High Voltage Banking Transformer.
In this video Mr Rusty gives a brief overview of "Transformer Banks". It covers several different types, their uses, and the importance of proper wiring and types of connections. In future videos we will be getting more in depth on the specific types of banks.
This video was created with the Lamar Institute of Technology at the Frank Robinson Center in Silsbee, TX with the help of Russell Koenig (Program Director).
We all know that a power plant is a very important part of the power transmission and power substation systems. There are many types of electrical equipment in a power plant, and we can say that the most important one is the power transformer.
How many types of substations are there?
Based on voltage class, there are super high voltage power substations, high voltage power substations, medium voltage power substations, and low voltage power substations. Voltage over 330kV is called super high voltage, between 10kV and 330kV is called high voltage, from 1kV to 10kV is called medium voltage, and less than 1kV is called low voltage.
According to the difference in power supply objects, it can be divided into village and town substations, industrial substations, and agricultural substations.
According to its position and function in the power system, it can be divided into hub substations, intermediate substations, regional (local) substations, enterprise substations, and end (user) substations.
Learn more: Power Transformer In Substation
Hub substation: The hub substation is located at the pivot point of the power system. The voltage level is generally 330kV and above. It is associated with multiple power sources, and there are many loops and large substation capacity. It plays the most important role in the stability and reliability of the power system operation.
Intermediate substation: The intermediate substation is located at the interface of the main ring line of the system or the main trunk line of the system. The voltage level is generally 330 ~ 220kV, and 2 ~ 3 power sources and several lines are converging. After the power outage of the whole station, it will cause the disconnection of the regional power grid.
Regional substation: The regional substation is the main substation in a region and a medium or small city. The voltage level is generally 220kV. After the power failure of the whole station, the power supply in the region or city will be disrupted.
Enterprise substation: The enterprise substation is a special substation for large and medium-sized enterprises, with a voltage level of 35 to 220kV and 1 to 2 incoming lines.
Types and functions of power plant transformers
Main transformers for power plants
We know that the voltage emitted by the motor of the power plant is generally below 20kV. At this time, a transformer is required to increase the voltage to the system bus voltage to transmit power to the system to reduce the power in the transmission. Therefore, the main transformer used in the power plant is the power step-up transformer.
The output voltage of the generator is reduced to the bus voltage for high voltage in the power plant, and power is supplied to the high voltage motor and the transformer for low voltage in the power plant.
The high-voltage busbar voltage used in the power plant is reduced to the low-voltage busbar voltage, and power is supplied to low-voltage single-phase and three-phase loads.
Power Plant High Voltage Backup Transformer
As a backup power supply for high-voltage busbars used in power plants.
Power Plant Low Voltage Backup Transformer
As a backup power supply for the low-voltage bus of the power plant.
Isolation transformer
It plays the role of system isolation and safety and is generally used to supply power to remote mortar pumping stations.
There are also some special transformers, such as dust removal transformers, excitation transformers, running light transformers, rectifier transformers, and so on.
Read my article on: 10.5MVA-13.8/2.4kV Substation Transformer
Selection principle of the main transformer for power plant
1. Determination of the number of phases
The power step-up transformer has a single-phase transformer bank and a three-phase transformer. The single-phase transformer bank is composed of three single-phase transformers. Compared with the single-phase transformer group of the same capacity, the three-phase transformer has the advantages of low price, small size, and low loss. Generally, in the power system below 330kV, the three-phase transformer is generally used. Only when limited by the transformer manufacturing and transportation conditions,Only consider single-phase transformer banks.
Generally, the power plant should decide whether to choose a single-phase transformer group or a three-phase transformer according to economic arguments.
2. Determination of the number of power plant transformer windings
According to the number of windings, power transformers can be divided into three-winding, double-winding, self-coupling, and split-winding. When the maximum unit capacity of the power plant is 125kw and below if two elevated voltages are used to supply power to users or connect to the power system, a three-winding transformer is generally used, and the third winding is connected to the generator. However, the power passing through each winding should reach more than 25% of the capacity of the transformer, otherwise, it is not as economical and reasonable as using two double-winding transformers.
