Brief introduction of structure and equipment of Kuiwen substation: indoor power distribution, 220KV high-voltage room (upstairs): Pingdingshan high-voltage Kai Guanchang GIS, 1 10KV high-voltage room equipment (downstairs): products of Xi 'an High-voltage Electrical Apparatus Research Institute Co., Ltd., 10KV high-voltage room (downstairs south): Wuzhou ABB products, 1.
Summary of spring inspection in 2009: the gas pressure in 1 10KV GIS dropped, and the equipment gave an alarm at low voltage, indicating that the gas leakage rate in the whole year was not up to standard; 1 10KV GIS, 220KV GIS gas chamber micro-water exceeds the standard; 1 Oil leakage of main transformer. The maintenance report is provided by Weifang Power Supply Company. The maintenance task is undertaken by Weifang Power Transmission and Transformation Engineering Company (excluding the project warranty period, which belongs to the category of eliminating defects), manufacturers and Weifang Electric Power Bureau inspection field.
Fault analysis and preventive measures of gas pressure drop in internal air chamber of (1) totally enclosed SF6 combined insulator
1. Problem: This problem occurred in the totally enclosed SF6 combined insulation device produced in Gao Xi, including multiple outlet intervals and PT intervals. Mainly concentrated in the incoming air chamber and isolating switch air chamber on the equipment.
2. Fault detection means: The gas density standard of equipment in Kuiwen Station is 0.52MPa for the breaker gas chamber and 0.42MPa for other gas chambers without arc extinguishing function. When the gas pressure in the gas chamber drops, on the one hand, the gas pressure gauge on the equipment will drop unreasonably, that is, the annual gas leakage rate exceeds the national standard (the detection reliability is low, mainly due to the large measurement error and the great influence of environmental temperature change). On the other hand, the equipment can send out protection signals such as telemetry and telecommunication through the relay protection device (gas density relay), and the gas density can be monitored in time through the background machine. When instruments and other equipment parts with weak mechanical strength are damaged, resulting in a large amount of SF6 gas leakage, the gas alarm device installed in the high-pressure cabin will act to send an alarm signal.
3. Possible consequences of abnormal gas pressure in 3.GIS:
(1)SF6 gas, as an insulating medium with high insulation strength, is the main component of equipment insulation. When the gas pressure drops, the insulation strength of the equipment will decrease. The ability of GIS to withstand overvoltage is reduced. When the gas pressure drops beyond a certain threshold, GIS can't even guarantee the insulation strength of the power frequency voltage (because there is an automatic protection device, unless this will not happen in extreme cases), and the conductor inside the equipment will discharge the equipment shell, resulting in a short circuit fault to the ground. If the relay protection device does not act in time to remove the fault part, the fault may develop into an interphase fault, causing internal vibration of the system and huge electromotive force to destroy electrical equipment.
(2)SF6 gas is not only an important part of equipment insulation, but also the main arc extinguishing medium in the gas chamber of GIS circuit breaker. When the gas pressure in the circuit breaker gas chamber drops, its arc extinguishing ability will drop, and if the gas pressure in the circuit breaker gas chamber drops beyond a certain threshold, its arc extinguishing ability will drop seriously. At this time, the relay protection device will block the opening and closing function of the circuit breaker, resulting in the disappearance of the closing circuit capacity. If the protection device does not have the opening and closing function of locking the circuit breaker and the opening and closing operation of the circuit breaker occurs at this time, because the arc cannot be cut off in effective time and space, if the arc touches the equipment shell, it will cause the corresponding electrical equipment failure.
4. Theoretical analysis: (1) SF6 gas analysis under high pressure: According to Paschen's law curve, under high pressure, the gas density increases, the average free path of electrons shortens, and the probability of electrons accumulating enough energy between two adjacent collisions decreases, that is, the ionization difficulty increases and the discharge voltage increases. (2) The reason why SF6 is used as an insulating medium is analyzed: SF6 has strong electronegativity and is easy to absorb electrons with strong activity to form stable negative molecules, which weakens the charging process of gas and increases the discharge voltage; Stable chemical properties and high electrical insulation strength; SF6 gas has excellent arc extinguishing performance, and its arc extinguishing ability is 100 times that of air.
