With the rapid development of China's economy and modernization, the power load is increasing. In recent years, it has become a common phenomenon to replace busbars with busbars in developed countries, and China has also formed a directional development trend. However, due to some designers, users and quality supervisors, the most important safety technical parameters of the busway, the limit temperature rise value, the understanding and understanding are not deep, resulting in safety hazards and investment waste in the project. Let's talk about the busbar slot limit. Several problems with the temperature rise.
In China's fire accidents, the proportion of fire accidents caused by electrical electricity exceeds 60%, and the perpetrators of electrical fire accidents include: cables, wires, high and low voltage complete sets of equipment, transformers, busway, electrical components. Most of them are due to long-term temperature rise and heat, resulting in short-circuiting of insulation materials and causing fire accidents. The standard term for heat detection is the limit temperature rise.
Therefore, to ensure the safe operation of the power supply system and energy conservation and emission reduction, the limit temperature rise of the busway is an indispensable technical parameter for the evaluation of the busway product, which is enough to cause the design, supervision, Party A construction unit and the acceptance unit to pay attention.
First, why does the temperature rise determine the current carrying capacity of the busway:
Low-voltage power transmission trunk lines include wires, cables, branch cables, busbar trunks, bare conductive strips, and puncture cables. Due to the different heat dissipation of various products, the current carrying capacity per square millimeter is also different: the same product, the same conductor specifications, when passing the same current, the temperature rise is different; the same conductor cross-sectional area, due to the design structure Different, the temperature rise is also different. Of course, as the temperature rises, the resistance increases, the voltage drop increases, and the loss of electrical energy increases. For example, when the 35mm2 wire passes the 80A current, the temperature rise is lower. When passing the current of 100A, the standard is met. If the current is 120A or 150A, the temperature rise will exceed the standard, and the insulating material will quickly age, eventually causing a short circuit accident. If the 35mm2 wire passes 100A current, each mm2 is equivalent to 2.85A current, and the other 6mm2 wire passes 38A current, which is equivalent to 6.3A current per mm2. If the 6mm2 wire also passes 2.85A per mm2, then the 6mm2 wire passes at this time. The current is 18A, and its voltage drop and electrical loss are much smaller than 35mm2, because the temperature rise of the conductor decreases, and the loss of electrical energy also decreases. The busway is also the same, so the conductivity of the busbar conductor is calculated according to the conductivity per millimeter (current density), but different design structures and heat dissipation, skin effect, and impedance, inductive reactance and other factors. It is closely related to the current carrying capacity. Therefore, the national standard GB7251-2006 (equivalent to the international electrical standard IEC60439.2-2000) stipulates that the current carrying capacity of the busway is determined by the rated current passed under the limit temperature rise value.
Second, the busway standard for temperature rise requirements:
The international electrical standard IEC60439.2?2000 is the same as the national standard GB7251.2--2006 standard: the temperature rise of the busway is determined according to the heat resistance level of the insulating material. If the busway insulation is Class F, its heat resistance is ≥155°C, and its allowable temperature rise is 115K (155°C minus 40°C ambient temperature) under the conditions allowed by the surrounding environment. Therefore, the busbar slot is the full load test to determine the current carrying capacity of the busbar slot. The limit temperature rise is the most critical technical parameter of the busway. The national mandatory 3C certification test standard, the limit temperature rise of the busway is ≤70K, which is a safe and reasonable standard.
Third, the temperature rise involves the busway problem:
The busway is like a wire and cable, so it is also used as a trunking device for power transmission. The same wire 35mm2 can be used to carry 80A rated current and can also carry 125A rated current, the difference is that the rated current 80A and 125A temperature rise is completely different. The busway is also the same. When the limit temperature rise is 70K and 90K respectively, the same busbar slot has a current carrying capacity difference of more than 15%. At present, the busbar temperature rise value on the market is 55K, 70K, 90K, 100K, or even above, but the high temperature rise value involves the following problems. It is recommended that the user select the busbar slot for the ultimate temperature rise of ≤70K or ≤55K.
3.1 The increase in temperature directly reflects the increase in the loss of electrical energy.
3.2 The higher the temperature rise, the faster the insulation material ages, and the service life of the busway is shortened sharply.
3.3 The temperature rises, causing the aging of the surrounding insulation materials to accelerate, such as wire and cable adjacent to or connected to the busway; or electrical insulation supports, etc., which may even cause fire accidents.
