SYSTEM AND METHOD FOR ANALYZING DISSOLVED GAS IN ELECTRICAL INSULATING FLUID

Abstract

A gas analysis system for analyzing dissolved gas in electrical insulating fluid includes a trap that selectively captures or releases one or more gases, a temperature control device for controlling a temperature of the trap that determining whether the trap is in a gas capture mode or a gas release mode, and a gas sensor for analyzing the gas that was not selectively captured by the trap. The trap may be heated to a first temperature that enables the gas to be adsorbed by the trap, and the trap may be heated to a second temperature that enables the gas to be desorbed by the trap. The gas analysis system may further include a gas flow diverter for directing gas flow past the trap. The captured or released gases may be interfering matrix gases. A method and analyzer for analyzing dissolved gas in an electrical insulating fluid are also disclosed.

Claims

1. A gas analysis system for analyzing dissolved gas in electrical insulating fluid, comprising: a trap configured to selectively capture or release one or more gases; a temperature control device for controlling a temperature of the trap, the temperature determining whether the trap is in a gas capture mode or in a gas release mode; and a gas sensor for analyzing the gas that was not selectively captured by the trap.

2. The gas analysis system of claim 1, wherein the trap is configured to capture one or more of the gases using a process of adsorption.

3. The gas analysis system of claim 1, wherein the trap releases one or more of the gases using a process of desorption.

4. The gas analysis system of claim 2, wherein the trap is heated to a first temperature to enable the gas to be adsorbed by the trap.

5. The gas analysis system of claim 3, wherein the trap is heated to a second temperature to enable the gas to be desorbed by the trap.

6. The gas analysis system of claim 1 further comprising a gas flow diverter for directing gas flow past the trap.

7. The gas analysis system of claim 1, wherein the captured or released gases are interfering matrix gases.

8. A method for analyzing dissolved gas in an electrical insulating fluid comprising the steps of: receiving one or more types of gases; selectively capturing or releasing a subset of the received gases using a chemical trap; setting a temperature of the chemical trap, the temperature determining whether the chemical trap is in a gas capture mode of operation or a gas release mode of operation; and analyzing the gas not selectively captured by the chemical trap.

9. The method of claim 8, wherein the chemical trap captures gas using an adsorption process.

10. The method of claim 8, wherein the chemical trap releases captured gas using a desorption process.

11. The method of claim 9, wherein the adsorption process is controlled by setting the temperature of the chemical trap to a first predefined value.

12. The method of claim 10, wherein the desorption process is controlled by setting the temperature of the chemical trap to a second predefined value.

13. The method of claim 8, wherein the gas captured by the chemical trap includes mostly interfering matrix gas.

14. The method of claim 8, wherein the gas released by the chemical trap includes mostly interfering matrix gas.

15. The method of claim 10 further comprising the step of directing gas flow past the chemical trap.

16. An analyzer for analyzing dissolved gas in electrical insulating fluid, comprising: a trap configured to selectively capture one or more interfering matrix gases; a temperature control device configured to control a temperature of the trap at a first selected temperature, wherein the first selected temperature configures the trap to operate in a gas capture mode; and a gas sensor for analyzing the gas that was not selectively captured by the trap, enabling the gas sensor to perform direct measurement of an analyte gas substantially in absence of interfering matrix gas.

17. The analyzer of claim 16, wherein the temperature control device is further configured to control the temperature of the trap at a second selected temperature, wherein the second selected temperature configures the trap to operate in a gas release mode.

18. The analyzer of claim 16, wherein the first selected temperature enables the trap to adsorb interfering matrix gases.

19. The analyzer of claim 17, wherein the second selected temperature enables the trap to desorb interfering matrix gases.

20. The analyzer of claim 16 further comprising a gas flow diverter for directing gas flow past the trap.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The objects, features, and advantages of the present new and novel system will be apparent from the following detailed description with references to the following drawings.

[0022] FIG. 1 is a schematic of one exemplary embodiment of an overall analysis system that incorporates the subject thermal desorption system, a source of gases to be analyzed, and thermal control of the components.

[0023] FIG. 2 is an operational schematic of the thermal desorption system in an exemplary embodiment.

DETAILED DESCRIPTION

[0024] The presently described system is based on a new and novel application of the technique of analytical thermal desorption. Thermal desorption is applied in diverse areas such as air monitoring, polymer analysis, food analysis, breath analysis, and detection of chemical agents for security. However, thermal desorption has never been used to address the problem of matrix gas interference in dissolved gas analysis using spectroscopic gas sensing.

