Preventive test is an important link in the operation and maintenance of power equipment and one of the effective means to ensure the safe operation of power equipment. For many years, the high-voltage power equipment in the power sector and large industrial and mining enterprises has basically been tested in accordance with the requirements of the Preventive Test Regulations for Power Equipment issued by the former Ministry of Power (hereinafter referred to as the Regulations), which has played an important role in finding and diagnosing equipment defects in time.
1996 the former Ministry of electric power recently revised the specification, and the revised electric power industry standard DL/T 596- 1996 "Specification for Preventive Testing of Electric Power Equipment" was officially released and implemented in 1997.
Revision and Evolution of the Regulations
In the 40 years from 1950s to now, the code has been revised for five times, and the technology is relatively mature. The first two editions are "Soviet-oriented" in content and format, and the 1985 and 1996 editions are gradually "China-oriented".
The specification has a wide range of contents. In fact, some contents are beyond the scope of preventive testing. In terms of its nature, it belongs to the category of operation and maintenance. Therefore, it is suggested that the name be changed to "Electrical Equipment Maintenance Test Regulations". The word "maintenance" here includes preventive maintenance, predictive maintenance and defect elimination maintenance, which conforms to the actual content of the regulations. However, considering the narrow understanding of the word "maintenance" and the idiom of "preventive test", the old name is still used.
Summary of the Regulations
The "Regulations" stipulate the test items, test periods and technical requirements of various commonly used power equipment in chapters. These experimental projects integrate modern basic diagnostic techniques. Professionally, it belongs to electrical, chemical, mechanical and other technical fields, and most of them are electrical testing projects.
According to the nature of the examination, the examination items can be divided into four categories.
1. Periodic tests are preventive tests. This is a routine test of the equipment on a regular basis, so as to discover the potential defects or hidden dangers of the equipment in time. For example, chromatographic analysis of dissolved gas in oil, winding DC resistance, insulation resistance, dielectric loss factor, DC leakage, DC withstand voltage, AC withstand voltage, insulation oil test, etc.
2. Overhaul test refers to the inspection and test items carried out during or after overhaul. In addition to routine inspection items, insulation resistance of through-hole bolts, partial discharge, oil tank sealing test, opening and closing time and speed of circuit breaker, motor clearance and other tests are also needed, and some of them are purely mechanical inspection items.
3. Find out the fault test. In the specified periodic test or overhaul test, it is found that the test results are in doubt or abnormal, and it is necessary to further identify the fault or locate the fault, or it is called diagnostic test. This is a pilot project, only when necessary. For example: no-load current, short-circuit impedance, winding frequency response, vibration, water content and oil dielectric loss of insulating oil, power frequency reference voltage test of pressure releaser and zinc oxide arrester, etc.
4. Pre-test This is to identify the service life of equipment insulation, to know whether the insulation of the tested equipment can continue to be used for a period of time, or whether it is necessary to arrange replacement in the near future, such as insulation aging identification of generator stator winding or phase regulator, polymerization degree of transformer insulation paper (board), furfural content test in oil, etc.
As can be seen from the above, many testing items listed in the Regulations are indeed beyond the scope of routine preventive testing.
Determination of test items, cycles and sources of technical requirements
Test items and test periods of various equipment (such as transformers, capacitors, SF6 switchgear and supporting insulators). ) is determined by the reliability and safety of equipment operation, and whether it needs to be increased, decreased or modified.
The sources and basis of technical requirements can generally be divided into two categories:
1. Establish AC withstand voltage test standard based on insulation coordination design of power system;
2. Many technical requirements (such as dielectric loss, leakage current, absorption ratio, etc. ) are determined by the accumulation of experimental experience and statistical analysis, and gradually revised and improved in years of practice.
Analysis and judgment of test results
The Code emphasizes the comprehensive analysis and judgment of the test results. In other words, there are generally three steps: the first step is to compare with the results of various tests over the years; Step 2, compare with the test results of the same type of equipment; The third step is to comprehensively analyze the technical requirements of the code and other related test results, pay special attention to the development trend of defects and make judgments.
