These two days EP exhibition, Schneider, ABB coincidentally show the medium voltage Tubular Busbar Switchgear. 12kV and 40.5kV air insulated switchgear.
Abstract: With the development of carbon peak, carbon neutral, the green, energy-saving medium-voltage switchgear has become the trend of development, in which the use of materials can be saved, reduce non-ferrous metals, etc., the comprehensive benefits are significant;
The busbar, also known as the busbar, in the switchgear carries an important role in the conduction of current, while bearing the short-circuit current generated if a short-circuit fault occurs in the distribution system, the busbar current on the busbar working state involves electromagnetism, thermology and mechanics of the three physical phenomena, the most important of which are:
Current distribution and power loss in the conductor;
Heat dissipation and temperature rise in the conductor;
Electrodynamic and mechanical stresses on conductors and insulators during a short circuit;
The effect of conductor shape and arrangement on the electric field.
Busbar materials are mainly copper, aluminium and steel, except for the busbar used in special occasions, generally more copper busbar. The shape of the bus has a round, rectangular (including all rounded corners and right angles), tubular, D-type and many other shaped copper rows, rectangular cross-section of the most used;
Busbars can be used to determine the cross-section by the economic current density method. The so-called economic current density is calculated based on a combination of factors such as line investment, annual operating costs and savings in non-ferrous metals. Busbar in addition to meet the conductive cross-section requirements, but also need to short-circuit conditions for dynamic and thermal stability calibration calculations, in the dynamic stability calculations, the busbar in the bending of the calculated stress σjs is greater than the permissible stress of the busbar, it is necessary to change the design of the busbar, increase the distance between the phases, reduce the distance between the insulators spanning the busbar cross-section and other methods, to reduce σjs.
2 Application of tubular busbar
2.1 Application of high voltage switchgear
For medium-voltage metal-clad switchgear, especially 40.5KV air-insulated switchgear due to high rated voltage and high rated insulation level requirements, the selection of busbar should be more considerate of the overall insulation requirements of the switchgear cabinet, so give priority to the selection of tube-type copper busbar. The following figure is the air, different shapes of electrodes to the relationship between the distance between the plate and the lightning impact voltage, rod electrode is equivalent to the thick side of the rectangular copper row, 40mm diameter sphere is equivalent to the round tube, it can be seen that in the same distance, the round tube of the lightning impact resistance level is much higher, and the larger the diameter, the closer to the uniform electric field, therefore the higher the level of withstand the higher level of voltage, the round copper tube row is more suitable for 40.5kV switchgear cabinet as the busbar. Tube busbars are therefore widely used in 40.5 kV switchgear.
Schneider Electric’s (formerly AREVA) PIX-40.5, GE’s PV-40.5, TAMCO’s VH3, and Changzhou Pacific Electric Group’s KYN58-40.5 all use round copper tubes as branch or main busbar, and ABB’s ZS2 IEC36kV switchgear cabinet also adopts tubular busbars, which have a uniform electric field, effectively reduce the distance between phases and relative ground, and realise product quality. distance between phases and ground, realising the miniaturisation of the product.
2.2 Selection in case of high current
Under the case of high current, rectangular busbar needs to use more than one piece of conductor, resulting in additional loss, skin effect coefficient is large, the current distribution is not uniform, the current-carrying capacity decreases greatly, in the case of the same cross-sectional area, different cross-sectional shapes of the conductor affected by the skin effect is as follows, the more the number of pieces of copper rows, the same-phase busbar spacing can be reduced to reduce the impact of the skin effect.
With the substation capacity of the main transformer, industrial users such as petrochemical, metallurgy and other loads increase, the rated current of the switchgear is also increasing, in the previous project using multiple rectangular conductor practice has not been adapted to the circuit of the working current, and rectangular busbar in the technical and structural difficulties to meet the requirements of the busbar heat and electrodynamic requirements, which causes additional loss, skin effect coefficient increases, resulting in current-carrying capacity decline, uneven current distribution capacity decline, uneven current distribution.
When a single main transformer capacity of 180MVA or more, due to the transformer low-voltage side of the rated current, short-circuit current, as well as the single capacity in the system accounted for the proportion of the increase in the main transformer 10kV outgoing side of not only the busbar bridge itself electropneumatic problems, heating problems, as well as the busbar bridge pillar insulators, steel structure, and the busbar bridge near the concrete columns, the base of the steel bar in the alternating magnetic field caused by induced eddy currents in the heating problems, once the busbar short-circuit. Once the busbar is short-circuited, the open busbar, pillar insulators and transformer windings will be damaged, affecting the safe operation of the substation and the reliability of power supply.
3 Benefits of tubular busbar
3.1 Large current-carrying capacity
Copper tube busbar is hollow conductor with large surface area and uniform current density distribution on the surface of the conductor, the copper tube conductor is Φ100X5mm, cross-sectional area: 1491mm2, current carrying capacity: 4000A, current density: 2.68 (A/mm2); copper tube conductor is Φ100X10mm, cross-sectional area: 2826mm2, current carrying capacity: 6000A, current density: 2.12 (A/mm2). (A/mm2). Therefore, copper busbar is especially suitable for circuits with high working current.
3.2 Low skin effect, low power loss
Insulated copper busbar has low skin effect coefficient, Kf<1, small AC resistance, and thus small power loss of the busbar. If the use of multi-slice rectangular conductor, with the increase in the number of slices, skin effect coefficient is increasing, the unit cross-section of the effective flow-carrying capacity of the decline in the current distribution between the slices and slices of the uneven current distribution, the additional loss increases, poor heat dissipation conditions.
