WO2014194958A1

WO2014194958A1 - Method and device for determining an average parameter of a fluid in a closable container

Info

Publication number: WO2014194958A1
Application number: PCT/EP2013/061720
Authority: WO
Inventors: Axel Kramer; Hubert Brändle; Kai Hencken; Thomas Alfred Paul
Original assignee: Abb Technology Ag
Priority date: 2013-06-06
Filing date: 2013-06-06
Publication date: 2014-12-11
Also published as: EP3004832A1; CN105264352A; RU2015156300A

Abstract

A method for determining an average temperature of a fluid in a closable container (1) comprising: providing a duct (4) with first opening (11a) and second opening (11b), positioning a flow preventing element (7) in the duct (4), positioning a density sensor (3) or first pressure sensor (3) inside the duct (4) or container (1), attaching the duct (4) to a container outside wall, stabilizing a temperature of the duct wall (10) to a constant reference value, determining a difference between a first pressure of the fluid on one side and a second pressure of the fluid on the other side of the flow preventing element (7), measuring a local density of the fluid by the density sensor (3) and/or a local pressure by the first pressure sensor (3), and deriving the average temperature from the local differential pressure, the local density and the reference duct temperature.

Description

Method and device for determining an average parameter of a fluid in a closable container

Technical field

The invention relates to a method for determining an average parameter of a fluid in a closable container, a measurement device for carrying out the method and a measurement arrangement according to the independent claims. The measurement device can particularly be used for detecting fluid leakages, particularly gas leakages, in fluid containers and for example in fluid insulated switchgear in power transmission and distribution systems.

Background

Fluid insulated switchgear in power transmission and distribution systems are well known. These devices include gas insulated switchgear (GIS) like e.g. circuit breakers. The GIS devices have an outer encapsulation and the interior of this encapsulation is filled with an insulating gas like e.g. SF_e (sulfur hexafluoride ) . The gas quantity inside the GIS has to have a value above a critical level in order to avoid a deterioration of the dielectric insulation performance; however, due to material porosity, corrosion, faulty seals, etc., the gas may leak to the environment. This is undesired because of the reason mentioned above and because the gas may potentially be harmful to the environment, e.g. in case of SF₆ gas which has an extremely high global warming potential. Therefore, there is a need for monitoring the contents of the GIS and detecting potential gas leakages from the encapsulation. For leakage monitoring in gas insulated switchgear either the pressure, the temperature-normalized pressure or the density are tracked. However, a pressure measurement alone is unsuitable for detecting the leakage rates quickly (e.g. within one day or within a few days), because the pressure variations due to ambient and operational temperature changes are not taken into account. A temperature-normalized pressure measurement Pnorm is given by

Pnorm-Pmeas ^x

¾ieas assuming ideal gas behaviour, where T_norm is a chosen normalization temperature (e.g. 298 K) and p_meas and r_meas are the measured pressure and locally measured temperature, respectively. The normalization of the pressure is necessary in order to calculate reliably from this the total amount of gas (total mass) contained in the container. T_meas can also be an average of several temperature measurements (arithmetic average or adequately weighted average) . For the typically inhomogeneous temperature distributions within switchgear enclosures, an average temperature T_av is needed in order to calculate correctly the total mass, that is, the average density in the container. r_meas is normally not a good estimation for T_av. Therefore, a pressure measurement normalized using Tmeas is only of limited usefulness for high-precision leakage detection. A density measurement is at first sight independent of these pressure and temperature effects, but it also requires temperature normalization, because it is a local measurement and may not be representative of the average density within an enclosure. The ideal gas law is given by

Mp

wherein p is the density, M is the known molar mass of the gas, p is the pressure and R is the universal gas constant. Although the pressure is constant with a tolerance of approximately 100 Pa throughout the enclosure of the GIS, the density depends on the local temperature at the measurement location. A temperature distribution in the volume is therefore exactly correlated to a density distribution. Again, the density measurement can, in principle, be normalized by

= Px 7*

narm

Therefore, the same limitations as for a temperature- normalized measurement apply, preferably with the addition that the temperature measurement should ideally be at the point of the density measurement to at least obtain a good estimate for the local normalized density.

To overcome these complications, typically leakage rates are determined by averaging measurement values over long time intervals (weeks to years) to even out variations due to insufficient temperature compensation .

Temperature compensation of p- and p- measurements aims at compensating for the temperature- dependence of the pressure and of the local density. In both cases a local temperature measurement is used in the hope that this is representative of the average temperature in the compartment. However, during operation of the switchgear or when solar irradiation heats the encapsulation, large temperature gradients develop, resulting in local temperature measurements which are a poor estimation of the average gas temperature. The average temperature can be coarsely estimated by using multiple temperature sensors dispersed around the outside of the encapsulation. Averaging measurements over long durations, in order to make trends in the density or p/T visible, result in delays of leakage detection and hence unnecessary pollution of the environment.

