Aluminum alloy cable, will you really use it?

Compared with copper, aluminum alloy in the same section and equal length, the conductivity is 61.
8% of copper, the current carrying capacity is about 78% of copper, the specific gravity of copper is 8.
9 g/cm³, the specific gravity of aluminum is 2.
7g/cm³, the same cross-sectional cable, the weight of aluminum alloy cable is only more than
half of that of copper core cable.
Due to the light weight of aluminum alloy cable, it is especially suitable for large-span buildings, such as stadiums, convention and exhibition centers and other buildings of cable laying, the use of aluminum alloy cable can reduce the load on the building steel structure, save the cost
of steel structure.
In addition, the use of aluminum alloy cables for high-rise buildings can reduce the difficulty and workload of vertical cable laying, save labor costs, and also reduce the risk of
body damage due to cable construction.
Since the melting point of aluminum is 660 °C, aluminum alloy cables
must not be used for the fire line that requires continuous power supply in the event of a fire.

The International Electrotechnical Commission standard IEC287-3-2/1995 puts forward the view that the cable size selection is the best for the economy of the conductor section: the selection of the cable conductor section should not only consider the initial cost of the cable route, but also consider the cost of electrical energy loss during the life of the cable, so the cable section should be selected from the economic current density
.
According to the specification distribution of the cable section, the section of the aluminum alloy cable is increased by about 1.
5 times, and the electrical parameters such as ampacity and voltage drop are comparable
to copper.
Amperes of copper and aluminum alloy cables.
The kilometer voltage drop ratio and cross-section ratio are shown in Table 1

Under similar energy consumption, the cross-section of the aluminum alloy core cable is two specifications larger than the cross-section of the copper core cable, that is, the aluminum alloy cable is larger than the copper core cable two specifications to achieve similar ampacity
.
Increasing the cross-sectional area of the conductor has an impact
on cable routing, the structural size of the cable channel, and the electrical properties.

The choice of cable should also consider the maximum allowable temperature of the cable when normal and short circuit, and the maximum allowable temperature of aluminum alloy cable when short circuit is lower than that of copper core cable, which needs to be considered
when choosing.

When multiple cables are laid in parallel, the heat generated when running is more difficult to dissipate than a single cable, and the current carrying capacity is smaller than that of a single cable
.
The more parallel laying, the smaller
the allowable ampacity.
It can be seen from the following table that the cable parallel laying time is preferably greater than 300cm (cable diameter is more than 5 times) to ensure cable heat dissipation and reduce the impact
on other cables when it fails 。 Multiple power cables laid in the cable channel or directly buried in the soil its ampacity has many changes, in the same ambient temperature and soil thermal resistance conditions, if the aluminum alloy cable is selected, the net distance between the cables is reduced due to the increase in the cross-sectional area.
For example, the outer diameter of 240mm2 copper core cable is 83mm, and the outer diameter of 400mm2 aluminum alloy cable is 95mm, the net distance between cables is reduced by 21mm, nearly reduced by 20%, and the corresponding correction coefficient is also reduced
.
For example, for 400mm2 copper core cables, two 185mm2 aluminum alloy cables need to be selected, which increases the number of parallel direct buried cables, and the correction coefficient is reduced
.
Due to the use of aluminum alloy cable to increase the outer diameter of the cable, there will be a decrease in the distance between the lines, an increase in the number of cables, which will reduce the correction coefficient, that is, reduce the cable ampacity, in order to meet the working current requirements, it may also be necessary to choose a larger section of the wire
.
Therefore, the selection of aluminum alloy cable should also consider the problem
of increasing the cross-section, reducing the distance between cables, and reducing the allowable flow rate.

The capacitance current of the cable increases with the increase of the cable section, and the cable cross-sectional area increases by two specifications after the aluminum alloy cable is selected, and the capacitance value of the cable also increases
.
Most of our country is still based on arc suppression coil compensation, the determination of arc suppression coil capacity, depending on the size of the capacitor current in the power grid, the determination of capacitor current in the power grid mainly adopts two ways of actual measurement and theoretical estimation, for the power grid that has been operated, it can also be measured by capacitive current measurement, but in the design and construction stage, because the power grid has not yet formed, it is necessary to estimate by theoretical methods
.

When using 240mm2 copper cable at 10kV with a capacitance current of 1.
96A/km, and when using a 400mm2 aluminum alloy cable, the capacitance current is 2.
41A/km, and the capacitance current increases by 123%
after using aluminum alloy cable 。 The relevant technical standards and regulations of the power system stipulate: 3kV~66kV system, when the single-phase ground fault capacitor current does not exceed 10A, the ungrounded method should be adopted; When more than 10A and need to operate under ground fault conditions, the arc suppression coil grounding method should be adopted; 6kV~35kV is mainly composed of cable lines in the power distribution system, when the single-phase ground fault capacitor current is large, the low-resistance grounding method
can be used.
In the medium voltage power grid system, most of the arc suppression coil grounding method, when the aluminum alloy cable is selected, the arc suppression coil capacity will increase, and the corresponding construction cost and operating cost are also increasing
.
Domestic Beijing, Shanghai, Tianjin, Guangzhou and Shenzhen adopt low resistance grounding method, low resistance grounding method has a limit value on the grounding current, when the grounding capacitor current increases, it is also necessary to check the grounding current
.

With the development of urban distribution networks, the load density becomes higher and higher, and the density of the number of cables laid in the cable channel also increases
.
For the shortage of cable channel resources, the occasions with large short-circuit current can not only consider the direct cost of aluminum alloy cable when selecting aluminum alloy cable, but also consider the comprehensive cost, such as the selection of aluminum alloy cable to increase the size of the cable channel, over-dense laying makes the flow decrease with the reduction of cable spacing, the increase of capacitor current needs to increase arc suppression coil capacity and other construction and operating costs
.
In addition, the maximum allowable short-circuit temperature and thermal stability coefficient of aluminum alloy cable are lower than that of copper core cable, and thermal stability should be checked during design to ensure the safe and reliable operation
of the power grid.