Fuzzy Logic Based Differential Relay for Power Transformer Protection

ABSTRACT:

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.

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.

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.

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:

MonitorCPU

PIC MicrocontrollerIC-PIC16F877

Rs232

Power Transformer0-230/0-24V/10AMPS

Differential Relay

Relay Driver

Power Line

PT(V)

CT(A)

Full Wave Precision Rectifier

PT(V)

CT(A)

Full Wave Precision Rectifier

Mismatch between the

CT ratios and the

power transformer

ratio

Variable ratio of the power transformer

Phase shift between the power transformer primary and secondary currents

Magnetizing inrush currents

Transformer overexcitation

Current transformer saturation