油浸式变压器的使用和维护

Use and Maintenance of ELVIM Oil-immersed Distribution Transformers Technical Collection Building a New Electric World Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 1 The transformer is an electrical machine that allows the transmission and distribution of electric energy simply and inexpensively, since its efficiency is greater than 95%. Through the brief description of the use and maintenance of the oil-immersed distribution transformers, the present technical leaflet provides useful information for the engineers, who are involved in the selection, purchasing, installation, operation and maintenance of transformers. Use and Maintenance of ELVIM Oil-immersed Distribution Transformers Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 2 SECTION A: USE OF TRANSFORMERS A.1 Transformer Types A.1.1 Classification of transformers according to the use page 4 A.1.2 Classification of transformers according to the cooling method page 5 A.1.3 Classification of transformers according to the insulating medium page 5 A.1.4 Classification of transformers according to the construction of the magnetic circuit page 6 A.2 ELVIM Distribution A.2.1 General characteristics page 7 Transformers A.2.2 Advantages of ELVIM distribution transformers page 7 A.3 Transformer A.3.1 Magnetic circuit page 8 Manufacturing A.3.2 Windings page 8 Features A.3.3 Metallic parts page 9 A.3.4 Assembly page 9 A.3.5 Cooling medium page 9 A.4 Transformer A.4.1 Tank page 10 Components A.4.2 Cover page 10 A.4.3 Lifting lugs page 10 A.4.4 Rollers page 10 A.4.5 Draining and sampling oil valve page 10 A.4.6 Neutral earthing link page 10 A.4.7 High voltage bushings page 11 A.4.8 Low voltage bushings page 11 A.4.9 Low voltage connectors page 11 A.4.10 Tap changer page 11 A.4.11 Voltage selector page 11 A.4.12 Transformer thermometer page 11 A.4.13 Oil conservator page 12 A.4.14 Buchholz relay page 12 A.4.15 Air dehumidifier page 12 A.4.16 Filling valve page 12 A.4.17 Oil level indicator page 12 A.4.18 Rating plate page 13 A.4.19 Tank earthing point page 13 A.4.20 Accessories of sealed type transformers page 13 A.5 Transformer tests A.5.1 Type tests page 14 A.5.2 Routine tests page 14 A.5.3 Special tests page 15 A.6 Transformer A.6.1 Rated power page 16 electrical A.6.2 Temperature rise page 16 characteristics A.6.3 Ambient temperature page 16 A.6.4 Altitude of installation page 16 A.6.5 Short-circuit impedance page 16 A.6.6 No-load losses page 17 A.6.7 Load losses page 17 Contents Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 3 A.6 Transformer A.6.8 Rated voltage page 17 electrical A.6.9 Vector group page 17 characteristics A.6.10 Frequency page 18 A.6.11 Noise page 18 A.6.12 Efficiency page 18 A.6.13 Short-circuit current page 18 A.6.14 No-load current page 18 A.7 Transformer standards page 19 A.8 Tolerances page 19 A.9 Transformer A.9.1 Overloading page 20 operation A.9.2 Parallel operation page 21 A.9.3 Load distribution of transformers in parallel operation page 21 A.10 Transformer order form page 22 A.11 Transformer A.11.1 Electrical utilities page 23 selection A.11.2 Industrial users page 23 A.12 ELVIM A.12.1 Single-phase transformers, from 5 to 50 kVA, 20/0.231 kV page 24 transformers series A.12.2 Three-phase transformers, from 250 to 1600 kVA, 20/0.4 kV page 26 A.12.3 Three-phase transformers, from 250 to 1600 kVA, 20/0.4 kV, with low losses page 28 A.12.4 Three-phase transformers, from 250 to 1600 kVA, 20-15/0.4 kV page 30 A.12.5 Three-phase sealed type transformers, from 25 to 1600 kVA, 20/0.4 kV page 32 A.13 Examples A.13.1 Calculation of transformer efficiency page 34 A.13.2 Calculation of voltage drop page 34 A.13.3 Parallel operation of transformers page 35 A.13.4 Transformer selection page 35 SECTION B: TRANSFORMER INSTALLATION AND MAINTENANCE B.1 Dimensions of transformer installation area page 36 B.2 Instructions for transformer installation page 37 B.3 Instructions for transformer maintenance page 37 B.4 Instructions for thermometer connection page 38 B.5 Instructions for the connection of the Buchholz relay page 39 B.6 Instructions for the connection of the air dehumidifier page 40 Services of Schneider Electric page 41 Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 4 (a) Distribution transformers They are used in the distribution networks in order to transmit energy from the medium voltage (MV) network to the low voltage (LV) network of the consumers. Their power is usually ranging from 50 to 1600 kVA. (b) Power transformers They are used in the high-power generating stations for voltage step up and in the transmission substations for voltage step up or step down. Usually their power is bigger than 2 MVA. (c) Autotransformers They are used for voltage transformation within relatively small limits, for connection of electric energy systems of various voltages, for starting of