1. Select the cross-sectional area of the cable according to the long-term allowable load capacity.
1.1 To ensure the safety and service life of the cable, the temperature of the cable after energization should not exceed the specified long-term allowable working temperature. The temperature of polyvinyl chloride insulated cable is 70 degrees, and the temperature of cross-linked polyethylene insulated cable is 90 degrees. According to this principle, it is simple to check the table to select the cable.
1.2 Example: A plant transformer has a capacity of 2500 KVa and uses 10 KV power.
If the cross-linked polyethylene insulated cable is laid on 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 the YJV-8.7 /10KV-3X25 load capacity of 120AYJV-8.7/10KV-3X35 load capacity of 140A
Step 3: Select cable load capacity greater than 137A YJV-8.7/10KV-3X35, which theoretically can meet the requirements.
Note: This method does not take into account the requirements of dynamic stability and thermal stability.
2. Select the cable cross-sectional area according to the economic current density to understand the economic current density.
The cross-sectional area of the cable affects the line investment and power loss. To save investment, hope that the cable cross-sectional area to be smaller. To reduce the power loss, hope that the cable cross-sectional area to be larger. Based on the above considerations, determining a reasonable cable cross-sectional area 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 grid.
When we use the first and second methods to select the cross-sectional area of the cable.
If the cable is very long, there will be a certain voltage drop in the process of operation and start-up, and the voltage on the equipment side will be below a certain range, which will cause the equipment to heat up. 4.
4. Select the cable cross-sectional area according to the thermal stability factor (i.e., select the cable cross-sectional area according to the short-circuit current)
4.1. When a 0.4KV cable is protected with an air switch, the general cable can meet the thermal stability requirements, so there is no need to verify by 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 the thermal stability requirements are met according to the following formula. If it does not meet the requirements, then choose a larger cross-sectional area.
Formula: Smin = Id × √ Ti/C where Ti is the breaking time of the circuit breaker, that is, 0.25S, C is the thermal stability coefficient of 80, and Id is the system three-phase short-circuit current value.
For example: when the system short-circuits current is 18KA, how to choose the cable cross-sectional area. Smin = 18000 × times; √ 0.25/80 = 112.5 Conclusion: If the system short-circuits current reaches 18KA, even if the rated current of the equipment is small, the cable cross-sectional area should not be less than 120mm2.