Chapter 2-2 Electrical Equipment

Chapter 2-2-1 The Selection of the Conductor Cross-Section Area

development in 10 future years for overhead power line and after 20 years for underground cable line.

For short time load or short time repeatable load (total time for one cycle is up to 20 minutes and working time in the cycle is less than 4 minutes), in order to check cross section area of the conductor according to permissible heating, the calculated load is converted to the continuous working condition:

For copper conductor with cross section area up to 6mm2 and aluminum with cross sectional area up to 10mm2, the calculated load is taken equal to short time load and considered as continuous load.

For cooper conductor with cross sectional area over 6mm2 and aluminum with cross sectional area over 10mm2, calculated load is the short time load multiplied with the following coefficient:

0.875

tlv

Where, tlv is ratio between working time in the cycle and total continuous cycle time.

For short time working condition with closing time not excessive 4 minutes and off-time between two closings enough to cool conductor down to atmospheric temperature, the maximal permissible load is determined based on economic current density or voltage drop as well as other criteria.

For short time working condition with closingtime excessive 4 minutes and off-time between two closings not enough to cool conductor down to atmospheric temperature, the maximal load is considered as maximal continuous working load.

For two or more cables work permanent in parallel, when consideringthe development level of a heating for problems in the faults, a faults that is in the aerial cable lines in operation temporarily.

The neutral wire in the -phase,3 4-conductor power system must have electricity conductivity lot smaller than 50% of conductivity of the 3-phase conductor.

When calculating permissible continuous current for bare conductors, bare busbars, rigid and soft wires in atmospheric temperature conditions or other conditions, the adjusted coefficients in Table 72-

4 to Table 72-8 are used.

Permissible continuous current for conductors with rubber or PVC sheath, or soft conductors insulated with rubber, cable insulted with rubber or plastic covered lead, as mentioned inTable 68-1 to Table 68-8, is calculated according to core heating temperature of + 65°C when atmospheric temperature is +25°C or when earth temperature is +15°C or when calculating number of conductors placed in one

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pipe (or cores of multi-fiber conductor) without taking neutral wire into account for 3 phase, 4- conductor power system (or core grounded).

Table 68-1 Permissible current for copper core conductor covered with rubber or PVC

Table 68-2 Conductor with cooper core, rubber insulation in metal sheath and cable with copper core, rubber insulation in lead cover, PVC or rubber sheath, with or without steel bands

Table 68-4 Cable with aluminum core, rubber or plastic insulation, lead, PVC or rubber cover, with or without steel band

Table 68-5 Soft cable with cooper core, rubber insulation, movable used in underground mines

Notes: (1) For cables with or without core

Table 68-6 Soft cable with cooper core, rubber insulation, movable use in brown coal enterprises

Notes: (1) For cables with or without core grounded.

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Table 68-7 Soft cable with cooper core, rubber insulation, used for mobile electrical machines

Notes: (1) For cables with or without core grounded.

Table 68-8 Conductors with cooper core, rubber insulation, voltage of 1,3, 4kV, used in transport sector

Permissible continuous current of underground cables

The permissible continuous current of underground cables with voltage up to35kV, oiled paper insulation, aluminum, lead or PVC cover, is selected according tothe permissible heating temperature of the conductor core heating: +80°C for voltage up to 3kV, +60°C for voltage up to 6kV, +60°C for voltage up to 10kV, +50°C for voltage up to35kV. Permissible continuous current of underground cable will be selected according to manufacturer’s permissible parameters or concrete levels if such documents are provided by the manufacturer.

For underground cables, permissible continuous currents given Tablein 68-10, 12 to 14, 17 are calculated for cables placed in canals at the depth of 0.7- 1 m, earth temperature of +15°Cand earth thermal resistivity of 120 cm. °F /W.

If earth resistivity is other than the above value, the permissible continuous current of underground cable will be multiplied with the coefficients given in the following Table:

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For cable placed in water, permissible continuous currents given inTable 68-11, 14, 15 are calculated with water temperature of +15°C.

For cables placed in the air, permissible continuous currents given inTable 68-11 to 17 are calculated with distance between cables placed indoor, outdoorand in tunnel not less than 35mmand in canals not less than 50mm, for any number of cables and air temperature of +2°C.

The permissible continuous current of the cable placed in pipe buried under the ground without forced ventilation will be taken equal to the values of cables when placed in the air.

When cable line going through different environments, permissible current of the cable will be calculated for section with worst conditions if this cable section has length longer than 10m. In this case, cable of this section should be replaced with bigger cable.

When some cables are placed in the earthor pipe, the permissible continuous current will be reduced by multiplication with values inTable 68-18 to 21, not including standby cables. When cables are placed in earth, distance between cables will be not less than 100mm.

Permissible currents of oil, gas, XLPE and EPR cables, one coresteelbanded cables will be selected according to the manufacturer’s documents (see Annex II.2.1 (I.3A)).

Table 68-10 Aluminum core cable with pine resin impregnated insulating paper, lead or aluminum cover, placed in earth

Notes: (1) Permissible current of one core cable is DC current.

Table 68-11 Aluminum core cable with pine resin impregnated insulating paper, lead cover, placed in water

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Table 68-12 Aluminum core cable with pine resin impregnated insulating paper, lead or aluminum cover, placed in air

Notes: (1) Permissible current of one core cable is DC current.