AC (alternative current) motors, etc. According to their use, the transformers are classified into the following categories: The transformers are classified into various categories, according to their: (a) use, (b) cooling method, (c) insulating medium, (d) core construction. These categories are presented in the following subsections. (d) Test transformers They are used for the execution of performance tests with high or ultra-high voltage. (e) Special power transformers They are used for special applications, e.g. in furnaces and in welding. (f) Instrument transformers They are used for the accurate measurement of voltage or current. (g) Telecommunication transformers They are used in telecommunication applications aiming at the reliable reproduction of the signal in a wide range of frequency and voltage. A.1 Transformer types SECTION A: Use of Transformers A.1.1 Classification of transformers according to the use Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 5 ONAF: Oil Natural Air Forced. OFAN: Oil Forced Air Natural. OFAF: Oil Forced Air Forced. OFWF: Oil Forced Water Forced. The identification of oil-immersed transformers according to the cooling method is expressed by a four-letter code. The first letter expresses the internal cooling medium in contact with the windings. The second letter identifies the circulation mechanism for internal cooling medium. The third letter expresses the external cooling medium. The fourth letter identifies the circulation mechanism for external cooling medium. For example, if the internal cooling medium is mineral oil, which is circulated with natural flow, and the external cooling medium is air, which is circulated with natural convection, then this cooling method is coded as ONAN (Oil Natural Air Natural). In power transformers, various cooling methods are used including oil circulation by pumps, or forced air circulation by fans, or both of the above. As a result, the following cooling methods exist: Combinations like ONAN/ONAF, ONAN/OFAN or ONAN/OFAF are also applicable. A.1.2 Classification of transformers according to the cooling method (a) Oil-immersed type transformers The insulating medium is mineral oil or synthetic (silicon) oil. (b) Dry type transformers The cooling is implemented with natural air circulation and the windings are usually insulated with materials of H or F class. The materials of H class are designed in order to operate, in normal conditions, under temperatures up to 180ºC and the materials of F class under temperatures up to 155ºC. (c) Resin type transformers The resin type transformer is a dry type transformer insulated with epoxy resin cast under vacuum. According to their insulating medium, the transformers are classified into the following categories: A.1.3 Classification of transformers according to the insulating medium Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 6 (a) With three legs (vertical limbs) The magnetic flux of one leg must close through the other two legs and the flux also flows through the windings of the other phases, namely the transformer has non free return of the flux. (b) With five legs (vertical limbs) Free return of the flux through the external legs. The construction of the magnetic circuit of the three-phase transformers can be done, alternatively, as follows: (a) Stack core The layers of the sheets of the magnetic material are placed one over the other and the vertical and the horizontal layers are over lapped. (b) Wound core The magnetic circuit is of shell type and the sheets are wound. There are two different technologies for stacking the sheets of the magnetic material of the core: (a) Silicon steel sheet The silicon steel sheet that is used for the core construction is an alloy consisting of 97% iron and 3% silicon. This material has crystallic structure. The silicon steel sheets have thickness from 0.18 up to 0.5 mm. There are also silicon steel sheets for operation in high magnetic induction (Hi-B). (b) Amorphous metal sheet The amorphous metal sheet that is used for the core construction is an alloy consisting of 92% iron, 5% silicon and 3% boron. This material has not crystallic structure. It has 70% lower no-load loss than the silicon steel. The thickness of the amorphous metal sheet is 0.025 mm, namely it is about 10 times thinner than the typical thickness of the silicon steel sheet. Two different materials are used for core construction: A.1.4 Classification of transformers according to the construction of the magnetic circuit I 线条 I 线条 Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 7 A.2 ELVIM Distribution Trasformers At the industrial site oil of Schneider Electric AE, ELVIM distribution transformers are manufactured, with voltages up to 36 kV, having oil as cooling medium and the following technical characteristics: Single-phase transformers from 5 