[0025] A significant advantage of the newly described system includes the ability to provide accurate and repeatable correction for matrix gas interference by employing well-characterized adsorptive materials for matrix-gas trapping, not depending on the physicochemical characteristics of the fluid under analysis, completely removing interfering matrix gases from the gas sample, retaining a repeatable and measurable fraction of the analyte gases after matrix-gas trapping, and thermally regenerating the adsorptive material post-analysis. Other advantages may be realized as well, as described below.

[0026] As shown in FIG. 1, in the present exemplary embodiment thermal control of the spectroscopic cell is arranged such that the electrical fluid dissolved-gas extractor and the spectroscopic cell temperature may be controlled at operating temperatures that establish a sufficient positive thermal gradient at the spectroscopic cell and effect suppression of condensation of the less volatile matrix components.

[0027] In an additional step unique to this newly described system, the spectroscopic cell temperature may be elevated periodically while circulating gas through the cell and the absorptive trap such that condensed compounds in the spectroscopic cell that may evaporate from the cell surfaces are trapped in the first section of the adsorptive trap. After sufficient time has elapsed for the majority of the condensed material to be evaporated from the cell and absorbed in the trap, a trap cleaning cycle, as described above, is initiated and the trap is purged of the adsorbed compounds. It should be noted that these less volatile compounds will be completely trapped in the first, less absorptive, section of the trap.

[0028] Once the trap cleaning cycle is complete, the system is returned to its initial state in preparation for continuation of the normal analysis cycles. Such a cell cleaning cycle is initiated as required by the level of cell contamination build-up, which is determined by a background measurement in the absence of analyte compounds. One cycle per day is a practical maximum frequency for cell clean-up cycles; a frequency of one cycle per month represents a practical minimum, although the frequency of cycles may be varied as desired.

[0029] Continuing to refer to FIG. 1, an exemplary embodiment of the new and novel thermal desorption system is shown. It includes an oil pump 101, oil circulation path 105, heat exchanger 102 and gas extractor 112. There is also included a gas switching manifold 123, PAS gas measurement apparatus 122 and a chemical trap 20. The system further includes heaters 111, 112, 7 and insulation 118, 104, 110, 113.

[0030] In actual operation, beginning at a transformer 100 being monitored, transformer dielectric oil 115 containing fault gases 114 to be analyzed, is circulated via the oil pump 101 along the oil circulation path 105. The oil passes through the counter flow heat exchanger 102 to remove or add heat in the direction of the thermal set point Te of the gas extractor 112.

[0031] The system includes four distinct thermal zones. The first thermal zone contains the gas extractor 112 (maintained at Te), the second one contains the gas switching manifold 123 (maintained at Tg), the third one contains the PAS gas measurement apparatus 122 (maintained at Tcell) and the fourth one contains a chemical trap 20 (Ttrap). All zones are independently thermally controlled with heaters 111, 121, 7 and isolated by insulation 118, 104, 110, 113. In addition, the gas extractor thermal set point Te can be bi-directionally controlled with the thermal electric cooler 7, moving the heat from the oil exchanger 8 to the head sink radiator 6 and shed externally to ambient air. The thermal zones temperatures are controlled such that as the extracted sample gas sample travels through the system, it is exposed to increasingly higher temperatures. This prevents oil vapor partials contained within the sample from condensing on any of the PAS surfaces walls, windows and such until they have been either returned to the extractor or vented over board.

[0032] Turning now to FIG. 2, there is illustrated an exemplary configuration of the new and novel thermal desorption system directed to photoacoustic absorption spectroscopy (PAS) measurement for dissolved gas analysis (DGA) of electrical insulating fluids.

[0033] The system includes an incoming gas stream 12, a gas detection cell 1, a rotary valve 2, a gas pump 3, a variable restriction 4, a tubular trap 5, containing one or more suitable adsorbent materials, a pressure transducer 6, a heating means 7, a temperature sensor 8, two three-way electrical solenoid valves 9, 10, and an additional volume 11.

[0034] In operation, in the initial or standby state, sample gas enters the rotary valve at a first connection 12, exits the rotary valve 2 at a second connection 13, flows through a volume 11 to a three-way valve 9. It is then either vented or returns to the sample supply via fourth and fifth connections 14 and 15 on the rotary valve 2. The adsorbent trap 5 is heated to an initial temperature with the heater 7, where that temperature is suitable for the strong adsorption of interfering gases while only weakly adsorbing the analyte gases. By way of example and not limitation, the currently described configuration uses a temperature of 45-65 C.

