GB2102211A - Electrical transformer or reactor - Google Patents

Abstract

An electrical transformer or reactor, of the kind having one or more windings in an enclosure containing gas, is associated with a gas dryer (5) in which the gas is circulated upwards through a duct (10) continuously cooled by thermo- electric elements (11) at a rate such that the gas is progressively cooled to below its dewpoint but without freezing of the condensed moisture. By this means the accumulation of frost and ice which necessitates the periodic heating of the duct is avoided. <IMAGE>

Description

SPECIFICATION Electrical transformers and reactors This invention relates to electrical transformers and reactors (hereinafter referred to collectively as transformers for simplicity) which are of the kind having their electrical windings disposed within an enclosure containing gas, and has particular application to such transformers having their windings immersed in a dielectric liquid such as oil with a gas space above the liquid level.It is common for this space to be provided in what is known as a conservator and which is in the form of an auxiliary enclosure mounted above the main enclosure containing the winding or windings, and communicating with the main enclosure through a pipe or duct to allow movement of dielectric liquid between the enclosures which is necessary because of contraction and expansion of the liquid as a result of temperature changes caused by variations in electrical power of the transformer.

It is desirable to maintain the gas, usually air, above the dielectric liquid in a dry condition, and one way of achieving this has been to circulate it through a dryer comprising a duct which is cooled by a plurality of thermo-electric devices. Water from the air is deposited in the form of frost and ice on the wall of the duct so that progressive drying of the air above the liquid therefore occurs.

Water which may be in solution in the liquid is also transferred to the air and removed by the dryer. However because of the accumulation of frost and ice the duct has to be periodically heated, for example by reversal of the current through the thermo-electric devices, so as to melt the frost and ice and allow the water so formed to drain away through an outlet valve which can conveniently be operated electrically.

Such cyclic cooling and heating of the air is not, however, always convenient, and moreover the cooling of the air has been used to provide the energy for circulating the air through the system, but this is expensive.

An object of the invention is to provide a form of air dryer which avoids these disadvantages.

However the invention is not restricted to use with transformers incorporating a conservator, but may also find application with other transformers of the kind having windings within an enclosure containing gas as will be apparent from the following description.

According to the invention an electrical transformer having one or more windings disposed within an enclosure containing gas includes a closed circuit gas path comprising, in series, the enclosure and a gas dryer, the latter incorporating a cooling duct disposed at an appreciable angle to the horizontal, a plurality of thermo-electric elements disposed in thermal contact with the duct for cooling the duct by the Peltier effect when energised from an electrical supply, and gas circulating means for producing a flow of gas around said gas path in the direction which causes it to travel in an upward direction through said duct and at a rate such that the gas Is progressively cooled to below its dewpoint, but without freezing any moisture deposited on the walls of the duct.

The means for producing the circulation of the gas preferably comprises an electric heater disposed so as to heat the gas path above the cooling duct to a temperature sufficient to cause an upward flow of the gas through the cooling duct. The heater may, for example, take the form of resistance heating mat.

Preferably the thermo-electric elements are disposed so as to give a substantially uniform cooling of the gas as it rises in the cooling duct.

Then when the gas reaches its dewpoint, condensation will take place and water will run down the cooling duct by gravity to an exit at the bottom of the dryer. The exit may be through a trap, such as a U-bend, which prevents the passage of gas therethrough. It is essential that the exit should not become blocked due to freezing of water in the trap at low ambient temperatures, and where necessary heating means can be provided for heating the exit to prevent such freezing. Where the gas is air, as will commonly be the case, and the transformer winding or windings are immersed in a dielectric liquid a restricted opening may be provided in the gas path, preferably on the entry side of the dryer, to allow the transformer to "breath" as the liquid expands and contracts due to changes in the electrical load of the transformer.The breather opening should, of course, be of sufficiently small dimensions to provide a relatively high impedance to air flow, compared with the gas flow path, such that it does not interfere to any significant extent with the normal air circulation.

An alternative form of trap which incorporates a breather opening is in the form of an inverted funnel with the opening at its apex. In use water will collect around the funnel, and when it reaches the top positive surface tension will aid gravity in pulling it down away from the opening where, even if it does freeze, it will not block the opening. The need for associated heating means is therefore avoided.

Means may be provided for controlling the gasflow rate to maintain an approximately constant temperature of the gas leaving the cooling duct.

This can be achieved where an electric heater produces the circulation of the gas, by the use of an appropriately positioned temperature sensor controlling the operation of the heater, in a manner which tends to maintain a substantially constant gas-flow rate under varying ambients and transformer loading conditions.