Comprehensive analysis and judgment sometimes have certain complexity and difficulty, rather than simply and dogmatically comparing technical requirements (technical standards) item by item. Especially when the test results are close to the limit of technical requirements (not exceeding the standard), the influence of climatic conditions, possible errors of measuring instruments and even the technical quality of operators should be considered. The accuracy of comprehensive analysis and judgment depends to a great extent on working experience, theoretical level, analytical ability, understanding of the structural characteristics of the tested equipment, test methods adopted, measuring instruments and the quality of measuring personnel.
According to the comprehensive analysis, the equipment can generally be judged as qualified, unqualified or suspicious. The unqualified ones shall be repaired in time. In order to concentrate or speed up the treatment of defects, the parts should be tested in sections as far as possible according to the structural characteristics of the equipment, so as to further determine the location or scope of defects. For suspicious or abnormal equipment, it is not easy to determine whether it is qualified at one time. Measures should be taken to shorten the test period, or re-test when the weather is good or the temperature is high, so as to monitor the changing trend of suspected defects of equipment or verify the accuracy of previous measurements.
Research progress at home and abroad in recent ten years
In recent ten years, great progress has been made in the test methods, test items and test instruments of preventive tests of power equipment in China. Examples are as follows:
1. Basic insulation test items
Traditional basic insulation testing items, such as insulation resistance, DC leakage current, dielectric loss, DC withstand voltage and AC withstand voltage, remain basically unchanged, with only a few improvements:
(1) The insulation resistance test project found that the transformer absorption ratio test was not perfect. Many large-capacity transformers that are newly delivered or overhauled after drying have high absolute insulation resistance, but the absorption ratio (R60"/R 15 ") is too small, which is suspected to be unqualified. After research, it is easy to make a clear judgment by using the internationally widely used polarization index test (R600)/R60 "). Therefore, the polarization index test item has been added to the regulations.
According to the analysis of dielectric theory, the absorption time is shorter than the test time (only 60s), and the polarization process in the composite medium is just in the initial stage, far from forming the basic pattern, which can not fully reflect the real appearance of insulation, so the absorption ratio result is not accurate enough. The test time of polarization index is 600s( 10 minute). Although the dielectric polarization process has not been completed, it is close to the basic figure, so it can accurately reflect the insulation moisture. From the history of technological development, industrialized countries have been using polarization index test instead of absorption ratio test since 1940s.
(2) Improve the anti-interference method when measuring the dielectric loss of equipment under electric field interference. For example, new methods such as electronic shift and different frequency method are adopted to facilitate operation and improve work efficiency. However, when the interference is large, the error is still large by using another method of power supply inversion and automatic calculation.
(3) For 6-35kV medium-voltage rubber-plastic insulated power cables (PVC insulated, XLPE insulated and EPDM insulated cables), the DC withstand voltage test item after being put into operation is cancelled, and the insulation resistance of the outer sheath and inner liner is measured instead.
This is because the high amplitude DC voltage will reduce the insulation life of rubber-plastic cables macroscopically, and many rubber-plastic cables that have passed the DC withstand voltage test have breakdown accidents in operation, which has been confirmed in theory and operation practice at home and abroad. However, for paper-insulated cables of 35kV and below, years of experience show that DC withstand voltage test is still an effective preventive test project, and many defects can be found, so it should be continued.
(4) In the AC withstand voltage test, the power frequency series resonance method is increasingly used for large-capacity samples (such as SF6 combined appliances and large generators).
(5) Summing up decades of experience shows that the first periodic test item of power transformer should be chromatographic analysis of dissolved gas in oil. Most transformer defects are found by chromatographic analysis. When the specification was revised this time, chromatographic analysis was listed as the first test item of power transformer.