Attempts have been made to use distributed temperature sensors and computational fluid dynamics simulations to derive an average temperature in a container with as few T-sensing locations as possible. However, this method is not practicable as it is not generic nor easily extendable to any given geometry. The problem of determining a good estimate of the average gas temperature in a practicable and fast way, i.e. without a large number of sensors or over long periods of time, respectively, has remained unsolved so far.

Description of the invention

Thus, it is an objective of the present invention to improve the measurement accuracy and therefore the leakage detection time and reduce the costs for measurements of the type mentioned above. This objective is achieved by the subject-matter of the independent claims, that are herewith recited literally by reference. Embodiments arise from dependent claims and any claim combinations and the description and figures.

In a first aspect of the invention the objective is solved by a method for determining an average physical parameter of a fluid in a closable container, comprising the steps of:

a) providing a duct with a first opening and a second opening,

b) positioning a flow preventing element at a first level in such a way that the flow preventing element extends across an entire cross-section of the duct ,

c) positioning a density sensor and/or a first pressure sensor at a second level inside the duct or the container,

d) attaching the duct to the container, in particular to an outside wall of the container, in such a way that the first opening is connected to a first container opening and the second opening is connected to a second container opening, e) defining a reference duct temperature , f) determining, by means of a differential pressure sensor arranged between both sides of the flow preventing element or by means of a second and a third pressure sensor arranged on either side of the flow preventing element, a local differential pressure as a difference between a first pressure of the fluid on the one side of the flow preventing element and a second pressure of the fluid on the other side of the flow preventing element,

g) measuring a local density of the fluid by the density sensor and/or a local pressure by the first pressure sensor, and

h) deriving the average physical parameter from the local differential pressure, the local density or the local pressure, and the reference duct temperature .

In embodiments, the reference duct temperature is defined along a duct extension from the second opening up to the flow preventing element, or the reference duct temperature is defined along the whole duct.

In embodiments, in step e) defining a reference duct temperature is done by stabilizing a temperature of the fluid inside the duct to a constant reference value, and/or by stabilizing a temperature of the duct wall or duct walls to a constant reference value, and/or by determining an average fluid temperature inside the duct, and/or by determining an average duct wall temperature. Herein, as mentioned above, duct can mean duct extension between flow preventing element and second opening or can mean whole duct.

In embodiments, the reference duct temperature, in particular an average fluid temperature inside the duct and/or an average duct wall temperature, is or are determined from at least one temperature value, preferably from a plurality of temperature values, measured along the duct. Furthermore, the duct can be attached to an outside wall of the container. In all embodiments, duct can mean duct extension between flow preventing element and second opening or can mean whole duct .

In a second aspect of the invention the objective is solved by a measurement device for carrying out the method according to the invention. The device comprises a duct with at least one wall and a first and a second opening, a flow preventing element arranged at a first level of the duct, a density sensor and/or a first pressure sensor arranged at a second level of the duct, a differential pressure sensor or a second and a third pressure sensor arranged at the flow preventing element for measuring a local differential pressure between a first pressure of a fluid on the one side of the flow preventing element and a second pressure of the fluid on the other side of the flow preventing element.

In embodiments, the flow preventing element is for preventing a flow through the duct, in particular wherein the flow preventing element is for preventing a flow of the fluid out of or into the duct through the first opening.

The openings are advantageously adapted to be connected to respective openings of the closable container .

In one embodiment the duct is connected to the closable container via a thermal insulation element.

In another embodiment the flow preventing element is arranged inside the duct at the first level in such a way that the flow preventing element extends across an entire cross-section of the duct. The density sensor and/or the first pressure sensor is or are positioned inside the duct or inside the container at the second level.

In yet another embodiment the measurement device comprises an insulating layer for thermally insulating the duct from its environment, preferably the insulating layer covering substantially entirely the at least one wall of the duct.

In another embodiment the measurement device further comprises a temperature regulating device for regulating a temperature of the at least one wall of the duct .

In another embodiment the temperature regulating device is arranged between the at least one wall of the duct and the insulating layer.

In a third aspect of the invention the objective is solved by a measurement arrangement comprising a measurement device and a closable container. The duct of the measurement device is attached to an outside wall of the container in such a way that its first opening is connected to a first container opening and its second opening is connected to a second container opening. The measurement arrangement further comprises at least one data acquisition and processing device for retrieving sensor data from the density sensor and/or the first pressure sensor and/or the differential pressure sensor and/or the second pressure sensor and/or the third pressure sensor of the measurement device and for computing at least a value of an average physical parameter of the fluid in the container.

In embodiments, the average physical parameter can be an average temperature T_av of the fluid and/or an average density pnorm of the fluid and/or a temperature-normalized pressure p_n0rm of the fluid and/or an average particle density of the fluid and/or a fluid mass .

The measurement device according to the second aspect of the invention is particularly used for measuring, by means of the method according to the first aspect of the invention, a leakage of fluid out of or into a fluid-insulated switchgear, particularly a gas- insulated switchgear. The method according to the invention is generic and applicable to all situations or containers where the average temperature or average density or the mass of a confined gas volume needs to be known accurately and rapidly.

