Strategies for Reliability Enhancement of Electrical

Distribution Systems

Ph.D. Synopsis

Submitted To

Gujarat Technological University

By

Kela Kalpesh Bansidhar

(Enrollment No: 139997109005)

Guide : Co- Guide : Dr. B N Suthar, Dr. L D Arya Professor & Head Sr. Professor & Head

Electrical Engg Dept. Electrical Engg Dept.

(GEC,Bhuj) (Medi-Caps University, Indore)

Contents

1 Abstract 1

2 State of the art of research topic 2

3 Literature review 3

3.1 Motivation 5

4 Objectives of Research work 6

5 Original contribution of Research work 7

6 Achievements with respect to objectives 9

1

Synopsis

Strategies for Reliability Enhancement of Electrical

Distribution Systems

1. Abstract

The function of an electric power system is to satisfy the load requirement of the system with

proper maintenance of continuity and quality of service. The ability of the system to provide

electricity adequately is usually termed as reliability. The concept of power system reliability

is quite broad and contains various aspects of its ability to satisfy the requirements of

customers. The outages occurring in the system not only impact the revenue economy of the

system but also affects the customers in terms of interruption costs at their ends. In order to

reduce the frequency and duration of these events it becomes necessary to increase investment

either in the better design, operation or both of the system. In other words, reliability of the

system is required to be improved considering costs in mind as reliability and economics play

a major role in decision making.

In the proposed work different strategies have been adopted to enhance reliability of

distribution systems.

The electrical distribution systems are expected to provide continuous and quality electric

service to their customers at a reasonable rate by making economical use of available facilities

and alternatives. To maintain reliable service to customers, a utility may require to intensify

fault avoidance and corrective maintenance measures. This amounts additional budget

allocation. This enhances reliability of radial and meshed electrical distribution systems.

Further, additional fault tolerant measures are not always justified as they require substantial

budget. Depending on the budget allocation, optimized values of reliability indices have been

obtained. It has been observed that the customers look for value-added service from their

utilities. Failures in identifying customer needs may lead to drastic fall in the business of

utilities as the electricity selling market has started becoming competitive. It is a challenging

task for any utility to provide qualitative service to the customers keeping the cost on its

operation and maintenance such as to provide low cost services to them. In this thesis a balance

between the utility cost and cost incurred to the customers due to interruptions have been found

maintaining the required targets of reliability of the system. The optimum value of reliability

with least combined costs have been evaluated. Further, distributed generators (DGs) have been

added at certain load points. Optimum values of customer interruption costs, system

maintenance costs and additional costs on DGs have been found achieving the required

enhancement in reliability of the system. Proper locations of DGs keep significance in the

enhancement of reliability. Proper placements of DGs have been found in this thesis and then

reliability of the system has been optimized considering the above mentioned cost values. A

cost-benefit analysis has been made to verify the possibility of its execution. In the process of

optimizing reliability the additional costs have to be spent by any utility which can be justified

by rendering reward to it by the regulating authority. Optimized values of rewards have been

2

obtained considering customer interruption costs and costs on maintenance of the system. This

has been done attaining required reliability targets. Voltage sag at different load points due to

occurrence of symmetrical and unsymmetrical faults in the system may lead to momentary or

sustained interruption affecting the reliability of the system. The study of power quality

confined to voltage sag has been incorporated in the enhancement of reliability. The solutions

to these different strategies for reliability enhancement have been done by applying soft

computing techniques like Differential evolution, Flower pollination and Teaching learning

based optimization. Comparison has been made between the optimized results obtained by

them and in terms of statistical analysis too.

2. State of the Art of the Research Topic

Economics is an extremely important issue/constraint for deciding threshold values of

reliability indices. Enhanced investment is required to achieve acceptable reliability goals. It is

important that reliability and economics must be treated together in order to perform objective

cost – benefit studies. Reliability improvement of a system is an important problem not only at

the time of designing /planning but also when the system is currently in operation. During

system operation the reliability is improved by preventive maintenance which should be

executed by adequately trained technicians. This further may be improved by replacing

components of the system at suitable length of time. Power companies usually provide various

incentive schemes for the field workers to reduce failure rates and average repair times of the

components. This in turn improves the reliability indices. Cost is an important consideration in

operational reliability optimization of power network. By associating cost values to the

‘reliabilities’ of the system’s component it is possible to determine optimum parameter which

provides desired values of reliability indices at minimum cost functions. Hence one important

step is to obtain a relationship between cost of improvement and reliability. The preferred

approach would be to formulate the cost function from actual cost data. The complete

consideration of reliability economics includes two aspects now generally known as ‘reliability

cost’ and ‘reliability worth’. Reliability cost is considered to be the investment needed to

achieve a certain reliability level, whereas reliability worth is considered to be the monetary

benefit derived by the supplier and customer of such an investment. Reliability worth

assessment is an important function of reliability studies of power system at all stages.

