The function and type of busbar
1、 The function of busbar
Busbar: A conductor that connects large electrical equipment such as generators and transformers to various electrical devices in voltage distribution equipment at all levels of power plants and substations.
The function of the busbar is to collect, distribute, and transmit electrical energy.
Busbars include: the main busbar and equipment connection wires of the primary equipment, the AC busbar of the station power supply, the DC busbar of the DC system, and the small busbar of the secondary part.
2、 Structure type of busbar
（1） Open exposed busbar
1. Classified by the materials used for the busbar
Copper busbar: Copper has the advantages of high conductivity, high mechanical strength, and corrosion resistance, but it has many important industrial applications with low production and high price. Therefore, it is mainly used in areas prone to corrosion (such as near chemical factories or coastal areas).
Aluminum busbar: Aluminum has a conductivity second only to copper, and is lightweight, inexpensive, and high-yielding. It is widely used in indoor and outdoor power distribution equipment.
Aluminum alloy busbar: There are two types: aluminum manganese alloy and aluminum magnesium alloy. Aluminum manganese alloy busbars have a high current carrying capacity but poor strength, and can be widely used with certain reinforcement measures; Aluminum magnesium alloy busbars have high mechanical strength, but low current carrying capacity, difficult welding, and limited application range.
Steel busbar: Steel has high mechanical strength, but poor conductivity, and is only used in high-voltage small capacity circuits (such as voltage transformer circuits and the high-voltage side of small capacity plant transformers), low-voltage circuits with working current not exceeding 200A, DC circuits, and grounding device circuits.
2. Classify according to the cross-sectional shape of the busbar
Rectangular section busbar: commonly used in indoor power distribution equipment with a continuous operating current of 4000A or below and a voltage of 35kV or below. Advantages: Good heat dissipation, small skin effect, easy installation, and convenient connection. The side length ratio of rectangular busbars is usually 1:12,1:5,
The cross-sectional area of a single busbar should not exceed 10 × 120=1200mm. Under the same cross-sectional area and allowable heating temperature, rectangular busbars have a higher allowable working current than circular busbars. When the working current exceeds the allowable current of a single busbar with the maximum cross-section, two or three rectangular busbars can be fixed on the supporting insulator for each phase, and the distance between each busbar should be equal to the thickness of one busbar to ensure good heat dissipation. The number of rectangular busbars per phase should not exceed three.
Circular section busbar: used in outdoor power distribution equipment of 110kV and above to prevent corona discharge.
Slot shaped section busbar: commonly used in power distribution equipment with a continuous operating current of 40008000A and below 35kV. advantage: Uniform current distribution, low skin effect, good cooling conditions, high utilization rate of metal materials, and high mechanical strength.
Tubular section busbar: commonly used in power distribution equipment with a continuous operating current of over 8000A and a voltage of 110kV and above. Advantages: Small skin effect, high corona discharge voltage, high mechanical strength, and good heat dissipation conditions.
Twisted round flexible busbar: Steel cored aluminum stranded wire is composed of multiple strands of aluminum wire wrapped around the outer layer of single or multiple strands of steel wire, and is generally used in outdoor power distribution equipment of 35kV and above. The composite wire is composed of multiple aluminum stranded wires fixed on a ring, used for connecting the generator with indoor distribution equipment or outdoor main transformer.
（2） Closed busbar
Enclosed busbar: The busbar is enclosed by a shell and used for connecting wires between large generator sets with a single unit capacity of over 200MW, generators and transformers, as well as branch lines such as plant power and voltage transformers
1. Structural types of enclosed busbars
Divided by shell material: plastic shell busbar and metal shell busbar.
According to the structural form between the shell and the busbar:
Non isolated phase enclosed busbar: The three-phase busbar is installed in a common enclosure without phase plates, which can only prevent insulators from being contaminated and external objects from causing busbar short circuits, but cannot eliminate the possibility of phase short circuits, nor can it reduce the generation of phase electric force and steel structure heat.
Separated phase enclosed busbar: The three-phase busbar is installed inside a metal shell with metal (or insulation) partitions between phases, which can effectively prevent phase faults and reduce the electric force of the busbar and the heating of the surrounding steel structure to a certain extent. However, it is still possible to cause a phase short circuit due to burning through the phase partition caused by single-phase grounding.
Phase separated enclosed busbar: Each phase conductor is enclosed with a separate aluminum circular shell. According to the connection method of each section of the metal shell, it can be divided into two types: segmented insulation type and fully connected type.
2. Basic structure of fully connected phase separated enclosed busbar
Composition: current carrying conductor, supporting insulator, protective shell, fittings, sealing partition device, expansion compensation device, short-circuit board, shell support components.
Current carrying conductor: generally made of aluminum, using a hollow structure to reduce skin effect. When the current is high, water-cooled round tube busbars can be used.
Post insulator: Adopting a multi edge structure to extend the leakage distance, each support point can be supported by one to four insulators. Generally, a structure supported by three insulators is used, which has the advantages of good stress, easy installation and maintenance, and the ability to use lightweight insulators.
Protective shell: Made of 5,8mm aluminum plate in the shape of a circular tube, with maintenance and observation holes provided on the shell.
