Pavlos Georgilakis | National Technical University of Athens (original) (raw)

Papers by Pavlos Georgilakis

The scope of this study is the optimal siting and sizing of distributed generation within a power... more The scope of this study is the optimal siting and sizing of distributed generation within a power distribution network considering uncertainties. A probabilistic power flow (PPF)-embedded genetic algorithm (GA)-based approach is proposed in order to solve the optimisation problem that is modelled mathematically under a chance constrained programming framework. Point estimate method (PEM) is proposed for the solution of the involved PPF problem. The uncertainties considered include: (i) the future load growth in the power distribution system, (ii) the wind generation, (iii) the output power of photovoltaics, (iv) the fuel costs and (v) the electricity prices. Based on some candidate schemes of different distributed generation types and sizes, placed on specific candidate buses of the network, GA is applied in order to find the optimal plan. The proposed GA with embedded PEM (GA–PEM) is applied on the IEEE 33-bus network by considering several scenarios and is compared with the method of GA with embedded Monte Carlo simulation (GA–MCS). The main conclusions of this comparison are: (i) the proposed GA–PEM is seven times faster than GA–MCS, and (ii) both methods provide almost identical results.

This paper proposes a model for the computation of core losses and excitation current, in a lamin... more This paper proposes a model for the computation of core losses and excitation current, in a lamination by lamination method, for wound core distribution transformers. The model was developed based on the finite-element method (FEM). The results obtained by applying the proposed model were compared with the FEM results and with the measurements of the no-load test. The no-load losses obtained by the proposed model present a difference of 4% with respect to measured values, while they are almost the same with respect to FEM. The proposed model contributes in the research of new techniques that improve transformer design.

This paper proposes an efficient approach to transmission fixed cost allocation in a pool based e... more This paper proposes an efficient approach to transmission fixed cost allocation in a pool based electricity market that takes into consideration N-1 secure annual system operation. The optimal capacity of a transmission facility is defined as the maximum power flow the facility may face under a contingency situation for a certain system snapshot. In this pricing approach, for each time interval, the largest optimal capacity of a transmission facility is calculated for all N-1 conditions considering a security constrained optimal power flow. Charges for each facility are based on the facility usage of the largest optimal capacity for all time intervals during one or more years. Network usage is determined by generalized distribution factors and three variations of the MW-Mile method for pricing counter-flows are investigated for the proposed cost allocation method. The three proposed pricing methods are applied to the IEEE 24-bus reliability test system and compared with other pricing methods.

This paper proposes an efficient approach for probabilistic transmission expansion planning (TEP)... more This paper proposes an efficient approach for probabilistic transmission expansion planning (TEP) that considers load and wind power generation uncertainties. The Benders decomposition algorithm in conjunction with Monte Carlo simulation is used to tackle the proposed probabilistic TEP. An upper bound on total load shedding is introduced in order to obtain network solutions that have an acceptable probability of load curtailment. The proposed approach is applied on Garver six-bus test system and on IEEE 24-bus reliability test system. The effect of contingency analysis , load and mainly wind production uncertainties on network expansion configurations and costs is investigated. It is shown that the method presented can be used effectively to study the effect of increasing wind power integration on TEP of systems with high wind generation uncertainties.

This article compares the total mass and the total owning cost (TOC) of three-phase distribution ... more This article compares the total mass and the total owning cost (TOC) of three-phase distribution transformer banks with standard three-phase distribution transformers. The comparison is based on the minimum TOC. This is achieved through a field-validated distribution transformer design program that automatically minimises the objective function (TOC). In particular, 12 oil-immersed distribution transformers are designed: 6 three-phase transformer banks and 6 three-phase transformers; these designs meet all the requirements of a given transformer standard. As a result, curves of minimum TOC versus transformer rating are obtained for three-phase transformer banks and three-phase transformers. Moreover, similar curves from seven transformer manufacturers are collected; the advantage of this collection is that these different manufacturers have different types of transformers: oil immersed or dry type, core or shell type, various voltage classes and power ratings, and so on, and consequently more general conclusions can be drawn regarding the comparison of three-phase transformer banks and three-phase transformers. From these investigations, it was found that from the viewpoint of minimum total mass and minimum TOC, three-phase transformer banks should be recommended in case of small-size transformers (rating lower than 45 kVA). This is an important finding that is not emphasised in recommended practices reported in transformer textbooks.

