Abstract: Once a transformer fails, it will have a serious impact on the production of large-scale power outages and long periods. The timely understanding of the internal operation of the oil-immersed transformer and the discovery of faults is of great importance to ensure the safe, reliable and high-quality operation of the transformer. For oil-immersed transformers, the coils and iron cores are completely immersed in the transformer oil. It is impossible to judge the transformer's fault potential through naked eyes and direct measurement. Certain technical methods must be used to understand the operating conditions of the transformer.
A principle of gas chromatography Chromatography, also known as chromatography, is a physical separation technique. Its principle of separation is to distribute the components of the mixture between the two phases, where one phase is immobile, called the stationary phase, and the other is the fluid that pushes the mixture through the stationary phase, called the mobile phase. When the mixture contained in the active phase passes through the stationary phase, it interacts with the stationary phase. Due to the differences in the nature and structure of the components, the strength of the interaction also varies. Therefore, under the same driving force, the residence time of the different components in the stationary phase has a lengthy and short time, so that they can flow out of the stationary phase in an orderly manner. This separates the components of the mixture by the principle of two-phase distribution. The technology is called chromatographic separation or chromatography. When liquid is used as the active phase, it is called liquid chromatography. When gas is used as the active phase, it is called gas chromatography.
Chromatography has: (1) high separation efficiency, (2) rapid analysis, (3) low sample usage, (4) high sensitivity, (5) wide application range, and many chemical analysis methods cannot be compared with advantage.
The general flow of gas chromatography mainly includes three parts: carrier gas system, column and detector.
When a carrier gas carries a mixture of different substances through the column, a part of the gas phase will dissolve or adsorb to the stationary phase. As the molecules in the stationary phase increase, the sample evaporates from the stationary phase to the gas phase. Substance molecules also gradually increase, that is, the molecules of each substance in the sample are distributed in two phases and eventually reach equilibrium. The process of dissolution and volatilization of this substance between the two phases is called the distribution process. When the distribution reaches equilibrium, the concentration ratio of the substance in the two phases, called the distribution coefficient, also called the equilibrium constant, is expressed as K, K = the concentration of the substance in the stationary phase / the concentration of the substance in the active phase, at a constant temperature, The distribution coefficient K is a constant.
It can be seen that the principle of gas chromatography separation is to use different substances in the two phases have different distribution coefficients, when the two phases for relative movement, the components of the sample in the two phases are repeatedly allocated, making the original The components that have only a small difference in the distribution coefficient produce a large separation effect, thereby separating the components. Then enter the detector to identify the components.
The SP-3430 Gas Chromatograph Analyzer makes full use of this principle to quickly, efficiently, and accurately analyze the components and contents of gas in transformer oil. Based on the type and content of these gases, we can analyze them correctly. Determine whether the transformer is faulty, the nature of the fault, and the approximate location of the fault.
The gas generated by the failure of the second transformer and the type of fault 1 Gas components generated by the transformer-independent materials Oil and solid insulation materials Among various gases generated by the decomposition of electricity or heat, there is methane for gases that are of value for fault determination. Ethane, ethylene, acetylene, hydrogen, carbon monoxide, carbon dioxide. The gases produced during the normal aging process are mainly carbon monoxide and carbon dioxide. In the presence of partial discharges on the edges of oiled paper, the gases produced by oil cracking are mainly hydrogen and methane. When the fault temperature is higher than the normal operating temperature, the gas produced is mainly methane. As the fault temperature rises, ethylene and ethane gradually become the main features. At temperatures above 1000°C, for example at arc arc temperatures (above 3000°C), oil cracking occurs and the gas contains more acetylene. If the failure involves solid insulation materials, more carbon monoxide and carbon dioxide are produced.
In order to eliminate the major gas components produced by oil and insulation materials under different temperatures and energy, the following are included:
1) Evaporation vaporization and slower oxidation occur below 140°C.
2) Due to the oil's decomposition at 140°C to 500°C, the main decomposition of oil is the production of alkanes. Among them, methane and ethane are the main components. As the temperature rises (above 500°C), the decomposition of oil increases sharply, with the increase in olefins and hydrogen. Fast, especially ethylene, and when the temperature (about 800 °C) is higher, but also produces acetylene gas.
3) When an arc exists in the oil (temperature exceeds 1000 °C, the gas for cracking the oil is mostly acetylene and hydrogen, and there is a certain amount of methane and ethylene.
