Abstract: This paper briefly analyzes the relationship between the rated current and the cross-section, focusing on the cross-section and the economic current.
Keywords: rated current; Section; 1 Introduction The current capacity of the bus duct is large, which can reach more than 5000A, the working temperature is high, up to more than 130oC, and it is easy to tap.
The current capacity of the bus duct is large, which can reach more than 5000A, the working temperature is high, up to more than 130oC, and it is easy to tap. Therefore, it is an inevitable trend for high-current distribution trunks to adopt busway, and low-voltage busway is generally divided into intensive busway and resin cast busway.
1. The rated current of the bus duct
Both domestic and foreign busway samples are based on the rated current as the specification to provide users with various data: impedance, weight, dimensions, etc. However, not every manufacturer has a busbar section that is available to the user in the sample. The reasons why a busway cannot be selected solely based on the rated current of the busway are as follows:
(1) The rated current value indicates that in a laboratory environment of 20oC, the temperature rise of the bus duct does not exceed the allowable value (60K) specified in the national standard by passing this current. The actual use environment of the busway is not 20oC, for example, it is used in a 35oC environment, considering the long-term allowable operating temperature of the insulating material and the allowable voltage drop of the busway, the rated current value at this time must be lower than the data provided by the sample. Therefore, the temperature of the environment in which the busway is used must be taken into account when considering the rated current.
(2) The long-term allowable full-load current must be less than the rated current, and the different ventilation conditions directly affect the long-term allowable full-load current of the bus duct. The rated current values provided in the sample do not reflect this essential factor. Therefore, when considering the rated current, the general and dividing conditions of the bus duct use environment must be considered.
(3) There is impedance in the bus duct, and the voltage drop will inevitably occur when the load current passes, and the longer the bus duct, the greater the voltage drop, resulting in the terminal voltage drop exceeding the allowable value of the electrical equipment, which is undesirable. Under the same load current, the use of large-capacity bus duct, due to the relative reduction of impedance, the voltage drop will inevitably decrease. Therefore, when considering the rated current, the length of the bus duct and the allowable voltage drop must be considered.
(4) The power factor of the load directly affects the output voltage value of the bus duct. In this case, local compensation can be used, or a large capacity busway can be used to provide a high output voltage. Therefore, when considering the rated current, the power factor of the bus duct load must be considered.
Second, the cross-section of the bus duct
Since the national standard does not stipulate the cross-section of the bus duct, the cross-section of the same variety of bus duct marked with the same rated current value is often different for different manufacturers. There are several reasons for this:
(1) The cross-section of the routine test sample is different from the actual product. Previous routine test reports do not indicate the cross-section of the busway being tested. Because reducing the cross-section of the busway can reduce the cost and can produce a routine test report, the routine test of the busway is carried out by passing off the busway of a large section as a busway of a small section.
(2) The cross-section and specifications of the product are inconsistent. For samples that do not have a busway section, designers must be careful to beware of counterfeiting by the manufacturer.
3. Current density and economic cross-section
It is pointed out in the literature that the application of the economic current density values issued by the Ministry of Water and Power in 1956 (see Table 1) is not in line with the current reality, because the current prices, electricity prices, equipment depreciation rates and other factors are different from those of a few decades ago.
The literature also proposes a method for deriving the economic current density of the bus duct: the principle of small annual operating cost is composed of annual depreciation cost, large and small repair cost, maintenance and management cost and power loss cost after the operation of the bus duct. Let the material cost per meter of bus duct be M, the annual depreciation cost, repair cost and maintenance management cost per meter after the operation of the bus duct is Z, and the power consumption cost per meter is D, and then the economic current density of the bus duct is obtained by differential derivation, as shown in Table 2.
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