Utilization of phasor measurement units (PMUs) in the monitoring, protection and control of power... more Utilization of phasor measurement units (PMUs) in the monitoring, protection and control of power systems has become increasingly important in recent years. The aim of the optimal PMU placement (OPP) problem is to provide the minimal PMU installations to ensure full observability of the power system. Several methods, based on mathematical and heuristic algorithms, have been suggested for the OPP problem. This paper presents a thorough description of the state of the art of the optimization methods applied to the OPP problem, analyzing and classifying current and future research trends in this field.

Phasor measurement units (PMUs) are essential tools for monitoring, protection and control of pow... more Phasor measurement units (PMUs) are essential tools for monitoring, protection and control of power systems. The optimal PMU placement (OPP) problem refers to the determination of the minimal number of PMUs and their corresponding locations in order to achieve full network observability. This paper introduces a recursive Tabu search (RTS) method to solve the OPP problem. More specifically, the traditional Tabu search (TS) metaheuristic algorithm is executed multiple times, while in the initialisation of each TS the best solution found from all previous executions is used. The proposed RTS is found to be the best among three alternative TS initialisation schemes, in regard to the impact on the success rate of the algorithm. A numerical method is proposed for checking network observability, unlike most existing metaheuristic OPP methods, which are based on topological observability methods. The proposed RTS method is tested on the IEEE 14, 30, 57 and 118-bus test systems, on the New England 39-bus test system and on the 2383-bus power system. The obtained results are compared with other reported PMU placement methods. The simulation results show that the proposed RTS method finds the minimum number of PMUs, unlike earlier methods which may find either the same or even higher number of PMUs.

In this paper, the sensitivity analysis is used to select the core lamination thickness of single... more In this paper, the sensitivity analysis is used to select the core lamination thickness of single-phase distribution transformers rated from 5 to 50 kVA. Three different magnetic materials (M2, M3 and M4) with thicknesses of 0.18, 0.23 and 0.27 mm are considered. Transformer designs are compared based on the total owning cost as well as on the transformer bid price. The impact of the different laminations on total owning cost and bid price is calculated for a total of 144 transformers (72 for each criterion). All transformers fulfill all the operating and construction constraints. The paper considers the impact on core losses of the space factor (core-assembling pressure) and of the building factor and also describes how core losses are affected by core design parameters such as the number of laminations per step, air gap and overlap. It is concluded that for the analyzed power range, M3 lamination is the best choice since all of the studied cases have smaller bid price and 79% of the studied cases have lower total owning cost. This paper gives guidelines to select the appropriate thickness and can help transformer manufacturers to select the optimal thickness for distribution transformers.

The analysis and design of a small isolated power system (SIPS) with renewable energy sources (RE... more The analysis and design of a small isolated power system (SIPS) with renewable energy sources (RES) and storage can be challenging, because of the large number of design options and the uncertainty in key parameters. RES add further complexity because their power output may be intermittent, seasonal and non-dispatchable. Owing to this characteristic, reliability evaluation of a RES-based SIPS cannot be implemented using the traditional deterministic and analytical methods. Moreover, this evaluation has to be done within a cost-benefit framework. This study models and investigates the effect of customer worth of interrupted supply (customer damage cost) on the optimal design of SIPS with storage and increased RES penetration. The SIPS optimal design is implemented with a genetic algorithm combined with local search procedure. In addition, this study examines the effect of the forced outage rate of SIPS components on SIPS optimal design via Monte Carlo simulation. The performance of the proposed hybrid optimisation methodology is investigated for a large number of alternative scenarios via sensitivity analysis, which study the effect on the results because of the uncertainty on weather data, components efficiency and cost data. The results show that the optimal design of a RES-based SIPS depends largely on the consideration of customer damage cost as well as the inclusion of components forced outage rate. The method and results presented in this article should be valuable in planning and operating SIPS with RES.