4) During the operation of the equipment, due to thermal expansion and contraction caused by load changes, turbulence caused by circulating oil in the pump, and mechanical vibration caused by the hysteresis-stretch effect of the core, etc., can both cause voids and oil to form. Release dissolved gas. If the generated bubble is located in the high-voltage stress region of the device's insulating structure, air gap discharge (commonly referred to as partial discharge) will be caused under a high electric field, and the discharge itself can further cause the decomposition of the oil and the solid around. The decomposition of the edge material generates gases, which are more conducive to gas discharge under the action of electrical stress. This discharge causes the oil produced by the decomposition of the oil to be mainly hydrogen and a small amount of methane gas.
5) The solid insulation material, when heated at a relatively low temperature (below 140°C) for a long period of time, will gradually deteriorate and produce gases, mainly carbon monoxide and carbon dioxide, and the latter is the main component.
6) When solid superfluous materials are used at temperatures above 200°C, in addition to carbon oxides, hydrogen and hydrocarbon gases are also decomposed. At different temperatures, the ratio of carbon monoxide to carbon dioxide is different. This ratio is at a low temperature. Small and high at high temperatures.
7) Metal materials such as iron and steel play a catalytic role, and water reacts with iron to produce hydrogen. In addition, Austenitic stainless steel can contain hydrogen, which is dissolved in oil by contact with oil.
The following table shows the gas components produced by different types of faults:
Sometimes there are no faults in the equipment. For other reasons, the above gases will also appear in the oil, so it is necessary to pay attention to these sources of gas that may cause misjudgment. For example, the oil in the oil compartment of the on-load tap-changing transformer leaks to the transformer body or the floating potential discharge during a certain range of switching action: the equipment has had a fault, and after the fault is removed, the oil is not completely degassed. Some of the residual gas remains in the oil; the equipment tank has been repaired with oil; the originally injected oil contains certain types of gases. It should also be noted that the gas generated by the failure of the auxiliary equipment of the oil cooling system (such as submersible pumps, oil flow relays, etc.) will also enter the oil in the transformer body. When the internal oil content of the equipment exceeds the values ​​listed in the table below, it should be noted.
It is difficult to correctly judge the severity of a fault based solely on the exact value of the analysis result. It is necessary to examine the development trend of the fault, that is, the gas production rate at the fault point (if it exists). The gas production rate is directly related to the amount of energy consumed by the fault, the location of the fault, and the temperature of the fault point. If the relative production rate of total hydrocarbons is greater than 10%, care should be taken.
2 Determination of Carbon Monoxide and Carbon Dioxide When a failure involves solid insulation, it causes a significant increase in carbon monoxide and carbon dioxide. However, according to the existing statistical data, the normal aging process and failure conditions of solid insulation deteriorated and decomposed under the conditions of the carbon monoxide content in the oil. Generally, there is no strict limit, and the rule of carbon dioxide content is even less obvious. Therefore, in the investigation of these two kinds of gas content should be combined with the specific structural characteristics of the transformer (such as oil protection), operating temperature, load conditions, operating history and other conditions to be a comprehensive analysis.
The carbon monoxide content of open transformers is generally below 300ppm. If the total hydrocarbon content exceeds the normal range and the carbon monoxide content exceeds 300 ppm, consideration should be given to the possibility of overheating involving solid superheating; if the carbon monoxide content exceeds 300 ppm, but the total hydrocarbon content is in the normal range, it may generally be normal; Some transformers with twin pie coils with an additional outer sheathed edge, when the carbon monoxide content exceeds 300 ppm, even if the total hydrocarbon content is normal, there may be solids overheating failure.
For transformers with capsules or diaphragms in oil conservatories, the carbon monoxide content of oil is generally higher than that of open transformers.
In the case of sudden breakthrough failure, the content of carbon monoxide and carbon dioxide in the dissolved gas in the oil is not necessarily high and should be determined in conjunction with gas analysis in the gas relay.
(3) The location of faults inside the oil-filled equipment such as transformers The type of faults that may occur within the transformer is of great help when the results of gas chromatographic analysis are finalized. Failures within the oil-filled equipment, such as transformers, are mainly caused by:
1) Location where overheating faults occur 1 Overheating faults The most common parts in transformers are overheating faults caused by current-carrying conductors and defective connections. If the contact information of the tap-changer message is poor, the lead joints are inadvertently soldered, the short circuit between the coils, the lead wire is too long, or the dressing is damaged, the conductors are connected to each other to generate circulation heat. The overload operation heats up, the coil expands and the oil channel blocks. Caused by poor heat dissipation. The other is the magnetic circuit faults, such as multi-point grounding of the iron core, short circuit between the iron chips, short circuit between the iron core and the core screw, and local overheat of the fuel tank, clip, pressure ring and the like caused by magnetic leakage.