5. Basic treatment method: As the adopted strategy is still preventive maintenance. So the traditional method is to find out the leakage point of GIS shell and then repair it. In the process of maintenance, the traditional binding method is used to determine the air leakage area of the air leakage chamber of the combined electrical appliance. After determining the area, scan the air leakage area with SF6 leak detector probe to find out the air leakage point (in general, the position of the air leakage point can be confirmed by smearing soap bubbles). The leakage points are mainly distributed at the joint of air chamber, which is easy to leak when marked with green belt (the joint of two-phase ventilation chamber). This is easy to understand physically. Here, the metal joint surface is sealed by the sealing ring and sealant during installation. If the quality of the sealing ring is poor or the sealant is unevenly applied, and the torque on the fastening screw is uneven, it may lead to the emergence and development of the leakage point. Another place where leakage is easy to find is the joint between the instrument interface and the self-sealing valve body. This is caused by mechanical structure. In addition, during the maintenance, it was found that there was leakage at the cable terminal of one air chamber and leakage at the wall of the other air chamber. After the leakage point is found, the leakage point at the interface can be eliminated by retightening and replacing the sealing ring. The leakage point of the cable terminal is 1 10KV high-voltage cable, and the manufacturer is required to re-manufacture the cable terminal; For the leakage point on the cylinder wall, find a special manufacturer to repair the leakage point (the welding is completed by a special person to prevent the impurities from decomposing in the cylinder due to the reasonable change of the inside of the cylinder wall due to high temperature, which seriously damages the insulation environment inside the gas chamber).
6. Understanding and views on monitoring methods and preventive measures of air leakage fault in GIS.
Using gas density relay to transmit gas density signal to relay protection equipment can be regarded as a monitoring method. However, this monitoring method has some limitations, mainly because the density and activity of gas are affected by temperature or vibration. Especially the air chamber of the circuit breaker, the action mechanism will cause great vibration of the circuit breaker at the moment of opening and closing and tripping; When the temperature is different, the activity and expansion coefficient of gas are also different. It is pointed out that the installation position of gas density relay has certain influence on the measurement accuracy. I think, for the monitoring of gas leakage, we should focus on the following points:
(1) Consider the influence of external vibration or separation on measurement error and the method to correct this interference. The most basic thing is to compare the relevant parameters of non-operating circuit breakers when multiple circuit breakers are used.
(2) Considering the change of gas density and pressure at different temperatures and the temperature distribution of gas at different positions inside the circuit breaker; When the season and weather change, consider the gas density distribution.
(3) judging air leakage by combining other signals: for example, taking the current passing through the conductor (resistance heating) as a consideration factor; Synthesis of partial discharge signal and gas concentration signal during internal partial discharge. Of course, these depend on signal processing and its intelligent analysis process.
(4) Method of simultaneous detection and monitoring: At present, there is a laser camera SF6 gas leakage detector, which is said to have high sensitivity. From an economic point of view, it can be used as a supplement to online monitoring. In addition, the weak point of the air chamber also has certain characteristics, which provides a rapid processing method for this detection method.
(5) I think the most important thing is to diagnose the situation that rapid gas leakage may lead to serious failure. This diagnosis must be quick and accurate. For example, the gas leaks rapidly, the conductor has been discharged (temperature changes), and the equipment is not equipped with high-sensitivity quick-acting protection (such as longitudinal differential protection).
Regarding the prevention of such failures, I think we should focus on the following two aspects:
(1) Improve the processing accuracy of power system equipment and improve the materials used for GIS equipment. For example, using ultra-low temperature for electrical assembly.
(2) Improve the operation level of electrical construction operators and operate in strict accordance with perfect procedures.
(2) Fault analysis and preventive measures of gas micro-water exceeding the standard in the internal air chamber of fully enclosed SF6 combined insulator.
1. Problem: This problem is more serious in the 1 10KV GIS produced in Gao Xi, and the micro-water in the PT gas chamber of Pingdingshan high-pressure Kai Guanchang also exceeds the standard.
2. Fault monitoring means: the standard of micro-water has different regulations in different types of air chambers. You can refer to the relevant regulations. The station is tested by the maintenance site, and the micro-water in the air chamber is obtained by the micro-water tester. Judging from the configuration of relay protection remote communication, there is no micro-water remote communication signal. Therefore, in this overhaul and the normal operation of the station, the off-line monitoring method is used to monitor the micro-water. (Note: According to the regulations, the micro-water detection of newly injected gas should be carried out 24 hours after the completion of inflation)
3. Harm of micro-water exceeding the standard: Under normal circumstances, SF6 gas has good insulation performance and arc extinguishing performance, but when the moisture in the atmosphere invades the interior of the gas chamber or escapes from the medium in the wall of the gas chamber, the moisture in SF6 gas will increase. The subsequent consequence is that the electrical strength of the gas is significantly reduced. Especially in the air chamber with arc in the circuit breaker, SF6 gas will produce physical and chemical reactions under the combined action of arc and moisture, and finally produce highly toxic chemicals such as hydrofluoric acid and sulfuric acid, which will corrode the insulation materials or metal materials of the circuit breaker and deteriorate the insulation. In addition, when the micro-water seriously exceeds the standard, it may even cause the conductor to discharge to the cylinder wall and the inner surface of the cylinder wall to flashover. Failure to deal with it in time will eventually lead to electrical accidents.