[0035] In a first step (gas purge) of the analysis process, sample gas is diverted through the gas analysis cell 1 by energizing the three-way valves 9, 10. At the same time, the gas pump 3 is energized, drawing purge gas through the rotary valve 2 from the sixth connection 16 to the seventh connection 17. Purge gas is compressed in the pump 3, flows through the restriction 4, past the pressure transducer 6 and enters the rotary valve 2 at the eighth connection 18. Purge gas exits the rotary valve 2 at the ninth connection 19 and enters the adsorbent trap 5. Purge gas flows through the adsorbent trap 5 under impetus of the pressure drop created by the gas pump across the particulate packing contained therein, flows through the trap 5 and enters the rotary valve 2 at the tenth connection 20. Purge gas exits the rotary valve 2 at the position 21 and is vented. The purpose of this purge gas step is to remove any remaining gases from the trap and tubing prior to starting the adsorption of interfering gases in step 3. The flow rate through the trap 5 may be adjusted with the variable restrictor 4, and the pressure drop across the trap 5 may be observed via the pressure transducer 6. The measured flow rate in this exemplary configuration is 15-22 mL/min, depending on the trap temperature (gas viscosity increases as temperature increases).

[0036] In step 2 (gas measurement I) of the analysis process, the first three-way valve 9, and approximately 1 second later the second three-way valve 10, are de-energized, thereby pneumatically isolating the gas measurement cell 1 while bringing its internal pressure to equal ambient pressure. The gas pump 3 is de-energized, causing the pressure drop across the trap 5 to decay to zero and flow through it to cease after a delay. One or more gas measurements are performed at this time in the gas measurement cell 1. Gas measurements of non-interfered-with gases are complete at this stage. Measurements of interfered-with gases are subsequently performed after removal of interfering gases in the next steps.

[0037] In Step 3 (interfering gas removal) of the analysis process, after initial gas measurement has been completed, the three-way valves 9, 10 are energized. The rotary valve 2 is actuated to its alternate position causing connection 13 to communicate with position 17, 16 with 15, 14 with 19, 18 with 20, and 21 with 12. The gas pump 3 is energized and sample gas plus entrained purge gas are mixed and circulated from the measurement cell 1 through the volume 11, the gas pump 3, the restriction 4, the adsorbent trap 5, and back into the measurement cell 1. The adsorbent material in the trap 5 selectively retains up to one hundred percent of interfering matrix gases while passing through up to one hundred percent of analyte gases. The sample gas is diluted with the purge gas in the entirety of the volume contained in the communicating portions of the system. In addition, the circulating concentrations of the fully adsorbed matrix gases are reduced to nearly zero, while the circulating concentrations of the analyte gases are reduced in proportion to the degree of adsorption of the individual gases onto the adsorptive material in the trap 5 at its current temperature. The concentrations of gases in the gas stream will reach a steady state after multiple rounds of circulation through the trap 5. The adsorption of interfering matrix gases on the trap 5 must be sufficient so that the interfering gases do not break through the trap and re-enter the analytical gas stream during gas circulation.

[0038] In step 4 (gas measurement II), the gas pump 3 is de-energized and gas flow through the trap 5 decays to zero as the pressure at the transducer 6 approaches a steady state approximately equal to or greater than local ambient pressure. At or before achieving ambient pressure, the three-way valves 9, 10 are de-energized and the rotary valve 2 is actuated to its original position. The first three-way valve 9 is momentarily energized such that the pressure in the gas measurement cell 1 becomes equal to ambient pressure. With the first three-way valve 9 closed again, additional gas measurements are performed in the gas cell 1. The concentrations measured in this second series of gas measurements reflect analyte gas concentrations after dilution with purge gas and passage through the trap 5. The original gas concentrations may be calculated using equation 1.

[0039] There exists an optimum time and gas volume flow through the trap such that the circulating analyte gas concentrations reach maximum values after gas circulation begins. This maximum concentration time, t.sub.max is characteristic of the flow rate, trap temperature, trap material, and total volume of the communicating portions of the gas circulation. In the current configuration t.sub.max120 s. The achieved gas concentrations are a function of the degree of sample dilution with purge gas and the strength of adsorption of individual gases in the trap 5:

[00001]$\begin{array}{cc}{c}_g^o=\frac{c_{\mathrm{meas}}.Math.k_g}{f_{\mathrm{dil}}}& \left(1\right)\end{array}$

where c.sub.g.sup.o is the original gas measurement as without interference, c.sub.meas the measured gas concentration of gas g upon completion of this trapping step, f.sub.dil is the dilution factor or volumetric ratio of the sample and purge gas volumes, and k.sub.g is a frontal adsorption coefficient of gas g on the trap 5. The f.sub.dil coefficient can be calculated as:

