection may comprise a cable or a conductor passing through a bushing along said passage which at one side is connected within the liquid-filled transformer tank and at the other side is connected within a gas-filled enclosure of the switchgear.The passage and/or the connection and mounting parts thereof may be flexible to allow for relative movements e.g. during pressure and temperature changes Various arrangements are possible to avoid accumulation of gas around the connection in the transformer tank. Thus, the passage may be arranged in a side wall of the tank horizontally or preferably inclined to the horizontal so that the transformer side of the passage is higher than the other side thereof (e.g. the switchgear side). Alternatively the passage may be arranged at the top and an upper gas accumulation chamber or venting arrangement or the like may be provided. A further possibility is to arrange the passage at the bottom of the transformer tank, this being particularly convenient in the case where the transformer is to be connected with an underground gas-filled busbar system.

The electric power equipment of the second aspect of the invention may embody any or all of the features of equipment made in accordance with the first aspect of the invention as appropriate.

The invention will now be described further by way of example only and with reference to the accompanying drawings in which: Figs. 1 and 2 are respectively elevation and plan views of one form of a transformer filled with vaporisable liquid in accordance with the method of the first aspect of the invention; Figs. 3 and 4 are views similar to Figs. 1 and 2 of an alternative form of transformer; Fig.5 is a plan view of a transformer filled with vaporisable liquid and provided with electrical connections in accordance with one embodiment of the second aspect of the invention; and Figs.6 and 7 are elevations of transformers with alternative connection arrangements.

Figures 1 and 2 show an electric power transformer having a core 1 and windings 2 housed within a metal tank 3. One side wall of the tank has applied thereto outwardly directed corrugated sections 4. The corrugated sections 4 are formed from metal and have limited flexibility. The sections 4 need not be arranged as shown but may be applied to any one or more or all sides of the tank and/or on the roof of the tank 3 as desired.

The transformer may be single-phase or polyphase.

With the core 1 and windings 2 located within the tank 3, during the concluding stages of manufacture, the enclosure defined by the tank 3 and the communicating corrugated sections 4, is filled under vacuum with a non-flammable vaporisable electrically insulating liquid which is partially or wholly degassed and dehydrated. The liquid may be a fluorocarbon, particularly the fluorocarbon sold by 3M Manufacturing Plc under the Trade Mark FLUORINERT which has a boiling point of 101 'Cat normal atmospheric pressure and this may be degassed and dehydrated prior to filling of the enclosure by distillation. After evacuation the tank is filled with the liquid and sealed.Sealing is effected with the temperature of the whole enclosure and transformer raised above ambient (but below the boiling point of the liquid) and with the liquid 5 completely filling the enclosure including the corrugations 4 and all crevices and secondary volumes linked with the inside of the tank 3. The enclosure is then hermetically sealed.

The enclosure, transformer and liquid are allowed to cool to atmospheric temperature and there is a general contraction, particularly in the volume of the liquid 5. The consequent tendency for the pressure within the enclosure to fall below that of the atmosphere causes the corrugated sections 4 to flex or collapse and thereby reduce the volume contained thereby thus contributing to the overall reduction in the required total internal volume of the enclosure.

In use, when loaded, the transformer core 1 and windings 2 generate heat which causes a general thermal expansion, particularly of the liquid 5 but also of any vapour or gas that may be evolved. The consequent tendency for the internal pressure to rise above that of the atmosphere causes the corrugated sections 4 to flex or expand so as to increase the volume contained thereby thus compensating for the overall increase in the required total internal volume of the enclosure. Gas production contributing to the pressure rise is reduced due to the use of partially or wholly dehydrated and degassed liquid. At low load and immediately following the application of heavy load the heat from the core 1 and windings 2 is transferred by simple convection currents in the liquid 5 to the walls of the tank 3 and corrugated sections 4 thence to be dissipated to the atmosphere.

At heavy loads and under stabilised conditions, the temperature of the windings 2, and possibly also the core 1, may rise to the point at which nucleate boiling of the liquid 5 commences. Heat is now removed by a combination of the aforementioned convection currents and the boiling off process, the vapour produced by the latter either condensing in the liquid or condensing on the inside walls of the tank 3 and corrugated sections 4.

Condensation is assisted by the use of partially or wholly degassed and dehydrated liquid.

The corrugated sections 4 are designed such that the volume provided by their flexing meets the thermal expansion requirements and the volume requirement of the vapour.

Figures 3 and 4 show an electric power transformer having a core 11 and windings 1 2 housed within a metal tank 1 3 connected by pipework 1 6 to a separate heat exchanger 1 7. The latter carries corrugated sections 1 4 which may be arranged on one or more or on all sides of the exchanger including the top and the underside. The transformer may be single-phase or polyphase.

With the core 11 and windings 1 2 located within the tank 13, and either during the concluding stages of manufacture or during the concluding stages of site erection, the enclosure defined by the tank 13, the pipework 16, the heat exchanger 1 7 and the corrugated sections 14, is evacuated then filled with a non-flammable vaporisable electrically insulating liquid which is wholly or partially dehydrated and degassed as described above with reference to Figures 1 and 2.After filling sealing is effected with the temperature of the whole enclosure and transformer raised above ambient (but below the boiling point of the liquid) and the liquid 1 5 completely fills the previously evacuated whole internal volume of the enclosure including all crevices and secondary volumes linked with the inside of the tank 1 3. The enclosure is then hermetically sealed.