Short description of the drawings

Embodiments, advantages and applications of the invention result from the dependent claims and from the now following description by means of the figures. It is shown in:

Fig. 1 a cross-sectional front view of an embodiment of a measurement arrangement according to the invention,

Fig. 2 a more detailed cross-sectional view of the measurement arrangement of Fig. 1, and in

Fig. 3 a diagram showing an exemplary curve of an average physical parameter derived by the method according to the invention and a curve determined by theoretical computation.

Ways of carrying out the invention

The invention is described for the example of a high voltage circuit breaker, but the principles described in the following also apply for the usage of the invention in any other gas-enclosing containers or devices or switching devices. The terms "top", "upper", "bottom", "lower" and similar terms are relative to a vertical direction as given by the gravitational force (analoguous to the top and the bottom of the figures) . The term "closable" in relation to the container denotes, in the context of this disclosure, that the container may have openings, e.g. for introducing an insulating fluid, however these openings are gas-tightly sealable and in particular sealed during usage of the container. Such openings are used to connect the measurement device according to the invention with the container. The term "pressure sensor" refers to absolute pressure measurement. The term "level" relates to the direction z (indicated in Fig. 2, e.g. vertical) starting from a level ho. If it is used to describe objects or containers 1 elongated in the direction z the level h₀ can refer to a lowermost extremity of these objects or containers 1.

In general, the first level refers to the level where the flow preventing element is positioned, and the second level refers to the level where the density sensor and/or the first pressure sensor are positioned.

In embodiments, the first level is the upper level h, where the flow preventing element is positioned, and the second level is the lower level ho, where the density sensor and/or the first pressure sensor is or are positioned, or vice versa.

In alternative or additional embodiments, the first level refers to the level where the first opening of the duct is positioned, and/or the second level refers to the level where the second opening of the duct is positioned .

In alternative or additional embodiments, the first level refers to the level where an opening of that duct part not having the reference duct temperature is positioned, and/or the second level refers to the level where an opening of that duct part having the reference duct temperature is positioned.

For example, the first level refers to the level where the flow preventing element is positioned, and the second level refers to the level where an opening of the duct part having the reference duct temperature is positioned. In another example, the first level is the upper level h, where the flow preventing element is positioned, and the second level is the lower level h₀, where the lower opening of the duct is positioned. Fig. 1 shows a sectional front ^' view of an embodiment of a measurement arrangement according to the invention, comprising a simplified gas-insulated switchgear (GIS) 1 connected to a simplified measurement device la. The GIS 1 is an example for a closable container 1 and comprises an outer enclosure 8a and an inner busbar 8b (i.e. the electrical conductor 8b) . It is noted that the majority of elements of the GIS 1 are not shown here for reasons of clarity. The measurement device la comprises a duct 4 with a first and a second opening 11a, lib connected to the enclosure 8a of the GIS 1. The cross section of the duct 4 is round in this case, such that the duct 4 has a single duct wall 10, but it may also have another shape, e.g. rectangular, and may therefore have a plurality of duct walls 10. The openings 11a, lib are adapted to be connected to respective openings of the closable container 1. In this example the first opening 11a will be referred to as top opening 11a and the second opening lib will be referred to as bottom opening 11a for reasons of clarity.

A volume of the duct 4 is between 10 times and 1' 000' 000 times smaller than a volume defined by the enclosure 8a of the GIS 1. However, the dimensions of the duct 4 can vary depending on the shape and/or volume of the GIS 1.

The connection of the duct 4 to the GIS 1 is done via a thermal insulation element 5 for each connection. In the present example the insulation elements 5 are PTFE adapters, however other materials suitable to thermally insulate the duct walls 10 from the enclosure 8a may also be used. The measurement device la may further comprise a valve or a self-sealing valve (not shown) for each one of the first and the second opening 11a, lib of the duct 4 for avoiding a fluid escape from the enclosure 8a to an environment atmosphere.

A differential pressure sensor 2 is provided at a first level on the side of the top connection between the duct 4 and the container 1 and a density sensor 3 is provided at a second level on the side of the bottom connection between the duct 4 and the container 1. In this example the second level is lower than the first level; however, the location of the two sensors 2, 3 may also be exchanged. The function and attachment of said sensors 2, 3 will be explained in more detail in connexion with Fig. 2.

The measurement device la further comprises a temperature regulating device 12 for regulating the temperature of the wall 10 or walls 10 of the duct 4, which temperature-regulating device 12 is supplied with power by a source 6.

Fig. 2 shows a more detailed sectional view of the measurement arrangement of Fig. 1.

A flow preventing element 7, preferably a flexible membrane 7, is arranged such that a fluid circulation between the duct 4 and the container 1 is only suppressed by the flexible membrane 7. The membrane 7 extends across an entire cross-section of the duct 4. In this example the membrane 7 entirely covers the top opening 11a of the duct 4. However, the membrane 7 may also be arranged at another location inside the duct 4. The arrangement according to the present example is preferred, because it simplifies the setup of the differential pressure sensor 2.

