During this period, with the development of power capacitor insulation technology and material science, the design and manufacturing technology of CVT abroad has been greatly improved, and some new products have appeared. We should summarize the experience of CVT manufacturing and operation at home and abroad in time, further promote the development of domestic CVT, and provide high-quality and reliable products for the construction and transformation of urban and rural power grids and the construction of EHV power grids in China. The high voltage of CVT is mainly borne by capacitive voltage divider, so the selection of capacitor dielectric material is very important. In the late 1980s, polypropylene film and capacitor paper were impregnated with organic synthetic insulating oil medium almost simultaneously at home and abroad to replace capacitor paper impregnated with mineral oil medium. Because the dielectric strength of the film is four times that of oil-impregnated paper, the dielectric loss is reduced to110 of the latter, and the gas absorption performance of synthetic oil is good, the film-paper composite dielectric can improve the capacitance of CVT, reduce the dielectric loss, improve the partial discharge performance and improve the insulation margin. At the same time, because the capacitance-temperature characteristics of film and oil-immersed paper are complementary, the reasonable combination of film and paper can greatly reduce the capacitance-temperature coefficient of capacitor. All these have created conditions for improving the accuracy, rated output power and operational reliability of CVT.
In recent years, the performance of dielectric materials used has been continuously improved, and the manufacturing technology and insulation structure design of capacitors have also been continuously improved. The collocation of film and paper abroad is divided into 1 film and 2 paper, 2 film and 1 paper, 2 film and 2 paper, etc. Insulation impregnants include alkylbenzene (ABB called FARADOL300), M/DBT and SAS-40, etc. The solid media used in China are generally 2 films and 3 papers, and there are also 2 films 1 paper. Impregnating agent is mainly alkyl benzene, and some products use PXE. In special cases, M/DBT is also useful. The development of medium structure and impregnant application at home and abroad is basically synchronous. In order to reduce the edge field strength of capacitor elements, a new structure of folded aluminum foil is adopted abroad. Some use thicker aluminum foil instead of the traditional copper lead sheet as the lead wire of the unit electrode, which can prevent the lead sheet from damaging the medium and make the edge field strength uniform. The bracket of capacitor core has long been made of electrical cardboard instead of plastic cardboard. Referring to foreign advanced technology, aluminum foil is used to lead out electrodes in China, and electrical cardboard is used instead of traditional phenolic cardboard, which is beneficial to vacuum drying and impregnation of bracket insulation, fully eliminating bubbles and moisture, and eliminating the possibility of internal failures. CVT includes capacitance and nonlinear inductance elements (intermediate transformer, compensation reactor). Under the action of overvoltage caused by sudden closing or secondary short circuit elimination, some internal subharmonic ferromagnetic resonance may occur, endangering the safety of CVT itself and affecting the normal work of secondary measurement and protection. Therefore, it is a key technology of CVT to adopt appropriate shock absorber. Foreign CVT has adopted fast saturable reactor damper since 1980s. In normal operation, the impedance is very large and the power consumption is very small, which does not affect the measurement accuracy. When resonance overvoltage occurs, the reactor is saturated, and a large current passes through the resistor in series with it, consuming a lot of power, which can quickly damp ferromagnetic resonance within 10 power frequency cycle. Xirong Factory took the lead in the research and application of various dampers in China, and experienced the development process from resistance damper, resonance damper to speed saturation damper. The velocity saturation damper developed by 1990 has been widely used in the whole industry in China.
