Charis Demoulias - Academia.edu (original) (raw)

Papers by Charis Demoulias

— This paper proposes a new hybrid control system for an AC microgrid. The system uses both centr... more — This paper proposes a new hybrid control system for an AC microgrid. The system uses both centralised and decentralised strategies to optimize the microgrid energy control while addressing the challenges introduced by current technologies and applied systems in real microgrid infrastructures. The well-known 3-level control (tertiary, secondary, primary) is employed with an enhanced hierarchical design using intelligent agent-based components in order to improve efficiency, diversity, modularity, and scalability. The main contribution of this paper is dual. During normal operation, the microgrid central controller (MGCC) is designed to undertake the management of the microgrid, while providing the local agents with the appropriate constraints for optimal power flow. During MGCC fault, a peer-to-peer communication is enabled between neighbouring agents in order to make their optimal decision locally. The initial design of the control structure and the detailed analysis of the different operating scenarios along with their requirements have shown the applicability of the new system in real microgrid environments.

—Microgrids composed of solemnly AC-coupled distributed energy resources can be found in many rea... more —Microgrids composed of solemnly AC-coupled distributed energy resources can be found in many real-life applications while their control has not been researched nearly enough to address some fundamental challenges, the most important of which is overall system reliability and fault tolerance. This paper proposes a droop-based coordinated control scheme for microgrids with AC-coupled units, a method that enables distributed energy resources units to hot swap between current source and voltage source grid-supporting control modes for satisfying load demand and ensuring energy storage systems will constantly be able to form the grid during islanded operation. The proposed control scheme has been realised in MATLAB/Simulink simulation model of a small-scale microgrid of AC-coupled units that corresponds to a real testbed in Northern Greece. Preliminary simulation results, in islanded mode, demonstrate the effectiveness of the proposed control scheme regarding power-sharing accuracy among the resources and state-of-charge balancing among storage units. Keywords—microgrids, hybrid power systems, maximum power point tracking, energy storage, droop control

Electric Power Systems Research, Apr 2015

Because of the increasing penetration of distributed generation (DG) units, distribution system o... more Because of the increasing penetration of distributed generation (DG) units, distribution system operators are faced with significant challenges such as voltage and congestion problems. This has led to continuously evolving grid interconnection requirements for DG. One of these requirements is the voltage support by DG units during voltage dips. In this paper, the grid support, in case of voltage dips, by two three-phase control strategies is investigated. One of the controllers is the positive-sequence control strategy and the second is the three-phase damping control strategy, which emulates a resistive behaviour towards the negative and zero-sequence voltage components and has originally been developed for mitigating unbalance in three-phase systems. First, the behaviour of both control strategies under voltage dips is analytically studied showing that the damping control strategy provides voltage support by injecting higher currents in the phase(s) where the fault is presented. Second, the positive effect on the residual grid voltage is validated by means of simulation and experimental results.

Electric Power Systems Research, 2010

A new simple analytical method for the calculation of the optimum inverter size in grid-connected... more A new simple analytical method for the calculation of the optimum inverter size in grid-connected PV plants in any location is presented. The derived analytical expressions contain only four unknown parameters, three of which are related to the inverter and one ...

Over the last decade, environmental concerns and the liberalization of the electric and gas marke... more Over the last decade, environmental concerns and the liberalization of the electric and gas markets have led to a reform of the power distribution networks towards the increasing use of distributed generation units. These units use power electronics to interface with the network, thus causing distortion of the network current and voltage shapes. This affects the power quality and may cause damage to electric equipment. This paper aims to identify the levels of the harmonic content and distortion caused in a distribution network due to the connection of distributed generators with power electronics interface at the low-voltage side of the grid. More specific, the level of harmonic distortion in networks with distributed generators using PWM is examined and the impact of installing harmonic filters at the output of power electronic devices is evaluated. The network under study is an underground medium voltage distribution line of an urban area in Northern Greece. All the simulations as well as the evaluation of the results have been conducted using the NEPLAN© software.

2015 IEEE Eindhoven PowerTech, 2015

ABSTRACT In this paper, a new software tool is presented for the simulation of electrical network... more ABSTRACT In this paper, a new software tool is presented for the simulation of electrical networks under steady-state conditions. Its distinct advantage is the robust integration of distributed generation droop controls, while offering the ability to simulate extended networks fast and reliably. The proposed simulation tool is based on the combination of two well-known software products, namely MATLAB and OpenDSS. The latter is employed as an unbalanced power flow solver, whereas the former implements the droop control of DG units. Simulation results for a simple and extended low-voltage network show the effectiveness of the proposed tool and mainly the reduction in the execution times over other conventional time-domain-based software products.

