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Busbar Design Basics PowerPoint Presentation
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Busbar Design Basics

Busbar Design Basics

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Busbar Design Basics

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    1. Busbar Design Basics Welcome to this LE webcast. It discusses the design of busbar systems, including the choice of material, how to size busbars for a particular current rating and how to optimise the lifetime costs. Welcome to this LE webcast. It discusses the design of busbar systems, including the choice of material, how to size busbars for a particular current rating and how to optimise the lifetime costs.

    2. High electrical conductivity only silver is better than copper Good thermal conductor heat reaches surface quickly Strong (at working temperature) to withstand short circuit stresses low creep Easy to joint resistant to corrosion Materials for Busbar Systems Busbar current ratings are based on how much current can be carried by the bar without it becoming too hot. Using a material with high electrical conductivity reduces the amount of heat generated and allows a higher current rating. Apart from silver, copper has the best conductivity of any material. Copper also has excellent thermal conductivity, so heat can reach the surface easily to be dissipated into the surrounding air, again reducing temperature and increasing current rating. Copper retains its properties at busbar working temperatures, capable of withstanding short circuit stresses and with low creep. Finally, it is easy to bend and joint. Copper forms a very thin oxide layer that is relatively slow to form, easily removed, and is not an insulator. Joints are easily made by bolting or clamping.Busbar current ratings are based on how much current can be carried by the bar without it becoming too hot. Using a material with high electrical conductivity reduces the amount of heat generated and allows a higher current rating. Apart from silver, copper has the best conductivity of any material. Copper also has excellent thermal conductivity, so heat can reach the surface easily to be dissipated into the surrounding air, again reducing temperature and increasing current rating. Copper retains its properties at busbar working temperatures, capable of withstanding short circuit stresses and with low creep. Finally, it is easy to bend and joint. Copper forms a very thin oxide layer that is relatively slow to form, easily removed, and is not an insulator. Joints are easily made by bolting or clamping.

    3. Busbar ratings are determined only by the maximum desired working temperature for the material and its surroundings. In the past, working temperatures as high as 110 degrees centigrade were often used, but now, energy efficiency is much more important and lower temperatures are chosen. Ways to optimise working temperature and energy costs are covered later. Whatever the chosen temperature, at that temperature the amount of heat generated in the bar by current flow is exactly equal to the amount of heat lost to the atmosphere. This apparent simplicity is deceptive - the heat loss process is highly non-linear and requires an iterative approach. The process is described in detail in the next few slides.Busbar ratings are determined only by the maximum desired working temperature for the material and its surroundings. In the past, working temperatures as high as 110 degrees centigrade were often used, but now, energy efficiency is much more important and lower temperatures are chosen. Ways to optimise working temperature and energy costs are covered later. Whatever the chosen temperature, at that temperature the amount of heat generated in the bar by current flow is exactly equal to the amount of heat lost to the atmosphere. This apparent simplicity is deceptive - the heat loss process is highly non-linear and requires an iterative approach. The process is described in detail in the next few slides.

    4. The busbar rating depends on several things: - the maximum desired working temperature, taking into account the maximum permitted temperature for all cables and components that are connected directly to it and energy efficiency considerations. - the maximum ambient temperature likely to be encountered under ‘worst case’ summer conditions, inside any cabinets or enclosures. - the way heat is lost by convection - which requires free circulation of air - and by radiation. Conduction can be ignored because for any segment of busbar, as much heat is gained from adjacent segments as is lost to them. - and the heat generated in the busbar - simply calculated from the size and the material conductivity, corrected for the working temperatureThe busbar rating depends on several things: - the maximum desired working temperature, taking into account the maximum permitted temperature for all cables and components that are connected directly to it and energy efficiency considerations. - the maximum ambient temperature likely to be encountered under ‘worst case’ summer conditions, inside any cabinets or enclosures. - the way heat is lost by convection - which requires free circulation of air - and by radiation. Conduction can be ignored because for any segment of busbar, as much heat is gained from adjacent segments as is lost to them. - and the heat generated in the busbar - simply calculated from the size and the material conductivity, corrected for the working temperature

    5. This is an empirical equation describing the relationship between the power lost by convection per unit area from each single vertical face, the temperature difference and the face height. There are similar equations for horizontal faces and cylindrical surfaces. Because hot air rises, the air flowing over the upper part of the surface is already warmed and is not so effective at cooling. This is an empirical equation describing the relationship between the power lost by convection per unit area from each single vertical face, the temperature difference and the face height. There are similar equations for horizontal faces and cylindrical surfaces. Because hot air rises, the air flowing over the upper part of the surface is already warmed and is not so effective at cooling.

