Method and apparatus for continuous monitoring of quality and moisture parameters of liquids

Abstract

This application relates to a method and a system for determining aging of a liquid. The method includes steps for measuring a relative water content (rS.sub.1) of the liquid in a first measurement step at a first temperature (T.sub.1), measuring the relative water content (rS.sub.2) at a second temperature (T.sub.2) in a second measurement step, the first and second measurement step are performed such that absolute water content (w) stays essentially unchanged between these two measurements, and based on these at least two measurement values (rS.sub.1, T.sub.1, rS.sub.2, T.sub.2), determining a first water in oil solubility coefficient (B) for the liquid. In accordance with the invention the first water in oil solubility coefficient (B) is monitored essentially continuously in order to determine the liquid quality.

Claims

1. A method for continuous monitoring of quality and moisture parameters of a liquid, comprising: a. measuring relative water saturation of the liquid at a first thermodynamic temperature; b. measuring relative water saturation at a second thermodynamic temperature, provided that absolute water content at the first thermodynamic temperature and absolute water content at the second thermodynamic temperature are essentially equal; c. determining the absolute water content of the liquid, according to the formula: $w=\frac{\mathrm{rS}\exp \left(A-\frac{B}{T}\right)}{100},$ or similar mathematical function comprising the same variables, 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=B+, or similar mathematical function of B, where and are constants known or experimentally obtained for the liquid, and wherein B is 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 temperatures.

2. The method according to claim 1, wherein the liquid quality is determined by a liquid quality index (LQI) as a function of B water in liquid solubility coefficient, according to the formula $\mathrm{LQI}=1-\frac{B_{max}-B}{B_{\mathrm{ma}x}-B_{min}},$ or similar mathematical function of one or more of the same variables, where Bmax and Bmin are a maximum value and a minimum value of B, respectively, known for the liquid.

3. The method according to claim 1, wherein the Henry's law constant k.sub.H for water in the liquid is determined according to the formula k.sub.H=exp(AB/T)/p.sub.s or similar mathematical function of the same variables, where p.sub.s is the saturated water vapour pressure, a function of thermodynamic temperature T.

4. The method in accordance with claim 1, wherein the measurements are performed with two relative saturation sensors along with temperature sensors located in positions of the measurement object such that during measurement there is temperature difference between the sensors.

5. The method in accordance with claim 1, wherein the measurements are performed with two relative saturation sensors along with temperature sensors located in positions of the measurement object such that during measurement the absolute water content (w) remains essentially the same at both locations.

6. A system for continuous monitoring of quality and moisture parameters of a liquid, wherein the system comprises: a tank for holding a quantity of a liquid; a cooling device; a first cooler pipe extending between the tank and the cooling device; a second cooler pipe extending between the tank and the cooling device; wherein the second cooler pipe is located a distance away from the first cooler pipe; a top moisture and temperature probe provided in the first cooler pipe, wherein the top moisture and temperature probe includes an embedded first temperature sensor and first moisture sensor; a bottom moisture and temperature probe provided in the second cooler pipe, wherein the bottom moisture and temperature probe includes an embedded second temperature sensor and second moisture sensor; wherein the first moisture sensor is utilized to take a first measurement of a relative water saturation of the liquid at a first thermodynamic temperature; wherein the second moisture sensor is utilized to take a second measurement of the relative water saturation at a second thermodynamic temperature; performing the first and second measurements such that absolute water content stays essentially unchanged between the first and second measurements; and utilizing at least one of the first and second measurements to determining the absolute water content of the liquid.

7. The system, according to claim 6, wherein the above system is also used to determine the liquid quality as a function of solubility coefficient B.

8. The system, according to claim 6, wherein the above system is also used to determine the Henry's Law constant for water dissolution in the liquid.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) FIG. 1 is a schematic view of a transformer equipped with moisture and temperature sensors which constitute the apparatus of the present invention.

(2) FIG. 2A is the graphical representation of a relationship between solubility coefficients A and B.

(3) FIG. 2B is another graphical representation of a relationship between solubility coefficients A and B.

(4) FIG. 3 is a schematic view of a transformer equipped with moisture and temperature sensors in accordance with another embodiment of the invention.