Reliability optimization is possible keeping balance between the two at minimum value of cost

function.

Electric power system may be divided in to functional zones of (i) generation (ii) transmission

and (iii) distribution. These zones have been combined to give three hierarchical levels for

reliability assignment.

The first level [HL I] relates to generation facility, the second level [HL II] involves generation

and transmission facilities and third level [HL III] refers to the complete system including

distribution network.

The HL structure implies that all generation delivers energy through the transmission system.

Whereas now a days significant role is played by an increasingly amount of individually small

scale generation embedded or distributed within distribution system. This affects voltage

3

profile and improves reliability and security of power system. This may affect the economical

operation of conventional generation. An optimum co-ordination must be established between

distributed generations (DG) and centrally located conventional generators. Many researchers

have evaluated reliability of combined generation as well as transmission system [HL II] in a

single problem formulation. It is impractical to evaluate combined reliability of generation,

transmission and distribution system using in a single problem formulation [HLIII]. Usually

distribution system reliability studies are performed separately because (i) distribution network

mostly connected to transmission system through one supply point and the load point indices

evaluated at [HL II] may be used if needed as input values for the reliability evaluation of

distribution network and (ii) on an average 90% interruption or unavailability of customer is

observed due to distribution network [1, 2].

The purpose of investigations in this thesis is to develop computationally efficient algorithms

for reliability optimization of distribution system.

3. Literature review

In its early stage, in 1990 a methodology was developed for evaluating optimal reliability

indices for distribution system by gradient projection method [3]. A new IEE Reliability Test

System (RTS-96) was developed and set of investigations about the bulk reliability

performance evaluation of it were presented by some researchers [4]. Later, an algorithm was

presented by researchers [5] for assessing reliability indices of general distribution system

which presents a practical reliability assessment algorithm for distribution systems of general

network configurations. In [6-12], various researchers have presented different methodologies

for optimum reliability enhancement of distribution network.

As the electricity selling market has started becoming competitive. It is a challenging task for

any utility to provide qualitative service to the customers keeping the cost on its operation and

maintenance such as to provide low cost services to them. The optimum value of system

reliability with least combined cost thus found may lead towards value based reliability

planning of distribution systems [13]. In [14-24], interruptions cost at the customer end have

been focused and also tried for their reduction. Estimation of customer damage function and

total reduction in reliability cost with reference to customer interruption cost have also been

discussed.

Distributed generations (DGs) are becoming the best alternatives for power distribution

companies to increase reliability of distribution systems. DGs enhance performance of the

systems by improving reliability, voltage profile and reducing losses of the system.

Reliability of the distribution system has been improved by DGs at predefined locations [25].

In many literatures the locations of DGs have been found minimizing the loss. But for certain

loads reliability may keep more worth than the loss and voltage profile. In recent years, many

researchers have worked to enhance reliability of distribution systems employing DG. In [26],

it has been proposed a method to determine best locations of DGs based on reliability indices

using sequential Monte Carlo simulation. In [27] authors have assessed the impact of

conventional and renewable distributed generation (DG) on the reliability of distribution

4

system. In [28-36], some authors have proposed allocation method for dispatchable DGs , some

have shown optimal sizing and siting of DGs in large scale distribution systems, studies have

been done regarding allocation of DGs in the presence of storage systems , study of DG

scheduling etc. with reference to reliability enhancement. Optimum cost for doing these have

also been found.

Due to introduction of restructuring in power systems, service quality regulation has become

very important in distribution system. A reward and penalty scheme (RPS) regulates and

ensures the service reliability. It is a financial tool implemented by regulator to maintain service

reliability. A reward and penalty scheme (RPS) penalizes the distribution company for poor

reliability and rewards it for better one in performance based regulation (PBR). In performance

based regulations incentives are decided for strong efficiency (in terms of profit) by the

companies. This may lead to deterioration of quality services to customers [37]. In order to

reach out such conditions, many performance based regulations are embedded with quality

regulations adopting direct or indirect quality controls [38]. In indirect quality control,

customers are provided information regarding quality of performance of distribution

companies while in direct control, the performance is evaluated in terms of financial incentives

provided by the regulators for maintaining adequate service quality [39]. Various researchers

have described different models of RPS with different approaches, their impact on service

reliability, their implementation in specific distribution system etc. in [40-47].

