Differential protection of three phase transformers based on wavelet theory

Other Title(s)

الحماية التفاضلية للمحولات الثلاثية الطور باستخدام تحويلة ربطة المويجة

Dissertant

al-Sad, Wad Nuri Tabur

Thesis advisor

Ulwan, Majid Abd al-Nabi
Ubed, Adil A.

Comitee Members

Thijil, Rabi Hashim
al-Saffir, Ala A.
Ali, Khearia Abd al-Jalil M.

University

University of Basrah

Faculty

Engineering College

Department

Department of Electrical Engineering

University Country

Iraq

Degree

Master

Degree Date

2012

English Abstract

The power transformers have a very important function in power systems, and their protection represents a critical task to maintain both the stability and reliability of these systems.

However, the protection of power transformer appears to be more difficult than the protection of any other part or element of the power system.

Such difficulties arise from the existence of the magnetizing inrush phenomenon, which is responsible for the occurrence of mal-functioning in the conventional protection techniques under high magnetizing current.

The differential protection is one of the important methods of protection which depends on the analysis of differential current between secondary and primary coils.

The differential current can be analyzed using many technologies such as harmonic restraint, transformer equivalent circuit model, Artificial Neural Network (ANN), etc.

These approaches depend on many features, the transformer parameters, harmonic contents and loading conditions.

These features may change due to many factors and may affect the performance of these approaches.

The present thesis proposes two approaches to discriminate inrush current from faults using Wavelet Packet Transform (WPT) and modified wavelet transform; S-Transform (ST).

In the first approach, the optimal for both mother wavelet and number of levels of decomposition have been selected, while in the second approach, no need for these selections.

A simple calculation for the energy and the Standard Deviation (STD) indices have to be calculated.

The data are mainly collected from a simulation for a 50MVA, 132 / 33KV, Y-Y and ∆-Y, 50Hz, power transformer.

The results from simulation contain many cases for inrush current, external fault currents and internal fault currents.

The WPT approach is fast in discrimination between internal fault and inrush current compared with ST approach.

The WPT based algorithm initiates a trip signal in less than quarter of a cycle and the ST algorithm initiates the trip signal in less than one cycle.

The WPT algorithm is tested off-line and on-line on a data collected experimentally from a portable 1KVA, 220/110V, ∆-Y three-phase transformer.

The results show that the mentioned algorithm detects a trip signal for internal fault in less than quarter of a cycle.

In on-line differential protection, and for internal faults, the transformer is disconnected from the supply after (10-15 msec) from the initiating of the trip signal because of the delay for converting the signal from analogue to digital and vice-versa and the delay in the contactor circuit as well as the personal computer used.

Main Subjects

Electronic engineering

Topics

• Electric power systems
• Security measures

No. of Pages

140

Table of Contents

• APA
• MLA
• AMA

Language

English

Data Type

Arab Theses

Record ID

BIM-317700