Small autonomous power systems (SAPS) that include renewable energy sources are a promising optio... more Small autonomous power systems (SAPS) that include renewable energy sources are a promising option for isolated power generation at remote locations. The optimal sizing problem of SAPS is a challenging combinatorial optimization problem, and its solution may prove a very time-consuming process. This paper initially investigates the performance of two popular metaheuristic methods, namely, simulated annealing (SA) and tabu search (TS), for the solution of SAPS optimal sizing problem. Moreover, this paper proposes a hybrid SA-TS method that combines the advantages of each one of the above-mentioned metaheuristic methods. The proposed method has been successfully applied to design an SAPS in Chania region, Greece. In the study, the objective function is the minimization of SAPS cost of energy (€/kWh), and the design variables are: 1) wind turbines size, 2) photovoltaics size, 3) diesel generator size, 4) biodiesel generator size, 5) fuel cells size, 6) batteries size, 7) converter size, and 8) dispatch strategy. The performance of the proposed hybrid optimization methodology is studied for a large number of alternative scenarios via sensitivity analysis, and the conclusion is that the proposed hybrid SA-TS improves the obtained solutions, in terms of quality and convergence, compared to the solutions provided by individual SA or individual TS methods.

This paper presents an analysis of the bibliography on transformers covering the period from 1990... more This paper presents an analysis of the bibliography on transformers covering the period from 1990 to 2000. It contains all the transformer subjects: a) Transformer design, b) Transformer protection, c) Transformer connections, d) Transformer diagnostics, e) Transformer failures, f) Transient analysis of transformers (overvoltages, overcurrents), g) Modeling and analysis of transformer using FEM (thermal modeling, losses modeling, insulation modeling, windings mod-eling). Several international journals were investigated including the following: Systems, and IET Generation Transmission & Distribution. Due to the high number of publication in journals, we are not considering publications of conferences and symposia. A total of 700 publications are analyzed in this paper. The research presented in this paper is important because it contains and analyzes the best research papers on transformers coming from many countries all over the world and published in top rated scientific electrical engineering journals.

This paper presents a method for the design of shell-type, single-phase distribution transformers... more This paper presents a method for the design of shell-type, single-phase distribution transformers to obtain the manufacturing specifications. The method is simple, efficient and accurate. By an exhaustive analysis, it is concluded that the obtained solution is the global optimum. The following constraints are imposed: excitation current, no-load losses, total losses, impedance and efficiency. The methodology of this paper requires only six input data: transformer rating, low voltage, high voltage, connection of low-voltage coil, connection of high-voltage coil, and frequency. These data are included in the transformer nameplate. In this paper, the minimization of the following four objective functions is considered: total owing cost, mass, total losses and material cost. The consideration of these four objective functions is implemented automatically by running the optimization algorithm four times without intervention of a designer. Consequently, transformer manufacturers save design man-hours and increase capacity. A design example on a 25 kVA transformer is presented for illustration. The optimized solutions of transformer design are validated with laboratory and process measurements.

Unit commitment (UC) is a very important optimization task, which plays a major role in the daily... more Unit commitment (UC) is a very important optimization task, which plays a major role in the daily operation planning of electric power systems that is why UC is a core research topic attracting a lot of research efforts. An innovative method based on an advanced memetic algorithm (MA) for the solution of price based unit commitment (PBUC) problem is proposed. The main contributions of this paper are: (i) an innovative two-level tournament selection, (ii) a new multiple window crossover, (iii) a novel window in window mutation operator, (iv) an innovative local search scheme called elite mutation, (v) new population initialization algorithm that is specific to PBUC problem, and (vi) new PBUC test systems including ramp up and ramp down constraints so as to provide new PBUC benchmarks for future research. The innovative two-level tournament selection mechanism contributes to the reduction of the required CPU time. The method has been applied to systems of up to 110 units and the results show that the proposed memetic algorithm is superior to other methods since it finds the optimal solution with a high success rate and within a reasonable execution time.

Improvements in energy efficiency of electrical equipment reduce the greenhouse gas (GHG) emissio... more Improvements in energy efficiency of electrical equipment reduce the greenhouse gas (GHG) emissions and contribute to the protection of the environment. Moreover, as system investment and energy costs continue to increase, electric utilities are increasingly interested in installing energy-efficient transformers at their distribution networks. This paper analyzes the impact of the environmental cost of transformer losses on the economic evaluation of distribution transformers. This environmental cost is coming form the cost to buy GHG emission credits because of the GHG emissions associated with supplying transformer losses throughout the transformer lifetime. Application results on the Hellenic power system for 21 transformer offers under 9 different scenarios indicate that the environmental cost of transformer losses can reach on average 34% and 8% of transformer purchasing price for high loss and medium loss transformers, respectively. That is why it is important to incorporate the environmental cost of transformer losses into the economic evaluation of distribution transformers.

