Symmetrical Faults

Symmetrical Faults
Fault studies form an important part of power system analysis. The
problem consists of determining bus voltages and line currents during
various types of faults. Faults on power systems are divided into three-
phase balanced faults and unbalanced faults. Different types of
unbalanced faults are single line-to-ground fault, line-to-line fault, and
double line-to-ground fault, which we will dealt with in the next unit.
The information gained from fault studies are used for proper relay
setting and coordination. The three-phase balanced fault information is
used to select and set phase relays, while the line-to-ground fault is
used for ground relays. Fault studies are also used to obtain the rating
of the protective switchgears.
 Symmetrical Faults
The magnitude of the fault currents depends on the internal impedance of the generators
plus the impedance of the intervening circuit. The reactance of a generator under short
circuit condition is not constant. For the purpose of fault studies, the generator behavior can
be divided into three periods: the subtransient period, lasting only for the first few cycles; the
transient period, covering a relatively longer time; and, finally, the steady state period. In this
chapter, three-phase balanced faults are discussed. The bus impedance matrix by the
building algorithm is formulated and is employed for the systematic computation of bus
voltages and line currents during the fault.
 Symmetrical Faults
BALANCED THREE-PHASE FAULT
This type of fault is defined as the simultaneous short circuit across all three
phases. It occurs infrequently, but it is the most severe type of fault encountered.
Because the network is balanced, it is solved on a per-phase basis. The other two
phases carry identical currents except for the phase shift. The reactance of the
synchronous generator under short-circuit conditions is a time-varying quantity,
and for network analysis three reactances were defined. The subtransient
reactance , for the first few cycles of the short circuit current, transient
reactance , for the next (say) 30 cycles, and the synchronous reactance ,
thereafter. Since the duration of the short circuit current depends on the time of
operation of the protective system, it is not always easy to decide which
reactance to use. Generally, the subtransient reactance is used for determining
the interrupting capacity of the circuit breakers. In fault studies required for
relay setting and coordination, transient reactance is used
 Symmetrical Faults
A short-circuit fault takes place when two or more conductors come in contact with each
other when normally they operate with a potential difference between them. The contact may
be a physical metallic one, or it may occur through an arc. In the metal-to-metal contact case,
the voltage between the two parts is reduced to zero. On the other hand, the voltage through
an arc will be of a very small value. Short-circuit faults in three-phase systems are classified
as:
i) Balanced or symmetrical three-phase faults.
ii) Single line-to-ground faults.
iii) Line-to-line faults.
iv) Double line-to-ground faults.
Generator failure is caused by insulation breakdown between turns in the same slot or
between the winding and the steel structure of the machine. The same can take place in
transformers. The breakdown is due to insulation deterioration combined with switching
and/or lightning over-voltages.
 Symmetrical Faults
. Over-head lines are constructed of bare conductors. Wind, sleet, trees, cranes, kites, airplanes, birds, or damage to
supporting structure are causes for accidental faults on overhead lines. Contamination of insulators and lightning over-
voltages will in general result in short-circuit faults. Deterioration of insulation in underground cables results in short circuit
faults. This is mainly attributed to aging combined with overloading. About 75 percent of the energy system’s faults are due
to single-line-to-ground faults and result from insulator flashover during electrical storms. Only one in twenty faults is due to
the balanced category.
A fault will cause currents of high value to flow through the network to the faulted point. The amount of current may be
much greater than the designed thermal ability of the conductors in the power lines or machines feeding the fault. As a result,
temperature rise may cause damage by annealing of conductors and insulation charring. In addition, the low voltage in the
neighborhood of the fault will cause equipment malfunction.
Short-circuit and protection studies are an essential tool for the electric energy systems engineer. The task is to calculate the
fault conditions and to provide protective equipment designed to isolate the faulted zone from the remainder of the system in
the appropriate time.
State fault current called Transient current and is donated by
Bus 1
 36880 − 34870
𝐷𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒= × 100=5.76 %
34870
The difference in fault current when the voltage behind subtransient reactance is considered and
when it is ignored is small and usually systems are assumed unloaded