2 The overheating fault accounted for a small proportion of faults in the low oil equipment (inductor transformer and capacitor bushing), and the main parts of the fault were: loosening of the once-lead fastening nut of the current transformer, loosening of the tapping tightening nut, etc.; The cable nose and the lead wire joints are badly welded, and the connection nuts of the conduits and general caps are not properly matched.
2) Location where discharge failure occurs 1 High-energy discharge (arc discharge) occurs in transformers, bushings, and transformers. Caused by the arc discharge fault is usually the breakdown of the winding between the layers, the internal flashover caused by the overvoltage, the arc caused by the breakage of the lead, the arcing of the tap switch and the breakdown of the capacitive screen. This type of fault gas generates violently and has a large amount of gas. Failure gas is too late to dissolve in oil and accumulate in the gas relay to cause gas action.
2 Low-energy discharges are generally spark discharges, which are intermittent discharge failures that occur in transformers, transformers, and casing. Between conductors and conductors of different potentials, between suspended body and instinct body and suspension of unfixed potential, spark discharge may be caused by extremely uneven or distorted electric fields and induced potentials.
3 Partial discharge refers to the air bubbles and the tip in the oil and solid insulation. Partial discharge occurs due to low compressive strength and electric field concentration. This kind of discharge continues to spread and develop, and it will cause damage (carbonization marks or perforation). For example, the capacitive core winding technology of the current transformer and the capacitor sleeve may cause partial discharge due to bad process or poor vacuum drying process.
3. To diagnose latent faults inside oil-filled equipment such as transformers When diagnosing latent faults inside oil-filled equipment such as transformers, the following three factors should be comprehensively considered to correctly determine the type of fault and the general location of the fault:
1 Failure of gas accumulation Accumulation of oil-filled electrical equipment The flammable gas produced by the latent malfunction will mostly dissolve in the oil. As failure continues, these gases accumulate in the oil until they saturate or even bubble out. Therefore, the content of fault gas in the oil and its cumulative degree are a basis for the existence and development of diagnostic faults.
2 The rate of gas production under fault Under normal circumstances, the oil-filled electrical equipment will also age and decompose into a small amount of flammable gas under the effect of heat and electric field, but the gas production rate is very slow. When there is a fault inside the equipment, the gas production rate of these gases is accelerated. Therefore, the gas production rate of fault gas is another basis for diagnosing the existence and development of faults.
3 Characteristics of gas production under fault The gas produced inside the transformer under different faults has different characteristics. For example, there will always be hydrogen in partial discharges; there will always be ethylene at higher temperatures; there will always be acetylene at arc discharges. Therefore, the characteristic of gas production under fault is another basis for diagnosing the nature of the fault.
IV. Example of Gas Chromatographic Analysis Example 1: Using the chromatographic analysis results to determine the presence of discharge in the transformer At the end of 2001, the voltage of the electrostatic precipitator #1 in the No. 3 boiler in the new plant area did not rise. Normally it should rise to 4 to 70,000 volts. Since the voltage of the electric field does not rise within the required range, dust removal efficiency is seriously reduced.
The process for this fault is: we first checked the control circuit, and after removing the factors of the control circuit, we then focused our attention on the step-up transformer of the electrostatic precipitator. Because the transformer is equipped with a high-voltage silicon stack, voltage equalization resistors, capacitors, and sampling circuits, all of which are immersed in the transformer oil. The structure is special and belongs to a special transformer. It is not allowed to be disassembled. In order to correctly determine the fault, it was first routinely tested, including the detection of transformer DC resistance, insulation resistance, etc. No problems were found. Gas chromatograph analysis was then used for further test diagnosis.
We took the transformer oil sample and got it on a chromatographic analyzer for analysis. The composition and content of the dissolved gas in the sample were measured as shown in the following table:
From the data in the table, it can be seen that the total hydrocarbon content of the gas in the transformer oil exceeds the value of 150ppm, and the hydrocarbon gas is the main component. The acetylene content far exceeds the note value of 5ppm, but due to the low content of hydrogen and methane The reason for the higher hydrocarbon gas content may be due to the long-term intermittent discharge of the transformer. According to the foregoing theory, it is determined that there is a serious metal arc discharge phenomenon in the transformer. The transformer should be immediately shut down and the core inspection should be recommended.