When the power plant only raises one level of voltage, a double-winding ordinary transformer can be used. When the maximum capacity of the power plant is more than 200MW, the two-winding transformer unit is generally connected to the system. If the power plant has two boosted voltages, the three-winding transformer (or autotransformer) is often used as the tie transformer. . The low-voltage winding is used as a factory starting power supply or a factory standby power supply, and its main function is to realize the connection of high and medium voltage. Its low-voltage winding is connected in a triangle to cancel the third harmonic component. When the medium voltage is the neutral point and is not directly grounded to the grid, only ordinary three-winding transformers can be used. Transformers with expanded unit wiring, suitable for transformers with low-voltage split windings, can greatly limit the short-circuit current.
Compared with ordinary transformers of the same capacity and voltage level, the economic benefits of autotransformers are very significant. However, since the autotransformer has an electrical connection between the high-voltage grid and the medium-voltage grid, it has the possibility of transferring the overvoltage from the grid of one voltage level to the grid of another voltage level, so its high-voltage side must be high-voltage The pressure side must be the neutral point directly grounded. Therefore, transformers with voltage levels of 220kV and above can choose auto-transformation.
Keep reading: How To Choose The Suitable 110KV Power Transformer?
3. Selection of tap changer-load-tap-changer-transformer-power-plant
In order to ensure the quality of power generation, the number of turns of the high-voltage winding of the transformer can be changed by switching the transformer split switch, thereby changing its transformation ratio and realizing voltage adjustment. The switching mode is divided into two types: the on-load (excitation) voltage regulation mode with load switching and the no-load (non-excitation) voltage regulation mode without load switching. The no-load adjustment range is generally only within 10% (ie ±2X2.5%), and the on-load voltage adjustment range can reach 30%, but its structure is complex and the price is high. Generally, the no-load voltage regulation mode is adopted when the normal voltage fluctuation can be met. For the main transformer of the power plant whose connection and output voltage vary greatly, and the secondary voltage of the transformer is required to be maintained at a certain level, the on-load voltage regulation method should be adopted. When the generator often runs under a low power factor, an on-load voltage regulating transformer should be selected.
4. Determination of winding connection method
The connection mode of the transformer windings must be consistent with the system voltage phase, otherwise, the parallel operation is not possible. The transformer windings in the power system are connected in two ways: star and delta. The symbol Y is used to indicate that the high-voltage winding is star-connected, the neutral point is used to indicate YN, and the medium and low voltage windings are represented by y and yn, and the triangle is used.
When connecting, use D to represent the high-pressure side; used to represent the low-pressure side. The transformer with an all-star connection group is used, that is, the high, middle, and low sides of the transformer are all connected in a star shape, which can eliminate the influence of the third harmonic. The zero-sequence group resistance of this wiring is large, which is conducive to limiting the short-circuit current and is also convenient for connecting the arc suppression coil at the neutral point. The disadvantage is that there is no channel for the third harmonic, which distorts the sine wave voltage waveform and interferes with the communication equipment, and also affects the accuracy and sensitivity of the relay protection setting.
Generally, for voltages of 110kV and above, the three-phase windings of the transformer are connected by “Yn”; For transformers from 35kV to 110kV, the "yn" connection is used, so that its neutral point is grounded through the arc suppression coil. Therefore, Yn, d11 wiring is generally used for ordinary double windings; YN, y, d11 or Yn, yn, d11 forms are generally selected for three-winding transformers.
5. Selection of Transformer Impedance
The impedance value of each side of the power plant transformer must be comprehensively considered from the aspects of power system stability, power flow direction, reactive power distribution, relay protection, short-circuit current, voltage regulation methods in the system, and parallel operation. The impedance of the transformer determines the structure of the transformer. The relative position of the transformer winding on the iron core determines the impedance of the transformer.
The three-winding transformer is divided into two types: boost structure and step-down structure. Considering the power system and the quality of the supply voltage and reducing the loss when transmitting power, the higher the transformer impedance is. The smaller the better, but the smaller the group resistance will increase the short-circuit current, making the selection of low-voltage equipment difficult. Therefore, the transformer connected to the generator should choose the step-up type.
