Week 11

Question 1

Why is it important to know the short circuit current

Answer 1

1. Switchgear breaking capacity
2. Cables(thermal withstand rating)
3. Protective devices( selective trip setting)

Question 2

What is bolted short circuit?

Answer 2

It’s a short circuit of zero impedance
The most severe and simplest to calculate
Worst case scenario

Question 3

In case of several transformers

Other factors taken into consideration

Answer 3

The value of fault current on an outgoing circuit immediately Dow stream the busbar can be estimated as the sum of short circuit currents from each transformer calculated separately

It is assumed that all the transformers are supplied from the same MV network, so the sum slightly higher than the actual value

Other factors: the impedance of the busbar & of the cable between transformer and circuit breakers.

Question 4

Busbars resistance

Answer 4

Resistance of bus bar negligible, so impedance is practically all reactive, approximate 0.15 m ohm/ meter length for LV busbar,
Doubling the spacing between the bars increase the reactance about 10% only,
It’s almost never possible to estimate the busbar length concerned by a short circuit downstream a switch board

Question 5

Motors under short circuit conditions

Answer 5

Running motor will act for a brief period as a generator and feed current into the fault, fault current can be ignored, but if the total power of motors running simultaneously is higher than 25% of the total power of transformers, the influence of motors must betaken into account
ISCM: 3.5 In for each motor ( only three phase motors, single phase motors not important)

Question 6

Each component (MV network,transformer , cable,busbar) characterized by their impedance Z

Answer 6

Consists of resistance R and inductive reactince X, capacitive reactance is not important in short circuit
Expressed in ohm
The method is dividing the network into sections, connected in series so all of them are added together then
Zt = sqrt(Rt^2 + Xt^2)