Afterwards, the maintenance staff carried out a core inspection of the step-up transformer. When the lid of the lid was opened, a pungent odor overflowed. The transformer core was lifted out of the box and the entire core was coated with ink. Arc Disruption The carbon deposits produced by the transformer oil are attached to the core windings and the silicon stack. Repeatedly flushing with transformer oil, after careful inspection, found that the transformer does exist arc discharge phenomenon, the fault location is located in the high voltage coil lead wire and high pressure casing screw connection. Due to the particularity of the transformer structure, manufacturers have to connect the high-voltage outlet wire through a saddle-type clamp to the through-hole screw for easy assembly. Due to long-time operation, this connection point has loosened, causing poor contact and causing the transformer. Running for a long time under arc discharge, saddle type copper clips have ablated an irregular hole. After the problem was found, we improved it. We removed the saddle clip and pressed the high-voltage lead directly on the screw through the wall. After a series of tests, the cartridge was replaced and the transformer was replaced. Oil, finished power test run, everything is normal.
Example 2: The use of chromatographic analysis to determine the existence of transformer overheating In February of this year, according to the operating staff reported that the new plant 4 # boiler electrostatic precipitator 2 # electric field can not run, as long as a boot, the high voltage control cabinet power switch trips, the computer Show as over activity.
After receiving the report, we checked all the control circuits, main power loops, and all internal control circuits of the electric field together with the operating staff. No problems were found. Finally, the problem was determined only in the transformer itself, and the high and low voltage leads of the transformer were removed. The conventional detection of the transformer was performed, the resistance was normal, the DC resistance was normal, and no problem was found. Then perform the second step of transforming oil chromatography to help check.
After retrieving the transformer oil sample, we placed it on a gas chromatograph analyzer for analysis. The results surprised us. The analysis results are shown in the following table:
From the data in the table, it can be seen that because of its acetylene content of 0 ppm, it can be basically concluded that there is no discharge phenomenon in the transformer, but the content of alkanes and hydrocarbon gases is very high, and the total hydrocarbon content is more than 100 times of the observed value of 150 ppm, and B Alkane and ethylene are the main components of total hydrocarbons, and the carbon dioxide content is also very high.
From this, it can be determined that there is a serious overheating fault in the transformer. The overheating causes the solid insulative material to decompose the above gas. The fault point temperature is approximately: 322Log (6887.47/7014.92) + 525 = 520°C. Therefore, it is suggested that the electric field should not be put into operation and the core should be immediately checked so as to prevent the failure from expanding.
The core of the transformer was then inspected. After the entire iron core was lifted, it was immediately visible that there was a trace of scorched material on the coil. The fault was a short-circuit between the low-voltage coils due to a short-circuit fault. A large short-circuit current causes the electric field to trip as soon as the power switch is turned on. Recall that the reason for not detecting the problem before hanging the core was that the low voltage coil was a short circuit between the turns, but it did not cause a short circuit between the low voltage coil and the high voltage coil and the low voltage coil, so the insulation resistance test was normal. It is reasonable to say that the DC resistance of the short circuit transformer in the coil turns should be reduced, but because the resistance value of the coil itself is very small, the DC resistance of some coil turns short after the short circuit, so the DC resistance is also a normal illusion, so the measuring transformer No problem was found in the DC resistance.
Because the coil windings of the transformer were burned out, the maintenance was difficult and the repair cost was relatively expensive. It was no longer worth repairing. Therefore, a new transformer was finally decided to be replaced.
Sixth, in the process of continuous detection of internal faults of oil-filled electrical equipment by gas chromatography, if one of the various gas contents in the oil reaches a range of noteworthy values, it should start attracting attention and take measures for other electrical tests. In order to analyze whether the abnormality of the equipment and determine. When one of the test results exceeds the maximum value limit, measures should be taken to stop the operation as soon as possible and use other tests to verify and further identify the points of failure to prevent major accidents from occurring.
Analysis of the gas content in the oil-filled electrical equipment oil by gas chromatography can identify equipment failures. More importantly, it analyzes the nature of the fault. The overheating fault or discharge fault and the approximate location of the fault is in bare metal. Some of them are still involved in solid insulation, so as to further assess the hazards of failures, so that timely measures can be taken to deal with them correctly and take preventive measures.
In summary, the use of gas chromatography to analyze the gas composition and content of transformer oil can enable the technical staff to fully grasp and monitor the operational status of the transformer, and can know in advance whether there is a potential fault within the transformer, that is, in the operation of the transformer (not In the case of a power outage or a non-detached core, the faults in the transformer can be diagnosed through routine inspection and chromatographic analysis, and the nature of the fault can be diagnosed. This is a key guide for the maintenance of the transformer. To ensure the safe operation of the power system.