6. Selection of power plant transformer cooling method
The cooling methods of transformers mainly include natural air cooling, forced air cooling, forced oil circulating water cooling, forced oil circulating air cooling, forced oil circulating guided cooling, etc. It varies with the type and capacity of the transformer. Small-capacity transformers generally choose natural air cooling and forced air cooling; Large-capacity transformers use forced oil circulation air cooling; Forced guide oil circulation cooling is generally used in large transformers. The main transformers of power plants with sufficient water resources, in order to save land, also adopt forced oil circulation water cooling.
Read on: How To Buy Power Transformers In Australia?
7. Selection of voltage on each side of the transformer
If a certain voltage level of the transformer is used as the power supply, in order to ensure the voltage quality of the power supply at the end of the line, that is, to ensure that the voltage at the end of the line is the rated value under the condition of 10% voltage loss, the voltage of this side is selected according to 120% of the rated voltage. If a certain voltage level belongs to the power grid, and the terminal is the load terminal, the voltage of this side should be selected according to the rated voltage of the power grid.
Which transformer is used in the power plant?
Above we have described the main transformer for the power plant in detail. Next, let's talk about the transformers used in other electric field plants. With the development of science, technology, and the economy, the world pays more and more attention to environmental protection and cleanliness. Therefore, solar power generation and wind power generation came into being, which is more environmentally friendly than traditional power generation methods. Box change and dry change are often used in solar power plants and wind power plants.
Dry-type transformers are commonly used in solar power plants for safe use as isolation. The dry transformer has no oil immersion in the transformer tank and has the characteristics of flame retardant and explosion-proof, so it has advantages in safe ration. However, according to our project experience, for the safety and personal safety of the transformer, we recommend that the dry-type transformer be installed with a protection cabinet. Protect the transformer from rain, wind, and small animals, and also protect people by keeping a safe distance from dry-type transformers.
High Voltage Banking Transformer.
In this video Mr Rusty gives a brief overview of "Transformer Banks". It covers several different types, their uses, and the importance of proper wiring and types of connections. In future videos we will be getting more in depth on the specific types of banks.
This video was created with the Lamar Institute of Technology at the Frank Robinson Center in Silsbee, TX with the help of Russell Koenig (Program Director).
We all know that a power plant is a very important part of the power transmission and power substation systems. There are many types of electrical equipment in a power plant, and we can say that the most important one is the power transformer.
How many types of substations are there?
Based on voltage class, there are super high voltage power substations, high voltage power substations, medium voltage power substations, and low voltage power substations. Voltage over 330kV is called super high voltage, between 10kV and 330kV is called high voltage, from 1kV to 10kV is called medium voltage, and less than 1kV is called low voltage.
According to the difference in power supply objects, it can be divided into village and town substations, industrial substations, and agricultural substations.
According to its position and function in the power system, it can be divided into hub substations, intermediate substations, regional (local) substations, enterprise substations, and end (user) substations.
Learn more: Power Transformer In Substation
Hub substation: The hub substation is located at the pivot point of the power system. The voltage level is generally 330kV and above. It is associated with multiple power sources, and there are many loops and large substation capacity. It plays the most important role in the stability and reliability of the power system operation.
Intermediate substation: The intermediate substation is located at the interface of the main ring line of the system or the main trunk line of the system. The voltage level is generally 330 ~ 220kV, and 2 ~ 3 power sources and several lines are converging. After the power outage of the whole station, it will cause the disconnection of the regional power grid.
Regional substation: The regional substation is the main substation in a region and a medium or small city. The voltage level is generally 220kV. After the power failure of the whole station, the power supply in the region or city will be disrupted.
Enterprise substation: The enterprise substation is a special substation for large and medium-sized enterprises, with a voltage level of 35 to 220kV and 1 to 2 incoming lines.