Generation expansion planning of Crete power system for high penetration of renewable energy sour... more Generation expansion planning of Crete power system for high penetration of renewable energy sources," Abstract. In this paper the Crete's Island power system, which is the largest isolated power system in Greece, is analyzed in two long term scenarios in order to estimate the corresponding costs and benefits associated with a significant high electricity production from renewable energy sources (RES) technologies in the period 2009-2020. In the first scenario, a 20% RES energy penetration in year 2020 is assumed, while in the second scenario the final RES energy penetration is increased to 50%, and it is achieved with the installation of hydro pumped storage systems. Long-range Energy Alternatives Planning (LEAP) software is used to develop the electricity demand model, as well as to estimate the gross electricity generation in Crete and the annual CO 2 equivalent emissions for the considered scenarios. This study demonstrates that substantial RES production till 2020 is technically feasible, and provides benefits in the forms of carbon emission reductions, energy adequacy and dependency.

Electricity market activities and a growing demand for electricity have led to heavily stressed p... more Electricity market activities and a growing demand for electricity have led to heavily stressed power systems. This requires operation of the networks closer to their stability limits. Cost effective solutions are preferred over network extensions. The flexible alternating current transmission system (FACTS), a new technology based on power electronics, offers an opportunity to enhance controllability, stability, and power transfer capability of ac transmission systems. This paper provides a comprehensive review and evaluation of FACTS controllers.

This paper introduces a new general methodology for the modeling and reliability evaluation of sm... more This paper introduces a new general methodology for the modeling and reliability evaluation of small isolated power systems, which include wind turbines, photovoltaics, and diesel generators, based on fluid stochastic Petri nets (FSPNs). The proposed methodology presents two major novelties in FSPN modeling, namely: 1) the introduction of a new kind of Petri net arc, called the database arc, which makes possible the direct import of real data in the simulation process; and 2) the selection of constant time intervals in FSPN modeling, instead of assuming continuous dynamics defined by the change of fluid level over time. Moreover, in order to construct the overall system model, this paper proposes a general framework for modular representation of the system under study following a number of well-defined steps. The obtained model is fully parameterized and compared to classical simulation methods, it provides to its user the additional advantage of graphical representation of system's components and attributes. Four scenarios, which describe power system's performance under different conditions, were implemented. For each one of the developed scenarios, nine reliability and performance indexes have been calculated and compared.

This paper proposes a stochastic optimisation method, based on Ant Colony Optimisation (ACO), for... more This paper proposes a stochastic optimisation method, based on Ant Colony Optimisation (ACO), for the optimal choice of transformer sizes to be installed in a distribution network. This method is properly introduced to the solution of the Optimal Transformer Sizing (OTS) problem, taking into account the constraints imposed by the load the transformer serves throughout its lifetime and the possible transformer thermal overloading. The possibility to upgrade the transformer size one or more times throughout the study period results in different sizing paths taking into account the transformer capital cost the energy loss cost during the study period.

The scope of this study is the optimal siting and sizing of distributed generation within a power... more The scope of this study is the optimal siting and sizing of distributed generation within a power distribution network considering uncertainties. A probabilistic power flow (PPF)-embedded genetic algorithm (GA)-based approach is proposed in order to solve the optimisation problem that is modelled mathematically under a chance constrained programming framework. Point estimate method (PEM) is proposed for the solution of the involved PPF problem. The uncertainties considered include: (i) the future load growth in the power distribution system, (ii) the wind generation, (iii) the output power of photovoltaics, (iv) the fuel costs and (v) the electricity prices. Based on some candidate schemes of different distributed generation types and sizes, placed on specific candidate buses of the network, GA is applied in order to find the optimal plan. The proposed GA with embedded PEM (GA–PEM) is applied on the IEEE 33-bus network by considering several scenarios and is compared with the method of GA with embedded Monte Carlo simulation (GA–MCS). The main conclusions of this comparison are: (i) the proposed GA–PEM is seven times faster than GA–MCS, and (ii) both methods provide almost identical results.

This paper proposes a model for the computation of core losses and excitation current, in a lamin... more This paper proposes a model for the computation of core losses and excitation current, in a lamination by lamination method, for wound core distribution transformers. The model was developed based on the finite-element method (FEM). The results obtained by applying the proposed model were compared with the FEM results and with the measurements of the no-load test. The no-load losses obtained by the proposed model present a difference of 4% with respect to measured values, while they are almost the same with respect to FEM. The proposed model contributes in the research of new techniques that improve transformer design.

