From January 03, 2020, What is the Maximum Allowable Short-Circuit Current in Vietnam?

The maximum permissible short-circuit current in the operation of the transmission power system in Vietnam is stipulated in Clause 5, Article 1 of Circular 30/2019/TT-BCT, which amends Article 12 of Circular 25/2016/TT-BCT. To be specific:

Illustration (source: internet)

- The maximum permissible short-circuit current value and the maximum fault elimination time by primary protection in the transmission power system in Vietnam is regulated in Table 6 as follows:

Table 6

Maximum permissible short-circuit current and maximum fault elimination time by primary protection

- Primary protection equipment is the main protection and is installed, calibrated to activate first, ensuring criteria for selectivity, reliability of operation, and operational time of the protection system when faults occur within the protection range of the equipment being protected;

- The maximum permissible short-circuit current for a 110 kV busbar of 500 kV and 220 kV substations in the transmission network is set at 40 kA.

This Circular stipulates that switching devices on the transmission power grid must have sufficient capacity to cut off the maximum short-circuit current passing through the switching device for at least the next 10 years from the planned time of putting the device into operation and must withstand this short-circuit current for a minimum duration of 1 second or more.

For hydro and thermal power units with a capacity greater than 30 MW, the total value of unsaturated sub-transient reactance of the generator (Xd’’-%) and the short-circuit reactance of the terminal transformer (Uk-%) calculated in per unit system (pu unit converted to the rated apparent power of the generator) must not be less than 40%. In cases where this requirement is not met, the investor is responsible for calculating, investing in, and installing additional reactors so that the total value of Xd’’, Uk, and reactors calculated in the per unit system (pu unit converted to the rated apparent power of the generator) is not less than 40%.

If the electrical works connected to the transmission power grid have a measured short-circuit current value at the connection point that exceeds the maximum permissible short-circuit current value stipulated in Table 6, the investors of these electrical works are responsible for implementing measures to reduce the short-circuit current at the connection point to less than or equal to the maximum permissible short-circuit current value stipulated in Table 6.

It should be noted that the transmission power unit has the responsibility to notify the maximum short-circuit current value at the connection point at present and as calculated for at least the next 10 years, so that the transmission power grid customers can coordinate during the investment and equipment installation process, ensuring that the switching devices have sufficient capacity to cut off the maximum short-circuit current at the connection point for at least the next 10 years from the planned time of putting the equipment into operation.

See the full text of the regulations in Circular 30/2019/TT-BCT, effective from January 3, 2020.

Thu Ba

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