2011 IEEE EUROCON - International Conference on Computer as a Tool, 2011

This paper deals with the modeling of a single-phase PV inverter using current control. Two contr... more This paper deals with the modeling of a single-phase PV inverter using current control. Two control methods are described; the hysteresis band and the zero-tolerance control. A study for the optimization of the Total Harmonic Distortion (THD) of the output current and for the minimization of the switching losses is being conducted. A new modulation method is also presented, defined

2011 IEEE EUROCON - International Conference on Computer as a Tool, 2011

ABSTRACT

International Journal of Electrical Power & Energy Systems, 2015

ABSTRACT In order to utilize Doubly Fed Induction Generators (DFIGs) as primary power source in a... more ABSTRACT In order to utilize Doubly Fed Induction Generators (DFIGs) as primary power source in an isolated system, they should be able to regulate the voltage and frequency of the system as well as ride-through faults. This paper proposes a new control strategy for a DFIG operating in an isolated power system, accomplished by a Dynamic Voltage Restorer (DVR) and a Supercapacitor Energy Storage System (SCESS), in order to ride through symmetrical and asymmetrical faults. During faults, the DFIG continues to operate normally, while the active power mismatch is handled by the SCESS. In particular, during asymmetrical faults, the DFIG and the DVR are properly controlled in order to feed the non-faulty phases uninterruptedly. When integrated in a power system with conventional synchronous generators, the proposed control strategy improves the Fault Ride-Through (FRT) capability of a DFIG, while providing frequency and voltage support to the system throughout the fault duration. Thus, the transient stability of the power system is significantly improved. The effectiveness of the proposed control method under different fault conditions is verified by detailed simulation results.

Sustainable Energy, Grids and Networks, 2015

ABSTRACT In order to widely integrate high power microgrids within the distribution networks, the... more ABSTRACT In order to widely integrate high power microgrids within the distribution networks, they should possess Fault Ride-Through (FRT) capabilities, as well as provide ancillary services during abnormal grid operation. This paper proposes a new control strategy for improving the ability of a microgrid to ride though balanced and unbalanced grid voltage sags. The microgrid consists of several inverter-interfaced Distributed Energy Resources (DERs), powered by Wind Turbine Generators (WTGs), each combined with a Supercapacitor Energy Storage System (SCESS). During balanced and unbalanced grid voltage sags, the proposed control strategy maintains the microgrid grid-connected, while provides the local loads with a high quality voltage profile. The developed control method is complemented by properly sized controlled series inductances, placed at the point of common coupling (PCC) with the main grid. Under fault conditions, the DERs operate collectively in order to support the voltage within the microgrid, by injecting additional reactive power, without the necessity of any physical communication among them. During unbalanced grid voltage conditions, the DERs are controlled to compensate the unwanted negative and zero sequence voltage components. Thus, the microgrid loads are supplied with a set of balanced three-phase voltages. The proposed control strategy is verified by detailed simulation results.

2014 IEEE 5th International Symposium on Power Electronics for Distributed Generation Systems (PEDG), 2014

ABSTRACT The increased penetration of converter-interfaced distributed generation in microgrids h... more ABSTRACT The increased penetration of converter-interfaced distributed generation in microgrids has emerged the serious problem of protection during faults, due to the lack of large current injection. This paper proposes a fault detection and clearing control method for three-phase symmetrical faults in a microgrid with looped topology. The fault detection method is based on measuring the microgrid impedance variation during the fault, through the injection of a slightly distorted current. When the fault is identified, the distributed energy sources (DERs) switch their control strategy from droop control to current source mode, in order to inject a current proportional to the measured microgrid impedance. In this way, the DER closer to the fault injects a relatively larger current. The fault clearing process is carried out with simple overcurrent relays, which have the same settings. The coordination of the protection means is implemented from the discrete current injection of the DERs. Furthermore, in order to ensure the current injection even in the case of lack of power from the primary renewable source, a supercapacitor energy storage system (SESS) is added on the DC-link. A significant contribution is the voltage recovery after the fault clearance with a seamless transient effect. The effectiveness of the proposed control strategy is evaluated through simulation tests, conducted in PSIM software environment.

