PowerTel and his associated factoreis have over 20 years manufacturing this power line transformers, up to 500 kV. In this post, we are trying to share with you all basic knowledge about this power line transformer.
First, Classifications for power line transformers
1. Classified by purpose
(1) Step-up transformer: used by power plants to transmit electricity to the outside.
(2) Step-down transformer: The substation of the power supply bureau is used for voltage conversion.
(3) Distribution transformer: used to supply power to users.
(4) Plant transformer: provides internal electricity for power plants.
(5) Station transformer: provides internal electricity for the substation.
(6) Converter transformer: used for DC transmission, with one side connected to AC power and the other side connected to a converter valve.
(7) Rectifier transformer: used for power supply of electrostatic precipitator in thermal power plants.
2. Classification by winding
(1) Double winding transformer: used for step-up transformer, step-down transformer, factory transformer, etc.
(2) Three winding transformer: used for step-down transformers, interconnection transformers, etc.
(3) Self coupling transformer: used for step-down transformers, interconnection transformers, etc.
(4) Split Transformer: There are two types of split transformers: axial split and radial split, used for auxiliary transformers and backup transformers.
3. Power line transformers Classified by structure
(1) Single phase transformer: used for 330-1000kV transformers.
(2) Three phase transformer: used for 10~500 kV transformers.
(3) Combination transformer: A transformer that is divided into several parts and assembled on site, used in areas with inconvenient transportation.
4. Classified by cooling method
(1) Oil immersed transformer: used for 10~1000kV transformers.
(2) Dry type transformer: used for 10~110 kV transformers.
(3) SF6 Transformer: Currently used for 110 kV transformers.
5. Model of power line transformer
(1) The meaning of the letters in the model
D-single-phase F-oil immersed air-cooled
O-self P-forced oil circulation
S-three-phase or triple coil J-oil immersed self cooling
Z-on-load voltage regulation L-aluminum coil
*Copper wire coils and double wire coils do not require symbols
(2) Example
SFPSL-12000/110: 110kV, 120MVA three-phase three coil forced oil circulation air-cooled aluminum coil transformer
OSFPSZ-240000/330:330 kV, 240MVA three-phase three coil on load voltage regulation, forced oil circulation air-cooled self coupling transformer
Second, Coils of power line transformers
The coil is the most important and complex component in power line transformers, consisting of copper (or aluminum) wires wound together with specialized insulation components.
1. Spiral coil
The main characteristic of spiral coils is that there are many parallel wires, and the wire coils are wound in a spiral shape, with each wire coil being one turn. Spiral coils have good mechanical stability, heat dissipation, and processability, and are widely used in low voltage and high current coils of transformers.
Spiral coils can be wound into three structures: single helix, double helix, and four helix, depending on the magnitude of the current.
2. Continuous coil
When a coil is composed of several axially distributed segments that do not require welding with each other, it is called a continuous coil.
The end support surface of a continuous coil is large, it can withstand large axial forces, has strong short-circuit resistance, and has a large heat dissipation capacity on each line segment. This type of coil is widely used in both voltage level and capacity range.
3. Tangled coil
A tangled coil is composed of several tangled line segments (cakes). A coil consisting entirely of tangled segments (cakes) is called a fully tangled coil and is widely used in transformers with voltages of 220kV and above. A coil composed of a portion of tangled line segments (pie) and a portion of continuous line segments is called a tangled continuous coil, which is applied to transformers with voltages of 66 kV and above.
Due to the insertion of non adjacent wire turns between adjacent wire turns of the coil, it forms staggered tangled line segments and forms a tangled coil, thereby increasing the longitudinal capacitance of the coil and improving the distribution characteristics of impulse voltage along the axial height of the coil. Therefore, it has been widely used in various high-voltage wire coils.
4. Internal shielded coil
The internally shielded continuous coil improves the distribution of impulse voltage by increasing the series capacitance between line segments. Its structural feature is that the additional capacitor turns are directly wound inside the continuous line segment, and the end of the capacitor turns is wrapped in insulation and suspended in the line segment. The capacitor turns do not carry current and only function under impulse voltage.
The internal shielded continuous coil has structural forms such as two section crossover, four section crossover, eight section crossover, and segmented connection.
Third, The iron core of a power line transformer
The iron core is also an important component of power transformers, which is composed of high magnetic silicon steel sheets stacked together, and then clamped with steel clips or tied with glass ribbon.
1. Silicon steel sheet
The silicon steel sheet used for power transformers is 0.3-0.5mm thick cold gadolinium silicon steel sheet. Currently, only Wuhan Iron and Steel Plant and Shanghai Baosteel can produce this cold gadolinium silicon steel sheet in China. However, the silicon steel sheets used for large transformers also need to be imported from Japan.
2. The structural form of the iron core
(1) Single phase two column iron core, used for various single-phase transformers.
(2) Single phase column side yoke type iron core, used for high voltage and large capacity single-phase transformers.
(3) Single phase two column side yoke type iron core, used for high voltage and ultra-high voltage large capacity single-phase transformers.