On cooling, the corrugations coilapse and subsequently can expand, in like manner to the corrugations of the embodiment of Figures 1 and 2. Evolved vapour condenses in the heat exchanger and is returned via the pipework 16 to the tank 13.

Figure 5 shows the tank of a transformer of the kind described with reference to Figures 1 and 2 or 3 and 4 with electrical connections 20, 21 (of two different kinds), passing through walls 22, 23 of the tank 24 so as to connect the transformer windings with adjacent gas-filled metal clad switchgear. The electrical connections may comprise cables 21, conductors extending through bushings 20 or the like and such connections pass through and are sealed relative to diaphragms or barriers between the transformer and the switchgear. As shown, such barriers are disposed within passages 25 along which the connections extend and these passages 25 are so inclined to the horizontal that gas and/or vapour within the tank 24 will not enter the passages 25 or accumulate in or adjacent to the connections 20, 21 where this could result in reduced electrical insulation strength.

With this arrangement it will be appreciated that the connections are insulated on one side by the liquid in the tank 24 and on the other side by the gas filling in the metal clad switchgear. The connections are not exposed to the atmosphere at any stage.

Figure 6 shows an arrangement in which connections 31 pass in passages 33 through the base of the transformer tank 32. With this arrangement it will be appreciated that there is little or no tendency for gas or vapour to accumulate alongside the connections 31 (since evolved gas or vapour rises). Conveniently the connections may connect with gasfilled metal clad switchgear busbars in conduits beneath the transformer (e.g. underground).

Figure 7 shows an arrangement in which connections 41 pass along passages through the top of the transformer tank 42 but incorporate a chamber or chambers 43 above such connections in which gas and/or vapour may safely accumulate or from which gas and/or vapour may be removed.

The invention is not intended to be restricted to the details of the above embodiment which are described by way of example only.

Claims

1. A method of protecting electric power equipment housed within an enclosure which incorporates an expansible and contractable chamber, wherein said enclosure including said chamber is evacuated before being filled with a vaporisable electrically insulating liquid which is non-flammable or of low flammability, and said enclosure is then sealed, the liquid used for fillingsaid enclosure being wholly or partially degassed and dehydrated, and the sealing being effected at a temperature above ambient whereby after cooling to ambient the liquid and the said chamber undergo contraction.

2. A method according to claim 1, wherein the enclosure is wholly or substantially wholly filled with the vaporisable liquid.

3. A method according to claim 1 or 2, wherein the enclosure is wholly or substantially wholly degassed and dehydrated.

4. A method according to any one of claims 1 to 3, wherein provision is made for venting the sealed enclosure at an appropriate position so that any accumulation of unwanted gas can be removed.

5. A method according to any one of claims 1 to 4, wherein the vaporisable liquid comprises a fluorocarbon.

6. A method according to any one of claims 1 to 4, wherein the vaporisable liquid is distilled prior to filling of the enclosure in order to achieve the said degassed and dehydrated state thereof.

7. A method according to any one of claims 1 to 6, wherein the enclosure comprises a tank and the said expansible and contractable chamber is defined by corrugated or convoluted sections applied to any one or more surfaces of the tank.

8. A method according to any one of claims 1 to 6, wherein the enclosure comprises a tank and there is provided a separate heat exchanger connected to the tank by pipework so that evolved vapour flows from the tank to the heat exchanger to be condensed therein and returned as liquid to the tank, said expansible and contractable chamber being defined by corrugated or convoluted sections applied to any one or more surfaces of the tank and/or the heat exchanger.

9. Electric power equipment when protected in accordance with the method of any one of claims 1 to 8.

10. Protected electric power equipment according to claim 9, wherein said power equipment comprises a power transformer.

11. Electric power equipment which is housed within a tank filled with vaporisable electrically insulating liquid which is non-flammable or of low flammability, and has at least one electrical connection extending along at least one passage through at least one wall of the tank, the or each said passage being arranged so that vapour produced from said liquid in the tank tends to rise above said connection.

1 2. Equipment according to claim 11, which comprises an electric power transformer used in conjunction with gas-filled metal clad switchgear and wherein the or each said connection comprises a cable or a conductor passing through a bushing along said passage which at one side is connected within the liquid-filled transformer tank and at the other side is connected within a gas-filled enclosure of the switchgear.

1 3. Equipment according to claim 12, wherein the passage is arranged in a side wall of the tank so that the transformer side of the passage is higher than the other side thereof.

1 4. Equipment according to claim 12, wherein the passage is arranged at the top of the tank and an upper gas accumulation chamber or venting arrangement is provided.

1 5. Equipment according to claim 12, wherein the passage is at the bottom of the tank.

1 6. Equipment according to any one of claims 11 to 15, which comprises equipment according to claim 9.

17. A method according to claim 1, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

1 8. Equipment according to claim 9 or claim 11, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.