This paper proposes an efficient approach to transmission fixed cost allocation in a pool based e... more This paper proposes an efficient approach to transmission fixed cost allocation in a pool based electricity market that takes into consideration N-1 secure annual system operation. The optimal capacity of a transmission facility is defined as the maximum power flow the facility may face under a contingency situation for a certain system snapshot. In this pricing approach, for each time interval, the largest optimal capacity of a transmission facility is calculated for all N-1 conditions considering a security constrained optimal power flow. Charges for each facility are based on the facility usage of the largest optimal capacity for all time intervals during one or more years. Network usage is determined by generalized distribution factors and three variations of the MW-Mile method for pricing counter-flows are investigated for the proposed cost allocation method. The three proposed pricing methods are applied to the IEEE 24-bus reliability test system and compared with other pricing methods.

This paper proposes an efficient approach for probabilistic transmission expansion planning (TEP)... more This paper proposes an efficient approach for probabilistic transmission expansion planning (TEP) that considers load and wind power generation uncertainties. The Benders decomposition algorithm in conjunction with Monte Carlo simulation is used to tackle the proposed probabilistic TEP. An upper bound on total load shedding is introduced in order to obtain network solutions that have an acceptable probability of load curtailment. The proposed approach is applied on Garver six-bus test system and on IEEE 24-bus reliability test system. The effect of contingency analysis , load and mainly wind production uncertainties on network expansion configurations and costs is investigated. It is shown that the method presented can be used effectively to study the effect of increasing wind power integration on TEP of systems with high wind generation uncertainties.

This article compares the total mass and the total owning cost (TOC) of three-phase distribution ... more This article compares the total mass and the total owning cost (TOC) of three-phase distribution transformer banks with standard three-phase distribution transformers. The comparison is based on the minimum TOC. This is achieved through a field-validated distribution transformer design program that automatically minimises the objective function (TOC). In particular, 12 oil-immersed distribution transformers are designed: 6 three-phase transformer banks and 6 three-phase transformers; these designs meet all the requirements of a given transformer standard. As a result, curves of minimum TOC versus transformer rating are obtained for three-phase transformer banks and three-phase transformers. Moreover, similar curves from seven transformer manufacturers are collected; the advantage of this collection is that these different manufacturers have different types of transformers: oil immersed or dry type, core or shell type, various voltage classes and power ratings, and so on, and consequently more general conclusions can be drawn regarding the comparison of three-phase transformer banks and three-phase transformers. From these investigations, it was found that from the viewpoint of minimum total mass and minimum TOC, three-phase transformer banks should be recommended in case of small-size transformers (rating lower than 45 kVA). This is an important finding that is not emphasised in recommended practices reported in transformer textbooks.

Utilization of phasor measurement units (PMUs) in the monitoring, protection and control of power... more Utilization of phasor measurement units (PMUs) in the monitoring, protection and control of power systems has become increasingly important in recent years. The aim of the optimal PMU placement (OPP) problem is to provide the minimal PMU installations to ensure full observability of the power system. Several methods, based on mathematical and heuristic algorithms, have been suggested for the OPP problem. This paper presents a thorough description of the state of the art of the optimization methods applied to the OPP problem, analyzing and classifying current and future research trends in this field.

Phasor measurement units (PMUs) are essential tools for monitoring, protection and control of pow... more Phasor measurement units (PMUs) are essential tools for monitoring, protection and control of power systems. The optimal PMU placement (OPP) problem refers to the determination of the minimal number of PMUs and their corresponding locations in order to achieve full network observability. This paper introduces a recursive Tabu search (RTS) method to solve the OPP problem. More specifically, the traditional Tabu search (TS) metaheuristic algorithm is executed multiple times, while in the initialisation of each TS the best solution found from all previous executions is used. The proposed RTS is found to be the best among three alternative TS initialisation schemes, in regard to the impact on the success rate of the algorithm. A numerical method is proposed for checking network observability, unlike most existing metaheuristic OPP methods, which are based on topological observability methods. The proposed RTS method is tested on the IEEE 14, 30, 57 and 118-bus test systems, on the New England 39-bus test system and on the 2383-bus power system. The obtained results are compared with other reported PMU placement methods. The simulation results show that the proposed RTS method finds the minimum number of PMUs, unlike earlier methods which may find either the same or even higher number of PMUs.