The flow preventing element 7 prevents any exchange of matter from one side to the other side of the flow preventing element 7. Furthermore, for example, the flow preventing element 7 can be thermally non-conducting to better maintain the reference temperature of the duct on one side and the local temperature present in the container 1 on the other side. However, the case may be that both temperatures are identical and no thermal isolation across the flow preventing element 7 is needed. Across the flow preventing element 7 a differential pressure shall be maintained. This differential pressure stems from the duct reference temperature ΊΊ present on one side of the flow preventing element 7 and from a local container temperature present on the other side of the flow preventing element 7. The flow preventing element 7 may be arranged in any position along the duct 4 at a first level or height above the bottom or lower second level or height. In an embodiment, the average duct temperature can be present only in the lower part of the duct extending below the position of the flow preventing element 7, and the container temperature distribution can be present in the upper part of the duct extending above the position of the flow preventing element. In more specific embodiments, the flow preventing element 7 is arranged in the region of the first or upper opening 11a of the duct 4 and the whole duct 4 has the reference temperature Ti.

Instead of the membrane 7 also a rigid wall 7 may be used as flow preventing element 7. Furthermore, a first pressure sensor 3 may be used instead of or additionally to the density sensor 3. Furthermore, the differential pressure sensor 2 may be replaced by a second and a third pressure sensor (not shown) . In this case the second pressure sensor is arranged in such a way that it can measure the pressure on one side of the membrane 7 or wall 7. Accordingly, the third pressure sensor is arranged in such a way that it can measure the pressure on the other side of the wall 7. The differential value is obtained by computing the difference between the two measured pressure values.

In embodiments, the second level is lower than the first level, as seen in a gravitational vertical direction (z), or vice-versa.

In embodiments, the flow preventing element 7 entirely covers one of the first or the second opening 11a, lib, and the density sensor 3 and/or the first pressure sensor 3 is or are positioned at the other opening lib, 11a. The density sensor 3 is arranged in the area of the bottom opening lib. In this example the density sensor 3 is arranged at the boundary between the bottom opening lib of the duct 4 and the corresponding opening of the closable container 1. This is preferred in order to simplify the setup of the density sensor 3. The density sensor 3 may however also be arranged inside the duct 4 or inside the closable container 1, preferably at a level (with respect to a vertical axis z) which is different from the level of the differential pressure sensor 2. The level difference between the two sensors 2, 3 , i.e. the differential pressure sensor 2 and the density sensor 3, is denoted by Ah, which is equal to h - ho, in Fig. 2.

The duct 4 is enclosed by a thermally insulating layer 9, preferably consisting of an insulating sleeve, for thermally insulating the duct walls 10 from the environment. A temperature regulating device 12, which in this example is a heating band 12, is arranged between the insulating sleeve 9 and the duct wall 10. It is understood that other temperature regulating devices 12, e.g. a resistive wire located inside the duct 4 and controlled by the voltage source 6, may be used, as well. For example it is also conceivable to use the duct 4 itself as a resistive element for temperature regulation in case the duct 4 is made of metal and is electrically isolated from the container 1.

The PTFE adapters 5 and the insulating sleeve 9 serve to avoid as much as possible an influence of an outside temperature on the temperature of the duct 4. The temperature of the duct has to be as stable as possible for accurate measurements, because it is used as a reference temperature 2Ί .

In one embodiment the measurement device la further comprises a valve arrangement and/or a pump arrangement (not shown) for evacuating gases from the duct 4 prior to establishing a connection between the measurement device la and the closable container 1. This advantageously makes it possible to, increase the measurement accuracy by eliminating influences of the atmosphere present in the duct 4 and by minimizing contamination of the dielectric insulation gas inside the GIS with atmospheric gas.

The valve and/or pump arrangement and the self-sealing valve are particularly preferred when the measurement device la is used for servicing and diagnostics of filled gas-insulated switchgear, because these elements allow a connection between the measurement device la and the container 1, which connection can be carried out in such a way that the fluid atmosphere in the container 1 is not substantially changed with respect to density, temperature and pressure, when said connection is being established. Alternatively or additionally, the duct 4 can be prefilled with the same fluid as in the container 1 such that the density and pressure in the container 1 are even less affected.

In the following the method for determining an average physical parameter of a fluid within the enclosure 8a will be described by not only taking into account the actions to be performed but also the underlying mathematical and physical basis. For the present example the average parameter to be determined is an average temperature T_av of the fluid in the closable container 1. However, alternatively or additionally an average density of the fluid and/or a temperature- normalized pressure of the fluid and/or an average particle density of the fluid and/or a fluid mass may be derived as well. It is assumed that the steps a) to d) of the method have already been carried out, representing those steps that relate to the setup of the measurement arrangement. Thus, in the mounted state of the measurement arrangement the fluid has spread out into the duct 4 via the bottom opening lib.