up to 500 kVA. Three-phase transformers from 25 up to 2000 kVA. A.2.1 General characteristics More than 30 years of experience in transformer manufacturing (the manufacturing site is active since 1969). As a result, the best techniques and methods are used during transformer design and manufacturing. The application of the ISO 9001 quality assurance system in combination with the very careful monitoring of the whole industrial process lead in the manufacturing of high quality transformers. The application of the ISO 14001 environmental management system assures the protection of the environment and the reasonable use of natural resources during the transformer production. The use of the best materials for the transformer construction. The reliability of the suppliers of the transformer materials is systematically monitored and checked. The high automation of the industrial process has dramatically decreased the delivery time. For example, special transformers can be delivered within 3 weeks. A potential transformer user has a lot of reasons to choose ELVIM distribution transformers that are manufactured by the industrial site of Schneider Electric. Some of the most important reasons are the following: All the transformer offers are treated very carefully in order to finally give an offer, which fully satisfies the needs of the transformer user. The offer is technically complete and represents the optimum technical and economical solution for the specific transformer application. The wound core technology that is followed has the following advantages, in comparison with the stack core technology: Lower magnetization current. As a result, the transformer has lower current harmonics (better quality), lower consumption of reactive power and lower magnetization current. Less noise. A.2.2 Advantages of ELVIM distribution transformers Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 8 A.3 Transformer Manufacturing Features The wound core technology is followed and magnetic materials with low losses are used. The magnetic circuit is of shell type and the cores are wound. The production procedure of the wound core is as follows: the magnetic material is slit into sheets of standard widths. Next, the sheets are cut to predetermined lengths. Next, the sheets are wound on a circular mandrel and a circular core is created. Annealing treatment follows in order to recover the core’s physical and electrical properties. The quality control department checks the quality of the wound core. Figure 1 shows one wound core. A.3.1 Magnetic circuit The type of coil is rectangular concentric winding. For the low voltage coil, copper sheet or copper rectangular wire is mainly used. The high voltage coil is constructed from copper wire or copper rectangular wire. The combination of copper sheet in low voltage with copper wire in high voltage plus coated press paper with epoxy resin as interlayer insulation, increases the coil’s ability to withstand short-circuit. Important points during the production procedure are the following: Coil heat treatment at 100ºC so that epoxy resin is polymerized giving an extremely compact product, All coils pass through quality control. Figure 2 shows the assembled active part (cores and coils) of one three-phase wound core type transformer. A.3.2 Windings Figure 1: Wound core. Figure 2: Transformer active part. CORES COILS Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 9 For the construction of transformer metallic parts, the following basic mechanical equipment is used: CNC machines for cutting, punching and bending of steel sheets. Different types of welding machines (i.e. MIG- MAG, TIG, and electrode) for the welding of the metallic parts. Machines for construction and welding of corrugated panels and tanks. Certified technicians and welders are the operators of these machines. Stud welding equipment for stud welding on the transformer cover so that secure insulator placement is achieved. Equipment for oil leak detection of the transformer tanks. Modern painting shop for the painting of the metallic parts. The usual painting procedure includes the following steps: sandblasting, decreasing-phosphatizing Fe, painting with 4 coats (two primer coats and two final color coats) with total thickness of 160 Ìm. This painting procedure results in a durable corrosion protection and therefore lengthy life expectancy. A.3.3 Metallic parts For the transformer assembly, the following basic equipment is used: one crane of 35 tons and two cranes of 5 tons, one drying chamber to dry the active parts in order to remove the moisture, which is absorbed by the transformer insulating materials during the production procedure, two vacuum chambers, in which the transformers are filled with oil, machines for the processing of transformer oil, so that the oil obtains the appropriate