In this paper, the sensitivity analysis is used to select the core lamination thickness of single... more In this paper, the sensitivity analysis is used to select the core lamination thickness of single-phase distribution transformers rated from 5 to 50 kVA. Three different magnetic materials (M2, M3 and M4) with thicknesses of 0.18, 0.23 and 0.27 mm are considered. Transformer designs are compared based on the total owning cost as well as on the transformer bid price. The impact of the different laminations on total owning cost and bid price is calculated for a total of 144 transformers (72 for each criterion). All transformers fulfill all the operating and construction constraints. The paper considers the impact on core losses of the space factor (core-assembling pressure) and of the building factor and also describes how core losses are affected by core design parameters such as the number of laminations per step, air gap and overlap. It is concluded that for the analyzed power range, M3 lamination is the best choice since all of the studied cases have smaller bid price and 79% of the studied cases have lower total owning cost. This paper gives guidelines to select the appropriate thickness and can help transformer manufacturers to select the optimal thickness for distribution transformers.

The analysis and design of a small isolated power system (SIPS) with renewable energy sources (RE... more The analysis and design of a small isolated power system (SIPS) with renewable energy sources (RES) and storage can be challenging, because of the large number of design options and the uncertainty in key parameters. RES add further complexity because their power output may be intermittent, seasonal and non-dispatchable. Owing to this characteristic, reliability evaluation of a RES-based SIPS cannot be implemented using the traditional deterministic and analytical methods. Moreover, this evaluation has to be done within a cost-benefit framework. This study models and investigates the effect of customer worth of interrupted supply (customer damage cost) on the optimal design of SIPS with storage and increased RES penetration. The SIPS optimal design is implemented with a genetic algorithm combined with local search procedure. In addition, this study examines the effect of the forced outage rate of SIPS components on SIPS optimal design via Monte Carlo simulation. The performance of the proposed hybrid optimisation methodology is investigated for a large number of alternative scenarios via sensitivity analysis, which study the effect on the results because of the uncertainty on weather data, components efficiency and cost data. The results show that the optimal design of a RES-based SIPS depends largely on the consideration of customer damage cost as well as the inclusion of components forced outage rate. The method and results presented in this article should be valuable in planning and operating SIPS with RES.

Small autonomous power systems (SAPS) that include renewable energy sources are a promising optio... more Small autonomous power systems (SAPS) that include renewable energy sources are a promising option for isolated power generation at remote locations. The optimal sizing problem of SAPS is a challenging combinatorial optimization problem, and its solution may prove a very time-consuming process. This paper initially investigates the performance of two popular metaheuristic methods, namely, simulated annealing (SA) and tabu search (TS), for the solution of SAPS optimal sizing problem. Moreover, this paper proposes a hybrid SA-TS method that combines the advantages of each one of the above-mentioned metaheuristic methods. The proposed method has been successfully applied to design an SAPS in Chania region, Greece. In the study, the objective function is the minimization of SAPS cost of energy (€/kWh), and the design variables are: 1) wind turbines size, 2) photovoltaics size, 3) diesel generator size, 4) biodiesel generator size, 5) fuel cells size, 6) batteries size, 7) converter size, and 8) dispatch strategy. The performance of the proposed hybrid optimization methodology is studied for a large number of alternative scenarios via sensitivity analysis, and the conclusion is that the proposed hybrid SA-TS improves the obtained solutions, in terms of quality and convergence, compared to the solutions provided by individual SA or individual TS methods.

This paper presents an analysis of the bibliography on transformers covering the period from 1990... more This paper presents an analysis of the bibliography on transformers covering the period from 1990 to 2000. It contains all the transformer subjects: a) Transformer design, b) Transformer protection, c) Transformer connections, d) Transformer diagnostics, e) Transformer failures, f) Transient analysis of transformers (overvoltages, overcurrents), g) Modeling and analysis of transformer using FEM (thermal modeling, losses modeling, insulation modeling, windings mod-eling). Several international journals were investigated including the following: Systems, and IET Generation Transmission & Distribution. Due to the high number of publication in journals, we are not considering publications of conferences and symposia. A total of 700 publications are analyzed in this paper. The research presented in this paper is important because it contains and analyzes the best research papers on transformers coming from many countries all over the world and published in top rated scientific electrical engineering journals.

