Description
ABSTRACT
The importance of electricity in everyday life and demands to improve the reliability of distribution systems force utilities to operate and plan their networks in a more secure and economical manner. With higher demands on reliability from both customers and regulators, a big pressure has been put on the security of electricity supply which is considered as a fundamental requirement for modern societies. Thus, efficient solutions for reliability and security of supply improvements are not just of increasing interest, but also have significant socio-economic relevance. 33kv network or distribution system planning (DSP) is one of the major activities of distribution utilities to deal with reliability enhancement.
This study deals with developing optimization algorithms, which aim is to minimize customer interruption costs, and thus maximize the reliability of the system. This is implemented either by decreasing customer interruption duration, frequency of customer interruptions or both. The algorithms are applied on a single or multiple DSP problems. Mixed-integer programming has been used as an optimization approach.
It has been shown that solving and optimizing each one of the DSP problems contributes greatly to the reliability improvement, but brings certain challenges. Moreover, applying algorithms on multiple and integrated DSP problems together leads to even bigger complexity and burdensome. However, going toward this integrated approach results in a more appropriate and realistic DSP model.
The idea behind the optimization is to achieve balance between reliability and the means to achieve this reliability. It is a decision making process, i.e. a trade-off between physical and pricing dimension of security of supply.
List of Acronyms
DAS Distributed automated system
DG Distributed generator
DSP Distribution system planning
FPI Fault passage indicator
GA Genetic algorithm
MIP Mixed-integer programming
MILP Mixed-integer linear programming
MINLP Mixed-integer nonlinear programming
POC Point of connection
TABLE OF CONTENTS
TITLE PAGE
APPROVAL PAGE
DEDICATION
ACKNOWLEDGEMENT
ABSTRACT
CHAPTER ONE
INTRODUCTION
- Background of the project
- Problem statement
1.3 Research objectives
- Significance of the project
- Scope of the project
- Research contributions
- Research Ethics
- Definition of terms
- Thesis Organization
CHAPTER TWO
LITERATURE REVIEW
2.1 Definition security of electricity supply
2.1.1 Long-term Security of Supply
2.1.2 Short-term Security of Supply
2.2 Evaluating Security of Electricity Supply
2.2.1 Customer Outage Cost
2.3 Regulation of Energy Networks
CHAPTER THREE
3.1 IMPROVING SECURITY OF SUPPLY IN DISTRIBUTION SYSTEM
3.1.1 DSP Approaches for Improving Security of Supply
3.1.2 Optimal Feeder Routing
3.1.3 Optimal Switch Placement
3.1.4 Network Reconfiguration
3.1.5 Automation
3.1.6 Distributed Generation
- Optimization Algorithms
- MILP GA
- Optimality Gap
CHAPTER FOUR
- Simulation Results
- Interruption Frequency
- Cable Routing – Base Case
- Cable Routing and Switch Placement
- Tie-switch Placement
- Fault Passage Indicators (FPI) Placement
CHAPTER FIVE
- CONCLUSION AND RECOMMENDATION
- Conclusion
- Recommendation
Chapter one
1.1 Background of the study
Electricity is an essential good and it plays a vital role in everyday life. Today’s society is critically dependent on a power system that is able to provide electricity to its consumers in a reliable and economical way.
The growing share of electricity in final energy demand itself does not fully capture its importance. Electricity has critical linkages with other parts of the energy sector, particularly the oil and gas industry, and underpins the basic activities of the residential, commercial and industrial sectors. As electricity drives increased shares of heating, cooling, transport and many digital sectors of communication, finance, healthcare and others, so the need for adequate electricity security measures escalates.
Electricity security is often referred to using the term “security of supply” or the more literal phrase of “keeping the lights on”. The ultimate goal is to provide electricity to consumers reliably and at reasonable cost. Many threats exist to meeting this objective, ranging from equipment failure and fuel supply shortages, to operational planning failure, human error and deliberate attack. The IEA applies the following definition:
Electricity security is the electricity system’s capability to ensure uninterrupted availability of electricity by withstanding and recovering from disturbances and contingencies.
Electricity security brings together all actions taken – technical, economic and political – to maximise the degree of security in the context of the energy transition, cyber events and climate impacts, both short and long term.