A modern electric power system must be designed to supply acceptable levels of electrical

energy to customers. Simple power quality (PQ) disturbance events, such as voltage sags, may

cause considerable economic losses because industrial processes rely on electronic power-

control devices. Thus, the power supplied to utilities must be reliable and of good quality. In

[48] an analytical method for evaluating the voltage sag performance of a power-supply

distribution network has been presented. The severity of voltage sag, as a result of system fault,

is quantified by its magnitude and duration, and illustrated using a voltage sag density table.

Voltage sags due to power system faults such as single phase-to-ground, phase-to- phase, and

two-phase-to-ground faults are characterized by using symmetrical component analysis and

their effect on the magnitude variation and phase-angle jumps for each phase are examined in

[49]. In [50] statistical analysis is made regarding principal causes of faults in distribution

network. By using the available data, to know the relevant pattern of fault during specific period

is main motive. In [51-59], some papers have discussed regarding power quality issues and

their mitigation. In some papers, network reconfiguration is suggested to improve reliability

and power quality in presence of DGs.

Large number of research papers have appeared in the area of power system reliability related

problems justifying further scope of studies in the same field. A representative literature survey

has been presented in chapter-1 of the thesis, based on which following sub-areas related to

power system reliability have been identified.

(i) Reliability evaluation of generation, transmission and distribution systems in terms

of relevant indices. Large numbers of algorithms have been developed for these

purposes.

(ii) The reliability indices represent the average reliable performance of the network.

5

Hence researchers have developed probability distribution functions of the indices

using MCS based methodologies.

(iii) Reliability cost and worth analysis.

(iv) System expansion planning based on the probabilistic models accounting various

operating constraints.

(v) Reliability enhancement based on configuration modifications and parameter

adjustments.

3.1 Motivation

It has been observed from literature survey that limited work has been done on the development

of quantitative technique for distribution system reliability evaluation and enhancement. As

distribution system provides final link between transmission network and ultimate customers,

it is one of the most important parts of the power system. Due to less cost and localized effect

of outages on it compared to transmission network, it has been given less importance. But the

statistical data in technical reports show that more than 80% of all customer interruptions occur

due to failure/outage in distribution systems only. This requires the distribution system to be

adequately reliable and the need to evaluate its reliability.

Several other aspects are also considered in the need to evaluate the reliability of distribution

systems. A given reinforcements scheme may be relatively inexpensive but large sums of

money are expanded collectively on such system. It is also necessary to ensure balance in the

reliability of generation, transmission and distribution. Another important point of

consideration is to select an option for reliability improvement among the number of

alternatives available. To achieve this, various methods have been developed by researchers

for reliability evaluation of distribution system.

In view of the above mentioned work done by various researchers in reliability evaluation and

enhancement of distribution systems so far, this thesis too covers the work in the same line. In

this thesis, reliability enhancement is done on a sample radial [6] and mesh distribution [63]

network and Roy Billinton Test System (RBTS-2) [64]. The motivation in the present work is

to develop computationally efficient algorithm for reliability optimization using soft computing

techniques [60, 61,62]. Literature survey reveals deficiency in the following aspect of

reliability studies of distribution systems.

(i) Limited research efforts have been made in the area of distribution system

reliability enhancement.

(ii) Inclusion of DG for reliability enhancement. Location of DGs from reliability

point of view. The effects of DG need to be incorporated in optimization

algorithm.

(iii) Inclusion of reward/penalty studies in an optimization algorithm.

(iv) Effect of voltage sag on reliability studies and its inclusion in an optimization

algorithm.

4. Objectives of Research work

The objectives of the proposed research work in this thesis are as follows

6

(1) Defining different methodologies by which reliability of electrical distribution systems

can be enhanced.

(2) Improvement in reliability indices below their target values considering the budget

allocated to achieve the same by developing an algorithm embedding metaheuristic

optimization techniques.

(3) Developing an algorithm to enhance reliability of the distribution system by achieving

proper balance between the cost incurred on customers due to interruptions and the

utility cost to achieve the desired reliability targets.

(4) Enhancing reliability by placing DGs at various locations. Deciding locations of DGs

from the point of view of enhancing reliability optimally by developing methodologies

for both the tasks.

(5) Incorporating reward/penalty imposed to the utility for achieving reliability targets

below/above certain target values. Deciding the optimum value of reward/penalty

corresponding to achieving the desired reliability targets.

