07-09-2013, 02:51 PM
Unbalanced Distribution System Short Circuit Analysis – An Object-Oriented Approach
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Abstract
A new Object-Oriented Design (OOD) for unbalanced
distribution system is proposed and employed for developing
short circuit analysis software. The difficulty associated with the
procedure-oriented implementation of short circuit analysis
program is overcome with a new object-oriented implementation.
The purpose of using Object-Oriented Methodology (OOM) for
unbalanced distribution system is to achieve a flexible software
model for the complex nature of the system involving three-
phase, two-phase and single-phase components. Considerable
reduction in the programming complexity can be obtained by the
proposed method. The method was successfully tested on
benchmark systems like IEEE 13-bus and IEEE 34-bus systems.
INTRODUCTION
Distribution system is one of the most complex systems in
power system domain. Distribution system analysis is
gaining more importance in recent days due to the
expansion of distribution system network and automation of
its operation [1]. Short circuit analysis is a vital tool for
determining the ratings of the protective devices in the
distribution system. Distribution systems are generally
unbalanced. A typical distribution system may be radial or
weakly meshed; balanced or unbalanced; single-phase, two-
phase or three-phase system[2]. Unbalanced distribution
system consists of several components of different types,
which require an efficient software modeling technique for
analysis. Shunt devices used for reactive power compensation
and loads in the distribution system may be either single-phase
or three-phase. Single-phase loads and shunts may be
connected either between a phase and neutral or between two
phases. Three phase loads and shunts may be either star
connected or delta connected. Transmission lines and cables
may be single-phase, two-phase or three-phase.
UNBALANCED DISTRIBUTION SYSTEM
SHORT CIRCUIT ANALYSIS
The computation of short circuit currents for unbalanced faults
can be performed using symmetrical components. However,
this method is not suitable for a system, which is inherently
unbalanced, such as a system with unequal mutual coupling
and unbalanced loads.
This is because the sequence
impedances in the unbalanced system are not decoupled as in
balanced system.
SHORT CIRCUIT ANALYSIS IN
PROCEDURE ORIENTED IMPLEMENTATION
Computation of the equivalent impedance between the fault
point and the source of fault current is the most involved part
in the short circuit analysis. The equivalent impedance
includes the total line impedance from the fault bus back to the
substation, substation transformer impedance and system
equivalent impedance as determined at the high voltage bus of
the substation. This calculation involves tracing the path
connecting the fault point and the substation. This path may
consist of several feeders, which are the aggregation of
branches such as transmission lines, transformers and
switches. The remaining part of this section explains the
method of tracing the path in the procedure-oriented
implementation, where the data is kept global and is passed as
arguments into the functions for performing the desired task.
OBJECT-ORIENTED DESIGN OF
UNBALANCED DISTRIBUTION SYSTEM
Unbalanced distribution system has three-phase, two-phase
and single-phase components, where the connected phases (a-
b, b-c, c-a for two-phase and a, b, c for single-phase) have to
be taken into consideration. Three-phase and two-phase
objects are modeled as a composition of three and two single-
phase objects respectively. In order to represent the connected
phases, an additional attribute is included in the single-phase
objects. The unbalanced distribution system has been modeled
as an object of class UBDistSys (UnBalanced Distibution
System), which is an aggregation of all three-phase feeders,
two-phase feeders, single-phase feeders, three-phase, two-
phase and single-phase buses etc [14]. Fig.3 shows the class
diagram of a three-phase unbalanced distribution system. Only
important base and derived classes are shown to improve the
clarity.
CASE STUDY AND RESULTS
The short circuit analysis program developed in C++, an
object-oriented programming language, has been tested using
4.16 kV IEEE 13-bus system[19] shown in Figure 2 and 24.9
kV IEEE 34-bus distribution system[19] shown in Figure 5.
Balanced and unbalanced fault analyses have been performed
on the IEEE 13-bus and IEEE 34-bus systems. The ratings of
both the systems are given in Table 2.
CONCLUSION
This paper proposed an object-oriented design for
unbalanced distribution system. The development of short
circuit analysis software using the proposed methodology was
demonstrated. A new method for tracing the path connecting
the faulted bus and the substation using the association
relationship between the objects has been introduced. The
flexibility of using the proposed method over traditional
procedure-oriented method has also been discussed. The
method was successfully tested for different benchmark
systems like IEEE 13-bus and IEEE 34-bus system.