    6. This chart shows the heat loss per metre length of surface plotted against the vertical height of the bar. The dotted line is an extrapolation of the loss rate at small heights and the difference between the curves shows how the ‘efficiency’ of convection as a heat dissipation mechanism falls off for larger heights. This chart makes it clear that doubling the size of the busbar does not double the heat dissipation ability, and so does not double the current rating.This chart shows the heat loss per metre length of surface plotted against the vertical height of the bar. The dotted line is an extrapolation of the loss rate at small heights and the difference between the curves shows how the ‘efficiency’ of convection as a heat dissipation mechanism falls off for larger heights. This chart makes it clear that doubling the size of the busbar does not double the heat dissipation ability, and so does not double the current rating.

    7. Radiation occurs in a direction normal to the surface. It is unaffected by the shape, size or orientation of the surface. The amount of heat radiated depends on the difference between the fourth power of the temperatures of the bar and the opposing surfaces, as shown in the equation. Note that the temperatures here are absolute temperatures. Normally, radiation accounts for only around 10% of the heat loss at busbar working temperatures. Radiation occurs in a direction normal to the surface. It is unaffected by the shape, size or orientation of the surface. The amount of heat radiated depends on the difference between the fourth power of the temperatures of the bar and the opposing surfaces, as shown in the equation. Note that the temperatures here are absolute temperatures. Normally, radiation accounts for only around 10% of the heat loss at busbar working temperatures.

    8. The emissivity, e, describes how effectively the surface radiates heat For a perfectly polished surface, the value is close to zero - a very poor radiator For a matt black surface, the value is close to 1 - a very good radiator The emissivity, e, describes how effectively the surface radiates heat For a perfectly polished surface, the value is close to zero - a very poor radiator For a matt black surface, the value is close to 1 - a very good radiator

    9. Bright copper has an emissivity of about 0.1During use as a busbar, the emissivity of the copper surface increases - and the current rating increases - as the copper gradually darkens over the first few months of use - naturally darkened copper can reach an emissivity of about 0.7 Tin plated copper has an emissivity of about 0.3 to 0.5 In some places it is common practice to paint busbars simply for the sake of appearance - partly to hide the fingerprints from the assembly process. This should be avoided. Firstly, it is not necessary - the surface will darken over time anyway, hiding any assembly marks. Secondly, paint has poor thermal conductivity, so the temperature of the bar will be increased, and the current rating reduced.Bright copper has an emissivity of about 0.1During use as a busbar, the emissivity of the copper surface increases - and the current rating increases - as the copper gradually darkens over the first few months of use - naturally darkened copper can reach an emissivity of about 0.7 Tin plated copper has an emissivity of about 0.3 to 0.5 In some places it is common practice to paint busbars simply for the sake of appearance - partly to hide the fingerprints from the assembly process. This should be avoided. Firstly, it is not necessary - the surface will darken over time anyway, hiding any assembly marks. Secondly, paint has poor thermal conductivity, so the temperature of the bar will be increased, and the current rating reduced.

    10. It is important to note that the amount of heat radiated from a surface depends on the temperature of the receiving surface. As illustrated here, there is no heat loss by radiation from the facing surfaces of these two bars. It is important to take this into account when dealing with laminated bars.It is important to note that the amount of heat radiated from a surface depends on the temperature of the receiving surface. As illustrated here, there is no heat loss by radiation from the facing surfaces of these two bars. It is important to take this into account when dealing with laminated bars.

    11. This chart shows the heat loss from one square metre of surface against temperature, assuming a 30 degrees centigrade (303 degrees Kelvin) ambient temperature. It is very strongly temperature dependent. This chart shows the heat loss from one square metre of surface against temperature, assuming a 30 degrees centigrade (303 degrees Kelvin) ambient temperature. It is very strongly temperature dependent.

    12. Adding the convection and radiation figures together gives this chart, showing that convection is a much more important heat dissipation mechanism than radiation.Adding the convection and radiation figures together gives this chart, showing that convection is a much more important heat dissipation mechanism than radiation.