In this state a temperature ΪΊ of the gas inside the duct 4 is equal to that of the duct wall 10 or walls and is stabilized to a constant reference value by e.g. heating up the duct wall 10 by means of the temperature regulating device 12. Alternatively, an average duct wall temperature may be determined from a plurality of temperature values measured along the duct 4. The reference temperature Γι is regulated to a value different from the temperature present or assumed to present in the closable container 1.

The difference between the average temperature of the gas within the volume of the closable container 1 and the average temperature of the gas within the volume of the duct 4 causes a pressure difference at the membrane 7 between the two volumes 1 and 4. After the stabilization of the reference temperature ΪΊ- in the duct 4 said local pressure difference Δρ is determined by means of the differential pressure sensor 2 as a difference between a first pressure of the fluid on one side of the membrane 7 and a second pressure of the fluid on the other side of the membrane 7. This is illustrated in Fig. 2 by the curved shape of the membrane 7.

The pressure of the fluid at the top of the duct 4, at height Ah above the lowest point ho of the duct 4 on the left side of the membrane 7, is

Mgj -hp)

p(h,T_l) = p(h_O)e ^ where h denotes the height as variable, Ti the reference temperature, h₀ the lowest point of the duct 4, M the known molar mass of the fluid, p the pressure as a function of height and/or reference temperature Ti, R the universal gas constant and g the gravitational acceleration. The fluid can be a pure substance or a mixture of substances.

According to the barometric equation, the pressure of the fluid at the top of the duct 4 on the right side of the membrane 7, i.e. inside the container 1, is p( ,T = p(h₀)e ^RT~ where T_av is the average temperature of the fluid inside the enclosure:

Hence, the difference pressure across the membrane 7, i.e. the pressure recorded by the differential pressure sensor 2, is:

Ap(h) = p(h,T_l ) - p(h,T_av) = p(h₀)(e ^ΛΓ< - e ^RT~ )

This expression can be solved for Tav, for example, if the expression is approximated by using the first two terms of the Taylor expansion e^x=l+x:

Solving for 1/T_av yields

1 _ RAp(h) 1

m p(h₀ )Mg(h - h₀ ) T By the above measurements and computations it is thus possible to derive the average physical parameter, in this case the average temperature of the fluid r_av in the closable container 1, from the local differential pressure Δρ, a local pressure measurement at h₀ and the reference duct temperature Γχ.

T_av may now be used to normalize the measured pressure to a chosen normalization temperature r_norm, thereby eliminating intrinsically all pressure variations due to changes in the average gas temperature.

Now, having the average temperature T_av of the fluid it is possible to derive also the average density of the fluid by compensating the local density p_meas of the fluid with the average temperature of the fluid. For this an equation of state of the fluid has to be considered, thus it is possible to choose between different models known by the person skilled in thermodynamics .

For example, assuming ideal gas behaviour with

Mp

p =—ⁱ-

RT the local density p_meas recorded by the density sensor 3 at the bottom of the duct 4 is:

Pmeas = PVk ) = _ρ

This density is now normalized to take into consideration the fact that it is a local density subject to the reference temperature ΪΊ .

Pnorm⁼ Pmeas ^' T\ / T_av

Substitution of 1 / T_av now leads to the convenient equation:

Pnorm is a good approximation of the real density in the container 1 and therefore represents the total mass of fluid in the container 1 . Thus, it can be used as a fast measure for leak detection. Hence, a local differential pressure measurement and a local density measurement can be used to derive the correct density of the fluid in the closable container 1 provided a constant reference temperature Γχ is present in the reference duct 4 . The quantity h -h₀ is known and constant. The temperature-stabilized duct 4 is used as a reference with respect to the container 1 and the temperature field is integrated yielding the average temperature by means of the barometric equation. To get an idea of the magnitudes of the differential pressures generated, representative values were calculated and are given in Fig. 3.

In another example it is possible to use a first Beattie-Bridgeman equation of state (EOS) , yielding : p = RTp + (RTK - M)p² - (RTKL - MN)p³ with K, L, M and N being fluid-specific parameters. This is well-known and will not be explained in more detail here.

In yet another example it is possible to use a truncated form of the Beattie-Bridgeman EOS, yielding: p = RTp + (RTB - A)p² with A, B = fluid-specific parameters. Other equations of state, e.g. the van der Waals EOS, the virial EOS, the Peng-Robinson EOS etc., may readily be used. Depending on the fluid in the container 1 a suitable EOS can be chosen that best approximates the fluid behaviour.

It is also possible to derive a mass of the fluid inside the container 1 from the average density and the known volume of the container 1, and/or a normalized pressure from the locally measured pressures using the derived average temperature.