Expansion compensation device: a welded expansion compensation device installed within a certain length range; Install screw joint expansion compensation devices at appropriate locations where connected to the equipment.
Sealed partition device: The enclosed busbar is located near the generator end, the main transformer terminal, and the plant high-voltage transformer terminal. A large-diameter insulation board is used as the sealed partition device, and a rubber ring is used for sealing to ensure the sealing and maintenance of micro positive pressure operation in the area.
3. Characteristics of fully connected phase separated enclosed busbar
advantage:
(1) The shells of each phase are separated for safe and reliable operation, and the busbar is enclosed in the shell, which is not affected by natural environment and external objects, and can prevent phase to phase short circuits. At the same time, the shell is grounded at multiple points, ensuring the safety of personnel coming into contact with the shell.
(2) The loss and heat generation in the steel structure near the busbar are significantly reduced due to the short circuit of the three-phase shell. The resistance of the aluminum shell is very small, and a circulating current similar in magnitude to the busbar current but opposite in direction is induced on the shell. The shielding effect of the circulating current reduces the magnetic field outside the shell to less than 10% of the exposed busbar, and the heat generation of the steel structure outside the shell can be ignored.
(3) When a short circuit occurs, the electric force between busbars is greatly reduced, and the span between insulators can be increased. When the busbar passes through a three-phase short-circuit current, the magnetic field generated by one phase current is weakened by the circulating current shielding of its outer shell, and the remaining magnetic field enters the outer shell of another phase. It will also be shielded by the eddy current of that phase's outer shell, causing the magnetic field entering the shell to be significantly weakened. The electric force acting on this phase's busbar can generally be reduced to about 1/4 of the exposed busbar electric force. At the same time, the electric power between the shells also decreases significantly.
(4) The current carrying capacity of the busbar can be large, and both the busbar and the casing can serve as pipelines for forced cooling.
Disadvantages:
(1) The consumption of non-ferrous metals has increased by about - times.
(2) The shell generates losses, and the power loss of the busbar increases by about - times.
(3) When the heat dissipation conditions of the busbar conductor are poor, the current carrying capacity of the busbar with the same cross-section decreases.
（3） Insulated busbar
Insulated busbar: composed of conductors, epoxy resin coated paper insulation, ground screen, end screen, end flange, and terminal blocks, it is most suitable for compact substations, underground substations, and subway substations, with reduced footprint and reliable operation.
Main advantages:
(1) The insulated busbar is fully insulated, and the phase spacing is not limited by the voltage level, but only depends on the installation size. The phase spacing is greatly reduced and the operation is reliable.
(2) A single insulated busbar can be designed according to the magnitude of the current passing through it, which can meet the requirements of any current and avoid the problem of current imbalance caused by using multiple cables together when the current is large.
(3) The mold free pouring of the insulation layer of the insulated busbar intentionally allows the shape and size of the busbar to be adjusted as needed to meet various needs.
(4) The use of insulated busbar connection devices makes the installation of insulated busbars very flexible, allowing for arbitrary segmentation and combination according to different spatial positions and installation dimensions, while also compensating for some deviations in installation dimensions caused by certain reasons.
3、 Installation and maintenance of busbars
1. Processing and production of busbars
(1) Straightening of hard busbar
Before using the hard busbar, the surface of the busbar should be checked for smoothness and flatness, and there should be no cracks, deformations, or distortions. If the busbar has a certain degree of bending and twisting, it needs to be straightened.
(2) Cutting of busbars
The specific size of the hard busbar is generally determined based on the on-site situation. Hand cutting is done using a steel saw, while mechanical cutting can be done using sawing machines, electric shearing machines, etc. Adequate margin should be left for the busbar during material cutting.
The construction of soft busbar requires meeting the sag value specified in the design and ensuring that the lowest point of the three-phase busbar is at the same level. When there is a scissor type isolation switch below the busbar, the requirements are even stricter. When cutting wires, the ends should be tied, and the cut end face should be neat, free of burrs, and perpendicular to the wire strand. When using a grinding wheel cutting machine for cutting, the wire should be clamped on the fixture of the grinding wheel cutting table. Hand saws are generally used for cutting aluminum strands, and it is strictly prohibited to damage the steel core during cutting. When sawing the aluminum strand to the innermost layer, only the 2nd and 3rd parts of the aluminum strand can be sawed, and then it can be cut off by hand.
(3) Bending of hard busbar
The joints and local areas of the hard busbar can be made into various shapes, mainly including flat bends, vertical bends, twisted bends, and duck neck bends. The maximum deviation of parallelism on both sides of the equidistant bend shall not exceed 3mm. The bent part shall be free of cracks and obvious wrinkles.
(4) Treatment of busbar contact surface
Handling methods: manual and mechanical.
The processed contact surface is brushed off with a wire brush to remove the surface oxide layer, and then coated with a layer of electric composite grease. The overlapping surface of the busbar with silver plating layer shall not be filed. The processing of the busbar contact surface must be smooth and free of oxide film. The allowable reduction in cross-section after processing should not exceed 5 for aluminum busbars and 3 for copper busbars.