The increased penetration level of distributed renewable energy sources (DRESs) into the existing distribution grids changes the dynamic properties of conventional systems, creating new challenges for power system operators. Therefore,... more
The increased penetration level of distributed renewable energy sources (DRESs) into the existing distribution grids changes the dynamic properties of conventional systems, creating new challenges for power system operators. Therefore, new dynamic equivalent models are required to evaluate the dynamic behavior and stability margins of active distribution networks. For this purpose in this paper, a generic, input-output equivalent model is formulated, suitable for dynamic simulations of active distribution grids with high penetration level of DRESs. The developed model employs variable-order transfer functions, enabling the accurate representation of complex power system dynamics. The optimal order of the transfer functions is automatically determined through an iterative procedure, while the required model parameters are identified from measurements using the Vector Fitting method. Various network topologies and a diverse range of DRESs are considered to validate the accuracy of the developed model and demonstrate its generic properties. Finally, the performance of the proposed model is thoroughly compared with other conventional equivalent models, using error indexes.
Research Interests:
Research Interests:
Research Interests:
Abstract This short communication introduces vector fitting for mode estimation from ringdown responses of power systems. The performance of the method is evaluated for different power system models and is found to be very accurate.
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
The integration of photovoltaics (PVs) in low-voltage (LV) grids is expected to rise within the following years posing technical challenges to the reliable operation of the electrical system. To tackle these challenges, distributed energy... more
The integration of photovoltaics (PVs) in low-voltage (LV) grids is expected to rise within the following years posing technical challenges to the reliable operation of the electrical system. To tackle these challenges, distributed energy storage systems (ESSs) coupled with PVs at prosumer side arise as a promising solution. Therefore, during the last years several control schemes have been developed to manage ESSs. To overcome disadvantages of conventional control strategies, a new localized control strategy is proposed in this paper. The proposed control aims to improve voltage profile along distribution feeders, by mitigating the peaks of the net injected/absorbed power at prosumers’ point of common coupling. Additionally, the new control aims to maximize prosumers’ self-consumption. To achieve peak mitigation at the net power profile, an exponential droop is introduced that charges/discharges ESSs with different rates based on the residual power between PV generation and load demand. Using this droop, ESS charging/discharging power is exponentially increased as the residual power becomes greater. To maximize the self-consumption of the installation, an optimization procedure is developed that properly adjusts the exponential droop parameters to prosumer's generation and consumption profile. The effectiveness of the control strategy is validated by both simulation and experimental results.
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
A novel education framework targeting energy professionals that work or aim to work in the Nearly-Zero Energy Building (NZEB) sector has been conceived under the Meeting of Energy Skills (MEnS) Horizon 2020 project. The main goal is to... more
A novel education framework targeting energy professionals that work or aim to work in the Nearly-Zero Energy Building (NZEB) sector has been conceived under the Meeting of Energy Skills (MEnS) Horizon 2020 project. The main goal is to upgrade the NZEB skills and qualifications of building professionals (engineers and architects) that have already finished their bachelor degrees. The framework comprises three pylons. The first pylon is the national educational courses at level-7 European Qualifications Framework, implemented in 10 EU countries, using a common structure and awarding 10 ECTS upon successful completion. The second pylon is the Front Meeting of Skills educational experience, which is a short-duration course using more practical and hands-on educational methods, allowing for participants to work on real case studies of NZEB retrofit and encouraging them to propose their own strategies and measures. The third pylon is the e-learning platform, where all activities of the other two pylons from all countries are included, but also it acts as a meeting place where professionals can learn from each other as well and benefit from usual distant learning approaches. This paper describes in detail the adopted educational framework and analyzes the first conclusions and lessons learnt from the first round of educational courses held in the participating countries.
Research Interests:
Research Interests:
The voltage distribution along the power cable cores and sheaths determines the insulation stress and the compliance with certain operational specifications defined by the international standards. Calculations of voltage profiles in cable... more
The voltage distribution along the power cable cores and sheaths determines the insulation stress and the compliance with certain operational specifications defined by the international standards. Calculations of voltage profiles in cable arrangements using cascaded frequency-dependent modalor phase-domain models may lead to error accumulations. Direct time-domain transmission line models using the Finite-Difference Time-Domain (FDTD) method can accurately calculate voltages and currents with a high spatial discretization along cable cores and sheaths. However, frequency-dependent FDTD models have been proven either significantly slow in computations or numerically unstable, whereas their application in underground cables is also limited. In this paper, an FDTD model is formulated for power cables with distributed constant parameters, taking into account the influence of the skin-effect and the imperfect earth. The proposed model provides accurate transient and steady-state responses in single-phase and multiphase overhead and underground cable arrangements. Sheath cross-bondings can be also handled by the proposed formulation. The model can also provide transient and steady-state voltage profiles along cables, useful in insulation coordination or power-line communication signal analysis studies.
