The advantages and principles of insulation of tubular busbars

With characteristics such as high current (12000A), high mechanical strength, good insulation, and wide applicability, it is increasingly recognized and accepted by more and more customers.

With the rapid development of our country's economy, the demand for electricity in the entire society is increasing day by day. At present, both 110kV and 220KV substations use voltage levels of 10kV and 35kV on the low-voltage side. As the capacity of the main transformer increases, the rated current on the low-voltage side of the transformer's incoming line is also constantly increasing.

Conventional rectangular busbars are becoming increasingly difficult to meet the requirements of busbar heating and short-circuit electrodynamics in terms of technology and structure, resulting in additional losses, increased skin effect coefficients, decreased current flow capacity, and uneven current distribution. Especially when the capacity of a single main transformer is 180MVA or above, the 10kV outgoing side of the main transformer not only has the electric power and heating problems of the busbar bridge itself, but also the heating problems caused by eddy currents induced in the electromagnetic field of the busbar bridge pillar insulators, steel frames, and reinforced concrete columns and foundations near the busbar bridge. Tube shaped busbars serve as a substitute for rectangular busbars.

With the emergence of modern engineering facilities and equipment, the power consumption of various industries has increased rapidly, especially with the emergence of numerous high-rise buildings and large factory workshops. Traditional cables, as transmission lines, can no longer meet the requirements in high current transmission systems. The parallel use of multiple cables has brought many inconveniences to on-site installation and construction connections. Plug in bus duct, as a new type of distribution wire, has emerged. Compared with traditional cables, it fully demonstrates its superiority in high current transmission. At the same time, due to the use of new technologies and processes, the contact resistance and temperature rise at the connection points of the bus duct at both ends and the junction of the distribution port are greatly reduced. High quality insulation materials are used in the bus duct, thereby improving the safety and reliability of the bus duct and making the entire system more complete.

Tube type busbar is a product after rectangular busbar. It not only has the advantages of rectangular busbars, but also has its own advantages. Tube type busbars are widely used in various power plants and transmission projects. The insulation of tubular busbars is one of the obvious advantages of Z. Do you know the principle of insulation of tubular busbars?

One is the fully insulated and fully shielded tube type busbar: medium and low voltage tube type busbars of 6kV and above are usually equipped with insulation barriers, while some low-voltage busbars do not have barriers. The barrier layer is set between the insulation layers outside the conductor, and the barrier layer on the outer layer of the insulation layer is grounded through copper braided fabric, so that the potential of the tubular busbar is zero.

The barrier layer eliminates the increase in external electric field strength caused by the uneven exterior of the conductor and inner sheath. The tubular busbar is equipped with a grounding wire, which is important for covering insulation breakdown faults or large fault currents of the busbar. The induced voltage of the metal sheath will cause the insulation layer to rupture, generate an arc, and burn through the metal sheath.

Secondly, semi insulated tubular busbar: insulated barrier free tubular busbar. In normal times, copper tube conductors are only used for insulation treatment and do not have a barrier layer. The busbar is fixed by insulation to provide sufficient insulation gap between the tubular busbar and the grounding.

High purity cold drawn copper tube is used as the main conductor for power transmission. The characteristics of copper tube are large cross-sectional area, low resistivity, high conductivity, etc. These advantages have been proven in power transmission and distribution applications.

It is composed of three-phase four wire and five wire, suitable for industrial and mining areas, and even for equipment renovation near high-rise buildings or workshop equipment renovation. Dense tubular busbar. But how to equip the tubular busbar? In fact, its equipment is not as difficult as imagined. It is divided into three processes on average, careful planning is enough. Equipment for dense tubular busbars: For the connection of busbars, please use high-strength insulation boards with high insulation, arc resistance, and flame retardancy as connection accessories, and tighten them with bolts with insulation sleeves. According to the standard requirements, in order to meet the overlapping area of the busbar, the gap between the bolt holes and bolts is small, because the insulation sleeve is added to the tubular busbar bolts, thereby narrowing the gap; Therefore, when connecting the main busbars of two segmented tubular busbars, the axes of the two busbars should be strictly kept on the same centerline, and the bolt holes of the busbars and insulation plates of the two tubular busbars should be consistent. Align. When tightening bolts, each bolt must be applied evenly in sequence. This is to prevent the insulation sleeve on the bolt from being damaged by the shear force during the equipment process. During the construction process, several reasons have been found for the decreased insulation state of the tubular busbar caused by the stress damage of the bolt insulation sleeve. The grounding connection can meet the grounding requirements by using busbar shells, but when spraying all busbar shells, the two parts of the tubular busbar must be connected to the wire through grounding bolts.

