Patent Description:
A presence of water (also known as moisture) in insulating liquid is one of the greatest concerns for reliable operation of electrical equipment including large, medium and small liquid filled transformers. The water is harmful for both liquid and solid insulation; it accelerates aging and severelly compromises dielectric integrity of any electrical apparatus. Therefore an assessment of water content in both liquid and solid insulation is critical and an essential part of any condition based monitoring programs.

When a power transformer is manufactured its insulation system has low water content. As the transformer ages, water is transferred into the insulation either by ingress from outside the transformer or by chemical decomposition of the insulation materials. The solubility of water in insulating material changes considerably as the insulating liquid ages. The water parameters, including absolute and relative water content in the insulation along with its solubility characteristics, may therefore be used to indicate the condition of the insulating liquid and health of the transformer as a whole.

Traditionally, for determination of water content in the insulating liquid chemical methods, such as Karl Fischer titration, have been used. These methods are labor intensive, use dangerous reagents and can only be used in the laboratory, thus are not suitable for continuous online assessment. Another drawback of the laboratory methods is a possibility for error as a result of measurement uncertainty, cumbersome sampling procedures and human errors.

During the last decade online moisture sensors, based on capacitive polymer thin film technology, have been successfully used for determination of relative saturation of water in oil measured in percentage. These sensors generally do not provide for the determination of absolute water content measured, for example, in mg/kg, and therefore can't be utilized for benchmarking, trending and moisture assessment conforming to relevant industry standards and guidelines.

<CIT> describes a method and apparatus for measuring the water content of a liquid, where the properties of the liquid are measured using both relative-value and absolute-value measurement method. To accomplish this task two different methods are used simultaneously, one is directly measuring relative to saturation value of water content of a liquid and another is a prediction of absolute value of water content by measuring the liquid dielectric constant.

It is well known that dielectric constant is dependent on type, chemical composition, acidity and moisture content of insulating liquid. The invention describes a method to calculate dielectric constant of dry liquid and therefore absolute water content is possible to calculate but as the water content of insulation liquids are low a needed performance of the dielectric constant measurement is difficult to achieve. For those reasons an implementation of the described invention is very limited.

<CIT> describes a method and apparatus for measurement of total water content in a liquid. According to the method, the relative water content (aw) of the liquid is measured in a first measurement step at a first temperature (T1). According to the publication, the temperature of the liquid under measurement is altered from the first temperature (T1) and the relative water content is measured in a second measurement step at the second, altered temperature (T2), whereby, based on these at least two measurement values (aw(<NUM>)T <NUM> aw(<NUM>)T <NUM>), the total water content is determined from the temperature dependence of water dissolution into the liquid under measurement.

<CIT> describes a method where measurements of dielectric characteristics of liquid are used for calculating a quality parameter of the liquid. The method includes a means for measuring opacity of the said liquid along with the means of measuring liquid conductivity. It is known that conductivity of a liquid is dependent on water content, chemical composition, acidity and other contaminants. It is difficult if not impossible to distinguish effects of various parameters on oil quality and degree to which extend liquid is affected by any particular parameter.

Because of their relatively high cost and complexity, none of these methods and devices has achieved any substantial degree of commercial success and there accordingly exists a need for a reliable low cost, simple, method and apparatus for measuring the quality of insulating liquid along with absolute water content and water solubility characteristics of the insulating liquid.

It is, therefore, an objective of the present invention to provide a practical method and apparatus for identifying the absolute water content of insulating liquid, water solubility coefficients, including Henry's law constant for said insulating liquid.

Another objective of the invention is to provide a measure for identifying the condition of insulating liquid, such as liquid quality index.

The present invention is based on the objective for determining quality of a liquid by temperature and humidity measurements.

These and other objects are achieved by the present invention, as hereinafter described and claimed. Thus the invention concerns which method for continuous monitoring of quality and moisture parameters of a liquid, comprising steps for measuring a relative water saturation (rS<NUM>) of the liquid at a first thermodynamic temperature (T<NUM>), measuring a relative water saturation (rS<NUM>) at a second thermodynamic temperature (T<NUM>), provided that absolute water content w<NUM> at the first thermodynamic temperature (T<NUM>) and absolute water content w<NUM> at the second thermodynamic temperature (T<NUM>) are essentially equal, determining the absolute water content from equation <MAT>, where where rS is relative water saturation of the liquid at temperature T; wherein the water in liquid solubility coefficient A is determined according to the formula A = αB + β , where α and β are constants known or experimentally obtained for the liquid and the water in liquid solubility coefficient B is determined as the function of said relative water saturation of the liquid at the first thermodynamic temperature, the relative water saturation of the liquid at the second thermodynamic temperature, and the respective thermodynamic temperatures.