(4) Three phase three pillars are iron cores used in various three-phase transformers.
(5) Five column iron core, used for large capacity three-phase transformers.
Fourth, Oil tank of oil immersed transformer
1. The cylindrical oil tank is mainly used for various small oil immersed transformers and large-sized oil immersed transformers.
2. Bell type oil tank, widely used in 110-500kV oil immersed transformers.
3. The fully sealed oil tank, which is about to be welded out, has only recently been used for oil immersed transformers of 110kV and above.
Fifth, Oil storage tank for oil immersed transformers
The oil storage tank of a transformer has two functions: one is to provide space for thermal expansion and contraction of the transformer oil in the tank; Additionally, it is important to isolate the transformer oil from the external atmosphere to prevent aging of the transformer oil.
1. Capsule type oil storage tank, inside which rubber capsules are used to separate the transformer oil from the external atmosphere and provide space for thermal expansion and contraction of the transformer oil.
2. Diaphragm type oil storage tank, which uses a rubber diaphragm to separate the transformer oil from the external atmosphere and provides space for thermal expansion and contraction of the transformer oil.
3. A corrugated oil storage tank is a metal expander composed of metal corrugated sheets that separates transformer oil from the external atmosphere and provides space for thermal expansion and contraction of transformer oil. The corrugated oil storage tank is divided into two types: internal oil type and external oil type. The internal oil type has better performance, but it has a larger volume.
Six , Cooling methods for oil immersed transformers
1. Symbol for cooling method
The first letter: O – mineral oil, K – synthetic insulating fluid, L – insulating gas.
The second letter: N – Natural convection circulation, F – Forced oil circulation, D – Forced guided circulation.
The third letter: A – air, W – water.
The fourth letter: N – Natural convection, F – Forced circulation (fan, pump).
2. Example
ONAN – Natural Cooling
ONAF – Wind Cooling
OFAF – Forced Oil Circulation Air Cooling
ODAF – Forced Oil Circulation Guided Cooling
Seven, Bushings for power line transformer
1. Pure ceramic insulation sleeve for 40kV and below
This type of casing has two types of structures: guide rod type and cable threading type. Guide rod type low-voltage bushing for transformers; Cable type is used for 10-20kV high-voltage outgoing lines.
2. 40kV and below high current bushing
This type of casing has two structures: guide rod type and capacitor type. Guide rod type pure ceramic bushing is used for low-voltage winding outgoing lines of medium capacity generator transformers; Capacitive bushings are used for low-voltage winding outgoing lines of large generator transformers.
3. 66kV and above oil paper capacitive bushing
The internal insulation of this type of sleeve is a capacitor core formed by alternating winding of insulating paper and aluminum foil. The capacitor core is filled with insulating oil between the ceramic sleeve, and the connection between the sleeve and the winding has two types of structures: guide rod type and cable type. The oil paper capacitor core is alternately wound on the conductive tube by 0.08-0.12mm thick cable paper and 0.01mm thick aluminum foil.
4. 66 kV and above adhesive tape capacitive bushing
The internal insulation of this type of sleeve is a capacitor core formed by alternating winding of adhesive paper and aluminum foil. The insulation oil is filled between the capacitor core and the porcelain sleeve, and the lower part of the sleeve does not require a porcelain sleeve. But the tan of this type of casing δ The adhesive paper is prone to cracking and partial discharge, and production has been stopped.
5. Resin cast capacitive sleeve
The main insulation of this type of casing is also a capacitor core formed by alternating winding of insulation paper and aluminum foil, which is then poured with epoxy resin to form a solid insulation casing. This type of casing can be used as an oil gas casing, with the upper part embedded in the GIS pipeline and filled with SF6 gas between them; The lower part is immersed in transformer oil.
Eight, Voltage regulation methods for power line transformers
1. Voltage regulation method
There are two voltage regulation methods for transformers: excitation voltage regulation and load voltage regulation. Unexcited voltage regulation, also known as no-load voltage regulation, is carried out when the transformer is stopped and unloaded; On load voltage regulation refers to the operation and regulation of a transformer under load during operation. The voltage regulating device without excitation is called an no-load tap changer; The device for on load voltage regulation is called an on load tap changer.
2. On load voltage regulation position
There are three types of on load voltage regulation positions for transformers: neutral point voltage regulation, medium voltage line end voltage regulation, and high voltage coil line end voltage regulation. The structure and process of neutral point voltage regulation are relatively simple, and there are also many applications.
3. On load voltage regulating switch
The voltage regulating switch is also a tap changer. At present, the quality of on load tap changers produced in China is not good enough, and most of them rely on imports, with more imported from MR in Germany and ABB in Sweden.
Nine, Oils used for power line transformers
1. Composition of transformer oil
Transformer oil is a mineral oil, which is a mixture of many hydrocarbon molecules with different molecular weights. It is a hydrocarbon compound mainly composed of alkanes, cycloalkanes, and a small amount of aromatic hydrocarbons.
