DETECTION OF BRANCH FAILURES IN THE ROTATING 
RECTIFIER OF A BRUSHLESS EXCITATION SYSTEM 
 
 
 
 
 
 
J.B.A.Nishantha 
 
 
 
The dissertation is submitted in partial fulfillment of the requirement for degree of M. Eng.  in 
Electrical Engineering 
 
 
 
Department of Electrical Engineering.  
Univercity  of Moratuwa. 
Sri lanka 
May 2001 
 
 
 
 
 
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CONTENTS 
 
l.lntroduction to the project 
 
1.1 Need for a branch failure detector 
1.2 Structure of the brushless excitation system that uses the detector 
1.3 Rotating rectifier 
2. Branch failure detection principles 
 
2. 1 Types of failures 
2.1.1 Inoperative of Diode 
2.1.2 Breakdown of Diode 
2.2 Boosts of field current harmonics 
2.2.1 Normal operating condition 
2.2.2 One branch failure 
2.2.3 Two branches failure 
2.2.4 Short circuit of one branch 
2.2.5 Line- line short circuit 
2.2.6 Line-neutral short circuit 
2.3 failure detection criterion 
Construction of the detector 
 
3.1 Overall arrangement  
3.2 Detector circuit.  
3.2 .1 Band pass filters 
3.2.2 Peak detector 
3.2.3 Comparator 
3.2.4 Logic circuit  
3.3 Application notes. 
3.3.1 Tuning of the detector 
 
Conclution.  
 
Appendix A : Line currents in 3- phase un cont rolled rectifier under normal 
operation and branch failure conditions. 20 
Appendix B : Harmonics on exciter field due to 3-phase rectifier failure 
conditions and due to system external faults. 
References 
 
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PREFACE __________________________________ __ 
This research is about the principal of operation and the 
construction of a detector circuit to identify the branch failure 
conditions in the rotating diode bridge of a brushless excitation system. 
Brushless excitation is used in high-speed synchronous generators. 
The principal of detection is based on the harmonics present in 
the exciter field current. Different failure conditions give different 
contents of harmonics.By identifying harmonics it is possible to 
interpret the type of fault .. 
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The circuit that detects exciter harmonic following a failure in a 
diode branch gives an alarm or indication. This circuit can be applied 
to any brushless excitation system. 
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ACKOWLEDGMENTS 
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I wish to express my gratitude to my Supervisor, 
Dr.J.P.Karunadasa of the Department of Electrical Engineering at 
University of Moratuwa and Co-Supervisor, Chief Engineer 
Mr.A.K.Samarasinha at Ceylon Electricity Board for their great guidance, 
support and encouragement through out the project. 
I should like to thank to many people who directly or indirectly 
have supported me and Thanks to Professor J.R.Lucas and Or. 
H.Y.R.Perera of the Department of Electrical Enginneering at University 
ofMoratuwa for their support. l 
Finally I must express my deep appreciation to my wife, Wasantha 
for her encouragement to completion of my Research. 
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J .B .A.Nishantha. 
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CONTENTS 
l.lntroduction to the project 
1.1 Need for a branch failure detector 
1.2 Structure of the brushless excitation system that uses the detector 
1.3 Rotating rectifier 
2. Branch failure detection principles 
2.1 Types of failures 
2.1.1 Inoperative of Diode 
2.1.2 Breakdown of Diode 
2.2 Boosts of field current harmonics 
2.2.1 Normal operating condition 
2.2.2 One branch failure 
2.2.3 Two branches fai lure 
2.2.4 Short circuit of one branch 
2.2.5 Line-line short circuit 
2.2.6 Line-neutral short circuit 
2.3 failure detection criterion 
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3. Construction of the detector 
3.1 Overall arrangement 1 1 
3.2 Detector circuit. 12 
3.2 .1 Band pass filters 
3.2.2 Peak detector 
3.2.3 Comparator 
3.2.4 Logic circuit I 
3.3 Application notes. 17 
3.3.1 Tuning of the detector 
.t.Conclution. 19 
Appendix A : Line currents in 3- phase uncontrolled rectifier under normal 
operation and branch failure conditions. 20 
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Appendix B : Harmonics on exciter field due to 3-phase rectifier failure 
conditions and due to system external faults. 
References 
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