A principle of gas chromatography Chromatography, also known as chromatography, is a physical separation technique. Its principle of separation is to distribute the components of the mixture between the two phases, where one phase is immobile, called the stationary phase, and the other is the fluid that pushes the mixture through the stationary phase, called the mobile phase. When the mixture contained in the active phase passes through the stationary phase, it interacts with the stationary phase. Due to the differences in the nature and structure of the components, the strength of the interaction also varies. Therefore, under the same driving force, the residence time of the different components in the stationary phase has a lengthy and short time, so that they can flow out of the stationary phase in an orderly manner. This separates the components of the mixture by the principle of two-phase distribution. The technology is called chromatographic separation or chromatography. When liquid is used as the active phase, it is called liquid chromatography. When gas is used as the active phase, it is called gas chromatography.
Chromatography has: (1) high separation efficiency, (2) rapid analysis, (3) low sample usage, (4) high sensitivity, (5) wide application range, and many chemical analysis methods cannot be compared with advantage.
The general flow of gas chromatography mainly includes three parts: carrier gas system, column and detector.
When a carrier gas carries a mixture of different substances through the column, a part of the gas phase will dissolve or adsorb to the stationary phase. As the molecules in the stationary phase increase, the sample evaporates from the stationary phase to the gas phase. Substance molecules also gradually increase, that is, the molecules of each substance in the sample are distributed in two phases and eventually reach equilibrium. The process of dissolution and volatilization of this substance between the two phases is called the distribution process. When the distribution reaches equilibrium, the concentration ratio of the substance in the two phases, called the distribution coefficient, also called the equilibrium constant, is expressed as K, K = the concentration of the substance in the stationary phase / the concentration of the substance in the active phase, at a constant temperature, The distribution coefficient K is a constant.
It can be seen that the principle of gas chromatography separation is to use different substances in the two phases have different distribution coefficients, when the two phases for relative movement, the components of the sample in the two phases are repeatedly allocated, making the original The components that have only a small difference in the distribution coefficient produce a large separation effect, thereby separating the components. Then enter the detector to identify the components.
The SP-3430 Gas Chromatograph Analyzer makes full use of this principle to quickly, efficiently, and accurately analyze the components and contents of gas in transformer oil. Based on the type and content of these gases, we can analyze them correctly. Determine whether the transformer is faulty, the nature of the fault, and the approximate location of the fault.
The gas generated by the failure of the second transformer and the type of fault 1 Gas components generated by the transformer-independent materials Oil and solid insulation materials Among various gases generated by the decomposition of electricity or heat, there is methane for gases that are of value for fault determination. Ethane, ethylene, acetylene, hydrogen, carbon monoxide, carbon dioxide. The gases produced during the normal aging process are mainly carbon monoxide and carbon dioxide. In the presence of partial discharges on the edges of oiled paper, the gases produced by oil cracking are mainly hydrogen and methane. When the fault temperature is higher than the normal operating temperature, the gas produced is mainly methane. As the fault temperature rises, ethylene and ethane gradually become the main features. At temperatures above 1000°C, for example at arc arc temperatures (above 3000°C), oil cracking occurs and the gas contains more acetylene. If the failure involves solid insulation materials, more carbon monoxide and carbon dioxide are produced.
In order to eliminate the major gas components produced by oil and insulation materials under different temperatures and energy, the following are included:
1) Evaporation vaporization and slower oxidation occur below 140°C.
2) Due to the oil's decomposition at 140°C to 500°C, the main decomposition of oil is the production of alkanes. Among them, methane and ethane are the main components. As the temperature rises (above 500°C), the decomposition of oil increases sharply, with the increase in olefins and hydrogen. Fast, especially ethylene, and when the temperature (about 800 °C) is higher, but also produces acetylene gas.
3) When an arc exists in the oil (temperature exceeds 1000 °C, the gas for cracking the oil is mostly acetylene and hydrogen, and there is a certain amount of methane and ethylene.
