Abstract:
Voltage sag is one of the power quality problems in power systems. Voltage sag is defined
as a temporary RMS reduction in voltage typically lasting from a half cycle to several seconds.
As the primary cause of voltage sag is due to faults that may occur anywhere in transmission and
distribution systems, a Monte Carlo simulation and a phase coordinate method are proposed as
the main tool for voltage sag prediction in this research.
The Monte Carlo simulation method is used to capture stochastic behavior of fault, such as
fault type, fault location, fault phase and fault impedance. The phase coordinate method is
employed to calculate voltage magnitude for various types of fault and multiple locations of fault.
The prefault voltage of a faulted bus is obtained from a conventional three phase Newton-
Raphson power flow algorithm developed for analysis in a transmission system, and a networktopology-
based three-phase power flow algorithm developed for analysis in a distribution system.
Variance checking is also introduced for a stopping criterion to reduce computation time in Monte
Carlo simulations.
The IEEE-57 bus system and a distribution system with 34 load points are tested in the
case studies. With the proposed methodology, the expected value of voltage sags as well as
voltage unbalance, and their probability distribution can be obtained. This information is useful
for the utilitys customers to select appropriate equipment specifications to assure the optimum
operation of their production facilities.