I. Classification of Power Transformers
Classification by Purpose
Step-up Transformer: Used by power plants to transmit electricity outward.
Step-down Transformer: Used in the substations of power supply bureaus to transform voltage.
Distribution Transformer: Used to supply power to users.
Station Service Transformer: Used to provide internal power for power plants.
Substation Service Transformer: Used to provide internal power for substations.
Converter Transformer: Used in DC power transmission, with one side connected to AC power and the other side connected to the converter valve.
Rectifier Transformer: Used to supply power for the electrostatic precipitator in thermal power plants.
Classification by Windings
Two-winding Transformer: Used for step-up transformers, step-down transformers, station service transformers, etc.
Three-winding Transformer: Used for step-down transformers, tie transformers, etc.
Autotransformer: Used for step-down transformers, tie transformers, etc.
Split Transformer: There are two types, axial split and radial split, used for station service transformers and starting and standby transformers.
Classification by Structure
Single-phase Transformer: Used for 330~1000kV transformers.
Three-phase Transformer: Used for 10~500kV transformers.
Combined Transformer: The transformer is divided into several parts and then assembled on site, used in areas with inconvenient transportation.
Classification by Cooling Method
Oil-immersed Transformer: Used for 10~1000kV transformers.
Dry-type Transformer: Used for 10~110kV transformers.
SF6 Transformer: Currently used for 110kV transformers.
Model of Power TransformerMeaning of Letters in the Model
D-Single-phase; F-Oil-immersed air-cooled
O-Autotransformer; P-Forced oil circulation
S-Three-phase or three-winding; J-Oil-immersed self-cooled
Z-On-load tap changer; L-Aluminum winding
There is no need to add symbols for copper windings and two-windings.
Examples
SFPSL-120000/110: 110kV, 120MVA three-phase three-winding forced oil circulation air-cooled aluminum winding transformer.
OSFPSZ-240000/330: 330kV, 240MVA three-phase three-winding on-load tap changer forced oil circulation air-cooled autotransformer.
II. Windings of Power Transformers
The winding is the most important and complex component in a power transformer. It is wound with copper (or aluminum) wires and is equipped with special insulating components.
Spiral Winding
The main feature of the spiral winding is that there are a large number of parallel wires, and the winding discs are wound in a spiral shape, and one winding disc is one turn of the winding. The spiral winding has good mechanical stability, good heat dissipation performance, and good processability. It is widely used in the low-voltage and high-current windings of transformers.
According to the magnitude of the current, the spiral winding can be wound into three structures: single spiral, double spiral, and quadruple spiral.
Continuous Winding
When the winding is composed of several segments distributed along the axial direction and the segments do not need to be welded to each other, it is called a continuous winding.
The continuous winding has a large end support surface, can bear a large axial force, has strong short-circuit resistance, and each segment has large heat dissipation capacity. This type of winding is widely used regardless of the voltage level or capacity range.
Interleaved Winding
The interleaved winding is composed of several interleaved segments (discs). A winding composed entirely of interleaved segments (discs) is called a fully interleaved winding, which is widely used in transformers with a voltage of 220kV and above. A winding composed of a part of interleaved segments (discs) and a part of continuous segments is called an interleaved continuous winding, which is applied to transformers with a voltage of 66kV and above.
Since it inserts non-adjacent turns between adjacent turns of the winding, an interleaved segment is formed and an interleaved winding is composed, which increases the longitudinal capacitance of the winding and improves the distribution characteristics of the impulse voltage along the axial height of the winding. Therefore, it is widely used in various high-voltage windings.
Inner Shielded Winding
The inner shielded continuous winding aims to improve the distribution of the impulse voltage by increasing the series capacitance between segments. Its structural feature is that the additional capacitance turns are directly wound inside the continuous segment, and the ends of the capacitance turns are insulated and suspended in the segment. The capacitance turns do not carry current and only function under impulse voltage.
The inner shielded continuous winding has forms such as two-segment bridging, four-segment bridging, eight-segment bridging, and segmented connection in structure.
III. Iron Core of Power Transformers
The iron core is also an important component of a power transformer. It is stacked with high-permeability silicon steel sheets and then clamped with steel clamps or bound with glass fiber tape.
Silicon Steel Sheets
The silicon steel sheets used in power transformers are cold-rolled silicon steel sheets with a thickness of 0.3~0.5mm. Currently, only Wuhan Iron and Steel Plant and Shanghai Baosteel in China can produce such cold-rolled silicon steel sheets. However, the silicon steel sheets for large transformers still need to be imported from Japan.
Structural Forms of the Iron Core
Single-phase Two-column Iron Core: Used for various single-phase transformers.
Single-phase Segmented Column and Side Yoke Iron Core: Used for high-voltage and large-capacity single-phase transformers.
Single-phase Two-column Side Yoke Iron Core: Used for high-voltage and extra-high-voltage large-capacity single-phase transformers.