4) During the operation of the equipment, due to thermal expansion and contraction caused by load changes, turbulence caused by circulating oil in the pump, and mechanical vibration caused by the hysteresis-stretch effect of the core, etc., can both cause voids and oil to form. Release dissolved gas. If the generated bubble is located in the high-voltage stress region of the device's insulating structure, air gap discharge (commonly referred to as partial discharge) will be caused under a high electric field, and the discharge itself can further cause the decomposition of the oil and the solid around. The decomposition of the edge material generates gases, which are more conducive to gas discharge under the action of electrical stress. This discharge causes the oil produced by the decomposition of the oil to be mainly hydrogen and a small amount of methane gas.
5) The solid insulation material, when heated at a relatively low temperature (below 140°C) for a long period of time, will gradually deteriorate and produce gases, mainly carbon monoxide and carbon dioxide, and the latter is the main component.
6) When solid superfluous materials are used at temperatures above 200°C, in addition to carbon oxides, hydrogen and hydrocarbon gases are also decomposed. At different temperatures, the ratio of carbon monoxide to carbon dioxide is different. This ratio is at a low temperature. Small and high at high temperatures.
7) Metal materials such as iron and steel play a catalytic role, and water reacts with iron to produce hydrogen. In addition, Austenitic stainless steel can contain hydrogen, which is dissolved in oil by contact with oil.
The following table shows the gas components produced by different types of faults:
Sometimes there are no faults in the equipment. For other reasons, the above gases will also appear in the oil, so it is necessary to pay attention to these sources of gas that may cause misjudgment. For example, the oil in the oil compartment of the on-load tap-changing transformer leaks to the transformer body or the floating potential discharge during a certain range of switching action: the equipment has had a fault, and after the fault is removed, the oil is not completely degassed. Some of the residual gas remains in the oil; the equipment tank has been repaired with oil; the originally injected oil contains certain types of gases. It should also be noted that the gas generated by the failure of the auxiliary equipment of the oil cooling system (such as submersible pumps, oil flow relays, etc.) will also enter the oil in the transformer body. When the internal oil content of the equipment exceeds the values ​​listed in the table below, it should be noted.
It is difficult to correctly judge the severity of a fault based solely on the exact value of the analysis result. It is necessary to examine the development trend of the fault, that is, the gas production rate at the fault point (if it exists). The gas production rate is directly related to the amount of energy consumed by the fault, the location of the fault, and the temperature of the fault point. If the relative production rate of total hydrocarbons is greater than 10%, care should be taken.
2 Determination of Carbon Monoxide and Carbon Dioxide When a failure involves solid insulation, it causes a significant increase in carbon monoxide and carbon dioxide. However, according to the existing statistical data, the normal aging process and failure conditions of solid insulation deteriorated and decomposed under the conditions of the carbon monoxide content in the oil. Generally, there is no strict limit, and the rule of carbon dioxide content is even less obvious. Therefore, in the investigation of these two kinds of gas content should be combined with the specific structural characteristics of the transformer (such as oil protection), operating temperature, load conditions, operating history and other conditions to be a comprehensive analysis.
The carbon monoxide content of open transformers is generally below 300ppm. If the total hydrocarbon content exceeds the normal range and the carbon monoxide content exceeds 300 ppm, consideration should be given to the possibility of overheating involving solid superheating; if the carbon monoxide content exceeds 300 ppm, but the total hydrocarbon content is in the normal range, it may generally be normal; Some transformers with twin pie coils with an additional outer sheathed edge, when the carbon monoxide content exceeds 300 ppm, even if the total hydrocarbon content is normal, there may be solids overheating failure.
For transformers with capsules or diaphragms in oil conservatories, the carbon monoxide content of oil is generally higher than that of open transformers.
In the case of sudden breakthrough failure, the content of carbon monoxide and carbon dioxide in the dissolved gas in the oil is not necessarily high and should be determined in conjunction with gas analysis in the gas relay.
(3) The location of faults inside the oil-filled equipment such as transformers The type of faults that may occur within the transformer is of great help when the results of gas chromatographic analysis are finalized. Failures within the oil-filled equipment, such as transformers, are mainly caused by:
1) Location where overheating faults occur 1 Overheating faults The most common parts in transformers are overheating faults caused by current-carrying conductors and defective connections. If the contact information of the tap-changer message is poor, the lead joints are inadvertently soldered, the short circuit between the coils, the lead wire is too long, or the dressing is damaged, the conductors are connected to each other to generate circulation heat. The overload operation heats up, the coil expands and the oil channel blocks. Caused by poor heat dissipation. The other is the magnetic circuit faults, such as multi-point grounding of the iron core, short circuit between the iron chips, short circuit between the iron core and the core screw, and local overheat of the fuel tank, clip, pressure ring and the like caused by magnetic leakage.