(6) Assessment and enhancement of reliability of distribution system considering power

quality (PQ) disturbance events, such as voltage sags for different kind of faults in the

system.

In all the objectives mentioned above, optimized values of primary reliability indices and

customer and energy based reliability indices are to be found so as to set targets for distribution

companies for achieving them.

5. Original contribution of Research work

The thesis includes the work done organised in the following way.

Chapter - 1 presents a critical survey of the past works concerning power system

reliability and clearly spells out the motivations and objectives of the research work carried out

in this thesis.

Chapter - 2 describes a computationally efficient algorithm for reliability optimization

of radial and mesh distribution systems modifying the values of the two decision variables

(failure rate and repair time) of different section of the distribution systems. Here optimization

has been done considering the constraint of allocated budget to enhance reliability. As customer

oriented and energy based reliability indices are in terms of primary indices, they too are

optimized. The optimization has been done by differential evolution (DE) [62], teaching

learning based optimization (TLBO) [61] and flower pollination (FP) [60]. The developed

algorithm has been implemented on a sample radial distribution network, sample mesh

distribution network and Roy Billinton Test System-Bus-2 (RBTS_2) and the results thus

obtained by the three methods have been compared.

Chapter – 3 presents a proposed methodology which shows enhancement of reliability

by optimizing total reliability cost of electrical distribution systems. The total reliability cost

consists of cost incurred by utility and customers both. An objective function in terms of failure

7

rates and repair times i.e. primary reliability indices has been formulated which depicts both

these costs . Hence, optimization of the objective function will give a balance between these

costs with optimized values of primary reliability indices. This optimization has been done

considering the constraints of achieving customer and energy based reliability indices below

threshold/target values. The methodology has been applied on a sample radial network, sample

mesh network and Roy Billinton Test System- Bus 2 (RBTS-2).

Chapter – 4 provides the development of an algorithm for reliability optimization of

electrical distribution system accounting the effect of distributed generation (DG) connected at

load points. Here, an algorithm finding out proper locations for connecting DGs from reliability

point of view has been presented. A cost function which accounts cost of failure rate and repair

time modification and customer interruption cost along with additional cost of expected energy

supplied by DG has been constructed. The effect of DG on reliability and parameter

modifications have been obtained by implementing the developed algorithm on the three

sample systems as before and results have been obtained using DE,TLBO and FP strategies.

Chapter – 5 represents the development of algorithm for reliability optimization of

electrical distribution system incorporating reward/penalty scheme (RPS). Here the cost

function formulated includes cost of reward/penalty. Optimized values of reward/penalty have

been found for the set value of target reliability indices. Optimized values of maintenance cost,

customer interruption cost and additional cost required to be spent by DGs to achieve the

reliability targets have been found by the computational methods in consideration and this

algorithm has been applied on all the systems as considered so far.

Chapter – 6 depicts the algorithm for reliability enhancement considering effect of

power quality disturbance such as voltage sag . For different kinds of fault, the voltage sag

occurring at different load points and substantially its effect on reliability of system has been

considered and optimized values of reliability indices and power quality index have been found

considering the constraints imposed. It has been implemented on the distribution systems

under consideration.

Chapter – 7 highlights the main conclusions and significant contributions of this thesis

and presents scope for future work in the area of distribution system reliability evaluation and

optimization. The methodologies developed in the contributory chapters 2-6 have been

implemented on sample radial and meshed distribution networks and Roy Billinton Test

System -Bus-2 (RBTS-2). The vital findings, contribution of the thesis and future scope are

described in this chapter. The dissertation contains seven chapters namely:

Chapter-1: Introduction

Chapter-2: Application of Metaheuristic Optimization Methods for Reliability

Performance Enhancement of Electrical Distribution Systems

Chapter-3: A Value Based Reliability Optimization of Electrical Distribution

Systems considering Expenditures on Maintenance and Customer

Interruptions

8

Chapter-4: Cost Benefit Analysis for Active Distribution Systems in Reliability

Enhancement

Chapter-5: Optimal Parameter Setting in Distribution System Reliability

Enhancement with Reward and Penalty

Chapter-6: Reliability Enhancement of Distribution Systems considering System

Voltage Sag Performance

Chapter-7: Conclusions and guidelines for future work

6. Achievements with respect to objectives

Some core sample results are tabulated as under

Current and optimized reliability indices and corresponding value of objective function

for meshed distribution system

Sr.

No

.