characteristics, according to the international standards. A.3.4 Assembly Transformer oil according to IEC 296 specifications is used as cooling medium. The initial filling of transformer with oil is done under high vacuum in order to assure the high penetration of oil everywhere and to remove air bubbles or moisture that could cause dielectric failure of coil. Oil can also be filled later on without vacuum under the prerequisite that the oil level covers the active part and the oil has been filtered. In agreement with the customer, the oil can be supplied from Schneider Electric or another company provided that the oil is according to the given standard. A.3.5 Cooling medium Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 10 This link ensures the neutral earthing of the three-phase winding with the transformer tank. A.4.6 Neutral earthing link A.4 Transformer Components The transformer tank consists of the bottom plate, frame, and the tank sides. The tank sides are made of corrugated panels in order to increase the total cooling area. The tank of sealed type transformers (without oil conservator) is filled with oil and is sealed. The corrugated panels do not allow the creation of significant increase of pressure internally, which is caused by the increase of oil temperature during transformer’s operation. The transformer tank has two earthing points. The rolling system or the base skid is welded to the tank bottom plate. A.4.1 Tank There are two lifting lugs on the tank cover, which are used for lifting and carrying the transformer. On request, the thermometer pocket and the thermometer with two electrical contacts are placed on the cover. Moreover, a neutral earthing link is also placed on the cover. A pressure relief device is usually placed on the cover of the sealed type transformers. A.4.2 Cover The lifting lugs are used for lifting and carrying the transformer. A.4.3 Lifting lugs The transformers up to 160 kVA are usually manufactured as pole-mounted. The transformers above 160 kVA are equipped with bi-directional rollers. A.4.4 Rollers In the lower part of the tank side there is a draining and sampling oil valve, which allows the oil sampling in order to test the oil dielectric strength. A.4.5 Draining and sampling oil valve Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 11 For medium voltage of 6, 10, 20, 30 kV, porcelain bushings according to DIN 42531 are used. Alternatively, on request, plug-in bushings can be used. A.4.7 High voltage bushings Low voltage bushings of 1 kV series, according to DIN 42530, are used in the low voltage. A.4.8 Low voltage bushings Low voltage connectors, according to DIN 43675, are used. A.4.9 Low voltage connectors The applying medium voltage to the primary winding of transformer is not stable and depends upon the transformer position in the distribution network. Therefore, taken the primary voltage as granted, the tap changer is used in order to keep the secondary voltage of the transformer as stable as possible. The tap changer is placed into the transformer tank. The control interface of the tap changer is placed on the cover. The handling of the tap changer must be done when the transformer is out of voltage, as follows: initially, the handle of the tap changer is pulled upwards so that the pin is released and entered into the fixed annulus. Then we turn the handle right or left so that the pin is placed to the desirable tap position. If it is desirable to switch from one position (e.g. position 1) to another (e.g. position 5), then the handling is implemented step by step, through all intermediate positions (e.g. positions 2, 3, 4). The taps positions are inscribed on the rating plate of the transformer. For example, when the transformer is designed to operate in two voltage levels, e.g. 20 kV and 15 kV, then using a 5-position tap changer, the regulation of the primary voltage can be ±2x2.5 % for medium voltage 20 kV (i.e. voltages 19.0, 19.5, 20.0, 20.5, and 21.0 kV) and ±2x3.3 % for medium voltage 15 kV (i.e. voltages 14.0, 14.5, 15.0, 15.5, and 16.0 kV). A.4.10 Tap changer The voltage selector (changeover switch) is used for the change of the transformer operating voltage from one voltage level to another (e.g. from 15 kV to 20 kV and vice-versa) in proportion with the voltage of the network that the transformer is connected. The handling of the voltage selector is the same with the handling of the tap changer, the only difference is that the annulus has two positions (e.g. 15 kV or 20 kV). For example, if we want a 20-15/.4 kV transformer to operate with primary voltage 19.5 kV, we set the voltage selector at the 20 kV position and the tap changer at the -2.5% position. A.4.11 Voltage selector The thermocouple of the thermometer is set at the higher oil layer, in order to measure the maximum oil temperature. The electrical contacts