Like the application of any new technology, some new technical problems have been encountered and solved in the application of fast saturation damper. The problem is that some products that have passed the ferromagnetic resonance test according to IEC standards and national standards will still have internal persistent ferromagnetic resonance when put into operation, accompanied by output voltage waveform distortion and abnormal noise. To this end, we have carried out technical research and adopted a series of technical and quality control measures: First, the characteristic parameters of permalloy iron core for fast saturation reactor have been strictly tested, and the magnetic density of intermediate transformer has been appropriately reduced. Then, after in-depth study, we have a new understanding and summed up a new set of ferromagnetic resonance testing methods. The tested products can ensure that ferromagnetic resonance can be effectively prevented under any electric shock (including sudden voltage at rated voltage on site). This test method re-selects the parameters of shock absorber and is used for CVT factory inspection. Through the field verification in Tianjin Yangliuqing Power Plant and Yichang Power Supply Bureau, and the practice that more than 500 CVT produced by 1998 have no resonance, it shows that the improvement measures are effective, which is a great technological progress of domestic CVT, and also denies the saying that "domestic CVT can't solve the resonance problem". As a matter of fact, foreign CVT has had resonance problems during its operation in China, which has been improved in recent years. Whether it can guarantee that resonance will not happen again has not been verified in China.
Using advanced fast saturated reactance damper can not only effectively damp ferromagnetic resonance, but also obviously improve the transient response characteristics of CVT. After the primary side of CVT with resonant damper is short-circuited, the residual voltage on the secondary side is less than 10% after 20ms, and the residual voltage can be reduced to less than 5% after using fast saturation damper, which can meet the requirements of modern fast relay protection. Foreign CVT generally pays attention to beautiful appearance and good corrosion resistance. The exposed metal parts are made of aluminum alloy or hot dip galvanized, the fuel tank is made of aluminum alloy welded or cast, and the fasteners such as bolts and nuts are made of acid-resistant steel, so it is not necessary to repaint on site. The internal structure also has many unique features. The transformer adopts annular iron core, the reactor adopts C-shaped iron core, and the medium voltage bushing is made of epoxy resin by vacuum casting. The medium voltage terminal is connected with the electromagnetic device through the spring piece, and pressure contact can be realized without connecting wires. For the convenience of use, CVT of ABB company leads the pressure regulating tap out of the fuel tank. When the secondary load changes in operation, the accuracy can be kept unchanged or improved by adjustment, and even the electromagnetic device and the capacitive voltage divider can be reassembled on site. In order to save installation area and cost, CVT abroad is generally allowed to install line arrester at the top.
Domestic CVT has a certain gap in this respect. Rough processing, poor anticorrosion performance, and insufficient consideration for users in structure. In recent years, things have begun to change. The factory purchased CNC machine tools, machining centers and other precision equipment to improve the machining accuracy, and began to use vacuum casting epoxy medium-pressure casing. Some products, such as (TYD4500), have introduced error adjustment terminals, developed integrated devices with wave arresters at the top of CVT coupling capacitors (including combined filters), and passed the seismic test. In terms of anti-corrosion, the quality of domestic hot dip galvanizing is still relatively poor. We adopted a new technology of plastic spraying for fuel tank, and the fasteners were galvanized, which significantly improved the corrosion resistance. ① High insulation reliability. As a capacitive voltage divider that bears high voltage, its dielectric strength is the most important factor. Dielectric breakdown will not only affect the measurement accuracy of CVT, but also cause a vicious accident of capacitor explosion and fire. The working field strength of capacitor dielectric in domestic CVT is generally less than 70% of that of imported products. In addition, the improvement of insulation structure and strict control of trace moisture in oil have reduced the dielectric loss and partial discharge of capacitors, and their insulation characteristics are obviously higher than those of imported products, which has been verified by a large number of domestic operating experiences. Recently, the 500kV and 220kV CVT imported by a foreign company in China suffered serious explosion accidents and capacitor dielectric breakdown, and were replaced by CVT produced by Xirong Factory and put into normal operation. Therefore, just as the domestic shunt capacitor for reactive power compensation has high reliability, the insulation reliability of domestic CVT is higher than that of imported products, which can better meet the needs of the domestic market.
② It can reliably damp ferromagnetic resonance. After a lot of application research and improvement of the speed saturation damper, and quality control according to strict factory standards, it is ensured that each CVT can effectively damp the ferromagnetic resonance of various frequencies at any voltage from low to high to rated voltage factor, which has been confirmed in a large number of operation practices.
