Environmental Engineering Reference
In-Depth Information
X
g
R
l
X
l
Fuse
I
sc
Short
E
g
circuit
Generator
Line
Figure 5.14
Single-phase equivalent circuit of a synchronous generator feeding a symmetrical short-
circuit fault
Short - circuit faults can occur in a number of ways, including line - to - ground, line - to - line,
etc., but, for simplicity, it is usual to start by considering the worst case of symmetrical three-
phase faults. Fortunately this case is also the easiest to analyse.
5.7.2 Symmetrical Three - phase Fault Current
A symmetrical three-phase fault occurs when all three conductors of a three-phase line are
shorted together. This amounts to the same thing as if they were all shorted to ground, but,
because it is symmetrical (balanced), no current will fl ow in the ground and a simple single-
phase analysis can be used.
In the simple case shown in Figure 5.14,
E
g
is the internal phase-to-neutral EMF of a syn-
chronous generator,
X
g
is its internal per-phase reactance and
R
l
and
X
l
are parameters of the
line. By inspection, the fault current
I
sc
is given by
E
XR X
g
(5.9)
I
=
sc
j
++
j
g
l
l
Obviously, if the fault occurred nearer to the generator, the total impedance would be less
and the fault current would be greater. This equation gives
I
sc
as a scalar value, which is
normal practice. For some purposes, however, the
X
- upon -
R
ratio of the impedance is also
calculated.
5.7.3 Fault Currents in General
In a large power system, of course, there are many synchronous generators and many trans-
mission lines interconnecting them. The
prospective fault current
, at any point on the network,
will be the sum of contributions from all the generators. This prospective fault current will
be different at different points of the network. Additionally, in practice, many faults are not
symmetrical and a full analysis requires calculation of the unbalanced currents, usually
employing symmetrical components.
The calculation of fault currents is mathematically similar to load fl ow and most power
system simulation software packages will include
fault analysis
as standard.
