Environmental Engineering Reference
In-Depth Information
5.7.4 Fault Level (Short - circuit Level) - Weak Grids
In systems operating at various voltages, it is useful to multiply the fault current
I
sc
by the
nominal operating voltage (before the fault). On a per-phase basis this is
V
ph
I
sc
, where
V
ph
is
phase - to - neutral voltage. In a balanced three - phase system,
VV
ph
3
, where
V
is line - to -
=
line voltage. Multiplying by 3 gives the three-phase fault level
S
k
:
S
=
3
VI
(5.10)
k
sc
where
V
is the nominal line-to-line voltage (before the fault) and
I
sc
is the symmetrical three-
phase fault current, as introduced above.
The fault level is a slightly abstract quantity since the voltage and current being multiplied
together are not occurring at the same time. It does
not
indicate the amount of power avail-
able. Its units are VA, kVA or MVA, not kW or MW. The usefulness of the concept is that
it gives the rating of a circuit breaker capable of withstanding the full voltage when the system
is normal and of interrupting the highest prospective fault current at the point at which the
fault level is calculated. As a consequence the numerical value of the fault level and therefore
the rating of circuit breakers is very large indeed. Table 5.2 gives typical values of fault levels
at various system voltages.
The fault level at a particular point will vary as generators are brought on or taken off line
elsewhere in the network. Also, it will vary if the network is reconfi gured. Given that the
entire protection system is designed around calculated fault levels, network operators must
be aware of changes in fault levels and manage these appropriately.
The fault level is an important design parameter, not only for predicting currents under
fault conditions but also for predicting performance under normal operating conditions. The
Thévenin equivalent circuit in the following section illustrates this concept.
5.7.5 Th é venin Equivalent Circuit
Consider again the Icelandic national power system shown in Figure 5.12. It is often useful
to be able to calculate performance at one particular node, without having to perform a full
load fl ow or fault analysis of the entire network. This can be achieved by representing the
network by its
Th é venin equivalent
, as explained in the Appendix.
The values in the Thévenin equivalent circuit shown in Figure 5.15 must be calculated for
the particular node in question. The magnitude of
Z
th
can be found from the fault level at the
node by
Table 5.2
Typical fault levels
Nominal system voltage (kV)
Fault level (MVA)
132
5 000 - 25 000
33
500 - 2 500
11
10 - 250
