Device for purifying liquid in a liquid reservoir and a transformer provided with such a device

A device for purifying liquid in a reservoir and including a vacuum chamber (1) having an inflow opening (2) and an outflow opening (3) adapted to communicate with the liquid in the reservoir through a feed conduit (4) and a return conduit (5), respectively. The return conduit (5) is provided with a pump (6) and the vacuum chamber is connected with a vacuum pump (7). The device further includes a filter cartridge (8) provided with a plurality of filter elements, each having a flow side communicating with a central passage (26) and a flow side opening into the periphery of the filter cartridge (8). The filter cartridge (8) is arranged inside the vacuum chamber, the central passage (26) of the filter cartridge communicating with the inflow opening (2) of the vacuum chamber.

TECHNICAL FIELD

The invention relates to a device for purifying liquid in a liquid reservoir, said device including a vacuum chamber having an inflow opening and an outflow opening adapted to communicate with the liquid in the reservoir through a feed conduit and a return conduit, respectively, a pump arranged in the return conduit, a vacuum pump connected with the vacuum chamber, and a filter cartridge provided with a plurality of filter elements, each having a flow side communicating with a central passage and a flow side opening into the periphery of the filter cartridge, and where the filter cartridge is arranged inside the vacuum chamber, the central passage of the filter cartridge communicating with the inflow opening of the vacuum chamber.

A high-voltage transformer is a well-known component, which is widely used as part of the power supply network and as a vital part of the high-voltage electricity grid. High-voltage transformers may for instance also be used in factories to step up or step down the supply voltage according to need. A high-voltage transformer is formed of five essential components, ie. an iron core, a winding, an insulating material, a coolant and a casing. It further comprises a number of other components which, however, do not determine the basic function. The iron core is typically a silicon iron alloy rolled into thin sheets of a thickness of between 0.3-0.5 mm, said plates being stamped into a suitable shape and assembled into an iron core. Usually the winding in the high-voltage transformer is a copper winding, but an aluminium winding may also be used even though this is an unusual choice. The winding in the high-voltage transformer is insulated. The insulating material may be made different types of materials, but it is typically made of paper wound round the conductors in the winding. Various materials may be used as coolant in a high-voltage transformer, but either air or oil is commonly used. Air is a poor coolant due to its low specific heat capacity and low heat transfer coefficient for which reason air is usually only used in small high-voltage transformers or under special conditions. Oil is far more often used as coolant. It is very effective coolant, the specific heat capacity and the heat transfer coefficient thereof being superior to those of air. The object of the casing of a high-voltage transformer is of course to enclose the iron core and the winding, but in addition thereto the casing serves as a tank for the transformer oil. Furthermore bushings are provided in the casing for the inlet and outlet lines of the winding as well as various pressure relief valves. Gauges may also be provided in the casing.

As mentioned above the transformer oil serves as a coolant for the high-voltage transformer. However, this is not the only function of the transformer oil, as the transformer oil also constitutes a vital part of the transformer insulation. As an example thereof, transformer oil has a dielectric constant, ∈r, between two and three unlike air or vacuum which have a dielectric constant of one. Another factor characteristic of the transformer oil is the voltage at which voltage breakdown occurs. A voltage breakdown is caused by an arc arising through the transformer oil between two conductors or between a conductor and the casing. The voltage of transformer oil at which voltage breakdown occurs is typically about 150 kV/cm. However many other factors influence the breakdown voltage size, eg. a water content of a merely 0.01% in oil reduces the breakdown voltage by 20%. Another factor, which may significantly reduce the breakdown voltage, is the presence of contaminating particles or suspended gasses (bubbles) in the transformer oil. These contaminating elements occur as electric dipoles and tend to create bridges between conducting parts at different electrical potentials. As a result the bridges form a possible path for an arc and thus contribute to reducing the breakdown voltage. The breakdown voltage in an extremely pure transformer oil is as high as 1000 kV/cm. However, in practice it is impossible to maintain such a transformer oil purity in a high-voltage transformer.

Several reasons exist as to why the oil in a high-voltage transformer becomes contaminated. Oil is filled into the transformer at the manufacture of the high-voltage transformer. If suitable precautions are not taken, the oil becomes contaminated. Even though contamination cannot be avoided completely, much is done to minimize contamination during all types of manufacturing processes of high-voltage transformers, eg by ensuring that the transformer is substantially free of particles and dry before being filled with oil. It is, however, also necessary inter alia to take both relative and the absolute humidity into account during the feeding of oil.