[00002]$\begin{array}{cc}{f}_{\mathrm{dil}}=\frac{V_A}{V_T}& \left(2\right)\end{array}$

where V.sub.A is the gas volume of the analysis portion of the system shown in FIG. 1 (i.e. the sum of the volumes of the sensor 1 plus the additional volume 11 plus the interconnecting tubing and communicating valve 2 volumes, and V.sub.T is the total gas volume of the system including V.sub.A plus the pump 3, the trap 5, the pressure sensor 6 and related interconnecting tubing and communicating the valve 2 volumes. The dilution factor f.sub.dil can be maximized towards 1.0 by increasing the additional volume 11 and decreasing the trap 5 volume. However this adjustment is limited by the corresponding increase in the volume of sample gas required. The dilution factor can be measured by performing this described procedure using a pure gas sample with no interfering matrix gases and with the trap 5 in a heated condition such that adsorption of the (single) pure gas in the sample does not affect its concentration significantly. The dilution factor is then:

[00003]$\begin{array}{cc}{f}_{\mathrm{dil}}=\frac{c_g}{c_g^o}& \left(3\right)\end{array}$

where c.sub.g is the measured single-gas concentration after the heated trap is introduced into the gas stream.

[0040] The frontal adsorption coefficient k.sub.g is a function of the trapping temperature, amount of adsorbent present, and adsorptivity of the gas on the adsorbent bed. It can be approximated from frontal chromatographic measurements for individual gases or directly from a trapping system by analyzing pure analyte gases without interfering matrix gases but with trapping, as in:

[00004]$\begin{array}{cc}{k}_g=\frac{c_g^o}{c_g^{\mathrm{tr}}}& \left(4\right)\end{array}$

Where c.sub.g.sup.tr is the measured pure gas concentration after trapping. The adsorption coefficient is also a function of trap temperature, where lower temperatures will cause stronger adsorption of analyte gases thereby increasing k.sub.g.

[0041] In step 5 (trap cleanup), the three-way valves 9, 10 are de-energized, the rotary valve 2 is actuated to its original position, the pump 3 is energized, and the heater 7 is energized so that the temperature of the trap 5 increases sufficiently to cause the desorption of adsorbed gases from the trap, including both interfering gases and target analytes. In the current configuration, the desorption temperature is 200 C. Trap desorption is made more efficient by reversal of the flow direction, or back-flushing, through the trap 5, such that interfering matrix gases which are preferentially adsorbed at the front of the trap are preferentially desorbed during the cleanup cycle. The temperature of the trap 5 is controlled by appropriate feedback means with the temperature transducer 8. After sufficient time to clear the trap of any residual gases, the heater 7 is deactivated and the system returns to the initial state.

[0042] In an alternate configuration of the adsorbent trap, a dual-adsorbent bed is utilized. A first adsorbent section occupies a defined and separate first fraction of the trap volume and a second adsorbent section occupies the remainder of the trap volume in a separately defined area.

[0043] The first adsorbent section starts at the entrance to the trap tube that first receives gas sample during Step 3, above. At the initial trap temperature the first adsorbent material is less adsorbing and has a smaller adsorption coefficient k.sub.g for the analyte and interfering matrix gases than the second adsorbent material, but the first adsorbent material does adsorb less volatile, condensable longer-chain hydrocarbons such as C.sub.7-C.sub.16 as well as other trace matrix components in the sample gas that may or may not interfere with the IR analysis of the target gases. Such materials may be present in the dielectric fluid under analysis in significant concentrations, and some fraction of these materials may be released from the dielectric fluid into the gas space undergoing DGA measurement. Such materials may also be released into the circulating gas stream during a PAS cell clean-up thermal cycle such as is described below.

[0044] The purpose of the first adsorbent material is to preferentially adsorb such less-volatile matrix components that, if adsorbed on the second more strongly adsorbing material, might not be completely desorbed upon trap heating and flow reversal. The first adsorbing material effectively increases the lifetime, or the number of sequential analysis cycles that may be performed, without depleting the effectiveness of the second adsorbing material for totally adsorbing the more volatile interfering matrix gases such as propane and propylene.

[0045] Considering the above described arrangement of spectroscopic analysis cell, gas pump, valving, interconnecting tubing, and thermally controlled trap, an additional spectroscopic cell clean-up thermal cycle can be realized. As previously stated, it is known in the art that less volatile compounds may be transferred through the gas phase by evaporation from a source such as the electrical insulating fluid under test, and such compounds may condense subsequently on the inner surfaces of the spectroscopic analysis cell thereby causing interference with the measurement of analyte substances. Although it has been shown in the prior art that such condensation can be reduced or controlled by establishing a positive thermal gradient from the source of the condensable compounds towards the spectroscopic cell, such an arrangement does not completely stop the condensation and interference. Condensable compounds may build up over an extended period of operation sufficiently to make the spectroscopic cell incapable of detecting analyte compounds with a desired sensitivity.

[0046] Although the present new and novel system has been described in terms of specific exemplary embodiments, it will be appreciated that various modifications and alterations might be made by those skilled in the art without departing from the spirit and scope of the invention as set forth in the following claims.