14-08-2012, 03:17 PM
Digital Differential Relay Reliability Enhancement of Power Transformer
2nd Harmonics Setting.pdf (Size: 413.2 KB / Downloads: 56)
Abstract
In this paper, an improvement of digital differential
relay reliability for protecting a large power transformer is discussed.
First, the Fourier sine and cosine coefficients required for
fundamental, second, third and fifth harmonics determination have
been calculated using rectangular transfer technique. Then, these
harmonics have been used in harmonics restrain and blocking
techniques used in differential protection system. Simulation testes
have been carried out on a variety of magnetizing conditions (normal
aperiodic inrush and over excitation conditions) using
Simulink/MATLAB. The obtained results shows that the developed
approach provides good discrimination between the magnetizing
current and the internal fault current.
INTRODUCTION
HE main purpose of power systems is to generate, transmit,
and distribute electric energy to customers without
interruptions and in the most economical and safe manner.
Power systems are divided into subsystems (generation,
transformation, transmission and distribution) which are
composed of costly components. Protection of these elements
is crucial.
The role of protection ensures that, in the event of a fault, the
faulted element must be disconnected from the system for
isolating the fault to prevent further damage to the components
of the system through which the fault currents were flowing.
A power transformer is mostly protected against internal fault
using a differential protection which is sensitive and a fast
clearing technique. This technique of protection detects
nonzero differential current, then activates a circuit breaker
that disconnects the transformer. However, this nonzero
differential current may be produced by transformer
magnetization, due to so called inrush current or overexcitation,
and may cause the protective system to operate
unnecessarily.
Differential Protection Methods
One of the most important means of protecting a power
transformer is differential protection based on the
comparison of the transformer primary and secondary
currents. When these currents deviate from a predefined
relationship, an Internal fault is considered and the transformer
is de-energized. However, during transient primary
magnetizing inrush conditions, the transformer can carry
very high primary current and no secondary current. The
resulting differential current can falsely trip the transformer.
The most common technique used for preventing false
tripping during the above condition is the use of second
harmonic restraint. If the second harmonic content of the
differential current exceeds a pre-defined percentage of the
fundamental, inrush is indicated and the relay is prevented
from tripping.
PROTECTION SYSTEM IMPLEMENTATION
The above discussed approach has been implemented using
Matlab/Simulink with the necessary tool box. The Matlab is
powerful software program used for any test and simulation.
The characteristics of the differential protection scheme that
has been used are plotted in Fig.2. Where there are two
straight lines given with a slope of K1=0.25 and a slope of
K2=0.6, which range from Irt0 to Irt1 and from Irt1 to Irt2,
respectively, and a horizontal straight line defining the relay
minimum pickup current, Iop0=0.3A. The relay operating
region is located above the slope, and the restraining region is
below the slope.
A dual-slope percentage characteristic provides further
security for external faults. It is represented as a dashed line in
Fig.4.
The dual-slope percentage pattern adds a restraint area and
avoids mal-operation caused by saturation. In comparison with
a single-slope percentage scheme, the dual-slope percentage
current differential protection can be regarded as a better curve
fitting of transformer operational principles [19, 20].
CONCLUSION
In this paper, an attempt has been made through the use of
MATLAB/SIMULINK to test a new approach applied to
digital differential protection relay for a large power
transformer. First, the Fourier sine and cosine coefficients
required for fundamental, second, third and fifth harmonics
extraction have been calculated using rectangular transfer
technique. Then, these harmonic components have been used
in harmonics restrain and blocking techniques which may be
utilized in differential protection system. Testes have been
carried out on a variety of magnetizing conditions (normal
aperiodic inrush and over excitation conditions due to external
fault) as well as internal fault. It can be noted that, from the
obtained simulation results using Simulink/MATLAB, the
developed scheme provides good discrimination between the
magnetizing current and the internal fault current.