Three-phase Three-column Iron Core: Used for various three-phase transformers.
Five-column Iron Core: Used for large-capacity three-phase transformers.
IV. Oil Tanks of Oil-immersed Transformers
Cylindrical Oil Tank: Mainly used for various small oil-immersed transformers and extra-large oil-immersed transformers.
Bell-shaped Oil Tank: Widely used for 110~500kV oil-immersed transformers.
Fully Sealed Oil Tank: That is, the oil tank is welded tightly. It has only been used for 110kV and above oil-immersed transformers in recent years.
V. Conservators of Oil-immersed Transformers
The conservator of a transformer has two functions. One is to provide a space for the thermal expansion and contraction of the transformer oil in the oil tank; the other is to separate the transformer oil from the external atmosphere to prevent the aging of the transformer oil.
Capsule-type Conservator: A rubber capsule is used inside to separate the transformer oil from the external atmosphere and provide a space for the thermal expansion and contraction of the transformer oil.
Diaphragm-type Conservator: A rubber diaphragm is used to separate the transformer oil from the external atmosphere and provide a space for the thermal expansion and contraction of the transformer oil.
Corrugated Conservator: A metal expander composed of metal corrugated sheets is used to separate the transformer oil from the external atmosphere and provide a space for the thermal expansion and contraction of the transformer oil. The corrugated conservator is divided into two types, internal oil type and external oil type. The internal oil type has better performance but a larger volume.
VI. Cooling Methods of Oil-immersed Transformers
Symbols Representing the Cooling Method
The first letter: O-Mineral oil, K-Synthetic insulating liquid, L-Insulating gas.
The second letter: N-Natural convection circulation, F-Forced oil circulation, D-Forced directed circulation.
The third letter: A-Air, W-Water.
The fourth letter: N-Natural convection, F-Forced circulation (fan, pump).
Examples
ONAN-Natural cooling
ONAF-Air cooling
OFAF-Forced oil circulation air cooling
ODAF-Forced oil circulation directed cooling
VII. Transformer Bushings
Pure Porcelain Insulation Bushings below 40kV
There are two structures, the stud type and the cable-passing type. The stud type is used for the low-voltage bushings of transformers; the cable-passing type is used for the 10~20kV high-voltage outlets.
High-current Bushings below 40kV
There are two structures, the stud type and the capacitive type. The stud type pure porcelain bushing is used for the low-voltage winding outlets of medium-capacity generator transformers; the capacitive bushing is used for the low-voltage winding outlets of large generator transformers.
Oil-paper Capacitive Bushings above 66kV
The internal insulation of this type of bushing is a capacitive core made of insulating paper and aluminum foil wound alternately. The space between the capacitive core and the porcelain bushing is filled with insulating oil. There are two connection structures between the bushing and the winding, the stud type and the cable-passing type. The oil-paper capacitive core is wound alternately with cable paper with a thickness of 0.08~0.12mm and aluminum foil with a thickness of 0.01mm on the conductive tube.
Resin-impregnated Paper Capacitive Bushings above 66kV
The internal insulation of this type of bushing is a capacitive core made of resin-impregnated paper and aluminum foil wound alternately. The space between the capacitive core and the porcelain bushing is filled with insulating oil, and the lower part of the bushing does not need a porcelain bushing. However, this type of bushing has a large tanδ, and the resin-impregnated paper is prone to cracking and partial discharge. Currently, its production has been stopped.
Resin-cast Capacitive Bushings
The main insulation of this type of bushing is also a capacitive core made of insulating paper and aluminum foil wound alternately, and then epoxy resin is cast outside to form a solid insulation bushing. This type of bushing can be used as an oil-gas bushing. The upper part is sleeved in the pipeline of the GIS, and SF6 gas is filled between them; the lower part is immersed in the transformer oil.
VIII. Voltage Regulation Methods of Power Transformers
Voltage Regulation Methods
There are two voltage regulation methods for transformers: no-load voltage regulation and on-load voltage regulation. No-load voltage regulation, also known as off-load voltage regulation, is carried out when the transformer is out of operation and without load; on-load voltage regulation is carried out when the transformer is in operation and with load. The voltage regulation device for no-load voltage regulation is called a no-load tap changer; the device for on-load voltage regulation is called an on-load tap changer.
On-load Voltage Regulation Positions
There are three on-load voltage regulation positions for transformers: neutral point voltage regulation, medium-voltage terminal voltage regulation, and high-voltage winding terminal voltage regulation. Among them, the structure and process of neutral point voltage regulation are relatively simple, and it is more widely used.
On-load Tap Changer
The tap changer is also called a tap switch. Currently, the quality of the on-load tap changers produced in China is not good enough, and most of the on-load tap changers rely on imports. Among them, those imported from German MR and Swedish ABB companies are relatively common.