    13. Having discussed how heat is dissipated from the busbar, we now turn to look at the amount of heat generated by the current flowing through the resistance of the busbar. The power dissipated is proportional to the square of the current and directly proportional to the resistance. Rho, the resistivity of the material, increases with temperature, so the correct value for the working temperature must be used. The resistivity of copper depends on the purity and the nature of the impurities; even small amounts of some impurities increase the resistivity considerably so high conductivity (HC) copper must be used for busbars. The conductivity of ‘pure’ copper was defined in 1913 as 100% of the International Annealed Copper Standard. This corresponds to resistivity of 1.724 micro ohm centimetres at 20 degrees Centigrade. Now, modern HC copper has an IACS value of around 102%, meaning that its conductivity is higher and its resistivity is lower than the IACS value.Having discussed how heat is dissipated from the busbar, we now turn to look at the amount of heat generated by the current flowing through the resistance of the busbar. The power dissipated is proportional to the square of the current and directly proportional to the resistance. Rho, the resistivity of the material, increases with temperature, so the correct value for the working temperature must be used. The resistivity of copper depends on the purity and the nature of the impurities; even small amounts of some impurities increase the resistivity considerably so high conductivity (HC) copper must be used for busbars. The conductivity of ‘pure’ copper was defined in 1913 as 100% of the International Annealed Copper Standard. This corresponds to resistivity of 1.724 micro ohm centimetres at 20 degrees Centigrade. Now, modern HC copper has an IACS value of around 102%, meaning that its conductivity is higher and its resistivity is lower than the IACS value.

    14. At some temperature, the heat generated in the bar, Pi, is equal to the sum of the heat lost from the bar by convection and radiation. This equilibrium temperature is the working temperature for the bar and the load current used. To determine the current rating of a busbar we need to find the size of the bar for which the heat generation and dissipation balance at the required working temperature. At some temperature, the heat generated in the bar, Pi, is equal to the sum of the heat lost from the bar by convection and radiation. This equilibrium temperature is the working temperature for the bar and the load current used. To determine the current rating of a busbar we need to find the size of the bar for which the heat generation and dissipation balance at the required working temperature.

    15. The steps to determine the required busbar size are: 1 - Decide on the maximum permitted working temperature and maximum likely ambient temperature. 2 - Knowing the maximum load current, and assuming a current density of 8 amps per mm2, calculate a starting cross sectional area for the bar. This size is too small, but is a good starting point for iteration. 3 - Select an appropriate bar from the standard sizes available. 4 - Calculate the heat dissipated in the bar, using the resistivity value for the working temperature. 5 - Calculate the heat lost from the bar, at the working temperature, by convection and radiation. 6 - If the heat generated is greater than the heat lost, increase the size of the bar and repeat step 4 onwards until the heat lost exceeds the heat generated. Now we know the smallest size of bar that could be used. The next step is to consider whether the size should be increased to reduce the working temperature further for better energy efficiency. The steps to determine the required busbar size are: 1 - Decide on the maximum permitted working temperature and maximum likely ambient temperature. 2 - Knowing the maximum load current, and assuming a current density of 8 amps per mm2, calculate a starting cross sectional area for the bar. This size is too small, but is a good starting point for iteration. 3 - Select an appropriate bar from the standard sizes available. 4 - Calculate the heat dissipated in the bar, using the resistivity value for the working temperature. 5 - Calculate the heat lost from the bar, at the working temperature, by convection and radiation. 6 - If the heat generated is greater than the heat lost, increase the size of the bar and repeat step 4 onwards until the heat lost exceeds the heat generated. Now we know the smallest size of bar that could be used. The next step is to consider whether the size should be increased to reduce the working temperature further for better energy efficiency.

    16. The ‘most economic’ size is the one which gives the lowest lifetime cost, I.e. the minimum total of material, installation and energy costs over the anticipated life of the installation. Reducing energy costs means using more conductor material in the installation, so a higher initial cost is traded against lower running costs. The ‘most economic’ size is the one which gives the lowest lifetime cost, I.e. the minimum total of material, installation and energy costs over the anticipated life of the installation. Reducing energy costs means using more conductor material in the installation, so a higher initial cost is traded against lower running costs.