A method preferably includes determining the liquid quality by a liquid quality index (LQI) as a function of B, according to the formula: <MAT> where Bmax and Bmin are the maximum and minimum value of B respectively known for the liquid.

A method preferably includes that the Henry's law constant kH for water in the liquid is determined according to the formula: <MAT> where ps is the saturated water vapour pressure, a function of thermodynamic temperature T.

A method preferably includes that regression method is used as a mathematical relationship.

A method preferably includes that the oil of a transformer is evaluated.

A method preferably includes that the measurements are performed with two humidity sensors including temperature sensors located in positions of the measurement object such that during measurement there is temperature difference between the sensors.

A method preferably includes locating the sensors in different heights relative to each other.

A method preferably includes locating the sensors in in-and outlet of a cooler for the liquid.

A method preferably includes locating the sensors in in- or outlet of an oil dryer.

Another aspect of the invention is a system for continuous monitoring of quality and moisture parameters of a liquid using the above method.

It is a novel approach to measure the insulating liquid quality by observing a change in water solubility characteristics of a liquid.

With help of the invention liquid quality of various objects may be monitored on-line. This is cost saving especially with large units like transformers, where aged liquid may cause serious damage to the transformer. With on line measurement cost are saved also compared to methods where the liquid is tested in laboratory based on samples collected on site from the transformer.

For purposes of illustration only and not to limit generality, the present invention will now be explained with reference to the model of an oil filled large power transformer. However, it should be recognized that the present invention is applicable to other types of liquidfilled electrical equipment, such as instrument transformers, autotransformers, rectifier transformers, reactors and tap changers.

For one skilled in the art it should not be difficult to see that the present invention could also be utilized in a laboratory environment where testing of dielectric liquid for moisture content and the liquid quality. When used in the laboratory environment it should also be recognized that the current invention can be used for non-insulating liquids such as lubricating and hydraulic oils.

<FIG> illustrates an example of a power transformer comprising a tank <NUM>, top cooler pipe <NUM>; top moisture and temperature probe <NUM> with embedded top temperature sensor 30A and top moisture sensor 30B; bottom moisture probe <NUM> with embedded bottom temperature sensor 40A and bottom moisture sensor 40B; and a bottom cooler pipe <NUM>. All said sensors are commercially available.

The insulating liquid is heated by electrical losses caused by alternating current flowing through the windings <NUM> in a way that the liquid is usually hotter at the top and it is cooler at the bottom of a transformer tank <NUM>. In such an arrangement the insulating liquid serves as a coolant as well as an insulator. The liquid is cooled by the cooling device <NUM>, which could be one of various designs, such as a radiator based on natural heat convection or assisted by a fan, or a water cooler assisted by a pump.

The moisture sensors 30B and 40B measure relative saturation of water in the liquid at above locations.

To determine the absolute water content of the liquid the following formula is well known in the art: <MAT> where w is the absolute water content (also known as water concentration) of the liquid, expressed in mg/kg; rS - relative water saturation in %, which could be measured by one of the said relative saturation sensor, T is a thermodynamic temperature in Kelvin measured by the temperature sensor at the same location as said relative saturation; and A and B are the specific water solubility coefficients. Unfortunately, these A and B coefficients change as the liquid ages, which create difficulty in continuous determination of absolute water content w.

The function of water content at saturation versus temperature is known as water solubility curve, which represents important information about composition of the insulating liquid and its quality. The solubility coefficients are not independent and a relationship between the two is normally observed as depicted in <FIG>.

It is well known in the art that by measuring temperature at two locations and relative water saturation at one of these locations (for example, bottom tank) the second (top) relative saturation value can be determined as: <MAT> where rSto is the relative saturation of water in liquid at the top cooler pipe location; rSbo is the relative saturation of water in liquid at the bottom cooler pipe location; Tto is the thermodynamic temperature at the top cooler pipe location and Tbo is the thermodynamic temperature at the bottom cooler pipe location. In case where water content of the liquid remains constant during temperature change from T<NUM> to T<NUM> equation (<NUM>) is also valid for a single location. In this case relative saturation at the second temperature rS<NUM> can be determined from the relative saturation at the first temperature rS<NUM> Then equation (<NUM>) can be rewritten as: <MAT>.

This is a core idea of the present invention to measure and monitor the water solubility coefficients for determination of absolute moisture content and quality of insulating liquid.

It is well recognized in the art that during a transformer operation the solubility of water changes as the liquid ages. Therefore by monitoring the change in water solubility it is possible to relate that change to a change in liquid quality.

