30-05-2012, 12:56 PM
Technique for contingency monitoring and voltage collapse prediction
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Introduction
The continuing increase in demand for electric power
has resulted in an increasingly complex interconnected
system, forced to operate closer to the limits of stability.
This has necessitated the implementation of techniques
for analysing and detecting voltage collapse in
bus bar or lines prior to its occurrence. Voltage collapse
occurs when a system is heavily loaded and is not
able to maintain its generation and transmission schedule.
It is observed by sudden decline or ‘sag’ in systemwide
voltages. The increasing incidence of voltage collapse
in power systems due to voltage instability drew
the attention of many power system researchers.
Researchers have been trying to investigate the static
aspects of load flow solutions by applying various
methods for identification of the point of bifurcation
and to estimate the stability margin of the system.
Test system
The IEEE 24-Bus reliability test system is selected to
carry on the tests, adopting different criteria based on
the loading combination. A computer program was
developed to determine the stability measures
described. Several tests were carried out with different
combinations of loading. A sketch of the network and
line information are attached in the Appendix (Section
9). Testing criteria based on the loading pattern are single
load change, and multiple load change.
Single load change
The loading pattern in this case is chosen so that each
time the load is changed in only one particular node,
keeping the load at other nodes fixed at base load. Several
combinations of real, reactive and real, and reactive
load pattern are selected to accomplish this:
i) Single load change with real load only
ii) Single load change with reactive load only
iii) Single load change with real and reactive load
iv) Single load change with constant PF load.
Single load change with real power only:
The test is carried out at different nodes but only the
cases of node 6 and node 19 are illustrated. In both
nodes the real load is slowly varied to the level of voltage
collapse. The performance of all three stability
indicators are illustrated.
Node 6 is directly connected to node 2 (through line
5) and node 10 (through line 10). The addition of real
load at node 6, makes line 5 critical and hence near to
voltage collapse. Using our proposed method of line
stability indices, Table 1 is prepared showing the status
of lines for a particular heavy real power loading. The
Table clearly illustrates that line 5 is in a critical condition
and will be subjected to voltage collapse for any
further addition of load. The stressful condition of the
other lines is also indicated from their line stability
indices. The Table also illustrates that buses 2, 6, 8, 10
and 13 are in weak clusters and would be very sensitive
to loading.