    17. Obviously, the cost of raw material is roughly proportional to the cross sectional area for a given circuit length. Larger sized bars will cost a little more to install, requiring more and stronger supports, for example.Obviously, the cost of raw material is roughly proportional to the cross sectional area for a given circuit length. Larger sized bars will cost a little more to install, requiring more and stronger supports, for example.

    18. Increasing the cross section reduces energy losses exponentially - doubling the conductor cross section halves the resistance and reduces the losses. The first few increments make a big, but reducing, difference to the loss, but the cost of each increment increases.Increasing the cross section reduces energy losses exponentially - doubling the conductor cross section halves the resistance and reduces the losses. The first few increments make a big, but reducing, difference to the loss, but the cost of each increment increases.

    19. This chart shows the total cost for a hypothetical busbar. The red curve shows the material cost, rising with cross section. The blue curve shows the cost of losses for a particular current and duty factor over a life time of 5 years. The cost of losses falls exponentially as the busbar cross section is increased. The black curve shows the total cost. Note that the minimum overall cost is achieved with a much larger bar than would normally be selected for the current. Note also that the minimum is quite shallow - most of the benefit is gained by the first few size increases and the precise size is not too critical. In other words, if the duty cycle is significant, this is a safe bet - you always win. So far, this presentation has dealt with the principles of busbar size calculations. It is obviously an extremely tedious task. Now we’ll look at a software calculator that makes the task easier. The software is available for download free of charge - the link can be found by clicking the Attachments button in the top right.This chart shows the total cost for a hypothetical busbar. The red curve shows the material cost, rising with cross section. The blue curve shows the cost of losses for a particular current and duty factor over a life time of 5 years. The cost of losses falls exponentially as the busbar cross section is increased. The black curve shows the total cost. Note that the minimum overall cost is achieved with a much larger bar than would normally be selected for the current. Note also that the minimum is quite shallow - most of the benefit is gained by the first few size increases and the precise size is not too critical. In other words, if the duty cycle is significant, this is a safe bet - you always win. So far, this presentation has dealt with the principles of busbar size calculations. It is obviously an extremely tedious task. Now we’ll look at a software calculator that makes the task easier. The software is available for download free of charge - the link can be found by clicking the Attachments button in the top right.

    20. Busbar calculation software The Busbar calculation software runs on all versions of Windows since Windows 95. It will calculate the minimum size required for a specified current or the maximum current for a specified busbar size. It will also calculate the most economical size, taking into account the duty cycle, circuit lifetime, load growth, discount rate, and energy tariffs. The first step is to select the busbar configuration by clicking on the ‘Select configuration’ button and choosing the appropriate diagram. Now select whether the program should calculate the size for a specified current or the current carrying capacity of a specified size. Set the required parameters and the ambient and working temperatures and click on the ‘Calculate’ tab. The Busbar calculation software runs on all versions of Windows since Windows 95. It will calculate the minimum size required for a specified current or the maximum current for a specified busbar size. It will also calculate the most economical size, taking into account the duty cycle, circuit lifetime, load growth, discount rate, and energy tariffs. The first step is to select the busbar configuration by clicking on the ‘Select configuration’ button and choosing the appropriate diagram. Now select whether the program should calculate the size for a specified current or the current carrying capacity of a specified size. Set the required parameters and the ambient and working temperatures and click on the ‘Calculate’ tab.

    21. Busbar calculation software The results are displayed in a table with the top line showing the minimum busbar size, followed by increasingly energy efficient options until the most economic size, for the circuit lifetime specified, at the bottom. For each option, the size and predicted working temperature are given, together with the financial data, such as the relative lifetime savings and the payback period achieved for each incremental size. All the financial data is given in terms of net present value. At the bottom of the screen the financial and technical parameters used in the calculation are listed. These parameters can be entered on the set-up page.The results are displayed in a table with the top line showing the minimum busbar size, followed by increasingly energy efficient options until the most economic size, for the circuit lifetime specified, at the bottom. For each option, the size and predicted working temperature are given, together with the financial data, such as the relative lifetime savings and the payback period achieved for each incremental size. All the financial data is given in terms of net present value. At the bottom of the screen the financial and technical parameters used in the calculation are listed. These parameters can be entered on the set-up page.