Research Interests:
Research Interests:
The increased availability of measurements in modern power systems, due to the advent of smart grids and the installation of phasor measurement units, has favored the development of dynamic load models using online recorded responses.... more
The increased availability of measurements in modern power systems, due to the advent of smart grids and the installation of phasor measurement units, has favored the development of dynamic load models using online recorded responses. However, load model parameters are significantly affected by loading conditions and change considerably due to the time-varying and weather-dependent composition of load. Therefore, load model parameters obtained from in-situ measurements are valid only for a narrow range of operating conditions. Scope of this paper is to propose a systematic identification procedure to develop generic dynamic load models, valid for a wide range of discrete operating conditions. For this purpose, two different generic modeling approaches are considered. The first approach is based on statistical analysis, while the second employs Artificial Neural Networks (ANNs). Several simulation scenarios are performed using the NEPLAN software to investigate the accuracy of the derived models over a wide range of different loading conditions, while their generalization capabilities are evaluated using the cross-validation technique.
Research Interests:
In this paper the development of robust measurement-based load models for dynamic simulations is discussed. Load model parameters vary significantly, due to different loading conditions, thus, load models obtained from measurements are... more
In this paper the development of robust measurement-based load models for dynamic simulations is discussed. Load model parameters vary significantly, due to different loading conditions, thus, load models obtained from measurements are valid only for a specific operating condition and cannot be easily generalized. Scope of the paper is to develop a generic load model, suitable for dynamic simulations over a wide range of operating and loading conditions. In order to derive the proposed generic model, three methodologies are thoroughly investigated. Several simulation scenarios of different operational conditions are examined with NEPLAN software and are used to validate the accuracy of the proposed models.
Research Interests:
Research Interests:
Photovoltaic (PV) penetration in Low-Voltage (LV) networks is expected to increase in the following years, posing a series of problems in the operation pf low-voltage networks. Among the most important challenges imposed, is the reverse... more
Photovoltaic (PV) penetration in Low-Voltage (LV) networks is expected to increase in the following years, posing a series of problems in the operation pf low-voltage networks. Among the most important challenges imposed, is the reverse power flow caused by high injected energy from distributed PVs during peak irradiation hours. This is the reason of voltage rises above the permissible limits. Amongst other solutions, such as active power curtailment and reactive power absorption, the installation of distributed Energy Storage Systems (ESS) seems to be an attractive solution since it can absorb surplus power locally, avoiding curtailment methods, power flow losses on the feeder and grid reinforcements. As the cost of storage is decreasing, ESS are expected to be increasingly adopted by prosumers. This work evaluates the influence of such systems on the voltage profile of LV feeders and on the self-consumed energy of prosumers. Intensive simulations on a benchmark feeder assess the conflicted benefits of power system operators and ESS owners.
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
This paper examines voltage quality problems in Low Voltage (LV) Distribution Networks (DNs) due to high Photovoltaic (PV) and Electric Vehicles (EVs) penetration. The proposed methodology considers Battery Energy Storage Systems (BESSs)... more
This paper examines voltage quality problems in Low Voltage (LV) Distribution Networks (DNs) due to high Photovoltaic (PV) and Electric Vehicles (EVs) penetration. The proposed methodology considers Battery Energy Storage Systems (BESSs) along with the PV units in order to store the energy surplus that causes overvoltage during the high PV generation. This stored energy is utilized at night when the majority of the EVs is expected to be charged. Moreover, a Unified Particle Swarm Optimization (UPSO) algorithm is used in order to optimally schedule EVs charging during the night and under different charging modes. This scheduling is implemented under either an assumed continuous charging mode or an intermittent one. The objective function is set to be the optimal voltage profile. A real LV DN with real measured load data is examined. Variations have been taken into account about some EVs' parameters and the results indicate that the voltage profile is significantly improved under the proposed charging schedule by the UPSO. If this charging schedule is combined with efficient exploitation of the BESS stored energy then the improvement is even higher. The proposed approach also improves energy efficiency towards the implementation of the Nearly Zero Energy Buildings (NZEB) concept.