Fig. 3 shows a diagram showing example curves of an average physical parameter derived by the method according to the invention and by a state equation. The graph shows a plot of the average temperature T_av (i.e. T_av) versus the differential pressure Δρ. The curve "a" is obtained from an experiment performed in an apparatus similar to that shown schematically in Fig. 2 with SF_S as filling gas. In the experiment a left leg (duct) stands for the duct 4 according to the invention and a right leg (duct) stands for a container like the container 1 described herein. The left leg of the setup was temperature stabilized to a known temperature while the right leg was regulated to preset, known, average temperatures and Δρ was measured using a differential pressure sensor located at the top connection between the two legs. Curve "b" represents theoretical values computed using the Beattie Bridgeman EOS. As can be seen, the experimentally obtained values by using the claimed measurement method agree very well with theoretical computations, thus confirming the suitability of the method for deriving the average physical parameter.

By providing the claimed method for determining an average physical parameter of a fluid in a closable container 1 it is possible to derive an average physical parameter with great accuracy and considerably faster than the presently available methods. Furthermore, it is possible to reduce cost of the measurement device la, because only two sensors are used instead of arrays of sensors. This also results in an easier setup of the measurement .

In embodiments, the fluid used in the encapsulated container 1 or electric apparatus 1 can be SF6 gas or any other dielectric insulation medium, may it be gaseous and/or liquid, and in particular can be a dielectric insulation gas or arc quenching gas. Such dielectric insulation medium can for example encompass media comprising an organofluorine compound, such organo- fluorine compound being selected from the group consisting of: a fluoroether, a fluoroamine, a fluoro- ketone, an oxirane, a hydrofluorolefin, and mixtures thereof; and preferably being a fluoroketone and/or a fluoroether, more preferably a perfluoroketone and/or a hydrofluoroether . Herein, the terms "fluoroether", "fluoroamine" and "fluoroketone" refer to at least partially fluorinated compounds. In particular, the term "fluoroether" encompasses both hydrofluoroethers and perfluoroethers , the term "fluoroamine" encompasses both hydrofluoroamines and perfluoroamines , and the term "fluoroketone" encompasses both hydrofluoroketones and perfluoroketones . It can thereby be preferred that the fluoroether, the fluoroamine, the fluoroketone and the oxirane are fully fluorinated, i.e. perfluorinated .

In particular, the term "fluoroketone" as used in the context of the present invention shall be interpreted broadly and shall encompass both fluoromonoketones and fluorodiketones or generally fluoropolyketones . The term shall also encompass both saturated compounds and unsaturated compounds including double and/or triple bonds between carbon atoms. The at least partially fluorinated alkyl chain of the fluoroketones can be linear or branched and can optionally form a ring.

In particular, the fluoroketone can be a fluoromonoketone and/or may also comprise heteroatoms, such as at least one of a nitrogen atom, oxygen atom and sulphur atom, replacing one or more carbon atoms. More preferably, the fluoromonoketone, in particular perfluoroketone, shall have from 3 to 15 or from 4 to 12 carbon atoms and particularly from 5 to 9 carbon atoms. Most preferably, it may comprise exactly 5 carbon atoms and/or exactly 6 carbon atoms and/or exactly 7 carbon atoms and/or exactly 8 carbon atoms.

The dielectric insulation medium can further comprise a background gas or carrier gas different from the organofluorine compound, in particular different from the fluoroether, the fluoroamine, the fluoroketone, the oxirane and the hydrofluorolefin and preferably can be selected from the group consisting of: air, ¾, <¾, CO₂, a noble gas, H₂; O₂, NO, ₂O, fluorocarbons and in particular perfluorocarbons and preferably CF₄, CF₃I, SF₆, and mixtures thereof.

Throughout this application it is emphasized that the method and device are useful for determining an average physical parameter, in particular an average temperature, for any type of closeable container 1, independent of the application or the nature of the enclosed fluid, in particular enclosed gas. The application may be for example for large gas storage vessels, e.g. in the chemical engineering, petrochemical industry, gas manufacturing, electro-technical industry, etc .

While there are shown and described presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may otherwise variously be embodied and practised within the scope of the following claims. Therefore, terms like "preferred" or "in particular" or "particularly" or "advantageously", etc. signify optional and exemplary embodiments only.

List of reference numerals gas insulated switchgear

measurement device

differential pressure sensor

density sensor, first pressure sensor duct

thermal insulation element

voltage source

flow preventing element

enclosure of GIS

busbar (electrical conductor)

insulating layer

duct wall, duct walls

first opening of duct

second opening of duct

temperature regulating device first curve of average physical parameter second curve of average physical parameter height at the bottom of duct

height difference between sensors

vertical axis

T_av, T_av [K] = average temperature [in Kelvin] Δρ, dp [Pa] = pressure difference [in Pascal] .

