ABSTRACT
This work presents the reliability assessment of the Power sub-system of the North West Region
of Airtel network for a period of one year. The Power model was found to be an integrated setup
of major Power subcomponents such as transformer, automatic voltage regulator, generators,
rectifier system, battery bank, and Power control systems such as automatic transfer switch
and automatic main failure, which are all interfaced in a definite topological structure, with
unique redundancy model at each network site.The impact of critical power failure event at
different hierarchical stations on the subscribers was also assessed. The reliability data for power
sub-system was collected to determine the reliability indices of the power equipment. The data
were analysed using the MATLAB version 7.4 software. The two-generator system at the
integrated switch sites, integrated hub sites, terminal end sites and independent of PHCN power
supply on the network exhibited relatively high reliability with fault tolerance by the prevalence
of duty cycle availability indices of 51.83%, 56.00% and 56.5% respectively under a standard
configuration of 50.00% duty cycle absolute availability index for a generator unit. The rectifier
system with modules of failure rate of 0.040/103h installed on the network was deduced to
possess a high reliability index close to unity within a unit hour interval. Empirical findings
show that the use of battery units with voltage rating of 3V, 6V and 12V on the network of -48V
combinatorial battery bank system would yield relatively high reliability close to unity. The
study result showed that the efficiency of the network power system model depends on the
degree of automation of the subcomponents and the degree of ambient condition changes. It was
concluded that the reliability standards of the GSM power sub-system are dynamic and depends
on the redundancy mode of each subcomponents, the degree of redundancy, the failure ratings or
lifecycle and scalability of the subcomponents unit used in the system under optimal working
conditions of all the subsystems and ideal maintenance practice.
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TABLE OF CONTENTS
TITLE PAGE
TITLE PAGE – – – – – – – – – – i
DECLARATION- – – – – – – – – – ii
CERTIFICATION- – – – – – – – – – iii
DEDICATION- – – – – – – – – – iv
ACKNOWLEDGEMENT- – – – – – – – – v
ABSTRACT- – – – – – – – – – – vi
LIST OF FIGURES- – – – – – – – – – vii
LIST OF TABLES- – – – – – – – – – viii
LIST OF ABREVIATIONS- – – – – – – – – x
DEFINITION OF TERMINOLOGIES- – – – – – – xi
TABLE OF CONTENT- – – – – – – – – xii
CHAPTER ONE: INTRODUCTION- – – – – – – 1
1.1 BACKGROUND INFORMATION– – – – – – – 1
1.2 THESIS OUTLINE- – – – – – – – – 1
1.3 THESIS MOTIVATION– – – – – – – – 3
1.4 PROBLEM FORMULATION- – – – – – – – 4
1.5 AIMS OF THE STUDY- – – – – – – – – 7
1.6 MOTIVES FOR THE AIMS- – – – – – – – 8
CHAPTER TWO: LITERATURE REVIEW AND THEORETICAL BACKGROUND- 9
2.1 INTRODUCTION- – – – – – – – – 9
2.2 LITERATURE REVIEW- – – – – – – – 9
2.3 STRUCTURE OF THE NETWORK- – – – – – – 11
2.4 MAJOR SUBCOMPONENT OF THE NETWORK POWER SYSTEM- – 15
2.4.1 Generator Subsystem- – – – – – – – 15
2.4.2 Rectifier Subsystem- – – – – – – – 16
2.4.3Automatic Voltage Regulator (AVR) Subsystem- – – – – 17
2.4.4 Battery Bank- – – – – – – – – 18
2.5 THE NETWORK POWER SYSTEM MODEL- – – – – – 18
2.6 REDUNDANCY TOPOLOGY OF POWER SUBSYSTEM ON THE MODEL- 20
2.6.1Redundancy Topology of Generator – – – – – – 20
2.6.2Redundancy Topology of Rectifier System– – – – – 22
2.6.3 Redundancy Topology of Battery Bank System- – – – – 22
2.7 CHARACTERISATION OF NETWORK POWER OUTAGE- – – – 25
2.7.1 Nature of Free and Forced Outage at a Network Site- – – – 25
2.7.2 Nature of Partial and Total Power Outage at a Network Site- – – 25
2.8 THE CRITICAL POWER RELIABILITY EPISODE- – – – – 26
CHAPTER THREE: METHODOLOGY- – – – – – – 28
3.1 INTRODUCTION- – – – – – – – – 28
3.2 DATA COLLECTION TECHNIQUE- – – – – – – 29
3.3DESCRIPTION OF RELIABILITY DATA OF MAJOR POWER
SUBCOMPONENTS – – – – – – – – 30
3.3.1 Generator Reliability Data– – – – – – – 30
3.3.2 AVR Reliability Data- – – – – – – – 35
3.3.3 Rectifier System Reliability Data- – – – – – – 39
3.3.4 Battery Reliability Data- – – – – – – – 43
3.4 DATA EVALUATION TECHNIQUE- – – – – – – 46
3.5 DATA ANALYSIS- – – – – – – – – 47
3.5.1 Analysis of the Generator Reliability Data– – – – – 48
3.5.2 Analysis of the AVR Reliability Data– – – – – – 50
3.5.3 Analysis of the Rectifier Reliability Data- – – – – – 51
3.5.4 Analysis of the Battery Bank Reliability Data- – – – – 52
CHAPTER FOUR: RESULTS AND DISCUSSION- – – — – 55
4.1 INTRODUCTION- – – — – – – – – 55
4.2 ASSESSMENT OF POWER SUBCOMPONENT RELIABILITY STANDARDS- 55
4.2.1 Assessment of the Generator Reliability Standards- – – – 55
4.2.2 Assessment of the Rectifier System Reliability Standards- – – – 57
4.2.3 Assessment of the Battery Bank Reliability Standards- – – – 61
4.3 CRITICAL EVENT ANALYSIS AT HIERARCHICAL STATION- – – 65
CHAPTER FIVE: SUMMARY, CONCLUSION AND RECOMMENDATION- – 70
5.1 INTRODUCTION- – – – – – – – – 70
5.2 LIMITATIONS- – – – – – – – – – 70
