Method and apparatus for the quantitative determination of the dissolved gas content of water-containing fluid samples

Abstract

2.1. In the case of alternative synthetic and natural insulating fluids, which may have a high water content, the water content also extracted under vacuum leads to excessively high total gas contents of the dissolved gases, resulting in incorrect gas-in-oil analysis results. In water-containing fluid samples, the dissolved gas content should be determined by vacuum degassing without interference from the water content. 2.2. For this purpose, the fluid sample to be examined is dosed into an evacuated degassing vessel and simultaneously the pressure and moisture content in the gas space of the degassing vessel are recorded before and after dosing in order to subsequently correct the measured pressure increase by the moisture content measured in the gas. 2.3. The method according to the invention can be used in particular for the determination of gases in alternative fluids for use in transformers, for lubricating and engine oils, hydraulic fluids, vegetable oils and for measuring the degassing of battery electrolytes.

Description

[0001] The invention relates to a method and a device for the quantitative determination of the dissolved gas content of water-containing fluid samples.

[0002] To this day, mineral oil is predominantly used for insulation and cooling in oil-filled power transformers. For sustainability reasons, mineral oils are increasingly being replaced in power transformers by alternative insulating fluids such as natural or synthetic ester fluids (pentaerythritol tetraesters), silicone oils, or synthetic insulating fluids, e.g., produced using the gas-to-liquid (GtL) process.

[0003] The advantages of alternative insulating fluids lie particularly in their improved environmental compatibility and enhanced electrical properties (DE11 2011 104 579). Therefore, they are primarily used in compact transformers or in specific locations where higher demands are placed on temperature resistance or environmental compatibility. The further distribution of alternative insulating fluids is thus expected to increase in the future.

[0004] These alternative insulating oils are highly hygroscopic and, due to their chemical structure, can absorb a higher water content. Alternative oils can absorb several hundred ppm of water without significantly affecting their electrical properties. Synthetic ester fluids can contain water up to over 2000 ppm, and natural ester fluids up to over 1000 ppm at room temperature.

[0005] During the operation of high-voltage transformers, moisture is drawn from the insulating paper by the insulating fluids, so that even in hermetically sealed transformers a continuous increase in the water content of the oil is to be expected.

[0006] The significantly higher water solubility of the alternative insulating fluids, compared to conventional mineral oils which contain only about 1-20 ppm during operation (at a saturation of about 40 ppm), poses a major challenge for the analysis of dissolved gases.

[0007] The determination of gases dissolved in insulating oil is an important method for the early detection of faults in electrical equipment such as power transformers, chokes, and converters. The analysis of dissolved gases in oil is used for the diagnosis and evaluation of oil-filled electrical equipment in accordance with DIN EN 60567 and DIN EN 60599. It is applied to characterize the thermal or electrical condition and to identify acute and insidious faults.

[0008] For a complete condition assessment of the transformer, the following fault gases are analyzed: hydrogen, carbon monoxide, carbon dioxide, as well as the lower hydrocarbons methane, ethane, ethene, ethyne, propane, and propene, and the atmospheric gases oxygen and nitrogen. The targeted analysis of oxygen compared to nitrogen provides additional information on oxidation stability and the aging process. During transformer operation, other higher-chain hydrocarbons > C3 are also produced, e.g., butane, butene, butyne, butadiene, etc., which are also dissolved in the oil but are usually not individually detected analytically. Therefore, the total gas content is of great importance for the subsequent analysis.

[0009] To detect dissolved gases in the insulating fluid, the gas must first be separated from the insulating fluid so that it can then be analyzed, for example, by gas chromatography. The best-known methods for gas separation are the headspace method and vacuum degassing, in particular full vacuum degassing, as described, for example, in DE 102 52 652.

[0010] The headspace method is a pressureless equilibrium method, and the solubility coefficients of gases and water are crucial. Therefore, calibration is typically performed using liquid oil standards. However, only mineral oil-based standards without water are available. Different insulating fluids exhibit significant differences in their Ostwald coefficients. Consequently, the absolute concentrations of leakage gases cannot be determined with sufficient accuracy. Furthermore, aged insulating fluids have altered Ostwald coefficients. The release of water into the gas equilibrium is therefore dependent on the type of fluid and its state of aging, making correction impossible.

[0011] For partial vacuum degassing, the oils are degassed until an equilibrium of the gas components with the overlying atmosphere is reached. This gas phase is then separated and analyzed by gas chromatography. However, since it cannot be assumed that the water content in the oil affects the gas equilibrium in the same way for every type of insulating oil and at every concentration, no conclusion can be drawn about the contribution to the total gas content from measuring the water content in the oil.

[0012] In full vacuum degassing, a predefined volume of the oil sample is dosed into a vacuum-sealed extraction vessel. Under vacuum, the gases are completely separated from the oil. Simultaneously, the pressure increase in the vacuum vessel is measured. From this pressure increase, the total volume of dissolved gases can be determined, which, relative to the oil volume, indicates the total gas content in the oil.

[0013] The gases extracted under vacuum are then collected and analyzed in a gas analyzer, such as a gas chromatograph. The concentration of the gases in the gas mixture, determined by gas chromatography, is then converted to the concentration of the gases dissolved in the oil using the total gas content.