2. The function and grade of transformer oil
The insulation oil for oil immersed transformers is transformer oil. Transformer oil not only has insulation function, but also has heat dissipation function.
Transformer oil is divided into 25 # oil and 45 # oil according to its different solidification points. The freezing point of No. 25 oil is minus 25 ℃; The freezing point of No. 45 oil is minus 45 ℃.
25 # transformer oil belongs to paraffin based oil, and 45 # transformer oil belongs to naphthenic based oil. In the past, No. 45 transformer oil was imported from abroad, but now Karamay Refinery in Xinjiang can also produce it.
Ten, Manufacturing process of power line transformers
The power transformer consists of two main parts: the body and accessories. The body is composed of a coil, insulation parts, iron core, tap changer, transformer oil, and oil tank. The accessories of transformers include oil storage tanks, coolers, bushings, gas relays, pressure release devices, and thermometers. The cooler, insulation oil, sleeve, tap changer, gas relay, pressure release device, and thermometer are all purchased from outside. Below is a brief introduction to the manufacturing processes of several main components.
1. Coil winding: Install the winding framework – coil winding – wire welding – insulation wrapping – coil shaping – coil testing.
2. Iron core assembly: cutting silicon steel sheets, removing burrs, stacking iron cores, installing pull plates and shielding, binding iron cores, conducting iron core testing, and installing iron core clamps.
3. Insulation processing: Insulation cutting – deburring – chamfering – moisture-proof treatment.
4. Oil tank and storage tank processing: steel plate cutting – welding of oil tank and storage tank – rust removal – sandblasting – primer painting – painting – mechanical strength test.
5. General assembly: Install iron core – Install oil tank pipeline – Install coil – Stack iron yoke – Install tap changer – Weld lead – Wrap lead insulation – Semi finished product test – Dry body – Organize body – Oil tank assembly – Accessory assembly – Oil injection – Sealing test – Hot oil circulation – Static release.
Eleven, Factory testing of power line transformers
The factory test of power transformers is divided into three types: routine (factory) test, type test, and special test. Routine testing is a test item that every transformer must undergo before leaving the factory, usually also known as factory testing; Type test is a test project conducted by sampling 1-2 transformers from a type of product; Special tests are test items proposed by the user and agreed upon with the manufacturing plant.
1. Basic requirements and regulations for high-voltage insulation testing
The transformer winding is inspected according to the highest operating voltage Um and the corresponding insulation level. The following table shows the insulation test items specified in the national standard GB1094.3-2003 “Power Transformers Part 3: Insulation Level, Insulation Test, and External Insulation Air Gap”.
2. Routine (factory) test items
(1) Winding DC resistance measurement: a measurement taken at all tap out terminals.
(2) Ratio measurement: Measure at all tap positions.
(3) Wiring group detection: Test at the rated tap position.
(4) Insulation resistance, absorption ratio, and polarization index measurement: Polarization index measurement is only performed on transformers with a voltage of 220kV and above.
(5) Winding tan δ And capacitance testing: 35kV and above transformers must undergo tan testing δ Measurement
(6) Sleeve tan δ And capacitance testing: capacitor type bushings of 66kV and above must undergo tan testing δ And capacitance testing
(7) Testing of transformer oil: Oil chemical analysis, insulation strength, tan δ、 Chromatographic analysis and other projects, as well as particle size testing in oil for transformers of 750kV and above. Moreover, oil chemical analysis and oil chromatography analysis need to be repeated throughout the entire experimental process.
(8) Measurement of no-load loss and no-load current: Conduct tests under rated voltage wiring.
(9) Load loss and short-circuit impedance measurement: Conduct tests at rated voltage wiring.
(10) Partial discharge test: The discharge amount is not used as an assessment, but only as a reference for whether a high-voltage test can be conducted.
(11) Lightning full wave impulse test: 220kV and above, 120MVA and above transformers
(12) Operation impulse test: 330 kV and above transformers.
(13) Induced voltage withstand test with partial discharge measurement: 110 kV and above transformers.
(14) External power frequency withstand voltage test for low-voltage winding and neutral point.
(15) Partial discharge test: This test is an assessment test of the factory test values.
(16) Oil flow live measurement: Transformers with oil pumps at 330 kV and above.
(17) Partial discharge test for rotating oil pump: Transformer with oil pump at 330 kV and above.
2. Type test items
(1) Temperature rise test.
(2) Thunderbolt cutoff has been tested.
(3) Lightning full wave impulse test for neutral point.
(4) Radio interference test
3. Special test items
(1) Sound level measurement.
(2) Measurement of zero sequence impedance of three-phase transformers.
(3) Harmonic measurement of no-load current.
(4) Measurement of power absorbed by fan motor and oil pump motor.
(5) Measurement of transient voltage transmission characteristics.
(6) Short circuit withstand capacity test.
PowerTel and his dedictaed engineers are able to provide you a custom design of a specific type power line transformer for your projects, referring to standards of IEC, IEEE & ANSI etc.