5.3 SUMMARY OF MAJOR FINDINGS- – – – – – – 71
5.4 CONCLUSION- – – – – – — – – – 72
5.5 RECOMMENDATIONS- – – – – – – – 72
REFERENCES- – – – – – — 74
APPENDIX1: ELECTRIC, THERMOMETRIC AND RELIABILITY DATA OF
POWER SUBCOMPONENT AT HIERARCHICAL NETWORK STATION- 76
APPENDIX 2: MATLAB SCRIPTS FOR THE COMPUTATION OF SUBSYSTEM
RELIABILITY INDICES- – – – – – – 83
CHAPTER ONE
INTRODUCTION
1.1 BACKGROUND INFORMATION
Since the advent of global system for mobile communication (GSM) in Nigeria in 2001, the
telecommunication industry has been experiencing unprecedented expansion in the number of
subscribers. However, this increase in the use of GSM network have put enormous pressure on
the network operators to ensure reliability and quality of service desired by the customers, but
the reality on ground is that many of the network operators have not achieved the desired results
[1, 2].
Reliability and quality of service can be affected by either traffic congestion or network failure
due to equipment malfunction. Many studies have been carried out to ascertain the performance
of GSM operators in Nigeria based on the size of traffic carried by the network operator and the
causes of congestion in the network [3]. However, much attention has not been devoted in the
past to network failure due to power outages caused by equipment failures. These outages affect
the reliability and quality of service rendered by the network operator. Hence, it is essential to
determine the number of customers affected by these outages when such situations occur in order
to establish the best methodology in the allocation of resources to minimise such occurrences.
1.2 THESIS OUTLINE
Chapter one presents the formulation of the problem, the aims of the study, and the motives that
motivated the aims. The outline of the activities done to achieve the aims of the study was
presented.
Chapter two presents the literature review and the structure of the hierarchical network under
study. In this chapter, the major subcomponents of the network power system are outlined, and
the features of each of the power subsystem are described. The operating power system model is
presented, and the redundancy model of each of the major power subsystems is described with
the presentation of their related reliability equations. The chapter concludes with the discussion
of the characterization of the network power outages and the description of the events associated
with critical power episodes when the network load is supported by only battery power.
Chapter three presents the methodology and strategies adopted for the sourcing and collection of
reliability data. In this chapter, the primary reliability data collected for the major power
subsystems are briefly described. The evaluation techniques used for the computation of
secondary reliability data are presented and the data were analyzed using central tendency
measure.
Chapter four presents the reliability results which were evaluated based on the redundancy model
of each of the major power subcomponents on the power system model. Assessments of the
reliability standards of each major power subsystem for the impacts of practices and
considerations that causes variations of parameters under certain constant conditions are
discussed. The impacts of critical episode events on network outage and customers are also
discussed.
Chapter five presents the limitations observed during the course of the study. The summary of
the findings discovered upon the assessments of the efficiency of the operating model; the
operational conditions; and the reliability results, were highlighted. Conclusions on the
deductions made on the reliability study were also stated. Recommendations on practices to
enhance reliability standards on a GSM power system were outlined.
1.3 THESIS MOTIVATION
Wireless networks such as GSM network have become critical telecommunication infrastructure
as million of people depend on these networks for daily communication. More so, many
thousand more new customers are subscribing to wireless service every day. As networks grow,
network operators face tremendous challenges not to compromise network dependability such as
reliability, availability and maintainability [3, 4, 5].
However, this situation is complicated the more, since power supply from Power Holding
Company of Nigeria (PHCN) is rarely reliable or available. Therefore, network operators are left
with the option of generating power from alternative power sources and integrating them with
other power subcomponents such as rectifiers, automatic voltage regulator and battery to ensure
the reliability of GSM telecommunication network.
The operational reliability of the transmission and base station subsystem loads at each site is
dependent on the efficiency and performance of the power system model at each site as well as
the conditions of their operating environment. The stability of the power subsystem is technically
required to eliminate failures in the form of current and voltage surges that could lead to
equipment malfunction or failure due to device overheating and distortion which could impair
network reliability.