4. Theoretical analysis.
From the perspective of insulation design, we hope that the insulation of the main equipment is as uniform as possible. For SF6 gas, its excellent insulation performance can only be fully realized in a uniform electric field. When gas contains moisture, it is excited by arc and partial discharge, and SF6 is thermally decomposed to produce sulfur and fluorine. These impurities react with oxygen and hydrogen produced by water cracking to produce hydrofluoric acid, sulfuric acid and metal fluoride. These impurities will corrode the inner cylinder wall, destroy the uniformity of electric field and destroy insulation. Therefore, GIS has strict requirements on the control of moisture and impurities.
Personal understanding: The analysis of SF6 containing water can draw lessons from the breakdown theory of liquid dielectric, such as the "small bridge theory" analysis. Impurities caused by internal moisture will build an uneven area of insulation in the original almost uniform insulation structure, which looks like a "small bridge" that reduces the insulation level, and this "small bridge" area is the short board in the "wooden barrel short board".
5. Cause analysis of micro-water exceeding the standard:
(1) The quality of SF6 gas product is unqualified. That is, the new gas injected into the equipment is unqualified, which is mainly caused by the gas factory's lax detection of new gas, unqualified transportation and storage environment and long storage time.
(2) When the circuit breaker is filled with SF6 gas, moisture is brought in, which is mainly caused by the staff not operating according to the regulations and maintenance operation requirements.
(3) Moisture brought by heat preservation. The manufacturer failed to dry the insulating material before assembly or failed to dry the insulating material. During maintenance, the insulation is exposed to air and gets wet.
(4) Infiltrate moisture through the sealed joint. The external water pressure is higher than the internal water pressure of the air chamber. Moisture seeped out from the joints of the pipe wall.
(5) Water seeps into the leakage point. Leakage points such as air inlet, pipe joint, flange and sand hole of aluminum castings are channels for water to penetrate into circuit breakers, and water vapor in the air gradually penetrates into equipment.
(6) The electrical installation process was not carried out in accordance with the temperature and humidity specified in the regulations.
(7) The gas hygroscopic agent is affected by moisture. This generally has little impact, because the intact adsorbent is vacuum packed, and when the vacuum packing is abnormal, this adsorbent will not be used.
6. Basic method of micro-water exceeding the standard: directly discharge the gas in the air chamber with micro-water exceeding the standard into the atmosphere (it should be recovered by SF6 recovery device according to regulations, but it is limited to illegal operation due to the high cost of recovery and purification); Displacement adsorption
Agent (new and intact adsorbent is packed in vacuum, and it is best to heat it in the oven before replacement); Vacuum with a vacuum pump until the negative pressure of the air chamber reaches the specified standard (because the vacuum degree is measured by Maxwell vacuum gauge, which is inaccurate, and the indication number of the vacuum gauge on the vacuum pump is unreliable. Therefore, the vacuum degree is greater than the margin specified in the regulations. In addition, when measuring the vacuum degree with Maxwell vacuum gauge, the operation should be standardized to prevent mercury in the vacuum gauge from entering the cylinder through the self-sealing valve, causing insulation accidents); Further drying the air chamber by injecting dry nitrogen; Then vacuumizing until reaching the standard; Inject new SF6 gas (pay attention to the gas brand when injecting gas, try not to mix the gas from different manufacturers, and try not to mix the new gas with the old gas).
7. Understanding and views on monitoring methods and preventive measures of micro-water exceeding the standard in GIS.
Limited by my own understanding and practice, I know little about the monitoring methods of GIS micro-water exceeding the standard. In my opinion, GIS micro-water online monitoring only refers to the detection methods similar to transformer oil moisture or hydrogen humidity of hydrogen-cooled generators. Micro-water detection, the usual off-line detection means is to use dew point meter. Monitoring can be achieved by installing a dew point sensor in the instrument, that is, a humidity sensor, into the equipment, but there are also feasibility and economic considerations. These also depend on the development of newer sensor technology and the progress of communication technology.
Regarding the prevention of such failures, I think we should focus on the following two points:
(1) Improving the production quality and technology of electrical products and related products, such as adopting self-sealing inflation valves in GIS is a good example.
(2) It is very important to improve the operation level of electric power construction personnel.
(3) Improve the automation level of power grid and focus on developing on-line monitoring technology of electrical equipment.
Conclusion: From the process of this spring inspection, the following points must be paid attention to in the construction and operation of power system: 1. Reasonable selection of electrical products. In this overhaul and operation, the product quality of Gaoxiyuan is relative to that of the four high-pressure Kai Guanchang (Shen Kai and Kathy). Poor product quality; 2. Improve the quality level of power system operators and strictly manage them. Many failures are caused by the illegal operation of operators during construction or operation; 3. Study the fault detection technology in the process of power system operation to improve the automation level of power system operation.