IX. Transformer Oil
Composition of Transformer Oil
Transformer oil is a mineral oil, which is a mixture composed of many hydrocarbon molecules with different molecular weights, mainly hydrocarbon compounds such as alkanes, naphthenes, and a small amount of aromatics.
Functions and Grades of Transformer Oil
The insulating oil used in oil-immersed transformers is transformer oil. Transformer oil not only has an insulating function but also a heat dissipation function.
According to the difference in the freezing point, transformer oil is divided into No. 25 oil and No. 45 oil. The freezing point of No. 25 oil is -25°C; the freezing point of No. 45 oil is -45°C.
No. 25 transformer oil belongs to paraffin-based oil, and No. 45 transformer oil is naphthenic-based oil. In the past, No. 45 transformer oil needed to be imported from abroad, and now Xinjiang Karamay Refinery can also produce it.
X. Manufacturing Process of Power Transformers
Power transformers are composed of two major parts: the core assembly and the accessories. The core assembly is composed of windings, insulating parts, iron core, tap changer, transformer oil, and oil tank. The accessories of the transformer include conservator, cooler, bushing, gas relay, pressure relief device, and thermometer, etc. Among them, the cooler, insulating oil, bushing, tap changer, gas relay, pressure relief device, and thermometer are all purchased from outside. The following briefly introduces the manufacturing processes of several main components.
Winding Winding: Install the winding skeleton-wind the winding-weld the wires-wrap the insulation-shape the winding-test the winding.
Iron Core Assembly: Cut the silicon steel sheets-remove the burrs-stack the iron core-install the pull plates and shields-bind the iron core-test the iron core-install the iron core clamps.
Insulating Part Processing: Cut the insulating parts-remove the burrs-round the corners-carry out moisture-proof treatment.
Oil Tank and Conservator Processing: Cut the steel plates-weld the oil tank and conservator-remove rust-sandblast-apply the primer-paint-carry out the mechanical strength test.
Final Assembly: Install the iron core-install the oil tank pipes-put on the windings-stack the upper yoke-install the tap changer-weld the leads-wrap the lead insulation-carry out the semi-finished product test-dry the core assembly-arrange the core assembly-assemble the oil tank-assemble the accessories-fill with oil-carry out the sealing test-carry out the hot oil circulation-let it stand.
XI. Factory Tests of Power Transformers
The factory tests of power transformers are divided into three types: routine (factory) tests, type tests, and special tests. Routine tests are the test items that every transformer must undergo before leaving the factory, and are usually also called factory tests; type tests are the test items carried out by sampling 1~2 transformers in a type of product; special tests are the test items proposed by the user and agreed upon by the manufacturer through negotiation.
Basic Requirements and Regulations for High-voltage Insulation Tests
The windings of the transformer are 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 Levels, Insulation Tests and External Insulation Air Clearances".
Routine (Factory) Test Items
Measurement of DC Resistance of Windings: Measured at all tap lead-out ends.
Measurement of Transformation Ratio: Measured at all tap positions.
Detection of Winding Connection Group: Tested at the rated tap position.
Measurement of Insulation Resistance, Absorption Ratio and Polarization Index: The polarization index is measured only for transformers of 220kV and above.
Measurement of tanδ and Capacitance of Windings: tanδ is measured for transformers of 35kV and above.
Measurement of tanδ and Capacitance of Bushings: tanδ and capacitance are measured for capacitive bushings of 66kV and above.
Tests of Transformer Oil: Items such as oil chemical analysis, insulation strength, tanδ, chromatographic analysis, etc. For transformers of 750kV and above, the particle size in the oil also needs to be tested. And the oil chemical analysis and oil chromatographic analysis need to be carried out repeatedly during the whole test process.
Measurement of No-load Loss and No-load Current: Tested under the rated voltage connection.
Measurement of Load Loss and Short-circuit Impedance: Tested under the rated voltage connection.
Partial Discharge Test: The discharge amount is not used as an assessment, but only as a reference for whether the high-voltage test can be carried out.
Lightning Full-wave Impulse Test: For transformers of 220kV and above and 120MVA and above.
Switching Impulse Test: For transformers of 330kV and above.
Induced Voltage Withstand Test with Partial Discharge Measurement: For transformers of 110kV and above.
Applied Power Frequency Withstand Test on the Low-voltage Winding and the Neutral Point.
Partial Discharge Test: This test is used as the assessment test of the factory test value.
Measurement of Oil Flow Electrification: For transformers with oil pumps of 330kV and above.
Partial Discharge Test with the Oil Pump Running: For transformers with oil pumps of 330kV and above.
Type Test Items
Temperature Rise Test.
Lightning Chopped-wave Impulse Test.
Lightning Full-wave Impulse Test on the Neutral Point.
Radio Interference Test.
Special Test Items
Sound Level Measurement.
Measurement of Zero-sequence Impedance of Three-phase Transformers.
Measurement of Harmonics of No
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