Abstract:
Short-circuit studies provide the values of currents
and voltages in a power system network under the fault conditions.
On the basis of results of these studies, adequate
protective systems containing relays and circuit breakers
can be designed; Most of the faults that occur on a power
system are unsymmetrical faults and causes unbalanced currents
to flow in the system. The ratings of the circuit
breakers are to be determined for the severest duty and
consequently unsymmetrical fault occupies a place of importance
in short-circuit calculations, because this type of
fault often result in the highest value of the fault current.
In this work all important tJ~es of unsJ~metrical faults,
such as, single line-to-ground fault, double line-to-ground
fault and line-to-line fault together with three-phase-toground
fault calculations have been the subjects of investigation.
Both the existing BPDB system and the system planned
and proposed to be completed by June, 1990 are studied to
obtain sufficient short-circuit data for future use.
An efficient and time saving Dlothod of short-circuit study
using the ~usmatrix has been used here. A computer program
has been developed using the mathematical formulae developed
for different types of fault conditions. The program has been
used to compute the currents, voltages and the fault MVA at any
bus for any type of fault. Provisions for opening any line from
the faulted bus is also provided.
It is observed from the results of the fault calculations
that the fault MVA is the highest at a number of buses for
single line-to-ground fault whereas at some other buses the
fault level is the highest for three phase -to-ground fault.
Also at a few number of buses maximum fault MVA is observed for
double line-to-ground fault. The post-fault bus voltages at a
number of buses are found to increase above the pre-fault voltage
for double line-to-ground fault. The maximum fault level is
4343 MVA at Ashuganj-132KV bus of the existing system and that
for the 1990 system is 3560MVA at Ghorasal-230KV bus.
The procedure for selection of the MVA ratings of circuit
breakers on the basis of the short-circuit data of the system
has been presented. It has.been observed that at some location
of BPDB Power Network,circuit breakers of 5000MVA rating
have been used. The MVA rating can effectively handle the
short-circuit current at those location. On the other hand,
circuit breakers with lower MVA rating could have been
used at some locations. rhis would have cut down the cost
of the switchgear. resulting in considrable savings.
The results of the short-circuit study can be effectively
used for checking the ratings of the circuit breakers
and for possible modification of the protective system of
the existing power network and will provide the engineers
with useful information for planning and design of future
extension of the BPDB network.