(5) FIG. 4 is a schematic view of a transformer equipped with moisture and temperature sensors in accordance with another embodiment of the invention.

(6) FIG. 5 is a schematic view of an alternative embodiment in accordance with the invention.

DETAILED DESCRIPTION

(7) 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 liquid-filled electrical equipment, such as instrument transformers, autotransformers, rectifier transformers, reactors and tap changers.

(8) 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.

(9) FIG. 1 illustrates an example of a power transformer comprising a tank 10, top cooler pipe 20; top moisture and temperature probe 30 with embedded top temperature sensor 30A and top moisture sensor 30B; bottom moisture probe 40 with embedded bottom temperature sensor 40A and bottom moisture sensor 40B; and a bottom cooler pipe 50. All said sensors are commercially available.

(10) The insulating liquid is heated by electrical losses caused by alternating current flowing through the windings 60 in a way that the liquid is usually hotter at the top and it is cooler at the bottom of a transformer tank 10. In such an arrangement the insulating liquid serves as a coolant as well as an insulator. The liquid is cooled by the cooling device 70, 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.

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

(12) To determine the absolute water content of the liquid the following formula is well known in the art:

(13) $\begin{array}{cc}w=\mathrm{rS}\exp \left(-A-\frac{B}{T}\right)/100,& \left(1\right)\end{array}$
where w is the absolute water content (also known as water concentration) of the liquid, expressed in mg kg; rSrelative 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.

(14) 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. 2.

(15) 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:

(16) $\begin{array}{cc}{\mathrm{rS}}_{\mathrm{to}}={\mathrm{rS}}_{\mathrm{bo}}\exp \left(B\left(\frac{1}{T_{bo}}-\frac{1}{T_{\mathrm{to}}}\right)\right),& \left(2\right)\end{array}$
where rS.sub.to is the relative saturation of water in liquid at the top cooler pipe location; rS.sub.bo is the relative saturation of water in liquid at the bottom cooler pipe location; T.sub.to is the thermodynamic temperature at the top cooler pipe location and T.sub.bo 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.sub.1 to T.sub.2 equation (2) is also valid for a single location. In this case relative saturation at the second temperature rS.sub.2 can be determined from the relative saturation at the first temperature rS.sub.1. Then equation (2) can be rewritten as:

(17) $\begin{array}{cc}{\mathrm{rS}}_2-{\mathrm{rS}}_1\exp \left(B\left(\frac{1}{T_2}-\frac{1}{T_2}\right)\right),& \left(2a\right)\end{array}$

(18) 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.

(19) 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.

(20) According to the invention the determination of moisture parameters is conducted as follows:

(21) 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 (2) as:

(22) $\begin{array}{cc}B=\frac{\ln \left(\frac{{\mathrm{rS}}_1}{{\mathrm{rS}}_2}\right)}{\left(\frac{1}{T_2}-\frac{2}{T_1}\right)}& \left(3\right)\end{array}$

(23) The substantial temperature gradient is e.g. more than 5 C. 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 rS1, T1, rS2 and T2.

(24) Then the Liquid Quality Index (LQI) could be calculated as a function of B, e.g:

(25) $\begin{array}{cc}\mathrm{LQI}=1-\frac{B_{\mathrm{ma}x}-B}{B_{\mathrm{ma}x}-B_{min}},& \left(4\right)\end{array}$
where B.sub.max and B.sub.min are the highest and lowest values of the solubility coefficient B, which varies from B.sub.max, representing a new clean insulating liquid, to B.sub.min, representing very aged (end of life) liquid. For example, for transformer mineral oil these values are known to be 3900 and 3100 respectively. In accordance with the invention also LQI could be defined in other mathematical presentations, e.g., as a function of B

(26) According to the current embodiment the LQI varies between 0 and 1.1 (one) is assigned to a new clean, not contaminated liquid and 0 (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.

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

(28) (a) on-line, applying a linear relationship between A and B:
A=B+,(5)
where and are unique for a certain type of insulating liquid. For example, for mineral oil these could be obtained by performing a linear regression as shown in FIG. 2A. In other words the dots in FIG. 2 are laboratory measurements for values A and B and and are solved from this information by linear regression. In accordance with the invention also other, more complicated mathematical regressions could be used in order to determine the relation between A and B. e.g. as shown in FIG. 2B.
A=e.sup.B

(29) (b) off-line, using Karl Fisher (KF) titration method according to the equation

(30) $\begin{array}{cc}A=\ln \left(\frac{w}{\mathrm{rS}}100\right)+\frac{B}{T},& \left(6\right)\end{array}$
where w is water concentration of the liquid sample obtained by KF titration.