With the emergence of modern engineering facilities and equipment, the power consumption of various industries has increased rapidly, especially with the emergence of numerous high-rise buildings and large factory workshops. Traditional cables, as transmission lines, can no longer meet the requirements in high current transmission systems. The parallel use of multiple cables has brought many inconveniences to on-site installation and construction connections. Plug in bus duct, as a new type of distribution wire, has emerged. Compared with traditional cables, it fully demonstrates its superiority in high current transmission. At the same time, due to the use of new technologies and processes, the contact resistance and temperature rise at the connection points of the bus duct at both ends and the junction of the distribution port are greatly reduced. High quality insulation materials are used in the bus duct, thereby improving the safety and reliability of the bus duct and making the entire system more complete.

Principle:
(1) In one or more transmission units of the same electric drive system, sometimes the electric motor operates in a generating state and reflects the regenerated energy into the frequency converter. This phenomenon is called 'regenerated energy'. This situation generally occurs when the electric motor is inertia driven by mechanical equipment, or when the driving electric motor generates braking to supply satisfactory tension.

(2) Universal frequency converters do not have the function of regenerating energy reflected in the three-phase power supply in planning, so all the energy absorbed by the frequency converter from the motor will be stored in capacitors, after all, it will cause the DC bus voltage in the frequency converter to rise.

(3) Tube type busbar is one of the key equipment (materials) in the power transmission and transformation system, playing a crucial role in the safe and reliable operation of the transmission and transformation system and power equipment.

(4) It has the characteristics of high current (12000A), high mechanical strength, good insulation, and wide applicability, so it is increasingly recognized and accepted by more and more users

As a main power supply line, it has shown strong advantages compared to general cables, but over time, operational practice has shown that there are many irreparable shortcomings in the bus duct itself

Performance is reflected in∶

1) The tubular conductor (busbar) adopts a series of processes such as hot top casting seamless tube, with smooth surface, high dimensional accuracy, high tensile strength, and is not prone to discharge and deformation;

2) The product is made of rare earth and heat-resistant aluminum alloy materials independently developed by the company, which have excellent processing, welding, conductivity, and heat resistance properties. The product has been appraised by the technology department

3) Good convective heat dissipation conditions, low temperature rise, low losses, strong conductivity, and high current carrying capacity

4) High heat resistance, which can increase the working temperature of the busbar (up to 150 ℃ -200 ℃). The conductor has high mechanical strength at high temperatures, improving the safety and reliability of the transmission and transformation system

Material Research and Testing Center (Testing Report No. 2008343);

5) The installation area is small, and the composite tube busbar occupies one-third of the area of the flexible wire;

6) Corrosion resistance, less prone to icing, strong disaster resistance, long service life

The main types of copper aluminum composite bus ducts that are applicable include high-strength enclosure, dense type, and air type. It is widely used in power supply and distribution systems with AC frequency of 50-60Hz, rated voltage up to 660V, and rated working current of 200-4000A for transmitting electrical energy. The busbar trunking uses copper aluminum busbar as the conductor, with strong current carrying capacity, low loss, balanced reactance, and easy tapping. Its electrical performance is the same as copper busbar and can significantly reduce investment costs, making it a substitute for copper busbar products. The conductor configuration of the bus duct system adopts high-grade industrial conductive composite busbars. Using special processing techniques to form a relatively hard copper composite coating on the overall surface of the conductor, its mechanical and electrical properties can be equivalent to copper conductors. The soft tin plating layer generated on the surface of the copper aluminum composite further enhances the oxidation resistance and electrical quality of the conductor. The ultra lightweight copper aluminum composite conductor will significantly reduce the cost of the product and decrease the basic investment of engineering projects. Reduce the weight of the product itself and reduce the installation intensity for the majority of construction units.

It is mainly composed of an outer shell, an insulated plug, and a metal busbar. The outer shell plays a role in dust prevention and protection. There are ventilation holes on the shell to complete the heat dissipation function, and the built-in busbar can be made into a single or double pole according to the current size. If the shell is made of metal, it also has the function of grounding protection. From the perspective of insulation, sufficient distance should be left for safety; From the perspective of heat generation, sufficient space should also be left for heat dissipation to ensure that heat is dissipated into the air through convection. There are ventilation holes on the shell to complete the heat dissipation function, and the built-in busbar can be made into single or double according to the current size. If the shell is made of metal, it also has the function of grounding protection. Taking the three-phase bus duct as an example, the main structure.

