HV Substations and Installations

What is recommended rated duration for short circuit current of HV installations?

Based on AS 2067, section, rated duration for short circuit current is 1.0s. If a value other than 1s is appropriate, recommended values would be 0.5s, 2.0s and 3.0s. The rated duration should be determined taking into consideration the fault switching time.

As per IEEE 80, relay malfunctions, can result in fault duration in excess of primary cleaning times. Therefore for sizing the conductor (earthing), the backup clearing time is usually adequate. For smaller substations, this may approach 3s or larger. However, because large substations usually have complex or redundant protection schemes, the fault will generally be cleared in 1s or less.

What is the impact of altitude on equipment insulation?

Based on AS 2067, section, for installations situated at an altitude higher than 1000 m about sea level, the insulation level of external installation under the standardize reference atmospheric conditions shall be determined by multiplying the insulation withstand voltage required at the service location by a factor Ka in accordance with AS 1824.2.

Note that for internal pressurised insulation, the dielectric characteristics are identical at any altitude and no special precautions need be taken.

For low voltage auxiliary and control equipment, no special precautions need be taken if the altitude is lower than 2000 m above sea level.

Is there any restriction on depth of burring of HV cables?

HV cables installed underground shall be buried at a depth of not less than 0.75 m below the ground level, with mechanical protection as specified in AS/NZS 3000. If mechanical protection is not able to be provided, then the HV cables shall be installed at a depth of not less than 0.9 m below.

What should be the short circuit current rating of equipment connecting main busbar and current limiting device?

In circuits that have current-limiting protective devices, equipment may have rating that corresponds to the cut-off (let-through) currents of the current-limiting device. The current rating of equipment located between the busbar and the current-limiting device need to be at least equal to the through-fault current duty on the load side of the current limiting device.

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