Table 68-13 6kV copper 3-core cable with common lead sheath, oil impregnated insulation, placed in earth and air

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Table 68-14 6kV aluminum 3-core cable with common lead sheath, oil impregnated insulation, placed in earth and air

Table 68-15 Copper 3-core cable with separate lead sheath, oil and fire-resistant oil impregnated insulation paper, placed in earth, water and air

Table 68-16 Aluminum 3-core cable with separate lead sheath, oil and fire-resistant oil impregnated insulation paper, placed in earth, water and air

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Table 68-17 Copper one-core cable with oil impregnated insulation paper, lead sheath, without steel band, placed in air

Notes: (*) Numerator is for cables placed on the same plane in distance of 35-125mm. Denominator for cables placed in grazing points of equilateral triangle

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Table 68-18 Aluminum one-core cable with oil impregnated insulation paper, lead sheath, without steel band, placed in air

Notes: (*) Numerator is for cables placed on the same planein distance of 35-125mm. Denominator for cables placed in grazing points of equilateral triangle

Table 68-19 Coefficients for case in which many cables working in parallel and placed in earth with or without pipe (Coefficient k)

The values of permissible currents of oiled cables as given in the above tables, are calculated with air temperature of +25°C and water temperature of + 15°C.

When cables are placed in pipe buried in the earth the cables are considered as placed in the air with temperature of the earth.

When cables are placed in block, the permissible continuous current is determined by the following formula:

I = k* a * b * c * d * Io

Where:

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Io : the long-term current given in Table 68-1 to 18.

k: the coefficient of multiple parallel cable, for Table 68-19.

a:adjustment coefficient given inTable 68-20 depending on cross sectional area and arrangement of cable in the block.

b:adjustment coefficient according to nominal voltage of cable, given in Table 68-21

c:adjustment coefficient according to average daily load of the whole cable block, given inTable 6822.

d:the distance coefficient between the cable block, for Table68-23

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Article 69. Selection by Permissible Voltage Drop

All conductors (bare conductors, sheathed onductors, cables and busbars) shall meet permissible voltage drop conditions.

The longer length of a conductor becomes and the larger load current becomes, the higher voltage drop at the end of a conductor becomes. So the cross-sectional area of a conductor shall be selected in consideration of the length of a conductor and the load current.

Because the effect of voltage drop caused by load current is significantly highin low-voltage circuit, it shall be paid particular attention to the voltage drop in low-voltage circuit.

The permissible voltage drop for each case depends on requirement of load type, including starting of electrical motors and taking into account increase of load in future, especially with underground cable.

For some loads which require highly stable voltage, if conductor cross sectional area is selected according to permissible voltage drop, the conductor is too big and too expensive. In this case, it needs to make comparison with alternative to increase voltage of the power line togetherwith installation of low voltage transformers at the end of the line or alternative ensuring normal voltage drop together with installation of automatic voltage stabilizers at the end of the line.

The formula for calculation of the voltage drop is as below.

Vd=Ku*I*L*Z*10-3

Vd :Voltage drop [V]

Ku : Coefficient according to the power distribution scheme

Ku=2 ; between conductors for single-phase two-wire system and single-phase three-wire system

Ku=1 ; between a conductor and a neutral line for single-phase three-wire system and three-phase four-wire system

Ku=√3 ; between conductors for three-phase three-wire system and three-phase four-wire system

I : Load current [A]

L : Length of conductor [m] Z : Impedance [Ohm/km]

Permissible voltage drop for indoor wiring .

Article 70. The Selection Conductors of up to 1kV

As stipulated in Technical Regulation.

Article 71. Selection Conductors of exceeding 1kV

The cross-sectional area of conductors over 1kV is selected with respect to economical current density by adding to article 68 and 69 of Vol.1.

In some cases, when upgrading or rehabilitating, in order to avoid increasing number of power lines or number of circuits of the power line, the economic current density is allowed to be higher than and to level double of the limitation values given in the following table.

In technical economic calculation, total increased investment cost of lines and equipment in bays at both ends of the lines shall be included.

The alternative of upgrading power lines is also considered.

The above indications are also applied for thecase when upgrading with increasing conductor cross

Article 72. The Selection of Bare Conductors with large cross-section

The continuous permissible current of bare conductors and busbars are taken from the manufacturer’s documents or according to the values given in Tables 72-1, 72-2, 72-3, 72-4, 72-5, 72-6 and 72-7.

These values have been calculated with permissible heating temperature of +70°Cwhen air temperature is +25°C. In selecting conductors and busbars, it needs to check continuous permissible current at the working conditions of the conductors and busbars. The check calculation of continuous permissible current is performed according to guidelines given in the Annex II.2.2.

When arranging rectangular busbars in horizontally, the currents given in Table 72-2 and 72-6 must be reduced by 5% for busbar with width up to 60mm and 8% for busbar with width more than 60mm.

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Table 72-1 The continuous permissible currents by heating of copper, aluminum or steel- core-aluminum bare conductors (Permissible heating 70oC at Air 25oC)

Notes: (*): Conductors ACSR don’t have these sizes.

Table 72-2 Continuous permissible currents of copper or aluminum round busbars or pipes

Notes: (*) Numerator is the permissible AC current, denominator is permissible DC current

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Table 72-3 Continuous permissible current of copper rectangular busbar