Types and functions of power plant transformers
Main transformers for power plants
We know that the voltage emitted by the motor of the power plant is generally below 20kV. At this time, a transformer is required to increase the voltage to the system bus voltage to transmit power to the system to reduce the power in the transmission. Therefore, the main transformer used in the power plant is the power step-up transformer.
The output voltage of the generator is reduced to the bus voltage for high voltage in the power plant, and power is supplied to the high voltage motor and the transformer for low voltage in the power plant.
The high-voltage busbar voltage used in the power plant is reduced to the low-voltage busbar voltage, and power is supplied to low-voltage single-phase and three-phase loads.
Power Plant High Voltage Backup Transformer
As a backup power supply for high-voltage busbars used in power plants.
Power Plant Low Voltage Backup Transformer
As a backup power supply for the low-voltage bus of the power plant.
Isolation transformer
It plays the role of system isolation and safety and is generally used to supply power to remote mortar pumping stations.
There are also some special transformers, such as dust removal transformers, excitation transformers, running light transformers, rectifier transformers, and so on.
Read my article on: 10.5MVA-13.8/2.4kV Substation Transformer
Selection principle of the main transformer for power plant
1. Determination of the number of phases
The power step-up transformer has a single-phase transformer bank and a three-phase transformer. The single-phase transformer bank is composed of three single-phase transformers. Compared with the single-phase transformer group of the same capacity, the three-phase transformer has the advantages of low price, small size, and low loss. Generally, in the power system below 330kV, the three-phase transformer is generally used. Only when limited by the transformer manufacturing and transportation conditions,Only consider single-phase transformer banks.
Generally, the power plant should decide whether to choose a single-phase transformer group or a three-phase transformer according to economic arguments.
2. Determination of the number of power plant transformer windings
According to the number of windings, power transformers can be divided into three-winding, double-winding, self-coupling, and split-winding. When the maximum unit capacity of the power plant is 125kw and below if two elevated voltages are used to supply power to users or connect to the power system, a three-winding transformer is generally used, and the third winding is connected to the generator. However, the power passing through each winding should reach more than 25% of the capacity of the transformer, otherwise, it is not as economical and reasonable as using two double-winding transformers.
When the power plant only raises one level of voltage, a double-winding ordinary transformer can be used. When the maximum capacity of the power plant is more than 200MW, the two-winding transformer unit is generally connected to the system. If the power plant has two boosted voltages, the three-winding transformer (or autotransformer) is often used as the tie transformer. . The low-voltage winding is used as a factory starting power supply or a factory standby power supply, and its main function is to realize the connection of high and medium voltage. Its low-voltage winding is connected in a triangle to cancel the third harmonic component. When the medium voltage is the neutral point and is not directly grounded to the grid, only ordinary three-winding transformers can be used. Transformers with expanded unit wiring, suitable for transformers with low-voltage split windings, can greatly limit the short-circuit current.
Compared with ordinary transformers of the same capacity and voltage level, the economic benefits of autotransformers are very significant. However, since the autotransformer has an electrical connection between the high-voltage grid and the medium-voltage grid, it has the possibility of transferring the overvoltage from the grid of one voltage level to the grid of another voltage level, so its high-voltage side must be high-voltage The pressure side must be the neutral point directly grounded. Therefore, transformers with voltage levels of 220kV and above can choose auto-transformation.
Keep reading: How To Choose The Suitable 110KV Power Transformer?
3. Selection of tap changer-load-tap-changer-transformer-power-plant
In order to ensure the quality of power generation, the number of turns of the high-voltage winding of the transformer can be changed by switching the transformer split switch, thereby changing its transformation ratio and realizing voltage adjustment. The switching mode is divided into two types: the on-load (excitation) voltage regulation mode with load switching and the no-load (non-excitation) voltage regulation mode without load switching. The no-load adjustment range is generally only within 10% (ie ±2X2.5%), and the on-load voltage adjustment range can reach 30%, but its structure is complex and the price is high. Generally, the no-load voltage regulation mode is adopted when the normal voltage fluctuation can be met. For the main transformer of the power plant whose connection and output voltage vary greatly, and the secondary voltage of the transformer is required to be maintained at a certain level, the on-load voltage regulation method should be adopted. When the generator often runs under a low power factor, an on-load voltage regulating transformer should be selected.
