For the voltage of110kV, minimum cross section to limit corona generation is 70mm2, and for the voltage of 220kV, the said cross section is 240mm2.

For the voltage of 220kV and above, split in 2 or 4 smaller conductors can be applied to limit corona. Wireless communication interference by corona must be checked too.

Article 74. The Selection of Lightning Conductor

As stipulated in Technical Regulation.

In selection of lightning conductor, thermal stability conditions when short circuit of one phase occurs must be checked according to guidelines given in Annex II.2.3 (I.3C).

Chapter 2-2-2 The Selection of Electrical Equipment by Short Circuit Conditions

Article 75. General provisions

As stipulated in Technical Regulation.

Article 76. Short Circuit Currents calculation scheme

It must calculate short circuit currents for power generation and transmission development diagram at least for the period 10 years after putting equipment into operation (approximate calculation).

The following short circuit cases shall be considered:

1.Three phase short circuit for checking electrodynamic stability of equipment, busbars, conductors and support structures.

2.Three phase short circuit for checking thermal stability of equipment, busbars and conductors. At

generator voltage, selecting whichever causing higher heating from 3 phase or 2 phase short circuit currents for checking.

3.Three phase or one phase earth fault short circuit current, whichever bigger will be chosen for checking short circuit current breaking capacity of equipment. If circuit breaker has both -3phase

and one -phase current breaking values, both short circuit currents must be chosen.

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Article 77. Calculate the Time for Thermal Stability

When checking thermal stability, the calculation time is assumed to be equal to the total time required for the trip of the main protective relay and circuit breaker nearest to short circuit point and breaking time of circuit breaker itself including arcing time.

If the main protector has dead zone (by current, voltage, resistance etc.) the thermal stability shall be checked according to protector’s trip time when short circuit happens in the dead zone plus total trip time of the circuit breaker.

Article 78. Increasing Temperature in Fast Self-Reclosing Devices

When checking the thermal stability of equipment and power lineconductors with fast self-reclosing devices, the increase in temperature due to the increase in total short circuit time must be taken into account. When checking heating according to short circuit conditions, the conductors of one phase will be considered as one conductor with cross sectional area equal to total cross sectional area of all conductors in this phase.

Article 79. Short Circuit Currents

Equipment and conductor of circuitshall be selected according to maximum short-circuit current passing through.

They shall withstand the maximum short-circuit current for 2 seconds.

The short-circuit current is maximized when the maximum number of generators are connected with the power network and three-phase short-circuit fault happens

Article 80. Checking Items for Short Circuits

As stipulated in Technical Regulation.

Article 81. Electromotive Forces

Busbar, conductor, insulator and rigid support structure must withstand the electromotive force which generates in the maximum short-circuit current.

Electromotive force acting on soft conductors, insulators, terminals and conductor harness is calculated according to average square value of short circuit currents of two adjacent phases. For conductors in one phase and soft conductor harness, the acting force of-circuishort current in conductors in one phase is determined with respect to effective value of -threephase short-circuit

current.

Mechanical force of short-circuit current acts on busbar to support insulator and wall rancet insulator must not exceed 60% of minimum destructive force of insulator if insulator is single, and not exceed 100% of destructive force if insulator is double.

If conductors consist of several flat or -Ushaped bars, the mechanical stress is equal total stress generated by reciprocal forces between phases and elements of each bar.

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The maximum mechanical stress of materials of rigid conducting bars must not exceed 70% of instantaneous destructive force.

The detail of calculation example is as follows.

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Article 82. Permissible Values for Heated Conductors

The temperature of a heated conductor when a short circuit occurs must not exceed permissible values given in the following table.

Article 83. The Selection of Circuit Breakers

To select circuit breaker, it is necessary following requirements:

1.By breaking capacity: calculated breaking current is overall active short-circuit current (including acyclic elements) which is determined with tripping contact time of circuit breaker equals to total individual breaking time of the circuit breaker (from sending command onbreaking to releasing the contact for arc elimination) plus the time for extinguishment of the arc.

2.By closing capacity: at that time, the circuit breaker of the transformer installing at the side of

transformer voltage needs to be checked in case of asynchronous energization in phase opposition state.

Apart from being selected according to breaking capacity, circuit breaker shall alsobe selected based on transient recovery voltage (TRV) after short circuiting. The TRV capacity of a circuit breaker must exceed the calculated capacity in each location of circuit breaker inthe network. This criterion needs to be specially paid attention for500kV and 220kV circuit breakers at the terminals of long power lines, generator circuit breakers and circuit breakers placed near to reactor coils.

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