According to the invention the determination of moisture parameters is conducted as follows:
Firstly, provided that there is a substantial temperature gradient between top and bottom locations of the sensors 30A and 40A, the water in oil solubility coefficient B could be determined from (<NUM>) as: <MAT>.

The substantial temperature gradient is e.g. more than <NUM>. Of course, the water in oil solubility coefficient B could also be determined mathematically in several other ways and this formula is only an example of one embodiment of the invention. In other words the first water in oil solubility coefficient is defined as a function of said rS<NUM>, T<NUM>, rS<NUM> and T<NUM>.

Then the Liquid Quality Index (LQI) could be calculated as a function of B, e. g: <MAT> where Bmax and Bmin are the highest and lowest values of the solubility coefficient B, which varies from Bmax , representing a new clean insulating liquid, to Bmin , representing very aged (end of life) liquid. For example, for transformer mineral oil these values are known to be <NUM> and <NUM> respectively. In accordance with the invention also LQI could be defined in other mathematical presentations, e.g., as a function of B.

According to the current embodiment the LQI varies between <NUM> and <NUM>. <NUM> (one) is assigned to a new clean, not contaminated liquid and <NUM> (zero) is assigned to severely aged liquid, which needs to be replaced or reclaimed. Any other value of B coefficient is attributed to intermediate state of the liquid quality.

For determination of absolute water content of the said insulation liquid the calculation of solubility coefficient A is conducted following one of the methods:.

Once the solubility coefficients A and Bare determined the absolute water content of the liquid at any of the said locations could be calculated by applying the formula (<NUM>) as: <MAT> where rS and T are measured at the same location.

The Henry's law constant kH then for the said locations can be calculated by applying the formula: <MAT>.

The following World Meteorological Organization (WMO, <NUM>) formulation for the saturated vapor pressure could be used in (<NUM>): <MAT> where t is the temperature in Celsius, corresponding to thermodynamic temperature in (<NUM>).

The probes <NUM> and <NUM> may be positioned directly to the top and bottom parts of the transformer <NUM> tank <NUM>, because there is a temperature difference between the top and the bottom of the tank <NUM>.

The advantages of the present invention include, without limitation, that it is a method for determination of absolute water content of the insulating liquid and its water solubility characteristics, including Henry's law constant. Further, the method allows determination of liquid quality with the assistance of newly introduced liquid quality index.

Essentially continuous measurement means in this application e.g. regular measurements within predetermined intervals like minutes, hours or days. This could mean also e.g. <NUM> - <NUM> measurements with predetermined intervals for the liquid to be evaluated during its lifetime.

Mathematical equivalent in this application means a presentation using essentially the same variables forming the formula.

The liquids to be measured may be e.g. quenching oils used in metal heat treatment processes, refrigerant liquids and heat transfer liquids.

In this application the first rS<NUM> and second measurement step rS<NUM> in connection with formulas <NUM> and 2a are performed such that absolute water content w stays essentially unchanged between these two measurements. In practice this means with two probes typically a simultaneous measurement with the two probes <NUM> and <NUM>. Alternatively the measurement may be performed with short intervals with one probe. The intervals may be some minutes or even hours. In stabile conditions the absolute water content may stay unchanged for long periods, and therefore the method may be performed successfully also with rather long intervals between first rS<NUM> and second measurement step rS<NUM>, like intervals of hours or even days.

Monitoring coefficient B essentially continuously means in this application monitoring the coefficient B for weeks, months or even years, in other words long term monitoring, where the above two measurements (the first rS<NUM> and second measurement step rS<NUM>) are repeated much more frequently.

The coefficient B could be determined by the first rS<NUM> and second measurement step rS<NUM> e. g, daily or several times in a day and the value of B would be monitored continuously based on these measurements using one of the optimization technique.

Claim 1:
A method for continuously monitoring of quality and moisture parameters of a liquid, comprising:
(a) measuring a relative water saturation (rS<NUM>) of the liquid at a first thermodynamic temperature (T<NUM>),
(b) measuring the relative water saturation (rS<NUM>) at a second thermodynamic temperature (T<NUM>), provided that absolute water content (w<NUM>) at the first thermodynamic temperature (T<NUM>) and absolute water content (w<NUM>) at the second thermodynamic temperature (T<NUM>) are essentially equal;
characterized in that the method further comprises a step of
(c) determining the absolute water content of the liquid (w) according to the formula: <MAT>
where rS is relative water saturation of the liquid at temperature T,
wherein the water in liquid solubility coefficient A is determined according to the formula A = αB + β , where α and β are constants known or experimentally obtained for the liquid,
and the water in liquid solubility coefficient B is determined as the function of said relative water saturation of the liquid at the first thermodynamic temperature, the relative water saturation of the liquid at the second thermodynamic temperature, and the respective thermodynamic temperatures.