Additionally, network outages due to power failures that affect different categories of sites such
as the backbone Mobile Switching Center (MSC) sites, intermediate hub sites and terminal base
transceiver stations could impact network performance, undermine network robustness and
degrade the quality of service (QoS) offered by network operator. Hence, informed the need for
the evaluation of a power system driven network reliability.
1.4 PROBLEM FORMULATION
The poor availability and dependability of public power amidst mobile network
expansion compelled the exigency for a supplementary power infrastructure at GSM sites to
support the operation of network transmission and base station equipment so as to guarantee
network reliability and minimize the impact of network failures due to power outages on
customers. The supplementary power infrastructure is an integrated system with a collection of
different power subsystems, which include a transformer, an automatic voltage regulator, two
identical generators, a rectifier system, a battery bank, an automatic transfer switch, and
automatic main failure, all interfaced in a definite topological structure, with redundancy scale
that tolerates faults, allows for operation handover, and permits some degree of equipment
downtime before restoration to optimal efficiency. Not only is the assessment of the rated
capacity and the reliability standard of the major subsystems important to ascertain their
effectiveness, sufficiency or superfluity in the face of maintenance resource scarcity and
allocation challenges, but also the examination of the efficiency of the entire power system
model is imperative to minimize network vulnerability due to fault occurrence and power outage
on operating subsystems on the model. The investigation of the operational ranges of the power
subsystems and their survivability under dynamic nature of ambient and environmental
conditions is critical to assess the reliability status of each subsystem on the model in the context
of their survivability in the operating environment.
Meanwhile, the comprehensive assessment of this power system, in view of the diverse and
hierarchical nature of GSM network site, poses several challenges. Therefore, strategic
methodology needed to be developed to classify network sites in order of integration hierarchical
levels as integrated switch sites, integrated backbone sites, integrated hub sites and terminal end
site, all of which have power models of similar topological structure and functional subsystems,
but with possible variable redundancy scale. Afterward, the model of the dominant operating
power model on the network was formulated as shown in Fig 1.1.
The power infrastructure at KADMSS01 in Fig 1.1 below was adopted as the standard power
model and the representative of the dominant operating power system model on all the network
sites where the study was conducted. The power infrastructure shown below on Fig 1.1 consist of
a transformer, an automatic voltage regulator(AVR), two identical generators, a rectifier system,
a battery bank, an automatic transfer switch (ATS), and automatic main failure (AMF).The
function of each of the above equipment is fully discussed in chapter two under literature review
and theoretical background.
KADMSS01 station is an integrated switch site supporting Mobile Switching Centre (MSC),
Base Station Controller (BSC) and Base station Transceiver (BTS) loads on a common power
system infrastructure that allows for flexible switching and automatic disconnection of low
priority loads during critical power episode when the station loads are supported by limiting
battery power. The power infrastructure at KADMSS01 station is an integrated power system
consisting a transformer, an automatic voltage regulator(AVR), two identical generators, a
rectifier system, a battery bank, an automatic transfer switch (ATS), and automatic main failure
(AMF), all interfaced in a definite topological structure and installed in a unique environment to
support optimal operation and sustain their survivability. The impact of the variation in
redundancy scale for similar subsystems at different network sites was accounted for through the
collection of reliability data samples from a number of different hierarchical sites so as to
provide valid appraisal of the data with the aid of spatial data analytic tool. The examination of
the nature and reliability features of each power subsystem at different network sites was done to
identify similar features and develop strategy to account for the impact of variable features on
the reliability standard of each power subsystem at different hierarchical sites. The assessment of
the operational and ambient conditions of each subsystem was done to appraise the impact of
these conditions on subsystem reliability standards. The effectiveness of the redundancy model
of each of the power subsystem was verified by the evaluation and the appraisal of the reliability
indices of each major subsystem computed based on the nature of its redundancy topology. The
efficiency of the model was justified based on the appraisal of the reliability standards of each
subsystem and the general performance of each subsystem under the influence of their operating
and environmental conditions. The consideration of other potential factors that could flaw or
enhance the redundancy model performance was also used as a metric to rationalize the
efficiency of the model.
1.5 AIMS OF THE STUDY
The aims of the study include:
ï‚· To appraise the efficiency of the power system reliability model of the network and
ascertain the degree of its vulnerability to power outages; its degree of tolerance to fault
occurrence; and the impact of its vulnerability on network outage and customer
satisfaction.
ï‚· To assess the reliability standard of each power subsystems based on their redundancy
model and ascertain whether the reliability results justify the effectiveness of the
redundancy model of each of the power subsystem.
1.6 MOTIVES FOR THE AIMS
The main motives that motivated this aims include:
ï‚· The prevalent challenge of erratic public power supply and the potential impact it poses
on network vulnerability.
ï‚· The unprecedented expansion in network size amidst worsening state of public power and
the potential effect of this phenomenon on the quality of service.
ï‚· The practical challenge of balancing the twin demand of efficient resource allocation,
cost minimization and infrastructure reliability value maximization.
ï‚· The dynamic nature of ambient and environmental conditions and the shutdown impact
of such ecological instability on equipment critical operational
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