Moreover, demands to improve the reliability of distribution systems force utilities to operate and plan their networks in a more secure and economical manner. Therefore, to study and improve methods for the security of supply has a very high social, economic and energy relevance.
1.2 Statement of the problem
However, the structure of power systems has changed significantly over the past few years, resulting in new challenges for power systems’ planners and operators. As a result of deregulated electricity market scenarios, rapidly increasing demand for electricity and rise in levels of supply volatility due to the significant growth of electricity generation from renewable energy, distribution systems are being operated under very stressed conditions. One of the biggest concerns nowadays is the security of electricity supply. Security of electricity supply has become a fundamental requirement for modern societies. With higher demands on reliability from both customers and regulators, efficient solutions for reliability improvements are of increasing interest.
This work deals with apparatus for improving security of supply of mainly local power systems. The objective is to minimize customer interruption costs and other corresponding measures of reliability performance.
The thesis focuses on developing algorithms that take into account system reliability through distribution system planning (DSP). DSP problems and challenges that are addressed in this licentiate are:
- Cable routing and optimal network layout
- Optimal placement and number of sectionalizing and tie switches
- Effects of distributed automated systems
- Distributed generation
1.4 Scope of the study
The scope of this work covers developing optimization algorithms, which aim is to minimize customer interruption costs, and thus maximize the reliability of the system. This is implemented either by decreasing customer interruption duration, frequency of customer interruptions or both. The algorithms are applied on a single or multiple DSP problems. Mixed-integer programming has been used as an optimization approach.
1.5 Significance of the study
This study shall serve as a means of providing minimized customer interruption costs, and thus maximize the reliability of the system.
This study will serve as a means of deal with decreasing the frequency of customer interruptions and customer interruption duration per fault, using mixed-integer programming optimization.
Research contributions of this thesis can be summarized as follows:
- Defining the concept of Security of Electricity Supply.
- Identifying approaches within distribution system planning for reliability and security of supply
- Justifying use of mixed-integer programming (MIP) as an optimization algorithm over meta-heuristic
- Applying MIP on a single DSP problem with the aim to minimize customer interruption cost by improving interruption frequency.
- Applying MIP on multiple DSP problems with the aim to minimize customer interruption cost by improving interruption frequency.
- Applying MIP on a single DSP problem with the aim to minimize customer interruption cost by improving interruption duration.
Therefore, this study presents successfully developed algorithms for reliability and security of electricity supply improvement, that deal with decreasing the frequency of customer interruptions and customer interruption duration per fault, using mixed-integer programming optimization.
The aim of optimization is not to achieve perfect reliability no matter what, but rather to make decisions in order to allow the network business to maximize long- term profits, while delivering high service levels to the customers with acceptable and manageable risks. Balance between reliability and price is the key in this decision making.
Security of Electricity Supply is a subject that significantly affects society, economy and energy. The ethical aspect of this thesis is taken into account as far as the research leads towards minimizing customer interruption costs and other corresponding measures of reliability performance, using monitoring control devices. Moreover, optimization models are designed and modelled with certain simplifications and assumptions in order to respect power utility data privacy requirements and customer information. Research is conducted through computer simulations, causing no harm to the environment.
1.8 Definition of terms
Adequacy: The ability of the electricity system to supply the aggregate electrical demand within an area at all times under normal operating conditions. The precise definition of what qualifies as normal conditions and understanding how the system copes with other situations is key in policy decisions.
Operational security: The ability of the electricity system to retain a normal state or to return to a normal state after any is key in policy decisions.
Resilience: The ability of the system and its component parts to absorb, accommodate and recover from both short-term shocks and long-term changes. These shocks can go beyond conditions covered in standard adequacy assessments.