The oil is further contaminated during service of the high-voltage transformer. Despite being properly dried before the oil is filled into the transformer, the paper insulation of the transformer windings contains an amount of water. The actual water content in paper insulation depends on several factors including the temperature. In service, a high-voltage transformer suffers an energy loss, partly in form of a resistive drop in the transformer winding and partly as an iron loss caused by the conversion of electrical energy to magnetic energy and back to electrical energy. Ultimately the energy loss causes a temperature change in the high-voltage transformer. Since the size of this energy loss varies according to the load, the temperature changes over time. Consequently, when the high-voltage transformer switches from being loaded to running idle a situation may arise in which the temperature drops to a point at which the paper and the oil cannot contain the absorbed water, whereby free water is formed in the oil. Another factor is the variations in the temperature and the possible presence of oxygen which cause the transformer oil and the paper insulation to age. Ageing is the disintegration of a material and may thus cause the formation of decomposition products in the oil in form of particles, gases and water.

As mentioned above, the presence of water, particles or gas bubbles in the transformer oil is undesirable, for which reason filtering of the transformer oil is required. The oil in a high-voltage transformer is typically cleaned by means of one of the two methods below. In one method, which has been used for a number of years, the oil is drained out of the high-voltage transformer and moved to an oil treatment system or to move an oil cleaning system to the high-voltage transformer and to pass the oil through the system. However, this method is encumbered by the significant drawback that in order to treat the oil the high-voltage transformer has to be taken out of service for some time. In another method, which is used nowadays for the treatment of oil, a filter is secured to the high-voltage transformer and through which the oil circulates continuously, while the transformer is in service.

It is thus commonly known to filter a liquid, while the plant, in which the liquid is used, is in service. It is inter alia known to provide a filter for removing contaminat-ing elements from the lubricating oil in connection with a lubricating device of a combustion engine in a car. It is also known to use water traps to remove water for instance from diesel oil for a diesel engine. The necessary purification process of transformer oil differs from that of for instance lubricating oil by the degree of purity required in connection with transformer oil.

BACKGROUND ART

U.S. Pat. No. 3,249,438 discloses an apparatus for removing contaminants e.g. water from a fluid such as oil. The apparatus comprises filters placed inside a vacuum chamber, and the combination of the vacuum and oil film on the outside surface of the filters removes water from the oil in the form of a vapour. The vapour is thereafter removed from the vacuum chamber. There are not described means for controlling liquid foam in the vacuum chamber.

U.S. Pat. No. 6,224,716 discloses an apparatus much like U.S. Pat. No. 3,249,438 but it also comprises a heating circuit for the oil to speed up the evaporation of the water from the oil. There are described pumps and valves for feeding liquid to the vacuum chamber and removing liquid from the vacuum chamber. There are not described means for controlling liquid foam in the vacuum chamber.

U.S. Pat. No. 5,574,214 discloses an apparatus for the treatment of transformer oil, in which the oil is treated by being passed through a vessel containing a conventional filter material and a molecular sieve. A molecular sieve is a material which is able to retain molecules of a specific molecular weight. The conventional filter material thus removes the particles from the transformer oil and the molecular sieve removes the water. This method is disadvantageous in that over time the molecular sieve is exhausted and has to be replaced.

International publication No WO 00/52445 discloses an apparatus, in which a filter cartridge is used to filter the transformer oil. The filter cartridge contains a filter material such as Fuller's earth. Fuller's earth is a type of soil, which inter alia is used as cat litter, and which possesses the ability to absorb water very easily. This device is encumbered by the same drawback as the above US patent, ie that the filter material is exhausted and has to be replaced occasionally. The WO publication further discloses a vessel provided with a vacuum pump, which is used to minimise the gas suspension in the transformer oil.

U.S. Pat. No. 2,062,934 describes a filter system where the liquid flows through disk-like filter elements from the outside of the filter elements to the inside, and where the filter elements are placed inside a vacuum chamber. It is mentioned that the liquid is the oil from a transformer and that the system removes water from the oil.

U.S. Pat. No. 6,224,716 describes a filter system for lubricating oils where cylindrical, coalescent filter units are placed in a vacuum chamber and where heated oil is passed from the inside of the filter units to the outside. It is specifically mentioned that the system can remove water from the oil.

DK patent No 156542 discloses a filtering device comprising a plurality of stacked filtering elements forming a filter cartridge. The disc-shaped filtering elements each has a central aperture forming a central passage with apertures to the inner cavity of the individual filtering elements in the stacked position of the filtering elements. Typically, the central passage has an inlet opening at one end and an opening sealed by means of a plug at the other end. Each filtering element has a planar outer side adjacent the central aperture and a corrugate side. As a result each pair of filtering elements opens into the central passage on one side, while closely abutting the adjacent pair of filtering elements on its inner side.