2 The overheating fault accounted for a small proportion of faults in the low oil equipment (inductor transformer and capacitor bushing), and the main parts of the fault were: loosening of the once-lead fastening nut of the current transformer, loosening of the tapping tightening nut, etc.; The cable nose and the lead wire joints are badly welded, and the connection nuts of the conduits and general caps are not properly matched.
2) Location where discharge failure occurs 1 High-energy discharge (arc discharge) occurs in transformers, bushings, and transformers. Caused by the arc discharge fault is usually the breakdown of the winding between the layers, the internal flashover caused by the overvoltage, the arc caused by the breakage of the lead, the arcing of the tap switch and the breakdown of the capacitive screen. This type of fault gas generates violently and has a large amount of gas. Failure gas is too late to dissolve in oil and accumulate in the gas relay to cause gas action.
2 Low-energy discharges are generally spark discharges, which are intermittent discharge failures that occur in transformers, transformers, and casing. Between conductors and conductors of different potentials, between suspended body and instinct body and suspension of unfixed potential, spark discharge may be caused by extremely uneven or distorted electric fields and induced potentials.
3 Partial discharge refers to the air bubbles and the tip in the oil and solid insulation. Partial discharge occurs due to low compressive strength and electric field concentration. This kind of discharge continues to spread and develop, and it will cause damage (carbonization marks or perforation). For example, the capacitive core winding technology of the current transformer and the capacitor sleeve may cause partial discharge due to bad process or poor vacuum drying process.
3. To diagnose latent faults inside oil-filled equipment such as transformers When diagnosing latent faults inside oil-filled equipment such as transformers, the following three factors should be comprehensively considered to correctly determine the type of fault and the general location of the fault:
1 Failure of gas accumulation Accumulation of oil-filled electrical equipment The flammable gas produced by the latent malfunction will mostly dissolve in the oil. As failure continues, these gases accumulate in the oil until they saturate or even bubble out. Therefore, the content of fault gas in the oil and its cumulative degree are a basis for the existence and development of diagnostic faults.
2 The rate of gas production under fault Under normal circumstances, the oil-filled electrical equipment will also age and decompose into a small amount of flammable gas under the effect of heat and electric field, but the gas production rate is very slow. When there is a fault inside the equipment, the gas production rate of these gases is accelerated. Therefore, the gas production rate of fault gas is another basis for diagnosing the existence and development of faults.
3 Characteristics of gas production under fault The gas produced inside the transformer under different faults has different characteristics. For example, there will always be hydrogen in partial discharges; there will always be ethylene at higher temperatures; there will always be acetylene at arc discharges. Therefore, the characteristic of gas production under fault is another basis for diagnosing the nature of the fault.
IV. Example of Gas Chromatographic Analysis Example 1: Using the chromatographic analysis results to determine the presence of discharge in the transformer At the end of 2001, the voltage of the electrostatic precipitator #1 in the No. 3 boiler in the new plant area did not rise. Normally it should rise to 4 to 70,000 volts. Since the voltage of the electric field does not rise within the required range, dust removal efficiency is seriously reduced.
The process for this fault is: we first checked the control circuit, and after removing the factors of the control circuit, we then focused our attention on the step-up transformer of the electrostatic precipitator. Because the transformer is equipped with a high-voltage silicon stack, voltage equalization resistors, capacitors, and sampling circuits, all of which are immersed in the transformer oil. The structure is special and belongs to a special transformer. It is not allowed to be disassembled. In order to correctly determine the fault, it was first routinely tested, including the detection of transformer DC resistance, insulation resistance, etc. No problems were found. Gas chromatograph analysis was then used for further test diagnosis.
We took the transformer oil sample and got it on a chromatographic analyzer for analysis. The composition and content of the dissolved gas in the sample were measured as shown in the following table:
From the data in the table, it can be seen that the total hydrocarbon content of the gas in the transformer oil exceeds the value of 150ppm, and the hydrocarbon gas is the main component. The acetylene content far exceeds the note value of 5ppm, but due to the low content of hydrogen and methane The reason for the higher hydrocarbon gas content may be due to the long-term intermittent discharge of the transformer. According to the foregoing theory, it is determined that there is a serious metal arc discharge phenomenon in the transformer. The transformer should be immediately shut down and the core inspection should be recommended.