2013 International Conference on Renewable Energy Research and Applications (ICRERA), 2013

ABSTRACT The concept of microgrids is recently attracting considerable interest. However, in orde... more ABSTRACT The concept of microgrids is recently attracting considerable interest. However, in order to widely integrate microgrids within the distribution networks, a shift in the philosophy of interconnecting them with the utility grid seems necessary. A grid-connected microgrid is required to possess Faults Ride-Through (FRT) capabilities, as well as provide ancillary services during abnormal grid operation. In this paper, a control strategy for improving the ability of an inverter-based microgrid to ride though symmetrical and asymmetrical grid faults is proposed. The microgrid is formed of several Distributed Energy Resources (DERs), which utilize Wind Turbine Generators (WTGs) as primary renewable energy source, each combined with a Supercapacitor Energy Storage System (SCESS). During balanced and unbalanced grid voltage sags, aim of the proposed control strategy is to keep the microgrid connected to the grid, according to the FRT requirements, while maintaining an acceptable voltage profile within the common ac bus. Each DER is controlled to support the voltage within the microgrid by injecting reactive power, without any physical communication. During unbalanced utility voltage conditions, the DERs operate collectively in order to compensate the undesirable negative and zero sequence voltage components. Thus, a set of balanced three-phase voltages is provided within the common ac bus. Simulation results demonstrate that the microgrid can ride through heavily balanced and unbalanced utility voltage sags, while supplying its loads with a high quality voltage profile.

2013 International Conference on Renewable Energy Research and Applications (ICRERA), 2013

ABSTRACT The increase in the renewable energy penetration level imposes the microgrid concept, wh... more ABSTRACT The increase in the renewable energy penetration level imposes the microgrid concept, which can be consisted of several inverter-interfaced distributed resources (DERs) and loads, operating in dual state; either connected with the utility grid or isolated in island mode. The power sharing among the connected DERs is carried out by the combination of the droop characteristics of each DER according to the active and reactive power demand of the loads. When a fault occurs within the microgrid operating in island mode, it is very difficult to be detected due to the lack of large current production capacity. The fault situation can be further complicated, if the fault takes place between two phases or between a single phase and the earth. This paper proposes a fault detection method for the symmetrical and asymmetrical faults, which is independent of any further communication means. The three-phase faults can be detected through the impedance variation of the islanded microgrid, while the asymmetrical ones by the negative sequence components of the output voltage of each DER. A significant contribution is the voltage recovery after the fault clearance with a seamless transient effect. After the fault clearance, the microgrid will continue feeding its loads, through the implementation of a positive- and negative-sequence control strategy. The effectiveness of the proposed control strategy is evaluated through a set of simulation tests, conducted in PSIM software environment.

2012 47th International Universities Power Engineering Conference (UPEC), 2012

2012 47th International Universities Power Engineering Conference (UPEC), 2012

ABSTRACT The penetration of renewable energy sources in small-scale power production gives the op... more ABSTRACT The penetration of renewable energy sources in small-scale power production gives the opportunity parts of the grid to work as microgrids. The microgrid should be able to work both in grid-connected and island mode, while its voltage and frequency deviations follow the EN 50160 standard. The use of energy storage system is generally recommended in order to absorb the mismatches between the demand and the generation side and to preserve the quality of the microgrid voltage. While the up to day research is mainly concentrated on energy management based on communication, this paper proposes a wireless method for keeping the voltage and the frequency within the limits, using a battery as an energy storage system (ESS). An analytical expression for calculating the battery capacity is also proposed. The active and reactive power sharing among the parallel resources is achieved using the droop control method and an algorithm proportional to droop characteristic and the rated apparent power of each resource. According to the values of frequency and voltage and the State of Charge (SoC), the battery is connected in the microgrid, working in charging or discharging mode. A microgrid consisting of two inverter-interfaced power resources, a battery and a constant power load is investigated. Simulation results demonstrate that the proposed wireless control method provides the load with a high quality voltage in both grid-connected and islanded mode under several load scenarios.