(31) (c) off-line, adding known amount of water to the liquid sample. This method does not require KF titration and has an obvious advantage of not using the re agents/consumables.

(32) 0$\begin{array}{cc}A=\ln \left(\frac{w}{\mathrm{rS}}100\right)+\frac{B}{T},& \left(7\right)\end{array}$
where w and rS are the known amount of water in mg/kg of liquid added to the solution and the change in relative saturation respectively.

(33) Once the solubility coefficients A and B are determined the absolute water content of the liquid at any of the said locations could be calculated by applying the formula (1) as:

(34) $\begin{array}{cc}w=\frac{\mathrm{rS}\exp \left(A-\frac{B}{T}\right)}{100}& \left(8\right)\end{array}$
where rS and T are measured at the same location.

(35) The Henry's law constant k.sub.H then for the said locations can be calculated by applying the formula:

(36) $\begin{array}{cc}{k}_H=\frac{\exp \left(A-\frac{B}{T}\right)}{p_s}& \left(9\right)\end{array}$

(37) The following World Meteorological Organization (WMO, 2008) formulation for the saturated vapor pressure could be used in (9):
p.sub.s=6.112exp(17.62t/(243.12+t)),
where t is the temperature in Celsius, corresponding to thermodynamic temperature in (9).

(38) In accordance with FIG. 3 the probes 30 and 40 may be positioned directly to the top and bottom parts of the transformer 80 tank 10, because there is a temperature difference between the top and the bottom of the tank 10.

(39) Alternatively in accordance with FIG. 4 the probes 30 an 40 may be positioned at the opposite ends of the input pipe 130 leading to oil drying unit 90.

(40) Alternatively in accordance with FIG. 5 the liquid to be analyzed may be fed to a measurement pipe 140 (like pipes 20, 50 or 130 in FIGS. 1 and 4) provided by a bypass pipe 100 and two valves 110 and 120 for feeding the liquid alternatively to the by-pass pipe 100 or directly to the pipe 140. The by-pass pipe 100 has either cooling or heating function for the liquid, hence the parameters of equations 2 and 2a may be obtained by one probe 30 only by

(41) a) closing valve 120 and opening valve 110 for obtaining parameters at first temperature, and

(42) b) closing valve 110 and opening valve 120 for obtaining parameters at second temperature.

(43) 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.

(44) While the foregoing written description of the invention enables one skilled in the art to make and use what is considered presently to be the best mode thereof, those skilled in the art will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiment, method, and examples herein. The invention should therefore not be limited by the above described embodiment, method, and examples, but by all embodiments and methods within the scope and spirit of the present invention.

(45) 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. 100-1000 measurements with predetermined intervals for the liquid to be evaluated during its lifetime.

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

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

(48) In this application the first rS.sub.1 and second measurement step rS.sub.2 in connection with formulas 2 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 30 and 40. Alternatively the measurement may be performed with short intervals with one probe of FIG. 5. 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.sub.1 and second measurement step rS.sub.2, like intervals of hours or even days.

(49) 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.sub.1 and second measurement step rS.sub.2) are repeated much more frequently.

(50) The coefficient B could be determined by the first rS.sub.1 and second measurement step rS.sub.2 e.g., daily or several times in a day and the value of B would be monitored continuously based on these measurements.

LIST OF REFERENCE NUMBERS

(51) 10 tank 20 top cooler pipe 30 top temperature probe 30A top temperature sensor 30B top moisture sensor 40 bottom temperature probe 40A bottom temperature sensor 40B bottom moisture sensor 50 bottom cooler pipe 60 windings 70 cooling device 80 transformer 90 oil drying unit 100 cooling bypass pipe 110 first valve 120 second valve 130 input pipe 140 measurement pipe

(52) In this application humidity or humidity sensor means in connection with liquids relative saturation or relative saturation sensor.