2. Overhaul and fault detection test items
In this regard, some pilot projects have been added, such as:
(1) 35kV current transformer insulated by solid epoxy resin is added with partial discharge test;
(2) After the power transformer of 2)220kV and above is overhauled, the partial discharge test is carried out;
(3) After the outlet of the power transformer is short-circuited, do the frequency response test of the transformer winding to detect whether the winding is deformed;
(4) When necessary, test the water content in transformer oil, furfural content in oil and polymerization degree of insulating paperboard. The purpose of the latter two tests is to decide whether the insulation needs to be replaced;
(5) If the DC voltage test or AC resistive current test of zinc oxide arrester is unqualified, AC power frequency reference voltage test should be done for further judgment.
3. Improvement of measuring instruments and testing equipment
Over the years, the measuring instruments and test equipment produced in China have been greatly improved, and some of them have gradually moved towards digitalization, microcomputer operation, automation or semi-automation, which has improved the measuring accuracy and work efficiency and promoted the gradual upgrading of old instruments that have been used for decades. For example:
(1) digital megohmmeter appeared, which can automatically time, display absorption ratio and polarization index, and has automatic discharge function.
(2) The high-voltage DC voltage test equipment is more perfect. The power and voltage levels are improved, and digital and analog instruments are used, so the reading is convenient, accurate and easy to distinguish.
(3) There are many novel insulation loss angle testers (including new M-type test circuits and circuits for measuring phase angle difference between voltage and current). Most of them are controlled by microcomputers or automatically calculated and displayed digitally. The anti-interference performance has also been significantly improved, the measurement accuracy and working simplicity have been improved, and the elimination of QS 1 high voltage bridge has been promoted.
(4) The new digital AC /DC high voltage divider is widely used, which is convenient for directly measuring the high voltage side voltage on site and directly displaying the value or effective value of "AC voltage peak /√-2".
(5) A variety of series resonance test devices for AC withstand voltage test of large-capacity samples have been produced.
(6) The DC resistance measuring instrument for large power transformer windings solves the measurement problem of five-column triangular windings, and adopts microcomputer control to improve the current stability and significantly shorten the measurement time.
(7) The newly developed on-load tap-changer characteristic tester and high-voltage switch tester, using the principle of digital storage electronic oscilloscope, display waveforms and measured values, and print them out to become complete sets of special instruments.
(8) The domestic transformer winding deformation tester has good performance.
(9) The zinc oxide arrester, automatic tester for transformer ratio and wiring group, contact resistance tester and insulating oil medium strength tester have all been improved.
On the whole, there are few preventive tests in power companies in several industrialized countries, and some of them have a long test period. The basic test of insulation is similar to that in China, and these projects are generally done by power companies themselves. Some special test items (such as partial discharge location, winding deformation test, etc. ) used to identify faults are entrusted to professional testing units or manufacturers.
Some testing methods and projects adopted abroad are different from those adopted at home. For example, they are used to measure the dielectric loss of zinc oxide and ordinary valve (silicon carbide) arresters. In fact, it is to measure the resistive current loss of zinc oxide arrester at 5 ~ 10 kV AC voltage. This method is widely used. However, China is used to doing voltage test under DC 1mA. Some foreign countries have carried out partial discharge tests or radio interference measurements on lightning arresters, and found many defects. Some do impulse discharge voltage test for lightning arresters with gaps. For large motors, DC leakage and DC withstand voltage tests are widely used to replace AC withstand voltage tests.
At present, the experimental instruments used by the experimental teams in foreign countries are similar to those in China, but the advanced, computerized and automated instruments and experimental equipment in industrialized countries are superior to those in China, and the corresponding measurement accuracy is also higher. Some of them are also equipped with infrared cameras, portable communication equipment, notebook computers (some are equipped with "expert systems" for analyzing and diagnosing experimental data), mobile phones, fax accessories and printers, which can report important test results and found problems to superiors on the spot.
Foreign test teams generally have special test vehicles. Some heavy test equipment, such as AC and DC withstand voltage test equipment, dielectric loss meter, cable fault detection equipment, etc. Fixed to the car, can't move up and down. Use portable high-voltage copper shaft cable to lead to the equipment under test.
Looking at the progress of preventive tests in the electric power field at home and abroad, from the test items and test periods, all countries that produce electric power equipment with good quality, pay attention to maintenance and high reliability in operation have few test items and long test periods, and some even do not test some equipment.