3.4 The internal temperature of the busbar is raised and the voltage drop is increased.
3.5 The temperature rises, so that the mechanical strength of the busway is also reduced. After the metal conductor is heated, the stress begins to relax and the mechanical strength is reduced;
3.6 Reduced the safety factor, the shell high temperature is easy to burn people.
3.7 The temperature rises, so that the surrounding ambient temperature is significantly affected.
Fourth, the origin of temperature rise:
4.1 Copper bars have low copper content and high electrical resistivity.
It is often mentioned that the copper content of the copper row and the resistivity, etc., are indeed related to the current carrying capacity of the busbar. A copper row with a copper content of 99.95% or ≥99.93% and a resistivity of ρ ≤ 0.01777 (ohm·square mm/m) is a relatively good copper row in the busbar copper row. If the copper content is low, the resistivity is large, and the conductor specification can only be increased to ensure the current carrying capacity and temperature rise value. Otherwise, the temperature rise will be too high.
4.2 Insulation material and shell structure have poor heat dissipation.
The structure of the busbar is better, and the conductor of the busbar with better heat dissipation of the insulation material can meet the current-carrying requirements after being discounted according to the design manual or the electrician manual. However, some products of insulating materials are resin casting, or other insulating materials with poor heat dissipation, air-type busbar structure, and dense busbar structure with poor heat dissipation have more discounts. Some product structures and insulation materials have poor heat dissipation. The conductors are selected according to the 30 °C ambient temperature in the electrician's manual, which misleads the users. It is understood that some of these products can only reach 60%~70% of the interception capacity, which causes serious power supply to China. The safety hazards and huge power loss are worthy of attention.
4.3 overload operation.
Some projects, with the increase of equipment, the load increases, or the original design of the busbar can not meet the needs of the site, some projects use the varactor section to change the capacity when ordering, and do not take effective protection measures, the temperature rises during overload operation However, the switch at the beginning of the variable capacity cannot ensure the overload of the small current after the variable capacity, so there is a safety hazard.
4.4 The connection of the connector is unstable and the resistivity of the connector is increased.
Unstable connection of the connector, poor contact of the connector, and increased resistivity can cause the temperature rise of the busbar slot to rise.
4.5 Temperature rise and skin effect are not unrelated.
Inside the conductor, the heat generated by the resistor is not easily dissipated, the temperature is high, the rate of valence and electrons is high, and the line is not very flat, which results in a relatively narrow electronic path and high resistance. On the surface of the conductor, the heat dissipation is fast, the temperature is low, the rate of price and electron operation is low, and the line is flat. This leads to a relatively large electronic path, and thus the surface resistance of the conductor is small and the electrons run faster, which is also the cause of current skin collection. one.
For example, the busbar copper busbar conductor 6×100 and the 10×60 cross-sectional area are also 600mm2, but the former is 19% larger than the latter, which is the effect of the current skin effect. By the same current, the former ratio The latter operates with low temperature rise, low electrical losses, and a lower voltage drop than the latter, which means that at the same temperature rise, the latter is 19% less current-carrying capacity than the former. It can be seen that it is completely wrong to unilaterally determine the current carrying capacity of the conductor and the loss of electrical energy by the cross-sectional area.
4.6 Conductor calculation misleading:
Some technicians, regardless of the structure of the busway, use the conductor specifications according to the "Electrical Manual" (or "Electrical Design Manual") list, and the current capacity per mm2 to infer the service life of the busway. This is wrong. The conductor is copper or aluminum, and the service life of the busbar is critical depending on its operating temperature. The higher the operating temperature, the faster the aging rate (including copper, aluminum and insulation materials. Electrical copper and electrical aluminum for conductors, their creep strength, tensile strength and oxidation rate are closely related to temperature) . Because the busbars have different design and heat dissipation, the internal temperature is different. According to the conductor table of the design manual, the ambient temperature is 35 °C, and the dense busbar with better heat dissipation can reduce the current carrying capacity by 5%~15% to meet the temperature rise value of ≤70K. The dense busbar with poor heat dissipation should be reduced by 20%. About %, the air-carrying capacity of the air-type busbar is reduced more. According to the above aspects, the conductor current-carrying capacity of the busbar slot does not depend on the product structure, and the skin effect is not calculated. According to the cross-sectional area of the conductor and the current carrying per square millimeter. The ability to determine the choice of conductor specifications is wrong.
V. Temperature rise requirements of the relevant units on the conductor
5.1 Design Institute design and temperature rise
At present, most design institutes are designed without temperature rise, only rated current, and three-phase four-wire or three-phase five-wire, which is a relatively general design. If the busbar with the current specification of 1000A and the temperature rise value ≤55K is used in the 115K project with the F-class insulation material, the rated current sign with the mark above 1600A can be affixed, so it is important to design the temperature rise of the busway. of. It is recommended that the temperature rise of the busway limit ≤70K or ≤55K is the basis of engineering quality. If the temperature rise of all control equipment in China is below ≤55K (1000V), not only the line loss of electric energy will be greatly reduced, but also the electric accident caused by electrical failure. It not only saves energy and reduces emissions, but also benefits people's lives, property and safety.