Research Interests:
Research Interests:
Inter- and intra-area oscillations are inherent to large-scale power systems. Identification and analysis of these oscillations are vital for evaluating system stability margins and for determining control actions, aiming to enhance... more
Inter- and intra-area oscillations are inherent to large-scale power systems. Identification and analysis of these oscillations are vital for evaluating system stability margins and for determining control actions, aiming to enhance system damping levels. In this paper, a two-stage architecture is developed to facilitate identification of power system inter- and intra-area modes. In the first stage, ringdown responses, obtained from individual system buses, are analyzed using the Matrix Pencil (MP) method and modal estimates are derived. Subsequently, modal estimates are forwarded to a control center where they are grouped, analyzed and combined through clustering techniques to extract system properties, i.e., frequencies and damping factors of inter- and intra-area modes. The proposed approach is tested using simulated data, obtained from the Kundur power system, and found to be very accurate.
Research Interests:
Abstract Mode identification from post-disturbance ringdown responses is a valuable tool for the analysis of the dynamic performance and the stability margins of power systems. In this aspect, several techniques have been proposed,... more
Abstract Mode identification from post-disturbance ringdown responses is a valuable tool for the analysis of the dynamic performance and the stability margins of power systems. In this aspect, several techniques have been proposed, focusing mainly to single-signal analysis. However, considering large-scale power systems and especially future scenarios with high penetration of distributed energy resources, detailed network analysis at all voltage levels is required. As a result of these concerns, multi-channel mode identification algorithms have been developed. Scope of this paper is to evaluate the applicability and the performance of the most known multi-channel measurement-based identification approaches for the modal analysis of modern power systems incorporating active distribution networks. The algorithmic details and distinct characteristics of each method are briefly discussed. The examined methods are used to identify the dominant inter-area modes contained in ringdown responses at different levels of a combined transmission-distribution network.
Research Interests:
ABSTRACT Power engineers are expected to play a pivotal role in transforming buildings into smart and energy-efficient structures, which is necessary since buildings are responsible for a considerable amount of the total energy... more
ABSTRACT Power engineers are expected to play a pivotal role in transforming buildings into smart and energy-efficient structures, which is necessary since buildings are responsible for a considerable amount of the total energy consumption. To fulfil this role, a holistic approach in education is required, tackling subjects traditionally related to other engineering disciplines. In this context, USE Efficiency is an inter-institutional and interdisciplinary educational programme implemented in nine European Universities targeting energy efficiency in buildings. The educational programme effectively links professors, students, engineers and industry experts, creating a unique learning environment. The scope of the paper is to present the methodology and the general framework followed in the USE Efficiency programme. The proposed methodology can be adopted for the design and implementation of educational programmes on energy efficiency and sustainable development in higher education. End-of-course survey results showed positive feedback from the participating students, indicating the success of the programme.
Research Interests:
The total energy consumption of the buildings’ sector has been increasing steadily over the last decades, increasing the energy-related carbon dioxide emissions. By the transformation of buildings into nearly or net zero energy ones... more
The total energy consumption of the buildings’ sector has been increasing steadily over the last decades, increasing the energy-related carbon dioxide emissions. By the transformation of buildings into nearly or net zero energy ones (nZEBs), they will likely play a pivotal role in the smart grid context, being able to both produce and consume energy on-site and even acting as small nanogrids. In order to achieve that, a move towards smart and more energy efficient buildings with increased energy autonomy is mandatory for the existing and new properties. In this direction, the project "Enhancing storage integration in buildings with Photovoltaics" of "INTERREG Balkan Mediterranean 2014-2020" aims at both modelling and validating methods of optimal energy management in buildings equipped with Photovoltaics (PV) and storage systems. In this paper, a simple and accurate thermal model based on ISO 52016-1:2017 is proposed for the calculation of both heating and cooling energy needs in residential properties. The model has been tested in residential buildings in various European locations, but can be used in any type of building as well. From the comparison of the model with other simulation tools, it is concluded that slight deviations occur due to differences in climate data, the operative temperature, as well as the different approaches of ventilation and shading effects.
Research Interests:
Research Interests:
Research Interests:
Battery Energy Storage (BES) technology has a major role to play in the upcoming energy transition, especially when it is coupled with variable Renewable Energy Sources (RES) facilities such as solar photovoltaic (PV) systems. On the... more
Battery Energy Storage (BES) technology has a major role to play in the upcoming energy transition, especially when it is coupled with variable Renewable Energy Sources (RES) facilities such as solar photovoltaic (PV) systems. On the other hand, financial compensation and legislative barriers currently rendering storage technology, prevent it to become a profitable and reliable solution for energy system integration. This paper outlines the primary targets of StoRES project towards developing a technical solution for PV systems in conjunction with BES. For this purpose, coupled PV-BES residential storage systems were developed in six countries, aiming at optimizing the utilization of storage towards achieving higher PV penetration. Another focus of this paper is the implementation of a community storage system which was installed in Cyprus. This is connected to the same distribution feeder supplying the local residential pilot systems in order to make a credible comparison between t...