Index Current

Values

Optimized values Threshold

values

FP DE

1 SAIFI(interruptions/cus

tomer)

0.689895

0.33777

0.33787 0.5000

2 SAIDI(h/customer) 4.854797

1.17374

1.17422 4.0000

3 CAIDI(h/customer

interruption)

7.037003

3.47493

3.47536

5.0000

4 AENS(kW/customer) 20.53386

4.96135

4.96330

10.000

Objective function (J) 1.381518 0.42401 0.42414

Cost incurred(Indian

Rupees)

3844177.164

3807227.210

9

The balance between customer interruption cost and maintenance cost while reliability

optimization is as under.

Current and optimized values of Objective function (J) obtained by FP and DE for

sample radial distribution system

Sr.

No.

Current

Values(Indian

Rupees)

Optimized Values(Indian

Rupees)

FP DE

1 Maintenance cost

(∑ 𝛼𝑘 𝜆𝑘2⁄𝑁𝑐

𝑘=1 + ∑ 𝛽𝑘 𝑟𝑘⁄𝑁𝑐𝑘=1 )

133640 38271.185 37124.837

2 Customer interruption cost

(∑ 𝐶𝐼𝐶𝑁𝑐𝑘=1 )

450920 103905.846

105818.215

3 Objective function (J) 584560 142177.032

142943.053

While adding DGs at certain load points of the same network, the improvement from reliability

point of view can be seen as below.

Current and optimized values of Objective function (F) obtained by FP and DE for

sample radial distribution system

Sr. No.

Current

Values(Indian

Rupees) (((Indian

Rupees)

(Rs.)

Optimized Values(Indian

Rupees))

FP DE

1 Maintenance cost

(∑ 𝛼𝑘 𝜆𝑘2⁄𝑁𝑐

𝑘=1 + ∑ 𝛽𝑘 𝑟𝑘⁄𝑁𝑐𝑘=1 )

133640 31864.70 30338.75

2 Customer interruption cost

(∑ 𝐶𝐼𝐶𝑁𝑐𝑘=1 )

450920 61168.52 86759.55

Addditional cost to be paid while

generators are

connected(ADCOST)

-------- 44529.4021 41058.82

3 Objective function (J) 584560 137562.63 158157.13

10

For specific optimized values of customer and energy based reliability indices, the optimized

values of different segments of cost function while optimizing reliability are as under.

Current and optimized values of Objective function (F) obtained by FP for sample radial

distribution system (For SAIFI= 0.202724, SAIDI=0.90335, CAIDI=4.456051 and

AENS= 3.855498)

Sr. No.

Current

Values

(Indian

Rupees)

Optimized

Values(Indian

Rupees)

FP

1 Maintenance cost (∑ 𝛼𝑘 𝜆𝑘2⁄𝑁𝑐

𝑘=1 + ∑ 𝛽𝑘 𝑟𝑘⁄𝑁𝑐𝑘=1 ) 133640 38130.06

2 Customer interruption cost (∑ 𝐶𝐼𝐶𝑁𝑐𝑘=1 ) 138860 56826.49

3 Additional cost to be paid while generators are

connected(ADCOST) 35115 45109

4 Reward/Penalty (∑ 𝐶𝑅𝑃𝑁𝑐𝑘=1 ) 61688 -20345.2

5 Objective function (F) 369300 119720.4

Here negative value stands for reward and positive stands for penalty.

Analysis is done for all the systems in consideration.

11

List of papers published/communicated

(1) K. B. Kela, B. N. Suthar, and L. D. Arya, ‘Application of Metaheuristic Optimization

Methods for Reliability Enhancement of Meshed Distribution System based on AHP,”

International Journal of Advance Engineering and Research Development., vol.5, no.

1, pp 309-316

(2) K. B. Kela, B. N. Suthar, and L. D. Arya, “Reliability Enhancement of RBTS-2 by Jaya

Optimization Algorithm,”International Journal of Emerging Technology and Advanced

Engineering, vol. 8, no. 2, pp. 71–76, 2018.

(3) K. B. Kela, B. N. Suthar, and L. D. Arya, “Cost Benefit Analysis for Active Distribution

Systems in Reliability Enhancement” communicated to Electric Power Components

and Systems.

(4) K. B. Kela, B. N. Suthar, and L. D. Arya, “A Value Based Reliability Optimization of

Electrical Distribution Systems considering Expenditures on Maintenance and

Customer Interruptions “ communicated to International Journal on Electrical

Engineering and Informatics.

(5) K. B. Kela, B. N. Suthar, and L. D. Arya, “Optimal Parameter Setting in Distribution

System Reliability Enhancement with Reward and Penalty “ communicated to Journal

of Electrical Systems and Informatiom Technology, Elsevier.

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