Maximum short circuit of an assumed substation is related to network fault level which connected to it. The fault level of an electrical network is the capacity of the source of supply to feed the faulty circuit, and is represented in kVA or MVA. Some typical values are noted below:

To characterise the short-circuit infeed from one short-circuit source, a group of such sources or an entire system, the concept of short-circuit fault level is useful:

Short-circuit fault level(MVA) =√3VPhase–Phase(kV)Irms(kA)

where Irms(kA) is the rms short-circuit current infeed at the point of fault and

VPhase–Phase(kV) is the prefault phase-to-phase voltage at the point of fault. For example, for an rms short-circuit current of 54 kA and a 404 kV prefault voltage in a 400 kV system, the short-circuit fault level is equal to 37.786GVA.

Also the value of short circuit currents is calculated from the inductive reactances of the power system components and gives rise to different maximum short-circuit currents in the various system sections. These currents are very often ten to twenty times the continuous current rating and therefore the transitory heating effect must be taken into account. This effect can, in many cases, lead to dangerous overheating, particularly where small conductors are part of a large heavy current system, and must be considered when determining the conductor size. To calculate the temperature rise of the conductor during a short circuit it is assumed that all the heat generated is absorbed by the conductor with none lost by convection and radiation as for a continuous rated conductor. The temperature rise is dependent therefore only on the specific heat of the copper conductor material and its mass. The specific heat of copper varies with temperature, increasing as the temperature rises. At normal ambient temperatures it is about 385 J/kg K and at 300°C it is about 410 J/kg K.

For busbar short circuit (I = conductor current, kA) withstand estimation , you need to know:

t=maximum short-circuit time, s

A = conductor cross-section area, mm2

θ = conductor temperature rise, K

Author : Hamid - From: Iran