The cooling duct is preferably vertical, and in the case of a transformer having a conservator the lower end of the duct communicates via a first connection to one region of the air space in the conservator, and the upper end of the duct communicates via a second connection to another region of the gas space, and means are provided for heating the air within said second connection to produce the required air circulation in the upward direction through the cooling duct.

One transformer in accordance with the invention and a modification thereof will now be described by way of example with reference to Figures 1 and 2 of the accompanying schematic drawing.

The transformer comprises a tank 1 in which are disposed the transformer windings (not shown) immersed in insulating oil.

An oil conservator 2 is mounted above the tank 1 and is partially filled with an insulating oil, and a pipe 3 connected between the bottom of the conservator and the top of the tank allows movement of oil between the tank and conservator caused by expansion and contraction of the insulating oil as a result of temperature changes brought about by variations in the electrical load of the transformer. Above the oil in the conservator 2 there is an air space 4.

In accordance with the invention there is located adjacent the tank 1 a device 5 for drying the air in the space 4, and connected to the space by two pipes 6, 7. The first pipe 6 connects one side of the space to an inlet 8 at the lower end of the dryer 5, and the second pipe 7 connects the opposite side of the space to an outlet 9 at the top of the dryer 5.

In the lower part of the dryer there is disposed a vertical duct 10 surrounding along its length by a plurality of thermo-electric cooling elements 11 such as frigistors in thermal contact with its outer surface, and also with the outer wall 12 of the device, which is provided externally with cooling fins 13. Below the cooling elements the buttom end of the duct 10 constitutes the inlet 8 which is connected to the pipe 6, and the top end of the duct communicates with a chamber 14 formed of electrically and thermally insulating material and having around its bore a resistance heating mat 1 5. The top of the chamber has an opening, which constitutes the outlet 9 which is connected by the pipe 7 to the conservator space 4.

In use the thermo-electric cooling elements 11 and the heating mat 1 5 are connected to a suitable source of electrical power, derived for example from a winding on the transformer.

Heating of the air in the chamber 1 4 causes it to circulate upwards through the pipe 7 and into the conservator space 4; air is accordingly drawn upwards through the duct 10 to be replaced by air displaced from the conservator. On its passage upwards through the duct 10, the air is gradually cooled until it reaches its dewpoint. Condensation then takes place on the bore of the duct and the water runs down the duct to a drain outlet 1 6 which is conveniently in the form of a U-bend, as shown, to form a seal which prevents large quantities of external air being drawn into the system. However a restricted opening 1 7 is conveniently provided at the lower end of the duct to allow the transformer to "breathe" as the oil expands and contracts as a result of changes in the transformer loading.

It will be seen that the dewpoint of the air falls as the air rises in the duct 10, and the temperatures and air flow are arranged to be such that no freezing of the water takes place, which could lead to restriction of the air-flow, and in extreme cases to blockage of the system.

Moreover the removal of water in liquid form with the elimination of ice formation, avoids the requirements for the latent heat of freezing, so that the efficiency of operation of the equipment is improved compared with existing intermittently operated devices. In addition, since the dryer works on a continuous basis with no cycling, the components can be arranged to operate under substantially constant conditions.

Thus the rate of air circulation can be maintained substantially constant under varying ambients and transformer loading conditions by the provision, in parallel with the heater mat, of a positive temperature coefficient resistor responsive to the temperature of air in the pipe 7 such that a rise in temperature in the pipe, resulting from a slower air circulation, will result in an increased heater current, thereby producing a compensating increase in the airflow rate.

Alternatively a negative temperature coefficient resistor may be connected in series with the heater mat.

In some cases a temperature sensor may be arranged to monitor the temperature of the air entering the chamber 14 through the tube 10, and arranged to control the heating circuit to produce an increase in heater current, and thereby an increase in airflow if the temperature of air leaving the pipe falls below a predetermined minimum value.

In equipment used in places subject to low ambient temperatures an additional, similarly controlled, heater may be provided at the lower end of the duct 10 to heat the incoming air and prevent the frosting up of the duct in the event of the temperature falling to OOC or below.

The additional heater may be controlled by the same temperature sensor that controls the main heater 15, the heating current being similarly increased when the temperature leaving the duct 10 falls below said predetermined value.

Alternatively the additional heater may be brought into operation when the ambient temperature drops to the vicinity of OOC, and can in such a case be controlled by a separate suitably located temperature sensor.