DESCRIPTION OF THE INVENTION

The device according to the invention is characterised in that the filter cartridge is arranged inside the vacuum chamber, the central passage of the device communicating with the inflow opening of the vacuum chamber.

The resulting device ensures an effective continuous removal of contaminating particles from the liquid and removal of the water present in the liquid, the liquid being evacuated from the reservoir by means of the vacuum pump and into the central passage and then out passed through the filter elements and onto the surface of the filter cartridge, whereby a large liquid surface is formed. The particles in the liquid are filtered off by means of the filter cartridge. The flow of liquid onto the surface of the filter cartridge and the low pressure in the vacuum chamber reduce both the solubility of water in the liquid and boiling point of the water. As a result water is discharged as free water in the liquid and then boiled off from the liquid. All components of the device except the filter cartridge are not exhausted resulting in a long life span of the device. The liquid is pumped back to the reservoir from the bottom of the vacuum chamber.

The filter cartridge known from DK patent No 156542 may advantageously be used.

The boiling-off of the water from the liquid may cause the formation of foam due to the low pressure in the vacuum chamber. This is a disadvantageous effect, as the foam may inter alia escape to the vacuum pump. According to the invention a first sensor for monitoring the foam level has consequently been provided in the vacuum chamber. If the foam exceeds a specific level, a pressure equalizing valve is opened and the pressure in the vacuum chamber is increased due to the inflowing gas.

According to the invention the vacuum pump may be connected to a vessel adapted to accumulate the gas being drawn out of the vacuum chamber, said vessel communicating with the pressure-equalising valve through a conduit. The accumulated gas may advantageously be used for checking the condition of the transformer, especially if the oil stems from a transformer. This test may inter alia provide information about the expected remaining life of the transformer oil and of the insulating materials in the transformer or indicate the fault source at faults or breakdowns.

The invention also relates to a transformer characterised in that it includes a device of the above type, in which the oil reservoir is the oil in a transformer. As mentioned above a device of this type is suitable for removing impurities from a liquid such as a transformer oil. The transformer oil is thus cleaned continuously and any operation-interrupting contaminating elements are kept to a minimum resulting in increased system reliability.

BRIEF DESCRIPTION OF THE DRAWING

The invention is explained in detail below with reference to the drawing, which is a principle drawing showing all of the essential components of invention.

BEST MODES FOR CARRYING OUT THE INVENTION

The device shown on the drawing for purifying a liquid in a reservoir12includes a vacuum chamber1having an inflow opening2and an outflow opening3, a vacuum pump7, a filter cartridge8, a feed conduit4and a return conduit5. The filter cartridge8is of the type mentioned in DK patent No. 156542. As indicated in the drawing, the filter cartridge8includes a plurality of filter elements yi. The liquid is passed from the reservoir12through a feed conduit4to an inflow opening2in the vacuum chamber1and from there into the central passage26in the filter cartridge8. The vacuum pump7serves to maintain a low pressure in the vacuum chamber1, the low pressure evacuating the liquid from the reservoir12into the vacuum chamber1. The liquid then flows from the inner face of the filter cartridge8to the outer face of the filter cartridge8and drips to the bottom of the vacuum chamber1, from where a pump6pumps the liquid out of the vacuum chamber1through the outflow opening3and the return conduit5back to the reservoir12.

By passing the liquid from the inner face to the outer face of the filter cartridge8two features are obtained, viz. the filter material filters off the particle contamination from the liquid and a liquid film having a large surface is formed on the outer face of the filter cartridge8. The particles retainable by the filter cartridge8depend on the filter material, particles of a specific minimum size being allowed to pass freely therethrough. The provision of a large liquid film surface on the outer face of the filter cartridge8allows for the optimum liquid amount to be subjected to the low pressure in the vacuum chamber one. The water in the liquid on the surface of the filter cartridge8is boiled off of the liquid, even when the liquid temperature considerably below 100° C.'s at which temperature water boils at 1 ATM. The gaseous water is then evacuated from the vacuum chamber2by means of the vacuum pump seven.

The boiling-off of water from the liquid causes the formation of foam due to the gas bubbles in the liquid on the outer face of the filter cartridge8. Together with the liquid the foam drips to the bottom of the vacuum chamber1and accumulates on top of the liquid. This constitutes a problem, in particular if the foam reaches such a high level that it comes into contact with the vacuum pump7. A first sensor9has thus been provided in the vacuum chamber1for monitoring the foam level. A pressure equalising valve10is provided to reduce the foam level in the vacuum chamber1said valve equalising some of the pressure in the vacuum chamber1when an excessive foam level is recorded by the first sensor. The pressure equalisation is effected by the pressure equalising valve10allowing gas to flow into the vacuum chamber1, whereby the foam disintegrates.