This paper presents a method for the design of shell-type, single-phase distribution transformers... more This paper presents a method for the design of shell-type, single-phase distribution transformers to obtain the manufacturing specifications. The method is simple, efficient and accurate. By an exhaustive analysis, it is concluded that the obtained solution is the global optimum. The following constraints are imposed: excitation current, no-load losses, total losses, impedance and efficiency. The methodology of this paper requires only six input data: transformer rating, low voltage, high voltage, connection of low-voltage coil, connection of high-voltage coil, and frequency. These data are included in the transformer nameplate. In this paper, the minimization of the following four objective functions is considered: total owing cost, mass, total losses and material cost. The consideration of these four objective functions is implemented automatically by running the optimization algorithm four times without intervention of a designer. Consequently, transformer manufacturers save design man-hours and increase capacity. A design example on a 25 kVA transformer is presented for illustration. The optimized solutions of transformer design are validated with laboratory and process measurements.

Unit commitment (UC) is a very important optimization task, which plays a major role in the daily... more Unit commitment (UC) is a very important optimization task, which plays a major role in the daily operation planning of electric power systems that is why UC is a core research topic attracting a lot of research efforts. An innovative method based on an advanced memetic algorithm (MA) for the solution of price based unit commitment (PBUC) problem is proposed. The main contributions of this paper are: (i) an innovative two-level tournament selection, (ii) a new multiple window crossover, (iii) a novel window in window mutation operator, (iv) an innovative local search scheme called elite mutation, (v) new population initialization algorithm that is specific to PBUC problem, and (vi) new PBUC test systems including ramp up and ramp down constraints so as to provide new PBUC benchmarks for future research. The innovative two-level tournament selection mechanism contributes to the reduction of the required CPU time. The method has been applied to systems of up to 110 units and the results show that the proposed memetic algorithm is superior to other methods since it finds the optimal solution with a high success rate and within a reasonable execution time.

Improvements in energy efficiency of electrical equipment reduce the greenhouse gas (GHG) emissio... more Improvements in energy efficiency of electrical equipment reduce the greenhouse gas (GHG) emissions and contribute to the protection of the environment. Moreover, as system investment and energy costs continue to increase, electric utilities are increasingly interested in installing energy-efficient transformers at their distribution networks. This paper analyzes the impact of the environmental cost of transformer losses on the economic evaluation of distribution transformers. This environmental cost is coming form the cost to buy GHG emission credits because of the GHG emissions associated with supplying transformer losses throughout the transformer lifetime. Application results on the Hellenic power system for 21 transformer offers under 9 different scenarios indicate that the environmental cost of transformer losses can reach on average 34% and 8% of transformer purchasing price for high loss and medium loss transformers, respectively. That is why it is important to incorporate the environmental cost of transformer losses into the economic evaluation of distribution transformers.

Generation expansion planning of Crete power system for high penetration of renewable energy sour... more Generation expansion planning of Crete power system for high penetration of renewable energy sources," Abstract. In this paper the Crete's Island power system, which is the largest isolated power system in Greece, is analyzed in two long term scenarios in order to estimate the corresponding costs and benefits associated with a significant high electricity production from renewable energy sources (RES) technologies in the period 2009-2020. In the first scenario, a 20% RES energy penetration in year 2020 is assumed, while in the second scenario the final RES energy penetration is increased to 50%, and it is achieved with the installation of hydro pumped storage systems. Long-range Energy Alternatives Planning (LEAP) software is used to develop the electricity demand model, as well as to estimate the gross electricity generation in Crete and the annual CO 2 equivalent emissions for the considered scenarios. This study demonstrates that substantial RES production till 2020 is technically feasible, and provides benefits in the forms of carbon emission reductions, energy adequacy and dependency.

Electricity market activities and a growing demand for electricity have led to heavily stressed p... more Electricity market activities and a growing demand for electricity have led to heavily stressed power systems. This requires operation of the networks closer to their stability limits. Cost effective solutions are preferred over network extensions. The flexible alternating current transmission system (FACTS), a new technology based on power electronics, offers an opportunity to enhance controllability, stability, and power transfer capability of ac transmission systems. This paper provides a comprehensive review and evaluation of FACTS controllers.