③ Excellent transient response characteristics. Due to the reasonable selection of design parameters, the domestic CVT with fast saturation damper can ensure that its secondary residual voltage can be reduced below 5% within 20ms after short circuit. However, foreign CVT generally only guarantees the residual pressure of 10%, and only a few products can reach 5%. In this way, domestic CVT also has certain advantages in cooperation with fast relay protection.
④ It can be used to monitor the harmonics of power grid. Generally speaking, due to the complementary principle of inductance and capacitance at power frequency, there will be a big error in measuring the higher harmonic voltage at the secondary end of CVT. However, Xirong Factory installed a component on CVT and provided a pair of harmonic monitoring terminals, so it can accurately test the harmonics of power grid by using capacitive voltage divider, and its error meets the requirements of national standard GB/T 14549 "Power Quality-Harmonic of Public Power Grid". This technology has been approved as a national utility model patent. ① There is a big gap between design and manufacturing technology and foreign countries. The overall modeling is not beautiful enough, and the processing and assembly quality is rough, which is related to the low level of basic parts (such as the appearance quality of porcelain sleeves), backward technology and equipment, and lax quality control.
② Poor anticorrosion treatment of exposed metal parts. Needless to say, the quality of hot-dip galvanized aluminum alloy or aluminum alloy welded parts abroad is far better than that in China. In China, not only the quality of hot dip galvanizing is poor, but also the paint falls off and fades, so it needs to be painted many times in use.
③ Not paying enough attention to users' needs. Some parameters and characteristics have not yet provided test data, such as the influence of power grid frequency change on CVT measurement accuracy, the endurance test of capacitor 1A high-frequency current, and the curve of dielectric loss changing with temperature. Most CVT abroad are allowed to be equipped with line arresters, and only domestic 1 10kV products are assembled and used by users themselves in some cases. 4. 1 firstly, the design and manufacturing process of domestic CVT are improved according to the current gap, which fully meets the special requirements of IEC standards, national standards, electric power standards and regulations and user departments, reduces the failure rate, and truly realizes maintenance-free and less maintenance.
4.2 Develop new CVT products with modern new technologies. Foreign capacitive voltage divider photoelectric voltage transformer has been published, the draft standard of IEC electronic voltage transformer is under discussion, and the international power grid congress is also studying the development of photoelectric transformer as a new technology in the future. Foreign experts generally believe that due to the interface, capacity, habits and other problems, photoelectric transformers will not develop on a large scale in the next decade, but there will be a big development in the next decade, starting with low voltage and gradually developing to high voltage after gaining experience. Some people in China have invested in the research and development of electronic voltage transformers, and will gradually realize practical and large-scale production and application with the development of new technologies abroad in the future.
4.3 The products are developed to flame retardant, explosion-proof and higher voltage level. Silicone rubber synthetic sleeve, SF6 inflatable independent CVT and G 1S CVT have the advantages of pollution resistance, explosion-proof, flame retardant and reliable insulation, and are expected to be successfully developed in the near future. With the implementation of power transmission from west to east in China, the higher voltage level of 750kV in northwest power grid has been basically determined, and the development of 750kV CVT has become a top priority. In recent years, CVT has developed rapidly in China and accumulated rich experience in use and monitoring. However, because manufacturers can't keep up with the introduction of product performance and technical services, there are also some application technical problems to be solved. With the development of products and the improvement of power supply reliability requirements, the application technology of CVT will also develop and improve accordingly.
① Reasonable selection of secondary output. The secondary rated output of CVT is determined by the sum of the loads required by the secondary connected measuring and protection devices. The greater the CVT output, the higher the manufacturing cost and price. According to foreign experience, it is reasonable to determine the rated output of CVT according to the actual secondary load 1.3 ~ 1.5 times. The load of modern relay protection and measurement is not big. According to the standard, when the actual load is less than 25% of the rated output of CVT, the error may exceed the specified value of the corresponding precision level. Therefore, it is necessary to get out of the misunderstanding of "the greater the output, the better" and put forward requirements from the actual needs.
