How to choose a suitable cable's nominal cross sectional area?
1. Select the cable cross-sectional area according to the long-term allowable current-carrying capacity
1.1 In order to ensure safety and the service life of the cable, the temperature of the cable after power-on should not exceed the specified long-term allowable working temperature. The temperature is 70 degrees for polyvinyl chloride insulated cables and 90 degrees for XLPE insulated cables. According to this principle , It's very simple to look up the table and choose the cable.
1.2 Examples:
A certain factory has a transformer capacity of 2500KVa and uses 10KV power supply. If XLPE insulated cables are used to lay in the bridge, what is the cross-sectional area of the cable?
Step 1: Calculate the rated current 2500/10.5/1.732=137A
Step 2: Check the cable selection manual to know
YJV-8.7/10KV-3X25 current carrying capacity is 120A
YJV-8.7/10KV-3X35 current carrying capacity is 140A
Step 3: Choose YJV-8.7/10KV-3X35 with a cable carrying capacity greater than 137A, which can theoretically meet the requirements. Note: This method does not consider the requirements of dynamic stability and thermal stability.
2. Choose the cable cross-sectional area according to the economic current density
Simply understand the economic current density. The cross-sectional area of the cable affects the line investment and power loss. In order to save investment, it is hoped that the cable cross-sectional area should be smaller; in order to reduce the power loss, it is hoped that the cable cross-sectional area should be larger; based on the above considerations, determine a reasonable one The cross-sectional area of the cable is called the economic cross-sectional area, and the corresponding current density is called the economic current density.
3. Select the cable cross-sectional area according to the voltage drop of the power grid
When we use the first and second methods to select the cross-sectional area of the cable, if the cable is very long, a certain voltage drop will occur during operation and startup, and the voltage on the device side will be below a certain range, which will cause the device to heat up.
4. Select the cross-sectional area of the cable according to the thermal stability coefficient (that is, select the cross-sectional area of the cable according to the short-circuit current)
4.1 When the 0.4KV cable is protected by an air switch, the general cable can meet the thermal stability requirements, so there is no need to verify it according to this method.
4.2 For cables above 6KV, after selecting the cross-sectional area of the cable using the above method, you must verify whether it meets the thermal stability requirements according to the following formula. If it does not meet the requirements, choose a larger cross-sectional area.
Formula: Smin=Id×√Ti/C
Among them, Ti is the breaking time of the circuit breaker, which is 0.25S, C is the thermal stability coefficient of the cable, which is 80, and Id is the three-phase short-circuit current value of the system.
For example: when the system short-circuit current is 18KA, how to choose the cable cross-sectional area.
Smin=18000×√0.25/80=112.5
Conclusion: If the system short-circuit current reaches 18KA, even if the rated current of the equipment is smaller, the cable cross-sectional area should not be less than 120mm2.