Claims

1. Method for determining an average physical parameter (T_av, Pnorn Pnorm) of a fluid in a closable container (1), comprising the steps of:

a) providing a duct (4) with a first opening (11a) and a second opening (lib),

b) positioning a flow preventing element (7) at a first level (h) in such a way that the flow preventing element (7) extends across an entire cross- section of the duct (4),

c) positioning a density sensor (3) and/or a first pressure sensor (3) at a second level (h₀) inside the duct (4) or inside the container (1),

d) attaching the duct (4) to the container (1) in such a way that the first opening (11a) is connected to a first container opening and the second opening (lib) is connected to a second container opening, e) defining a reference duct temperature ( Ti ) , f) determining, by means of a differential pressure sensor (2) arranged between both sides of the flow preventing element (7) or by means of a second and a third pressure sensor arranged on either side of the flow preventing element (7) , a local differential pressure as a difference between a first pressure of the fluid on the one side of the flow preventing element (7) and a second pressure of the fluid on the other side of the flow preventing element (7),

g) measuring a local density of the fluid by the density sensor (3) and/or measuring a local pressure by the first pressure sensor (3) , and

h) deriving the average physical parameter from the local differential pressure, from the local density or the local pressure, and from the reference duct temperature (ΊΊ) .

2. Method according to claim 1, wherein the average physical parameter is an average temperature (T_av) of the fluid and/or an average density (p_norm) of the fluid and/or a temperature-normalized pressure (p_n0rm) of the fluid and/or an average particle density of the fluid and/or a fluid mass.

3. Method according to any one of the preceding claims, further comprising the step or steps of i) selecting the average physical parameter to be an average density (p_norm) of the fluid and deriving a mass of the fluid inside the container (1) from the average density (p_norm) and the known volume of the container (1) , and/or

j ) selecting the average physical parameter to be an average temperature (T_av) and/or a temperature- normalized pressure (p_norm) of the fluid and deriving the temperature-normalized pressure (p_norm) of the fluid from locally measured pressures using the derived average temperature (T_av) ·

4. Method according to any one of the preceding claims, wherein the average physical parameter is selected to be an average temperature (T_av) of the fluid, and the average temperature (T_av) of the fluid is derived from the local differential pressure, the local density or the local pressure and the reference duct temperature (Ti) by applying a barometric formula.

5. Method according to any one of the preceding claims, wherein the average physical parameter is selected to be an average temperature (T_av) and/or an average density (pnorm) of the fluid, and the average density (pnorm) of the fluid is derived by correcting the local density of the fluid with the average temperature (T_av) of the fluid.

6. Method according to any one of the preceding claims, wherein an average temperature T_av of the fluid is determined by the equation

1 RAp(h) 1

T_av p(h₀ )Mg(h - h₀ ) T_x

wherein R = universal gas constant, h = first level of the fluid where the flow preventing element (7) is positioned, h₀ = second level of the fluid where the first pressure sensor (3) is positioned, Ap(h) = differential pressure between both sides of the flow- preventing element (7), p(h₀) = local pressure measured by the first pressure sensor (3) , M = molar mass of the fluid, g = gravitational acceleration, h-h_Q = difference between the first level and the second level, and ΊΊ = temperature of the duct wall (10) or duct walls (10).

7. Method according to any one of the preceding claims, wherein an average or temperature-normalized density _n0rm of the fluid is determined by the equation

wherein h = first level of the fluid where the flow preventing element (7) is positioned, h_Q = second level of the fluid where the density sensor (3) is positioned, Ap(h) = differential pressure between both sides of the flow-preventing element (7), g = gravitational acceleration, h-h_D = difference between the first level and the second level, and meas = local density measured by the density sensor (3) .

8. Method according to any one of the preceding claims, wherein a temperature-normalized pressure p_n0rm of the fluid is determined by equation r_norm/2^, _av, with T_norm = normalization temperature, and 1/Tav being given, by 1 _ RAp(h) _

T_av ^~ p(h₀)Mg(h-h₀) ⁺ T] ' wherein R = universal gas constant, h = first level of the fluid where the flow preventing element (7) is positioned, h_D = second level of the fluid where the first pressure sensor (3) is positioned, Δρ(η) = differential pressure between both sides of the flow- preventing element (7), p(h_Q) = local pressure measured by the first pressure sensor (3), M = molar mass of the fluid, g = gravitational acceleration, h-h₀ = difference between the first level and the second level, and T_x = temperature of the duct wall (10) or duct walls (10) .

9. Method according to any one of the preceding claims, wherein

the first opening (11a) is an upper opening (lib) and the second opening (lib) is a lower opening (lib) of the duct (4), and/or

the flow preventing element (7) is arranged inside the duct (4), and/or

the first level (h) and the second level (h₀) are at different heights in the gravitational field.

10. Method according to any one of the preceding claims, wherein the reference duct temperature (Ti) is defined along a duct extension from the second opening (lib) up to the flow preventing element (7), or the reference duct temperature (ΊΊ) is defined along the whole duct (4 ) .

11. Method according to any one of the preceding claims, wherein in step e) defining a reference duct temperature (Ti) is done by stabilizing a temperature of the fluid inside the duct (4) to a constant reference value (Ti) , and/or by stabilizing a temperature of the duct wall (10) or duct walls (10) to a constant reference value (Ti) , and/or by determining an average fluid temperature (Τχ) inside the duct (4), and/or by determining an average duct wall temperature ( i) .