Furthermore as mentioned above, the gas present in the transformer oil is used for analysing the condition of the transformer, eg by indicating possible fault sources and estimating the remaining life of the transformer. A vessel25for receiving the gas from the vacuum chamber1is provided in connection with the vacuum pump7. As a result it is possible to continuously analyse the gas to detect any possible fault conditions or to analyse the gas in connection with a breakdown in order to locate the cause of the breakdown.

The mixture of water and gases evacuated from the vacuum chamber1may condense in the sump of the vacuum pump7. In order to prevent such a condensation air is supplied to the suction side of the pump7through the closing valve30and the paper filter31.

The mixture of water and gases is passed to a vessel25from the outflow opening of the vacuum pump7. This vessel is provided with a sampling valve33and a pressure equalising valve32. The pressure equalising valve32serves to maintain a specific differential pressure between the vessel25and the surroundings.

The vessel further communicates with the pressure control valve27and the pressure equalising valve10. The purpose of this connection is to return an amount of the evacuated gases to the vacuum chamber1. As the gases are not extraneous, the liquid in the vacuum1are not contaminated.

During the gas sampling process the mode of operation switches to sampling mode. The closing valve30is closed during sampling and only the gas being evacuated from the vacuum chamber1is pumped into the vessel25. After a specific period of time the gas present in the vessel25corresponds to the gas present in vacuum chamber1. The gas sample is extracted through the sampling valve33.

A second sensor11for measuring the liquid level is further provided in the vacuum chamber1. A certain minimum liquid level is required in the vacuum chamber1to avoid cavitation in the pump6. The second sensor11controls a level control valve20ensuring that specific minimum and maximum liquid levels are maintained.

A third sensor13is arranged on the vacuum chamber1and measures the pressure therein. This sensor further controls the pressure control valve27to ensure that a maximum vacuum is maintained in the vacuum chamber1. A vacuum gauge15is arranged adjacent the third sensor13to provide a reading of the pressure in the vacuum chamber1. As mentioned above, the pressure in the vacuum chamber1draws out the liquid from the reservoir12and through the filter cartridge8.

The control of the vacuum and the liquid flow is generally coordinated such that a state of equilibrium is generated in the vacuum chamber1at the same time as an acceptable liquid foam level is obtained and the pressure in the vacuum chamber1is sufficiently low to remove the water from the liquid.

In addition to the above pumps, sensors and valves a number of other components form part of the device. A throttle valve14is provided in the feed conduit4to reduce the liquid flow from the reservoir12to the vacuum chamber1. A closing valve21is further provided in the feed conduit4, said valve shutting-off the liquid flow from the reservoir12at the function “stop”. The flow in the feed conduit4also has to be restricted to be below the pump capacity of the oil pump6. A level control valve20is provided in the return conduit5for controlling the operation level of the liquid in the vacuum chamber1.

Further, a manually operated closing valve19is provided in the return conduit5. Jointly with a level control valve20the manually operated closing valve19may be used to shut off the liquid during removal of the pump6. A discharge valve18is used to evacuate liquid from the vacuum chamber1during maintenance. The vacuum pump7is provided with an oil level switch17switching off the vacuum pump7, if the oil level in the sump of the vacuum pump becomes too low. By shutting off the vacuum pump7the evacuation of liquid from the reservoir12is stopped. A float valve16is provided to allow the foam-reducing gas to enter the vacuum chamber1and further to allow another gas, eg atmospheric air, to enter the vacuum chamber1. Another function of the float valve16is to prevent liquid from flowing into the vacuum pump7. A pressure relief valve29is arranged between the vacuum chamber1and the return conduit5, said valve returning increasing amounts of the liquid to the vacuum chamber1, when the level control valve20is shut off during operation of the oil pump6. The return conduit5is further provided with a sampling point28allowing for a liquid sample to be extracted for analysis.

An embodiment of the invention is described above. Many modifications can be carried out without thereby deviating from the scope of the invention. The foam-reducing gas may for instance be atmospheric air or another gas applicable for that purpose. As mentioned above the gas in the transformer oil is used to check the condition of the transformer. If the foam-reducing gas is a known inert gas such as Argon, the gas is easily excluded in a future analysis. Another option is to use the gas already evacuated from the vacuum chamber, this gas having substantially the same composition as the gas to be tested and thus does not constitute a contamination.