At present, the quality and operation and maintenance level of power equipment in China are gradually improving. In the newly promulgated and implemented DL/T596- 1996, some test items have been appropriately simplified and the test period of some equipment has been extended, but there are still problems of too many test items and short period, which need to be further improved.
Working on electrical equipment, except for a few people who are charged in the operation of the equipment, is generally carried out in the state of power failure of the equipment, and technical measures such as checking electricity and hanging grounding wires are also taken to ensure personal safety of the power-off equipment. When conducting electrical tests on power-off electrical equipment, especially high-voltage electrical tests, in addition to cutting off all possible power supplies of the equipment, it is also necessary to use the test power supply to pressurize the tested equipment to generate high voltage, so as to achieve the purpose of the test. Because the tested equipment is often disconnected before and after pressurization; Equipment with large capacitance or equipment under test with electrostatic induction should be discharged or grounded after the test; The pressurization of the tested equipment is generally several times higher than the operating voltage, and the test leads are mostly exposed; Because other teams and groups often work at the same time or at the same time, high-voltage electrical test is more dangerous than general electrical equipment maintenance. Therefore, testers are required to seriously implement the technical measures and organizational measures to ensure personal safety in the electrical parts of power plants and substations in the Code for Safe Production in Electric Power Industry, and at the same time implement the relevant safety regulations of electrical tests to prevent high-voltage electric shock accidents in the tests and ensure the safety of testers and related staff. In the high-voltage electrical test, the following main safety precautions should be observed:
A, test personnel must be competent, test personnel shall not be less than two people, and should have a test director, formulate and implement safety measures.
The high-voltage tester must be clear about the purpose and method of the test (including being familiar with the performance and use of the test instrument, etc.). ) and the safety measures to be taken. Before work, the person in charge shall arrange the safety precautions in the test work for all testers in detail. Safety measures shall be formulated for live test according to site conditions. Important special tests, research tests and tests in operating systems must have test plans and be approved by relevant leaders. In this way, the test work can be carried out safely under the conditions of organization, leadership, safety measures and personnel at all levels. Failure to do so, especially if safety measures are not implemented, will lead to accidents.
For example, when the technicians in the electrical laboratory of a power supply station do the dielectric loss angle test of switches, they mix the test wiring in use with the unused wires, and clean them while pressurizing them, so that they come into contact with the terminal that has been pressurized to 3 kV and die of electric shock.
Second, make clear the scope of work, and clearly separate the tested equipment from other equipment and accept supervision.
When the equipment is powered off for high-voltage electrical test, the working ticket system should be implemented, and the work permit formalities should be handled with the operators to find out the power outage scope. According to the electrical part of power plants and substations in the Code for Safety Work in Electric Power Industry, fences or fences should be installed at the test site, and "Stop! High pressure danger "sign, supervised. When the two ends of the tested equipment are not in the same place, the other end should also be guarded. Its purpose is not to make a mistake about the scope of power outage. But judging from the accidents, some didn't set fences or fences, some didn't set guardians, and some didn't work at the end of fences. Here are a few examples: a high voltage test class of a substation tested the loss angle of 35 kV 3 12 switching medium. Because the work fence can't distinguish between power failure and live equipment, a tester turned off the switch and turned it on again without supervision, failed to understand the equipment under test, mistakenly boarded the nearby 3 12 switch and died of electric shock.
Third, we must adhere to the system of reviewing wiring before the exam.
During the test, the wires are often disconnected. Seriously implementing the system of rechecking wiring before test can correct wrong wiring in advance and avoid accidents caused by wrong wiring. Therefore, rechecking the wiring before the test is a basic system of the test work, and it is also an effective measure to prevent electric shock accidents and ensure personal safety. This system needs not only serious implementation, but also perseverance. The recheck of wiring for junior workers and interns should be focused, and the senior workers or simple wiring should not be relaxed, otherwise the purpose of rechecking wiring will not be achieved.