Any heating of the air by the additional heater, whilst being adequate to prevent frosting up of the cooling duct in the event of low ambient temperatures, should not, of course, be such as to affect the satisfactory drying operation of the dryer.

Water in the drain outlet 16 must also, of course, be prevented from freezing, whatever the ambient temperature, as this would block the passage of condensate from the dryer.

Accordingly means can be provided for heating the drain outlet, in the event of the ambient temperature falling to freezing point.

An alternative form of drain outlet is illustrated in Figure 2. This comprises, at the lower end of the duct 10, below the inlet 8, a nylon cone trap 19 in the form of an inverted funnel having a narrow opening 20 at the apex. When water 21 in the trap overflows the top of the funnel positive surface tension aids gravity in pulling the water down, away from the opening where, even if it does then freeze, it will not block the opening 20. The narrow opening 20, which can replace the opening 1 7 of the dryer shown in Figure 1, gives a high impedance to air flow compared with the inlet 8 so that the system is still essentially a closed recirculating one, although pressure equalisation with the atmosphere is aliowed.

If desired a solar umbrella, as at 18, may be disposed over the dryer to avoid violent temperature fluctuations due to solar radiation which might in some cases affect the operation of some of the components.

Claims (14)

1. An electrical transformer having one or more windings disposed within an enclosure containing gas including a closed circuit gas path comprising, in series, the enclosure and a gas dryer, the iatter incorporating a cooling duct disposed at an appreciable angle to the horizontal, a plurality of thermo-electric elements disposed in thermal contact with the duct for cooling the duct by the Peltier effect when enerqised from an electrical supply, and gas circulating means for producing a flow of gas around said gas path in the direction which causes it to travel in an upward direction through said duct and at a rate such that the gas is progressively cooled to below its dewpoint, but without freezing any moisture deposited on the walls of the duct.

2. A transformer according to Claim 1 wherein the means for producing the circulation of the gas comprises an electric heater disposed so as to heat the gas path above the cooling duct to a temperature sufficient to cause an upward flow of the gas through the cooling duct.

3. A transformer according to Claim 2 wherein the heater comprises a resistance heating mat.

4. A transformer according to Claim 1, 2 or 3 wherein the thermo-electric elements are disposed so as to give a substantially uniform cooling of the gas as it rises in the cooling duct.

5. A transformer according to any preceding Claim having an outlet for condensed water at the bottom of the duct.

6. A transformer according to Claim 5 wherein the outlet is in the form of a trap which prevents the passage of gas therethrough.

7. A transformer according to Claim 6 having the transformer winding or windings immersed in a dielectric liquid, wherein the wall of a part of the gas path has a restricted opening which permits the passage of air therethrough as the liquid expands and contracts due to changes in the electrical load of the transformer.

8. A transformer according to Claim 5 having the transformer winding or windings immersed in a dielectric liquid wherein the water outlet comprises a trap in the form of an inverted funnel having a restricted opening at its apex, the opening permits the passage of air therethrough ^ as the liquid expands and contracts due to changes in the electrical load of the transformer.

9. A transformer according to any preceding Claim including means for controlling the gas-flow rate to maintain an approximately constant temperature of the gas leaving the cooling duct.

1 0. A transformer according to Claim 9 in which an electric heater produces the circulation of the gas, in which there is provided a temperature sensor located at or adjacent the outlet end of the cooling duct and arranged to control the operation of the heater in a manner which tends to maintain a substantially constant gas-flow rate under varying ambients and transformer loading conditions.

11. A transformer according to any preceding Claim including a conservator wherein the duct is approximately vertical, and the lower end of the duct communicates via a first connection to one region of the air space in the conservator, and the upper end of the duct communicates via a second connection to another region of the gas space, and means are provided for heating the air within said second connection to produce the required air circulation in the upward direction through the cooling duct.

12. A transformer incorporating a closed circuit gas path substantially as shown in and as hereinbefore described with reference to Figure 1 of the accompanying drawing, or the modification thereof substantially as shown in or as hereinbefore described with reference to Figure 2 of the accompanying drawing.

13. A transformer according to any preceding Claim incorporating an electric heater adjacent the inlet end of the cooling duct, the heater being controlled by a temperature sensor operative to energise the heater or to increase the heating current when the sensed temperature falls below a predetermined value.

14. A transformer according to Claim 13 wherein the temperature sensor is operative to energise the heater when the ambient temperature is in the vicinity of OOC or below.