Afterwards, the maintenance staff carried out a core inspection of the step-up transformer. When the lid of the lid was opened, a pungent odor overflowed. The transformer core was lifted out of the box and the entire core was coated with ink. Arc Disruption The carbon deposits produced by the transformer oil are attached to the core windings and the silicon stack. Repeatedly flushing with transformer oil, after careful inspection, found that the transformer does exist arc discharge phenomenon, the fault location is located in the high voltage coil lead wire and high pressure casing screw connection. Due to the particularity of the transformer structure, manufacturers have to connect the high-voltage outlet wire through a saddle-type clamp to the through-hole screw for easy assembly. Due to long-time operation, this connection point has loosened, causing poor contact and causing the transformer. Running for a long time under arc discharge, saddle type copper clips have ablated an irregular hole. After the problem was found, we improved it. We removed the saddle clip and pressed the high-voltage lead directly on the screw through the wall. After a series of tests, the cartridge was replaced and the transformer was replaced. Oil, finished power test run, everything is normal.
Example 2: The use of chromatographic analysis to determine the existence of transformer overheating In February of this year, according to the operating staff reported that the new plant 4 # boiler electrostatic precipitator 2 # electric field can not run, as long as a boot, the high voltage control cabinet power switch trips, the computer Show as over activity.
After receiving the report, we checked all the control circuits, main power loops, and all internal control circuits of the electric field together with the operating staff. No problems were found. Finally, the problem was determined only in the transformer itself, and the high and low voltage leads of the transformer were removed. The conventional detection of the transformer was performed, the resistance was normal, the DC resistance was normal, and no problem was found. Then perform the second step of transforming oil chromatography to help check.
After retrieving the transformer oil sample, we placed it on a gas chromatograph analyzer for analysis. The results surprised us. The analysis results are shown in the following table:
From the data in the table, it can be seen that because of its acetylene content of 0 ppm, it can be basically concluded that there is no discharge phenomenon in the transformer, but the content of alkanes and hydrocarbon gases is very high, and the total hydrocarbon content is more than 100 times of the observed value of 150 ppm, and B Alkane and ethylene are the main components of total hydrocarbons, and the carbon dioxide content is also very high.
From this, it can be determined that there is a serious overheating fault in the transformer. The overheating causes the solid insulative material to decompose the above gas. The fault point temperature is approximately: 322Log (6887.47/7014.92) + 525 = 520°C. Therefore, it is suggested that the electric field should not be put into operation and the core should be immediately checked so as to prevent the failure from expanding.
The core of the transformer was then inspected. After the entire iron core was lifted, it was immediately visible that there was a trace of scorched material on the coil. The fault was a short-circuit between the low-voltage coils due to a short-circuit fault. A large short-circuit current causes the electric field to trip as soon as the power switch is turned on. Recall that the reason for not detecting the problem before hanging the core was that the low voltage coil was a short circuit between the turns, but it did not cause a short circuit between the low voltage coil and the high voltage coil and the low voltage coil, so the insulation resistance test was normal. It is reasonable to say that the DC resistance of the short circuit transformer in the coil turns should be reduced, but because the resistance value of the coil itself is very small, the DC resistance of some coil turns short after the short circuit, so the DC resistance is also a normal illusion, so the measuring transformer No problem was found in the DC resistance.
Because the coil windings of the transformer were burned out, the maintenance was difficult and the repair cost was relatively expensive. It was no longer worth repairing. Therefore, a new transformer was finally decided to be replaced.
Sixth, in the process of continuous detection of internal faults of oil-filled electrical equipment by gas chromatography, if one of the various gas contents in the oil reaches a range of noteworthy values, it should start attracting attention and take measures for other electrical tests. In order to analyze whether the abnormality of the equipment and determine. When one of the test results exceeds the maximum value limit, measures should be taken to stop the operation as soon as possible and use other tests to verify and further identify the points of failure to prevent major accidents from occurring.
Analysis of the gas content in the oil-filled electrical equipment oil by gas chromatography can identify equipment failures. More importantly, it analyzes the nature of the fault. The overheating fault or discharge fault and the approximate location of the fault is in bare metal. Some of them are still involved in solid insulation, so as to further assess the hazards of failures, so that timely measures can be taken to deal with them correctly and take preventive measures.
In summary, the use of gas chromatography to analyze the gas composition and content of transformer oil can enable the technical staff to fully grasp and monitor the operational status of the transformer, and can know in advance whether there is a potential fault within the transformer, that is, in the operation of the transformer (not In the case of a power outage or a non-detached core, the faults in the transformer can be diagnosed through routine inspection and chromatographic analysis, and the nature of the fault can be diagnosed. This is a key guide for the maintenance of the transformer. To ensure the safe operation of the power system.
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