12. Method according to any one of the preceding claims, wherein

the reference duct temperature (ΤΊ) , in particular an average fluid temperature (Ti) inside the duct (4) and/or an average duct wall temperature (ΊΊ) , is or are determined from at least one temperature value, preferably from a plurality of temperature values, measured along the duct (4), and/or

the duct (4) is attached to an outside wall of the container (1) .

13. Method according to any one of the preceding claims, wherein in step g) measuring the local density and/or the local pressure of the fluid comprises determining the local temperature of the fluid.

14. Measurement device (la) for carrying out the method according to any one of the preceding claims, the measurement device (la) comprising

a duct (4) with at least one wall (10) and a first opening (11a) and a second opening (lib) ,

a flow preventing element (7) arranged at a first level (h) of the duct (4),

a density sensor (3) and/or a first pressure sensor (3) arranged at a second level (h₀) of the duct (4) ,

a differential pressure sensor (2) or a second and a third pressure sensor arranged at the flow preventing element (7) for measuring a local differential pressure between a first pressure of a fluid on the one side of the flow preventing element (7) and a second pressure of the fluid on the other side of the flow preventing element (7) .

15. Measurement device (la) according to claim 14, wherein the flow preventing element (7) is for preventing a flow through the duct (4), in particular wherein the flow preventing element (7) is for preventing a flow of the fluid out of or into the duct (4) through the first opening (11a) .

16. Measurement device (la) according to any one of the preceding claims 14-15, wherein the first opening (11a) and the second opening (lib) of the duct

(4) are adapted to be connected to respective openings of a closable container (1), particularly wherein a volume of the duct (4) has a value in a range between 10 times and 1' 000' 000 times smaller than a volume of the closable container ( 1 ) .

17. Measurement device (la) according to claim 16, wherein the duct (4) is connected to the closable container (1) via a thermal insulation element

(5) , particularly via a PTFE adapter.

18. Measurement device (la) according to any one of the preceding claims 14 to 17, wherein the flow preventing element (7) is arranged inside the duct (4) in such a way that the flow preventing element (7) extends across an entire cross-section of the duct (4), and wherein the density sensor (3) and/or the first pressure sensor (3) is or are positioned inside the duct (4) or inside the container (1) .

19. Measurement device (la) according to any one of the preceding claims 14 to 18, wherein

the flow preventing element (7) is a flexible membrane (7) or a rigid wall (7), and/or

the second level is lower than the first level, as seen in a gravitational vertical direction (z), or vice-versa.

20. Measurement device (la) according to any one of the preceding claims 14 to 19, wherein the flow preventing element (7) entirely covers one of the first or the second opening (11a, lib), and the density sensor

(3) and/or the first pressure sensor (3) is or are positioned at the other opening (lib, 11a).

21. Measurement device (la) according to any one of the claims 14 to 20, further comprising an insulating layer (9) for thermally insulating the duct

(4) from its environment, particularly an insulation sleeve (9) covering substantially entirely the at least one wall (10) of the duct (4).

22. Measurement device (la) according to any one of the claims 14 to 21, further comprising a temperature regulating device (12) for regulating a temperature of the at least one wall (10) of the duct (4), particularly the temperature-regulating device (12) being a heating band (12) .

23. Measurement device (la) according to claims 21 and 22, wherein the temperature regulating device (12) is arranged between the at least one wall (10) of the duct (4) and the insulating layer (9).

24. Measurement device (la) according to any one of the claims 14 to 23, further comprising a valve for each one of the first opening (11a) and the second opening (lib) of the duct (4) for avoiding a fluid escape from the closable container (1) to an environment atmosphere of the closable container (1) and the duct (4), when the measurement device (la) is connected to the closable container (1).

25. Measurement device (la) according to any one of the claims 14 to 24, further comprising a valve arrangement and/or a pump arrangement for evacuating gases from the duct (4) prior to establishing a connection between the measurement device (la) and the closable container (1) .

26. Measurement arrangement comprising a measurement device (la) according to any one of the claims 14 to 25 and a closable container (1), wherein the duct (4) of the measurement device (la) is attached to an outside wall of the container (1) in such a way that its first opening (11a) is connected to a first container opening and its second opening (lib) is connected to a second container opening, further comprising at least one data acquisition and processing device for retrieving sensor data from the density sensor (3) and/or from the first pressure sensor (3) and/or from the differential pressure sensor (2) and/or from the second and/or the third pressure sensor of the measurement device (la) and for computing at least a value of an average physical parameter ( T_av, Pnorm_? Pnorm) of the fluid in the container (1) .

27. Measurement arrangement according to claim 26, wherein the average physical parameter (T_av, pnorm, Pnorm ) is an average temperature (T_av) of the fluid and/or an average density ( p_norm) of the fluid and/or a temperature-normalized pressure (p_norm) of the fluid and/or an average particle density of the fluid and/or a fluid mass.

28. Use of the measurement device (la) according to any one of the claims 14 to 25 for measuring, by means of the method according to any one of the claims 1 to 13, a leakage of fluid out of or into a fluid-insulated switchgear (1), particularly a gas- insulated switchgear (1) .