    22. Busbar calculation software On this page all the relevant financial data can be entered. Some data, such as material cost and energy costs vary considerably and they should be updated regularly - the date of last update is displayed to make this easier to track. The Load growth parameter allows the anticipated annual rate of increase of energy consumption to be entered so that the calculation can be based on current at the end of life. Note that if high load growth is expected, the initial busbar size will be oversized for the initial load - indicated by a long payback on increments. It is more realistic to perform the calculation on the final expected current. The appropriate discount rate - which accounts for the fact that a sum of money to be received some time in the future is not as valuable as the same sum received today - is often set by the financial management of a company. If no figure is available, it is usually close to the interest rate. The rate of increase in energy prices is the most difficult to assess. To be safe, perform the calculation at a few values. The last used values are saved. On this page all the relevant financial data can be entered. Some data, such as material cost and energy costs vary considerably and they should be updated regularly - the date of last update is displayed to make this easier to track. The Load growth parameter allows the anticipated annual rate of increase of energy consumption to be entered so that the calculation can be based on current at the end of life. Note that if high load growth is expected, the initial busbar size will be oversized for the initial load - indicated by a long payback on increments. It is more realistic to perform the calculation on the final expected current. The appropriate discount rate - which accounts for the fact that a sum of money to be received some time in the future is not as valuable as the same sum received today - is often set by the financial management of a company. If no figure is available, it is usually close to the interest rate. The rate of increase in energy prices is the most difficult to assess. To be safe, perform the calculation at a few values. The last used values are saved.

    23. Further considerations Having calculated the size of the bar, there are three further considerations: voltage drop skin effect increases apparent resistance by reducing effective area - important for: thick busbars high frequencies harmonics generated by non-linear loads short circuit current There are three further factors to be considered. The first is voltage drop. Although the bar has been sized for minimum energy loss, that does not mean - on long runs, for example - that the voltage drop will be acceptable. Skin effect must be taken into account for thick bars or at high frequencies. Skin effect calculations are quite difficult - a graphical method is given in CDA Publication 22 and that method is also implemented in the software. However, any special circumstance will require special care. Short circuit, or fault, current must be considered and this will be covered further in the next Busbar webcast. There are three further factors to be considered. The first is voltage drop. Although the bar has been sized for minimum energy loss, that does not mean - on long runs, for example - that the voltage drop will be acceptable. Skin effect must be taken into account for thick bars or at high frequencies. Skin effect calculations are quite difficult - a graphical method is given in CDA Publication 22 and that method is also implemented in the software. However, any special circumstance will require special care. Short circuit, or fault, current must be considered and this will be covered further in the next Busbar webcast.

    24. Summary of Busbar Material Characteristics High Conductivity low loss, low voltage drop >101.5 % IACS Easy Formability due to small grain size and advanced production technology easy to bend without surface deformation Good Flatness simple reliable jointing Good Straightness easy installation, lower joint stress Copper is the ideal material for busbars. From an electrical perspective, it has excellent conductivity, so that current carrying capacity is high and energy losses are low. In a future webcast, we will see that it is also easy to joint by a variety of methods because surface oxides are slow to form and easy to remove. Mechanically, copper is easy to form and bend to shape. The ‘as supplied’ material has good flatness and straightness and is therefore easy to install and joint. It is also strong, which is important for short circuit capacity. This webcast has covered the basics of calculating the current carrying capacity of busbars. In designing a system, the short circuit capacity, voltage drop and skin effect should also be taken into account and these subjects will be covered in future webcasts. Copper is the ideal material for busbars. From an electrical perspective, it has excellent conductivity, so that current carrying capacity is high and energy losses are low. In a future webcast, we will see that it is also easy to joint by a variety of methods because surface oxides are slow to form and easy to remove. Mechanically, copper is easy to form and bend to shape. The ‘as supplied’ material has good flatness and straightness and is therefore easy to install and joint. It is also strong, which is important for short circuit capacity. This webcast has covered the basics of calculating the current carrying capacity of busbars. In designing a system, the short circuit capacity, voltage drop and skin effect should also be taken into account and these subjects will be covered in future webcasts.

    25. Thank you for listening to this webcast. Further information on all aspects of PQ can be found at www.leonardo-energy.org. For links, please click the Attachments button in the top right.Thank you for listening to this webcast. Further information on all aspects of PQ can be found at www.leonardo-energy.org. For links, please click the Attachments button in the top right.