Comparison between bus duct and cable:

1. Multiple bus ducts can meet the rated current of 6300A, which is superior to cables.

The insurance cost for installing busbars is relatively low, while the cost for cables is relatively high.

3. The sub interface of the bus duct can add sub circuits, and the cables need to be laid from the distribution room.

4. The busbar is equipped with standard installation brackets and does not require any other support; Cables need to be laid using separate cable trays or pipelines.

5. Bus ducts can be controlled at the nearest location to the equipment, while cables need to be controlled in the distribution room.

6. The bus duct has a long service life and can be reused. And the cable has a short lifespan and cannot be reused.

Bus ducts have the following characteristics compared to cables:

(1) Bus duct is a flat conductor, which has the characteristics of good heat dissipation, low impedance, and high current carrying capacity compared to circular conductor cables or pre branch cables. Compared with cables or pre branch cables with a current carrying capacity of 630A or above, bus ducts with a current carrying capacity of 630A or above can save 10-35% of copper materials and reduce energy loss by more than 15% under the same current carrying capacity.

(2) The bus duct carries a large amount of current on weekdays, which facilitates the connection of feeders and reduces energy consumption together. It is very suitable for AC power supply and can withstand long-term operation at a given voltage value. Under the condition of alternating current within one minute, the bus duct will not be broken down and there will be no flashover. The withstand current and short-term peak current it can carry will meet the standards.

(3) The service life of bus ducts is approximately 2-3 times that of cables and branch cables. Hong Kong, Macau, and some European countries have clear regulations that the service life of cables is 12-15 years. In fact, the service life of bus ducts is as long as 30 to 50 years. Bus ducts can be divided into air insulated and fully insulated categories. In general, the inherent structure of bus ducts with air insulation is simple, covered with insulated bus ducts with heat dissipation characteristics and high current levels. The electrical shafts and bus ducts installed in high-rise buildings are equipped with a certain socket diameter, and the installation of such joints should consider multiple human disturbances. However, the initial capacity of the bus duct is large, and the structure is compact, which also has advantages in situations with high loads.

(4) The use of bus duct current carrying function ensures stability and avoids unnecessary power loss. The maximum temperature increment current (i.e. the rated current of the bus duct) that satisfies the standard rules for bus duct standards is very important. This current can control the current carrying function of the bus duct. The rated current of a cable is determined by its cross-sectional area. The calculation load varies, the number of cables in the cable tray varies, and different cable trays are used. The temperature inside the tray changes according to these factors, and the current carrying function of the cables also changes accordingly. So the current carrying capacity of the cable calculated by multiplying the reduction factor by the rated current of the cable is inaccurate. The installation method of this sealed cable tray also adds energy loss.

(5) It can be concluded that the capital is low, and there are copper and aluminum conductor materials for bus ducts. Copper conductor bus ducts are more expensive than copper cables, and the price of copper conductor bus ducts above 630A is similar to the cost of cables; Aluminum conductor bus ducts are 25% to 35% cheaper than copper conductor cables. The higher the current, the lower the cost of bus ducts compared to cables. For this reason, using bus ducts for transmission lines with a current carrying capacity greater than 630A is more economical and reasonable than using cables.

(6) Requirements for argon gas

The purity of argon gas should be controlled at 99.9% or above, with impurity oxygen below 0.005%, hydrogen below 0.005%, moisture below 0.02mg/L, and nitrogen below 0.015%. The increase in oxygen and nitrogen exacerbates the cathodic atomization effect. If the oxygen content exceeds 0.3%, the tungsten electrode will be burned more severely. If the oxygen content exceeds 0.1%, the weld surface will be dull or blackened. If the nitrogen content exceeds 0.05%, the fluidity of the molten pool will deteriorate, and the welding surface will be poorly formed.

(7) Joint form

Graphics and dimensions at the joint

(8) Welding requirements

To fix the tubular conductor and reduce deformation, the welding sequence should be spot welding first to fix the main pipe and reduce bending at the joint. Spot welding should be carried out symmetrically, and the argon flow rate is generally set at 30-50L/min

(9) Composition of welding wire (or welding rod)

(10) Joint strength coefficient