② Improvement of field test method. CVT is fully sealed and maintenance-free equipment. There is no need to test one by one according to the ex-factory test items before installation and operation and in the annual preventive test, let alone open the product to check the internal structure and conduct the test. The purpose of field test is to check whether there are any abnormal changes during transportation and operation. The test items and requirements have been given in the product manual, including measuring the capacitance and dielectric loss (tanδ) of the capacitor and measuring the insulation resistance of each winding of the intermediate transformer. When necessary, the accuracy comparison (comparison between products of the same model and specification) test can also be carried out.
When testing the lower capacitance of single-column CVT, only C and tanδ of the whole capacitance need to be tested, and C 1 and C2 need not be tested separately. Because C 1 and C2 are connected in series by the same capacitor element, encapsulated in the same porcelain sleeve, and the internal insulating oil is also connected, the series data of C 1 and C2 obtained in the overall test can be compared with the factory data or the last test data to judge the quality of the capacitor, which can completely reflect the breakdown or damp condition of C 1 and C2. In the past, the so-called "self-excitation method" was used to measure C 1 and C2 respectively from secondary pressurization. A lot of practice shows that there are many problems in "self-excitation method". Before the interview, during the exam, and after the exam, there are many points for attention, which are easy to make mistakes and cause damage to equipment and instruments; Secondly, the test voltage is very low, and the loop itself has a great error, which leads to a great error in the measurement results, and it is difficult to judge whether the product is normal or not. It is a simple and reliable method to measure only the whole capacitor C (that is, C 1 and C2 in series) and tanδ, which is a summary of practical experience of operation monitoring personnel in many areas.
③ Operation monitoring. CVT itself is a measuring instrument, and the amplitude, phase and waveform of secondary voltage can reflect the operation of the equipment itself, which can be said to have its own online detection function. We should make use of this advantage to monitor and compare the amplitude, phase and waveform of the secondary voltage of the same group or the same voltage product in operation. A sudden change in the voltage of a phase may be a signal of internal fault. In addition, during the operation of the product, the sudden rise of the open delta voltage or abnormal noise should also be paid great attention to, and the product should be further inspected as soon as possible.
④ On-site error adjustment and capacitor configuration. Generally, CVT with structure can not adjust the error on site; CVT with voltage and phase adjustment terminal lead-out structure can adjust the output voltage according to the change of field secondary load to ensure the accuracy of products or realize the reconfiguration of unit capacitors. When necessary, these works should be carried out with the participation of the manufacturer's professionals.
Differences and advantages between CVT and other transmissions;
1. automATic transmission (at):
Compared with AT, CVT has the advantages of stepless speed change, no shift impact, wide speed change range, faster start, lower engine speed AT high speed and lower fuel consumption 10% or more than AT.
2. Double clutch automatic transmission (DSG)
Compared with DSG, CVT has the advantages of simple structure, wide speed range and low fuel consumption at high speed. CVT uses hydraulic torque converter to drive power at first, and there is no friction between parts, so it can achieve the same life as a vehicle and is maintenance-free for life.
3. Chain drive CVT
CVT adopts steel belt drive, which is just the opposite of chain drive. For push transmission (the driving wheel pushes the driven wheel through the push plate of the steel belt), the lower half of the push plate side is in contact with the pulley, and the friction area is large, so the push transmission ratio pull transmission can transmit more torque, so the CVT made by Dongfeng Nissan can match the 3.5L large displacement engine.
4. Clutch CVT
CVT adopts hydraulic torque converter, which can increase torque and greatly improve the starting acceleration performance. The ultra-low speed of hydraulic torque converter can ensure the accurate operation when starting on the slope and reversing into the warehouse. CVT with hydraulic torque converter can be installed on off-road vehicles that need super torque. This combination improves the climbing ability of the car and greatly improves the ability of the car to drive on rough roads. In addition, the hydraulic torque converter uses transmission fluid to transmit power without friction, which prolongs the service life of CVT and can be maintenance-free for life.