This thesis is organized in seven chapters as follows:
- Chapter 1 provides the background, motivation, objective, scope and main contributions of the research
- Chapter 2 defines Security of Electricity Supply and means to evaluate
- Chapter 3 reviews DSP approaches for improving Security of Supply and ap- plied optimization algorithms. A base case of MIP algorithm that deals with Security of Supply improvement is presented in this chapter. The proposed MIP is also compared to one of the meta-heuristic algorithms to highlight its effectiveness within
- Chapter 4 presents refined versions of the developed algorithm for decreasing the frequency of customer interruptions within the cable routing problem, and focus on decreasing the customer interruption duration per fault by considering different DSP approaches for Security of Supply
- Chapter 5 concludes the work and outlines the future
CHAPTER FIVE
5.0 CONCLUSION AND RECOMMENDATION
5.1 CONCLUSION
This thesis deals with apparatus for improving security of electricity supply in distribution power systems. Developed algorithms take into account system reliability through different DSP approaches. The objective is to minimize customer interruption costs, either by decreasing customer interruption duration, frequency of customer interruptions or both.
DSP problem of cable routing and finding the optimal cable layout has been tackled in papers II, III and IV. It has been shown that it is possible to simultaneously update failure rate of every node while deciding on the cable outline. Base case of this problem has been shown in Paper II. Base case includes cable routing with power transfer capacity, radial and reliability constraints. Paper II also com- pares MIP with meta-heuristic GA optimization algorithm in solving this problem. It has been shown that GA can provide fast results, but has difficulties in handling constraints – therefore, optimal solution might not be obtained. Thus, Paper II justifies the use of MIP algorithm applied on DSP problems, since getting a more optimal result is much more important than getting a fast answer.
The idea of every algorithm is to create conditions that resemble the real network as much as possible. However, certain assumptions and simplifications are always presented in models. Nevertheless, algorithms can evolve by introducing more constraints and integrating multiple problems together.
The base case algorithm for cable routing problem has been refined in Paper IV by introducing different cable option and power quality constraints. In this aspect, more constraints have made the solution range narrower and tighter, enabling the optimization to find the optimal result fast.
Another refinement of base case algorithm has been presented in Paper III where two different DSP problems have been merged. Namely, cable layout has been simultaneously optimized with placement of reclosers. The combination of these two DSP problems has brought the biggest challenge, making the optimization more demanding, execution time longer and the convergence slower. Therefore, certain relaxation has been introduced that helped in achieving reasonable solution results within reasonable time, even for bigger system. Setting the minimum number of reclosers to a specific number minSwNo has limited the optimization to find the true optimal number of reclosers. However, this relaxation has enabled fast sensitivity analysis where different values of minSwNo have been included and tested.
The developed algorithms cannot support simultaneous update of both failure rate and restoration time. Rather, this can be done in stages, as presented in Paper
III. Usually, restoration time of every node needs to be decided when failure rate, and therefore cable layout, is known. Moreover, restoration time is affected by installing different equipment in the network, i.e. tie-switches (Paper III) or FPIs (Paper V). The most important part of the algorithm is finding interedependencies between nodes. These interdependencies are expressed through r(i, j) – restoration time of node j due to fault at node i. r(i, j) can be seen as a set of rules that differs for every optimization model and depends greatly on input network parameters, equipment that is going to be installed in the network, and therefore, on the nature of the DSP problem. However, once the interdependecies are defined, unavailability of each node, and therefore restoration time of each node can be updated.
5.2 Recommendation
Distribution utilities need to adhere to energy regulations in order to maintain the optimal level of security of supply. Recently, the revenue framework has made interruptions more costly [30]. Moreover, regulatory authorities are encouraging distribution automation enforcement through the economic incentives. Thus, it would be interesting to see the effects of distribution automation in the system on a more detailed level, with an accent on communication and remote control equipment.
The developed algorithm for improving customer interruption duration has posed new opportunities. Beside finding the optimal location of tie switches and FPIs, the algorithm can be used for optimal DG placement in the distribution network. However, beside reliability improvement, DG placement brings different challenges, such as multi-directional power flow, voltage stability, maintaining certain level of power quality, etc. Thus, it would be interesting to develop the algorithm that takes into account all mentioned issues and solves them accordingly. Nowadays, interesting shift from distribution utilities to customers is happening.
According to [31], by 2030, customer investment in distributed energy resources (DERs) is expected to increase tenfold to over $2 trillion, nearly three times greater than today’s global utility investment in generation, transmission, and distribution assets. Electric vehicles, peer-to-peer energy trading and block chain platforms are becoming popular in energy systems. Thus, it would be interesting to investigate the effects of these changes on security of electricity supply.