For example, a power plant uses a 100 kV high voltage tester to do a 6 kV porcelain insulator withstand voltage test in high voltage test room. Before the test, the connecting wire of the booster was connected without detailed inspection. When the booster was pressurized to 42 kV, it was found that a plastic wire was connected to the porcelain bottle at the high-voltage outlet of the booster, which went straight to 1 10 kV substation, about 50 meters. The test stopped immediately and the wire was removed. This line was originally dismantled after testing the 1 10KV switch more than ten days ago, and it was tied to the substation structure, with more than a dozen construction workers working near the structure. Fortunately, these workers did not approach or touch the conductor during the pressurization process.
Four, test work, should stand on the insulating mat or wear insulating shoes, this is to prevent electric shock accidents or reduce the degree of injury safety measures.
For example, when the M д- 16 bridge was tested in a repair shop of a power supply bureau, the tester only turned off the switch of the bridge and did not pull the power circuit breaker. When crossing the bridge, his right hand touched the live part of the power supply end of the bridge. Because the bridge was grounded, the worker put an insulating mat under his feet, and he left the power supply, which only caused electric shock and burns to the power supply circuit from the ring finger of his right hand to his left palm.
Five, before the pressure test, must inform the relevant personnel to leave the tested equipment or exit the scene rear can be carried out.
High-voltage tests are often carried out simultaneously with other maintenance teams or at the same time. Therefore, before pressurization, these teams must be informed to leave the tested equipment or leave the site, so that the tested equipment can be carried out in an unattended state to ensure the safety of personnel. These practices cannot be ignored, otherwise there will be serious consequences.
For example, in the transformer maintenance test of a substation, the relevant team members were informed before and after the transformer withstand voltage test, so that a repairman thought that the equipment was out of power and boarded the transformer twice to work. When the transformer was pressurized, the worker climbed on the transformer again. Fortunately, he was found and an electric shock accident was avoided.
Six, the device under test with capacitance or inductance must be fully discharged or grounded before and after the test.
After the power failure and the completion of the high-voltage DC test, the measured equipment has a large capacitance, such as buses, cables, capacitors and other equipment with electrostatic induction, which must be fully discharged or grounded to prove that the measured equipment has no charge before it can work. Because of the high residual voltage or induced voltage of these devices, insulating rods must be used when discharging, which can also prevent them from accidentally contacting live equipment during operation. Some units do not pay attention to discharge or grounding, resulting in electric shock accidents.
For example, technicians in the high voltage laboratory of a power plant conducted a DC 30 kV withstand voltage test on a 6 kV cable for 5 minutes. The numbering sequence of the two ends of five adjacent cables to be tested is actually reversed, leaving hidden dangers, such as one end numbered L and the other end numbered 5, which have never been detected. When testing a cable, after disconnecting the test power supply, before discharging, my hand touched the cable whose serial number was not the cable to be tested. In fact, I just finished testing, and the residual voltage was 25 kV, so that I was electrocuted by the residual charge.
Seven, pressure test work, closing, must echo each other, correctly convey the password.
The pressure test is frequently opened and closed. If the password is not heard through subjective assumptions or only by looking at the instrument, or the password is not conveyed correctly, an electric shock accident may occur.
For example, when the no-load test of transformer was carried out in the test class of a repair shop of a power supply bureau, the operator of the test power supply thought that the line had been connected and closed the test gate without informing the operator on the equipment. The informant found that the wiring was loose and he got an electric shock when moving the wiring.
Eight, pressure test rearrangement of wiring, voltage regulator must be back to zero, open the breaker in front of the test power supply.
When the voltage test works normally, the voltage regulator must be set to zero and the test power supply must be cut off. However, after the voltage test, problems were found, and it was found that the connection was not firm or wrong. The test power supply had both main circuit breaker and auxiliary circuit breaker, which was ignored by some testers and an accident occurred.
For example, the substation construction team of a power supply bureau found that there was something wrong with the test data when conducting AC withstand voltage test on the booster switch of a substation. When looking for the reason, the booster was not zeroed and the test power supply was not cut off. When it is found that the polarity of the booster is reversed, the tester is ready to change the polarity. Fortunately, he was out of power and only burned his hand, with no serious consequences.