3.3 Busbar insulation is good
Copper busbar electric field uniformity, can reduce the distance between phases, high voltage withstand value, especially 40.5kV switchgear, electric field uniformity, effectively reduce the cabinet size.
3.4 Allowable stress, high mechanical strength
The permissible stress of insulated copper busbar is 4 times that of rectangular busbar, which can withstand large short-circuit current and high mechanical strength, making the busbar support span increase.
3.5 Good heat dissipation conditions, low temperature rise
Copper busbar is a hollow conductor, the busbar inner diameter duct can naturally form hot air convection, ( the difference in air pressure between indoor and outdoor can naturally form hot air convection), good heat dissipation conditions.
3.6 Small contact resistance and low temperature rise of terminal connectors and intermediate joints
Intermediate joints are used ring hoop type and copper pipe connection, round and round between the force uniformity, contact surface area, contact surface is greater than the conductor cross-section of 10 times, the contact resistance is less than the conductor resistance, the connection part of the temperature rise is lower than the conductor.
3.7 Busbar architecture is simple, clear layout, easy to install, less maintenance workload.
4 switchgear tube busbar design
Tube bus bar has many advantages, especially in 40.5KV switchgear, the difficulty lies mainly in the connection between the copper rows, mainly in the following ways:
a. End connection, using the end face of the copper tube to make the connection.
b. Flattened connection, by flattening the copper tube connection and connecting it through the flat copper row transition.
c. Round face connection, by connecting through the outer round face of the round tube.
Tubular busbars have a high current-carrying capacity due to minimising the skin effect, and the current density can reach more than 2A/mm2, so in the current situation of high copper prices, it can save costs, and also reduce the consumption of non-ferrous metal resources, and the loss of energy in operation.
However, the connection of the tube busbar busbar is more difficult than the rectangular copper row, the bending fillet is large, and the arrangement is inconvenient, thus it is generally used in power plants and other busbar systems transmitting large currents or 40.5kV switchgear cabinets with a large space, due to the electric field uniformity, it can reduce the electrical gap and improve the insulation, and the switchgear cabinets are more reliable.
In fact, 12kV or low voltage cabinets can also use the tube type busbar, especially high current switchgear, can significantly reduce the amount of copper, performance improvement, there are many ways to connect the busbar, the typical end face connection, such as medium-voltage withdrawable circuit breaker primary contact arm connection is by the end face.
The hoop form of connection, through the hoop form of the two sections of busbar connected, previously said, not the lap area is bigger the better, but the effective lap, that is, lap only in the bolt connection area, through a certain amount of compression to achieve compression, too small will increase the resistance, and too big will produce cold current effect, resulting in permanent deformation.
Copper tube flattening method, that is, to the place where the connection is required, the press is used to flatten the copper tube, it can be realised like a rectangular copper row with a flat connection.
Welding flange form, the connection is welded disc flange, two sections of copper pipe busbar through the flange surface connection. Tubular busbars with large diameters can also be connected in the form of milled planes, which are realised through rectangular copper rows or soft braided wires.
All of the above are bolted connections. In order to realise a quick connection, the connection can be made in the form of a contact connector, which is connected by means of a self-supporting contact with an external round face.
Or through the spring contact finger connector with the inner circular surface connection, spring contact finger in the medium voltage gas-insulated switchgear is widely used, high current-carrying capacity, resistance to electric power, excellent performance, safety and reliability. Used for tube busbar connection also has the following benefits.
1. Tube to tube connection, electric field uniformity;.
2. No increase in diameter, does not occupy space.
3. Quick connection, no need for busbar opening, no need for bolting; and
4. Clearly defined current transfer, high current-carrying capacity, stable current-carrying capacity, not affected by the environment.
5. Constant contact pressure;
6. externally insulatable, connections do not require insulation treatment.
7. Low total cost of ownership – large range of assembly deviations between conductors, simple slot design, easy assembly
The benefits of tubular busbar are obvious, with the growth of copper prices, the shortage of non-ferrous metal resources, the need for energy saving and emission reduction, through the application of new technologies, new processes, innovative design, such as the use of spring-loaded contact finger busbar connectors, high current-carrying capacity, easy to install, reliable connection, to achieve the upgrading of the product, not only to reduce the cost, but also to improve the performance of the product, to win the customer and create profits.
Copper row as a precious non-ferrous metal, reduce the use of copper row can effectively save resources, through the design of multiple copper rows, can save a lot of copper rows, such as the original design of the SENPLUS cabinet 6300A 7-100×10, after splitting the multiple pieces of the design to enhance the copper rows changed to 12-40×10, saving 30% of the amount of copper rows; through the use of thin copper rows design, and the ACB By using thin copper row design and adding heat sink for ACB end, it can also achieve the natural air cooling of 6300A circuit breaker without the need of fan to force air cooling, which increases the reliability and reduces the cost. Since 40.5KV switchgear is very important, the product needs to be designed to be as reliable as possible. Tube busbar has the characteristics of good electric field and conductivity, etc. Through the high level of manufacturing and processing capacity now, it can be completely better applied to the product and really solve the switchgear reliability problem. More safe, reliable, low-carbon and environmentally friendly, contributing to the early realisation of the dual-carbon goals.