Research Interests:
This paper presents a Proton Exchange Membrane (PEM) Fuel Cell model. Mass balance and semiempirical equations are used in the modelling process. The model has been implemented in Matlab/Simulink and tested using various scenarios in... more
This paper presents a Proton Exchange Membrane (PEM) Fuel Cell model. Mass balance and semiempirical equations are used in the modelling process. The model has been implemented in Matlab/Simulink and tested using various scenarios in order to study the transient behaviour of a PEM fuel cell. The need for a Power Conditioning Unit (PCU) connecting the fuel cell to a Distributed Generation (DG) network has also been investigated.
Research Interests:
The determination of soil parameters is an important topic regarding the safe and efficient design of electrical grounding systems as well as the accurate calculation of the per-unit-length parameters of overhead transmission lines and... more
The determination of soil parameters is an important topic regarding the safe and efficient design of electrical grounding systems as well as the accurate calculation of the per-unit-length parameters of overhead transmission lines and underground cable systems. Scope of this paper is to investigate the seasonal variation of the two-layer earth structure parameters and propose simple generic models for this type of the analysis. For the determination of the two-layer earth parameters the application of different optimization methods is evaluated. Furthermore, soil resistivity measurements are conducted at three different sites during a period of a whole year. The obtained results of the two-layer earth structure are analyzed, in order to investigate the seasonal variation and their correlation to changes of rainfall. From the analysis of the results significant conclusions are drawn regarding the accuracy of the resulting soil parameters as well as their annual variation.
Research Interests:
Research Interests:
Three load matching indicators (self-consumption rate, self-sufficiency rate, loss of load probability) and the CO2 emissions were evaluated for 55 Cypriot households with 3 kWp rooftop photovoltaic (PV) generators. The calculations were... more
Three load matching indicators (self-consumption rate, self-sufficiency rate, loss of load probability) and the CO2 emissions were evaluated for 55 Cypriot households with 3 kWp rooftop photovoltaic (PV) generators. The calculations were performed using 30-minute generation and consumption data from a large scale smart meter project in Cyprus. To investigate the effects of recent advances in local legislation, an analysis for higher PV capacities (5 kWp and 10 kWp) was also performed. The PV generation profiles for 5 kWp and 10 kWp PVs were obtained by scaling the 3 kWp PV generation profiles. The results showed that the self-consumption of the analyzed households varied seasonally, as it was related to their heating and cooling demand. More interestingly, the ratio between the households’ annual electricity generation and demand, formally defined here as generation-to-demand ratio (GTDR), was found to be related to the value ranges of the studied load matching indicators. Hence, on...
Research Interests:
Research Interests:
Research Interests:
Distributed Generation (DG) units connected to the distribution network, contribute to the increase of the system fault level. Hence the knowledge of the exact contribution to the fault level by each DG unit is very important. In this... more
Distributed Generation (DG) units connected to the distribution network, contribute to the increase of the system fault level. Hence the knowledge of the exact contribution to the fault level by each DG unit is very important. In this paper the calculation of the short circuit current in distribution networks with DG units is carried out following two different approaches. The first approach is the IEC 60909 standard, which although adopts several simplifications and assumptions, in most cases provides results on the safe side. On the other hand dynamic simulations using dynamic tools such as the NEPLAN software are used for the accurate simulation of short circuit currents. Several parameters that influence the short circuit current, such as the length of the distribution feeders and the number of the integrated generators connected to the grid are investigated. From the comparison of the results by both approaches a better understanding of the influence of all parameters and assumptions used in short-circuit calculations is concluded.
Research Interests:
Online measurements can be used for aggregate load modeling, taking advantage of the advent of smart grids. In this paper initial load modeling results are presented using measurements recorded in a laboratory scale microgrid. The... more
Online measurements can be used for aggregate load modeling, taking advantage of the advent of smart grids. In this paper initial load modeling results are presented using measurements recorded in a laboratory scale microgrid. The examined microgrid configuration includes a load bank and an adjustable speed drive. A methodology for the identification of the model parameters from measurements is presented and the performance of a static and a dynamic load model is investigated in grid-connected and islanded operation modes. Numerical studies indicate that the dynamic model behaves satisfactorily enough in both modes of operation, while the static model can only be used in the islanded case. Generic load models suitable to simulate the real and reactive power for a wide range of load disturbances are proposed and their accuracy is evaluated using the cross-validation technique.