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1. Fuzzy Logic Based Differential Relay for
Power Transformer Protection

2. ABSTRACT:

3. Power Transformers or any CNC Machines or any electrical devices are
always subjected to Highvoltage and low voltage transients. Many devices will
get failed due to this. Hence we need to monitor the voltage constantly and if goes
below or above the range then we need to trip the supply immedieately. One
method of protecting power transformers is to use a differential relay. The
operation Diff Relay is to trip the equipment if it finds any difference in the
primary and secondary current.

4. Two basic requirements that the differential-relay connections must satisfy are:
(1) The differential relay must not operate for load or external faults; and
(2) The relay must operate for severe enough internal faults.
This is because the diff relay can act due to the following other factors also.

5. In the case of power transformer applications, possible sources of false differential
currents are:
 Mismatch between the CT ratios and the power transformer ratio
 Variable ratio of the power transformer caused by a tap changer
 Phase shift between the power transformer primary and secondary
currents Magnetizing inrush currents
 Transformer overexcitation
 Current transformer saturation
So, There should be an intelligent Mechanism to prevent the diff relay
from fault triggering. In this project we have employed fuzzy logic to
prevent this fault triggering. Our system when detects a current difference
between the primary and the secondary it also checks for other causes using
fuzzy logic and operates the relay only when essential. The other factors which
induces the fault triggering is given to the controller through switch inputs.
About Fuzzy:
Fuzzy Logic was initiated in 1965 by lofti A. Zadeh, professor for
Computer science at the university of California in Berkeley.
Fuzzy logic is a powerful new technology and has emerged as a
profitable tool for the controlling of subway systems and complex industrial
processes, as well as for household and entertainment electronics, diagnosis
systems and other expert systems. Although, fuzzy logic was invented in
United states the rapid growth of this technology has started from Japan and
has now again reached the USA and Europe also.

6. Fuzzy has become a keyword for marketing. Electronic articles
without fuzzy component gradually turn out to be a dead stock. As a gag,
that shows the popularity of fuzzy logic, there even exists a toilet paper with
‘Fuzzy Logic’ Printed on it. In japan fuzzy-research is widely supported
with a huge budget. In Europe and the USA efforts are being made to catch
up with the tremendous Japanese success. For instance, the NASA space
agency is engaged in applying fuzzy logic for complex docking-maneuvers.
Fuzzy logic is basically a multivalued logic that allows intermediate
values to be defined between conventional evaluations like yes/no,
true/false, black/white etc. Notions like rather warm or pretty cold can be
formulated mathematically and processed by computers
Block Diagram:
CPU
Monitor

7. Rs232
Mismatch between the
Magnetizi
ng inrush
currents
CT ratios and the
power transformer
ratio
Variable ratio
of the power
transformer
PIC EMBEDDED Microcontroller
IC-PIC16F877
Phase shift between
the power
transformer primary
and secondary
currents
Current
transformer
saturation
Relay
Driver
Full Wave Precision
Rectifier
(SIGNAL SENSING)
PT
(V)
Power Line
Transformer
overexcitatio
n
Differential
Relay(10A)
CT
(A)
Full Wave Precision
Rectifier
(SIGNAL SENSING)
PT
(V)
Power Transformer
0-230/0-24V/10AMPS
CT
(A)