4. Determination of winding connection method
The connection mode of the transformer windings must be consistent with the system voltage phase, otherwise, the parallel operation is not possible. The transformer windings in the power system are connected in two ways: star and delta. The symbol Y is used to indicate that the high-voltage winding is star-connected, the neutral point is used to indicate YN, and the medium and low voltage windings are represented by y and yn, and the triangle is used.
When connecting, use D to represent the high-pressure side; used to represent the low-pressure side. The transformer with an all-star connection group is used, that is, the high, middle, and low sides of the transformer are all connected in a star shape, which can eliminate the influence of the third harmonic. The zero-sequence group resistance of this wiring is large, which is conducive to limiting the short-circuit current and is also convenient for connecting the arc suppression coil at the neutral point. The disadvantage is that there is no channel for the third harmonic, which distorts the sine wave voltage waveform and interferes with the communication equipment, and also affects the accuracy and sensitivity of the relay protection setting.
Generally, for voltages of 110kV and above, the three-phase windings of the transformer are connected by “Yn”; For transformers from 35kV to 110kV, the "yn" connection is used, so that its neutral point is grounded through the arc suppression coil. Therefore, Yn, d11 wiring is generally used for ordinary double windings; YN, y, d11 or Yn, yn, d11 forms are generally selected for three-winding transformers.
5. Selection of Transformer Impedance
The impedance value of each side of the power plant transformer must be comprehensively considered from the aspects of power system stability, power flow direction, reactive power distribution, relay protection, short-circuit current, voltage regulation methods in the system, and parallel operation. The impedance of the transformer determines the structure of the transformer. The relative position of the transformer winding on the iron core determines the impedance of the transformer.
The three-winding transformer is divided into two types: boost structure and step-down structure. Considering the power system and the quality of the supply voltage and reducing the loss when transmitting power, the higher the transformer impedance is. The smaller the better, but the smaller the group resistance will increase the short-circuit current, making the selection of low-voltage equipment difficult. Therefore, the transformer connected to the generator should choose the step-up type.
6. Selection of power plant transformer cooling method
The cooling methods of transformers mainly include natural air cooling, forced air cooling, forced oil circulating water cooling, forced oil circulating air cooling, forced oil circulating guided cooling, etc. It varies with the type and capacity of the transformer. Small-capacity transformers generally choose natural air cooling and forced air cooling; Large-capacity transformers use forced oil circulation air cooling; Forced guide oil circulation cooling is generally used in large transformers. The main transformers of power plants with sufficient water resources, in order to save land, also adopt forced oil circulation water cooling.
Read on: How To Buy Power Transformers In Australia?
7. Selection of voltage on each side of the transformer
If a certain voltage level of the transformer is used as the power supply, in order to ensure the voltage quality of the power supply at the end of the line, that is, to ensure that the voltage at the end of the line is the rated value under the condition of 10% voltage loss, the voltage of this side is selected according to 120% of the rated voltage. If a certain voltage level belongs to the power grid, and the terminal is the load terminal, the voltage of this side should be selected according to the rated voltage of the power grid.
Which transformer is used in the power plant?
Above we have described the main transformer for the power plant in detail. Next, let's talk about the transformers used in other electric field plants. With the development of science, technology, and the economy, the world pays more and more attention to environmental protection and cleanliness. Therefore, solar power generation and wind power generation came into being, which is more environmentally friendly than traditional power generation methods. Box change and dry change are often used in solar power plants and wind power plants.
Dry-type transformers are commonly used in solar power plants for safe use as isolation. The dry transformer has no oil immersion in the transformer tank and has the characteristics of flame retardant and explosion-proof, so it has advantages in safe ration. However, according to our project experience, for the safety and personal safety of the transformer, we recommend that the dry-type transformer be installed with a protection cabinet. Protect the transformer from rain, wind, and small animals, and also protect people by keeping a safe distance from dry-type transformers.
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