In many medium-voltage single-core cables in foreign countries, in order to avoid eddy current loss, while ensuring that the cable can withstand a certain amount of tensile force, aluminum wire armoring is used for armoring. Because in the practical operation of cable installation, armor and shield are all grounded. Most foreign power systems belong to Class A systems, that is, the neutral point is directly grounded. In such a system, a large short-circuit current to the ground is required. That is, when a short circuit occurs in a cable, shielding and armoring can be relatively large. Short-circuit current, the current into the earth, and the cable from damage. Therefore, the short-circuit current calculation of aluminum wire armor is an essential skill for engineers and technicians. In the IEC standard, there is no formula for calculation of short-circuit current in aluminum wire armor, but only copper shield and copper wire shield. However, according to the IEC standard and other domestic and foreign data, the calculation and derivation of short-circuit currents in aluminum wire armoring can still be performed, and accurate short-circuit current data of aluminum wire armored steels that can withstand practical inspections can be obtained. Play an important role in the design and manufacturing process.
For any current-carrying portion of the cable, the method for calculating the rated short-circuit current is usually assumed to remain inside the carrier fluid (ie, adiabatically heated) during the duration of the short circuit. In fact, in the event of a short circuit, some of the heat will be introduced into the adjacent material. In fact, the short-circuit current may be larger, so-called non-adiabatic effects are considered. The non-adiabatic method is effective during the entire duration of the short circuit.
Compared with the adiabatic method, using the non-adiabatic method, the shielding layer, sheath, and conductors smaller than 10 mm2 (especially used as shielded wires) allow a large increase in the short-circuit current.
Aluminum wire armored non-adiabatic short-circuit currents are calculated as follows:
I. Calculation of correction factor considering non-adiabatic effect
Since the interior of the aluminum wire armor is a PVC sheath, and the outside needs to be bound with a nonwoven fabric before the PVC sheath can be squeezed, the surrounding media parameters need only consider the PVC sheath and the nonwoven fabric. In the formula, σ2, σ3 - the specific heat of the medium around the aluminum wire armor layer (J/Kom3)
Also: PVC sheath σ2=1.7×106 J/Kom3
Non-woven fabric σ3=2.0×106 J/Kom3
Also: ρ2, ρ3 Aluminium wire armored layer surrounding media thermal resistance (Kom/W)
PVC sheath ρ2=6.0Kom/W
Non-woven fiber ρ3=6.0Kom/W
F - imperfect contact factor when considering thermal imperfect contact between aluminum wire armor and surrounding non-metallic materials, F = 0.5
Σ1--specific heat of shield, sheath, or armor, J/Kom3 aluminum wire σ1=2.5×106 J/Kom3
All parameters are substituted into:
ε = 1.158 non-adiabatic coefficient.
Second, the adiabatic process short-circuit current calculation formula:
Among them, the cross-sectional area of the S--cable is YJV72 12/20kV 1x500. S=60*2.5^2*0.7854=295mm2
IAd aluminum wire armor shield short circuit current
The reciprocal of β temperature coefficient, 228
Θf final short circuit temperature, θf = 250°C
Θi initial short circuit temperature, θi = 90°C
Specific heat capacity of the conductor at σc 20°C, 2.5×106 J/Kom3
Ρ20 The resistivity of the conductor at 20°C, 2.8264x10-8Ω.m. t is 1S for short circuit time (S).
Then, the adiabatic process allows the short-circuit current (1 second) to be: = 28.87 kA
Third, the non-adiabatic effect of short-circuit current calculation
Based on the above calculation process,
The non-adiabatic process allows the short-circuit current (1 second) to be: 1.158*28.87=33.43 kA
From the above calculations, it can be seen that the non-adiabatic short-circuit current of the aluminum wire armor has actually increased significantly compared with the adiabatic short-circuit current. The IEC949 (1988) standard has assumed the worst case calculation conditions, ie, the margin has been actually taken into account in the calculation. Of course, the rated short-circuit current calculation result is generally safe. The above calculation is basically still based on the calculation formula of copper wire shielding in accordance with IEC949 (1988) standard, except that the surrounding medium of aluminum wire armoring is different from the surrounding medium of copper wire shielding, and the copper wire in the original formula will be considered. The corresponding parameters were replaced with aluminum wire.
In addition, with reference to foreign experience, the final short-circuit temperature of the metal shielding layer can reach 350° C. For safety, there is also a choice to 300° C. In order to improve the safety of the power grid in China, the actual calculation is calculated at 300° C. The above metal shields are for copper tapes or copper wires, and aluminum wire armoring should be different from copper shielding because aluminum materials are less resistant to high temperatures than copper. In actual calculation, we refer to the final short-circuit temperature of the aluminum conductor and calculate it at 250°C. The short-circuit current thus calculated can ensure that the sheathed aluminum wire does not cause safety problems due to overload during actual short circuit.