This paper introduces a new general methodology for the modeling and reliability evaluation of sm... more This paper introduces a new general methodology for the modeling and reliability evaluation of small isolated power systems, which include wind turbines, photovoltaics, and diesel generators, based on fluid stochastic Petri nets (FSPNs). The proposed methodology presents two major novelties in FSPN modeling, namely: 1) the introduction of a new kind of Petri net arc, called the database arc, which makes possible the direct import of real data in the simulation process; and 2) the selection of constant time intervals in FSPN modeling, instead of assuming continuous dynamics defined by the change of fluid level over time. Moreover, in order to construct the overall system model, this paper proposes a general framework for modular representation of the system under study following a number of well-defined steps. The obtained model is fully parameterized and compared to classical simulation methods, it provides to its user the additional advantage of graphical representation of system's components and attributes. Four scenarios, which describe power system's performance under different conditions, were implemented. For each one of the developed scenarios, nine reliability and performance indexes have been calculated and compared.

This paper proposes a stochastic optimisation method, based on Ant Colony Optimisation (ACO), for... more This paper proposes a stochastic optimisation method, based on Ant Colony Optimisation (ACO), for the optimal choice of transformer sizes to be installed in a distribution network. This method is properly introduced to the solution of the Optimal Transformer Sizing (OTS) problem, taking into account the constraints imposed by the load the transformer serves throughout its lifetime and the possible transformer thermal overloading. The possibility to upgrade the transformer size one or more times throughout the study period results in different sizing paths taking into account the transformer capital cost the energy loss cost during the study period.

The CIGRÉ C6 Study Committee (Distribution Systems and Dispersed Generation) considers the differ... more The CIGRÉ C6 Study Committee (Distribution Systems and Dispersed Generation) considers the different aspects of integration of distributed generation. In this context, the C6.19 Working Group (WG) is specifically focused on “Planning and optimization methods for active distribution systems”. This report assesses the various requirements to facilitate the transition towards active distribution systems (ADSs). Specifically, the report starts from a survey of requirements of planning methodologies and identifies a new framework and methodologies for short, medium and long term models for active distribution planning. Active distribution networks (ADNs) have systems in place to control a combination of distributed energy resources (DERs), defined as generators, loads and storage. With these systems in place, the AND becomes an Active Distribution System (ADS). Distribution system operators (DSOs) have the possibility of managing the electricity flows using a flexible network topology. DERs take some degree of responsibility for system support, which depends on suitable regulatory environments and connection agreements. Distribution operation and planning stages can no longer be considered as separate tasks in the distribution business since the exploitation of existing assets with Advanced Automation and Control may be a valuable alternative to network expansion or reinforcement. Information and communication technology (ICT) cannot be considered as a simple add-on of the power system and simultaneous analysis (co-simulation) of both power system and ICT system behavior is required for planning and reliability studies, as well as risk analysis. Smart meters and automatic meter reading (AMR) offer a huge amount of data for load profiling and modelling. Data analytics and big data are key drivers for planning, since essential information has to be captured from on line measurement and raw data be used directly in real world planning applications. The ADS planning asks for daily customers profile with a probabilistic representation to take account of uncertainties that characterize their behaviour. Finally, interesting methodologies and models have been extracted from relevant publications. Particular attention has been paid to the planning tools, which are ready to be transferred into scaled industrial applications by means of strong interaction with R&D distribution company centres and Academia.

Increasing competition in the global transformer market has put tremendous responsibilities on th... more Increasing competition in the global transformer market has put tremendous responsibilities on the industry to increase reliability while reducing cost. Spotlight on Modern Transformer Design introduces a novel approach to transformer design using artificial intelligence (AI) techniques in combination with finite element method (FEM). Today, AI is widely used for modeling nonlinear and large-scale systems, especially when explicit mathematical models are difficult to obtain or completely lacking. Moreover, AI is computationally efficient in solving hard optimization problems. On the other hand, FEM is particularly capable of dealing with complex geometries, and also yields stable and accurate solutions.

Many numerical examples throughout the book illustrate the application of the techniques discussed to a variety of real-life transformer design problems.

Spotlight on Modern Transformer Design is a valuable learning tool for advanced undergraduate and graduate students, as well as researchers and power engineering professionals working in electric utilities and industries, public authorities, and design offices.