Medium Voltage Application Guide

SWITCHGEAR
Resonant frequency
The busbar system must be designed to avoid resonance at the nominal system frequency and twice this value. The
calculations should include some tolerance:
Where:
f = resonant frequency (Hz)
E = modulus of elasticity:
copper = 1.3 x 10 6 daN/cm 2
aluminium = 0.67 x 10 6 daN/cm 2
m = linear mass of busbar (daN/cm)
I = moment of inertia of the busbar cross-section, relative to the
perpendicular vibrating plane (cm 4 )
l = distance between insulator stand-offs of the same phase (cm)
Exercise
Verify the resonant frequency of the busbar system in the Exercise above.
The resonant frequency is well away from 50 Hz and 100 Hz. The busbar solution is suitable.
Short Circuit Calculations
Short circuit fault currents at different points on a system are determined by the power feeding into the fault and the
equivalent short circuit impedance seen by the fault.
Power sources feeding a fault include supply networks, transformers, generators and motors. Impedance is a vital
factor in limiting the level of short circuit current. Sources of impedance include all electrical machines, as well as
cables, overhead lines, busbars and switching apparatus.
There are numerous methods to calculate short circuit current levels, such as impedance, per-unit and
point-to-point. The most commonly used and widely understood is the impedance method (see below). These
calculation methods were widely used before calculation software became available. These programs allow very
accurate results to specific conformance standards.
Short circuit calculations serve two main functions:
to determine the required make and break ratings of switchgear, and the mechanical withstand of all
equipment
to inform fuse selection and protection relay settings, in order to achieve adequate circuit discriminations
Page 156 Medium Voltage Application Guide 710-12280-00A