Annex II.2.2 (I.3B)

Checking power transmission capacity of conductor

Permissible power transmission current in electrical conductor according to heating conditions by transmission current and solar radiation is calculated in the following formula:

Where:

I : permissible transmission current (A). D: External diameter of conductor (cm).

θ: Permissible temperature increase on conductor (°C).

T: Air temperature corresponding to time point of checking transmission capacity and highest air temperature id selected.

α: Index of resistance increased by temperature (1/ °C). R20dc: Conductor resistance is 20°C for DC current (Ω/cm) Ws: Solar radiation (W/cm)

η: Thermal generation index (0.9).

β: Ratio of AC resistance to DC resistance v: Calculated wind speed (m/s)

hw: Convection heat dissipation factor calculated by experimental Rice formula as follows:

ν

d

hr: Radiation heat dissipation factor (Stefan-Boltzmann law) calculated by the following formula:

The values selected for calculation

1.Solar radiation energy: referring calculation data of countries in the region: Ws = 0.1 W/cm2

2.Calculation wind speed: v = 0.6 m/s

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3.Air temperature T is dependent on the time of checking permissible transmission current. In order to check permissible transmission current in the most unfavorable conditions, the air temperature of T = 40°C is selected as the highest air temperature, suitable forVietnam conditions.

4.Resistance increase factor due to temperature depends on material of conductor

1)For steel core aluminum conductor: α = 0.00403

2)For steel core aluminum alloy conductor: α = 0.00360

3)For copper conductor: α = 0.00393

5.Ratio β between AC resistance and DC resistance can be refer to values given in Table 1 for normal steel core aluminum conductor.

6.Permissible temperature increase on conductor θ depends on permissible temperature on conductor and air temperature T(= 40°C inVietnam conditions).

1)For steel core aluminum conductor, steel core aluminum alloy: permissible temperature on conductor can reach 90°C in normal conditions, that means:

θ= 90°C - 40°C = 50°C

2)For thermal superconductor (TAL): permissible temperature of conductor can reach 150 degrees Celsius in normal operation conditions, that means:

θ= 150°C - 40°C = 110°C

3)For thermal extra-super conductor (ZTAL): permissible temperature of conductor can reach 210°C in normal operation conditions, that means:

θ= 210°C - 40°C = 170°C

The power transmission capacity of conductor depends very much on constraint conditions at the time of test. For example, if test is carried out in night time, the air temperature is 25°C, solar radiation energy is Ws = 0, transmission capacity of conductor is significantly increased compared with test in daytime.

Table 1Ratio AC /DC (RAC /RDC) of reinforced steel core aluminum conductor (ACSR type)

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Annex II.2.3 (I.3C)

Selection of lightning conductor

Lightning conductors are selected to mainly meeting thermal stability ditionscon when one phase short circuit happens. Permissible short circuit current in lightning conductor is calculated in the following formula:

I = K.S

t

Where:

I:Permissible short circuit current.

t:time of short circuit

S:cross sectional area of lightning conductor (mm2).

K:constant dependent on material of lightning conductor: For steel core aluminum conductor: k = 93.

For zinc galvanized conductor: k = 56.

For aluminum covered conductor: k= 91-117.

The above conductors are usually used as lightning conductors in combination with optical fiber cables.

Lightning conductor’s capability to thermally stabilize when short circuit of one phase is compared to k.A2.s.

e.g. permissible short circuit in lightning conductor is calculated equal to I = 10 kA, duration of short circuit is t = 0.5 s, thermal withstanding capacity of lightning conductor will be:

(10kA)2.0.5s = 50 kA2.s

in practice, diagrams can be used for calculating permissible instantaneous current and comparing it with short circuit current NI(1) of one phase of power system in one check location. The thermal stability is ensured when I ≤ IN(1).

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Instantaneous permissible current of steel core conductor

Electric current intensity (kA)

Cross section area (mm2)

Values of instantaneous currents (kA)

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Instantaneous permissible current of steel core aluminum conductor and aluminum covered steel conductor

Electric current intensity (kA) Cross section area (mm2)

Values of instantaneous currents (kA)

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