[0014] With the mineral oils used so far, the existing water content in the low range < 20 ppm does not pose a problem during the vacuum extraction process, since although the water also evaporates under vacuum, its proportion is negligible compared to the dissolved gases. Therefore, it has not been necessary to consider the water content until now.

[0015] Experience with alternative synthetic and natural insulating fluids, which can have a 100 times higher water content, has shown that the water content also extracted in a vacuum leads to excessively high total gas contents of the dissolved gases, ultimately producing incorrect results in gas-in-oil analysis.

[0016] To complicate matters further, water from oils, especially operating oils, degasses only incompletely and uncontrollably under vacuum, meaning that a direct correlation between pressure increase and water content in the oil, particularly due to the oil's age, can no longer be considered constant. The undegassed water remaining in the oil, on the other hand, contributes nothing to the pressure increase and therefore has no effect on distorting measurements. The degassing of water from various alternative insulating fluids also varies considerably under vacuum.

[0017] This means that the existing vacuum degassing methods are not applicable to fluid samples containing water.

[0018] The invention specified in claims 1 to 10 is based on the problem of being able to determine the dissolved gas content in water-containing fluid samples by means of vacuum degassing without disturbing influence from the water content.

[0019] The problem is solved according to claims 1 to 7 by dosing the fluid sample to be examined into an evacuated degassing vessel and simultaneously recording the pressure and moisture content in the gas space of the degassing vessel before and after dosing in order to subsequently correct the measured pressure increase by the moisture content measured in the gas.

[0020] To carry out the method according to the invention, a device according to claims 8 to 10 is used, which consists of a conventional degassing vessel, which is equipped with a vacuum pump, an oil inlet valve for fluid metering and two or more sensors for measuring pressure and humidity in the gas space of the degassing vessel.

[0021] An advantage of the present method and apparatus is that the degassed fraction of water from the insulating fluids, which is registered via the humidity measurement in the gas, has an identical effect on the total gas content, even though the different fluid samples degasse water to varying degrees. This means that the determined correction function for the contribution of water vapor in the gas space is independent of the type of insulating fluid, which greatly simplifies its application to used oils or mixtures of insulating fluids.

[0022] Another advantage is that the corrected total gas content allows the determination of the dissolved gases actually contained in the water-containing fluid sample.

[0023] The oil dosing and degassing process, as well as the data recording process, are automatically controlled, thus eliminating external influences and measurement uncertainties due to asynchronous processes.

[0024] Before analysis, the degassing vessel is first degassed to remove residual air and moisture. The vacuum pressure and relative humidity in the degassing vessel are then recorded, and the fluid sample to be analyzed is metered in via the oil inlet valve. After metering, the pressure and moisture content of the gas in the degassing vessel are measured. The fluid sample is then removed from the degassing vessel.

[0025] The measured pressure increase is corrected for the proportion of the pressure increase caused by the moisture measurement in the gas, and the total gas content is calculated. The necessary correction function can be determined using gas-free fluid standards with known moisture contents. This calibration function is solely device-specific and not dependent on the type of fluid.

[0026] The sensors for pressure and humidity are vacuum-stable, contain no gas or water themselves and therefore do not contribute to measurement distortion.

[0027] The method according to the invention can be used in particular for determining gases in alternative fluids for use in transformers, for lubricating and engine oils, hydraulic fluids, vegetable oils and for measuring the degassing of battery electrolytes. Examples of implementation

[0028] They show: Fig. 1: Schematic diagram of an arrangement for degassing water-containing fluid samples with pressure and humidity sensor Fig. 2: Calibration function for various fluid samples Example 1

[0029] Analysis of the total gas content in water-containing fluid samples according to Fig. 1. To prepare for a determination, the degassing vessel 1 is evacuated. For this purpose, a vacuum pump is connected via valve 4 and evacuation continues until the pressure sensor 5 indicates a stable vacuum. The pressure and humidity content in the gas space of the degassing vessel are recorded.

[0030] Valve 4 is then closed and a known volume of the fluid sample is dosed via valve 2. Afterwards, valve 2 is closed and the pressure and humidity in the gas space of the degassing vessel are recorded again.

[0031] Using the previously created correction function according to Fig. 2. The pressure increase caused by the released water is determined from the moisture content of the gas.

[0032] The total pressure increase is reduced by the pressure increase caused by the released water, and thus the total gas content of dissolved gases in the fluid is calculated. Example 2

[0033] Fig. Figure 2 shows a correction function for converting the moisture content in the degassing vessel (measured as relative humidity rH) to the proportion of total gas content GG (in volume %) for the fluids Midel 1204, Midel 7131 and FR3.

[0034] The correction function is determined by dosing degassed fluids mixed with water into the vacuum apparatus used and measuring the pressure increase caused by the degassed water. List of reference symbols 1 degassing vessel 2 Valves for fluid dosing 3 Valve for fluid drainage 4. Valve for vacuum connection and connection to the analyzer 5 pressure sensor 6 humidity sensor Total gas content rH relative humidity QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] DE 11 2011 104 579

[0003] DE 102 52 652

[0009]

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