5.2 Engineering supervision, quality inspection station and power acceptance unit for temperature rise requirements.
At present, the current carrying capacity of the busway cannot be directly verified by most projects. According to the national standard GB7251.2 and the international standard IEC60439.2, if the environment allows, the limit temperature rise of the busway is determined by the heat resistance level of the insulation material. The design drawings of the design institute and Party A did not clearly indicate the temperature rise value, so the current carrying capacity of the busway could not be confirmed. Moreover, the user and the merchant do not have an established limit temperature rise value, so it is impossible to determine how much K of the busway temperature rises, which is considered to be a qualified product in engineering. It is recommended that the project supervision and quality inspection station and the power inspection personnel inquire about the 3C certificate and check whether the limit temperature rise of the conductor specifications of various currents in the 3C test report is consistent with the product, and at the same time, the conductor tester is used to detect the conductivity of the conductor, and the copper content is calculated. Resistivity, check whether the 3C certificate and technical parameters are consistent with the test report at the certification center or the test site. Ensure the current carrying capacity of the busway and operate at low temperatures.
5.3 National mandatory 3C certification verification of the limit temperature rise of the busway.
The mandatory 3C certification implemented in China verifies the limit temperature rise of the busway. Except that the fire-resistant busway is a special product, other busbars are uniformly tested according to the test value of ≤70K temperature rise value, but the current specifications of the busway are numerous. The test fee and certification fee of the product need several tens of thousands of yuan. Therefore, in order to reduce the burden on the enterprise, the certification center divides each unit according to the short-circuit withstand strength, and each unit can cover several rated current specifications. It is now uniformly required to test the maximum current specification in each unit. Other specifications are calculated by the company itself and reviewed by the test. The conductor specification for estimating the current within the current range covered by the certification is determined by the standard of how many A per mm2 of the test sample is carried. If it is less than the test sample, it must have been subjected to the temperature rise commission test, or other conductors with such specifications are allowed, otherwise it is not allowed to pass, ensuring the risk of certification. However, there are now a number of test sites that have passed the test of the conductor specifications of the covered currents that are not filled by the company and that are not based on the sample current and its current carrying capacity. For example, in the CCC type test report, the current covered by the 2500A unit is 2000A, 1600A, and 1250A. The manufacturer is to test the unit with the 2500A busbar. The conductor specification of the sample 2500A is 6×205, and the limit temperature rise is ≤ 70K, according to the sample to calculate the conductor specifications, should be 2.0325A per mm2 current carrier, 2000A conductor recommended should be 6 × 165, but the product description is written by the manufacturer to write 6 × 125, obviously only increase the temperature rise to reach 2000A current Capability, so it is difficult for the certification center to control the current conductor specifications covered. It is worthwhile to consider that the current per mm2 of the current covered by some certifications is larger than the per-mm2 of the certified test sample.
6. How to ensure the current carrying capacity and safety of the busway.
The China Quality Certification Center and the 3C testing unit recommend that the certification test be strictly controlled, and accurate and reliable data will be published on the Internet, such as the limit temperature rise of the certified products and the specifications of the current conductors and conductor materials, which is convenient for users to inquire and ensure people's lives. The security of the property and the interests of the users, mobilize the whole people to ensure good quality for the project.
6.1 design. When designing the busway, the limit temperature rise value should be marked on the drawing. At the same time, in the technical requirements of the drawing or on the design drawing, set a busbar protection instrument (or temperature controller) for monitoring the operating temperature. It is recommended to set it in each. The first connection of the current.
Note: The protector has two signal output points, the over-temperature alarm and the limit temperature cut-off current, which ensures the internal temperature rise of the busway during operation.
6.2 Party A and the supervisor can check the conductor specifications and temperature rise values in the 3C test report, or visit the China Quality Certification Center website for other relevant technical parameters. There are some technical parameter announcements on the 3C certificate and website, such as IP protection level, ICW=___KA (short-circuit withstand current intensity), rated current specification, etc., to ensure that the purchased busway is consistent with the certification.
6.3 limit temperature rise test.
6.3.1 In order to ensure that the quality of the purchased busway is fully capable of achieving low temperature, safe and low loss operation under full load conditions, the best method is to perform the limit temperature rise test.
6.3.2 The limit temperature rise detection position is also critical. Detect the temperature rise of the incoming section, conductor, plug conductor, connector, and housing of the busway. After running at full load current, the highest temperature stabilized, minus the ambient temperature, gives the temperature rise value, expressed in K. Other current specifications are calculated according to the per-mm2 current carrying capacity of the test sample. It is best to calculate from the maximum current to various specifications of small current, the current density per square millimeter that can be passed by the large current, and the same product structure. The small current busbar conductor is no problem.