The Journal of Engineering, 2014

ABSTRACT An enhanced role for the energy storage system (ESS), strategically placed at the point ... more ABSTRACT An enhanced role for the energy storage system (ESS), strategically placed at the point of common coupling (PCC) of the microgrid with the utility grid, is proposed. During island operation, the ESS ensures that the frequency and magnitude of the voltage will remain within the limits specified by the Standard EN 50160. By implementing an adjustable droop control method, the distributed energy resources (DERs) adjust their active and reactive powers in order to fulfil the load demand. When the grid is recovered, the ESS detects its presence and achieves a seamless synchronisation of the microgrid with the main grid, without any kind of communication. In grid-connected mode, the DERs deliver their available active power, whereas their reactive power is determined by a zero-sequence voltage. This voltage is injected by the ESS and aims to the zeroing of the amount of reactive power at the PCC. In this way, a reduction of power losses in the distribution lines of the microgrid is achieved. The effectiveness of the proposed control method in all operation modes, without any physical communication means, is demonstrated through detailed simulation in a representative microgrid with DERs fed by photovoltaics.

Since the penetration of photovoltaic (PV) systems in the Low Voltage (LV) distribution network i... more Since the penetration of photovoltaic (PV) systems in the Low Voltage (LV) distribution network is increasing, the need to register and model the contribution of these systems to the harmonic distortion of current and voltage waveforms is becoming an up-to-date issue. As PV systems incorporate power conditioning units, which are harmonic generating devices, they will have an influence on quality of supply, reliable operation of system equipment as well as component life expectancy. This paper investigates the harmonic impact of a 20 kWp PV system connected to the LV distribution network in Greece. The harmonic behavior of the PV plant as a function of the solar radiation under several weather conditions is analyzed. Measurements results are compared to those obtained from the power simulator packages PSIM© and Harmoniques©. The level of penetration of PV systems in the LV distribution network without harmonic limits been exceeded is investigated.

— This paper proposes a new hybrid control system for an AC microgrid. The system uses both centr... more — This paper proposes a new hybrid control system for an AC microgrid. The system uses both centralised and decentralised strategies to optimize the microgrid energy control while addressing the challenges introduced by current technologies and applied systems in real microgrid infrastructures. The well-known 3-level control (tertiary, secondary, primary) is employed with an enhanced hierarchical design using intelligent agent-based components in order to improve efficiency, diversity, modularity, and scalability. The main contribution of this paper is dual. During normal operation, the microgrid central controller (MGCC) is designed to undertake the management of the microgrid, while providing the local agents with the appropriate constraints for optimal power flow. During MGCC fault, a peer-to-peer communication is enabled between neighbouring agents in order to make their optimal decision locally. The initial design of the control structure and the detailed analysis of the different operating scenarios along with their requirements have shown the applicability of the new system in real microgrid environments.

—Microgrids composed of solemnly AC-coupled distributed energy resources can be found in many rea... more —Microgrids composed of solemnly AC-coupled distributed energy resources can be found in many real-life applications while their control has not been researched nearly enough to address some fundamental challenges, the most important of which is overall system reliability and fault tolerance. This paper proposes a droop-based coordinated control scheme for microgrids with AC-coupled units, a method that enables distributed energy resources units to hot swap between current source and voltage source grid-supporting control modes for satisfying load demand and ensuring energy storage systems will constantly be able to form the grid during islanded operation. The proposed control scheme has been realised in MATLAB/Simulink simulation model of a small-scale microgrid of AC-coupled units that corresponds to a real testbed in Northern Greece. Preliminary simulation results, in islanded mode, demonstrate the effectiveness of the proposed control scheme regarding power-sharing accuracy among the resources and state-of-charge balancing among storage units. Keywords—microgrids, hybrid power systems, maximum power point tracking, energy storage, droop control

Electric Power Systems Research, Apr 2015

Because of the increasing penetration of distributed generation (DG) units, distribution system o... more Because of the increasing penetration of distributed generation (DG) units, distribution system operators are faced with significant challenges such as voltage and congestion problems. This has led to continuously evolving grid interconnection requirements for DG. One of these requirements is the voltage support by DG units during voltage dips. In this paper, the grid support, in case of voltage dips, by two three-phase control strategies is investigated. One of the controllers is the positive-sequence control strategy and the second is the three-phase damping control strategy, which emulates a resistive behaviour towards the negative and zero-sequence voltage components and has originally been developed for mitigating unbalance in three-phase systems. First, the behaviour of both control strategies under voltage dips is analytically studied showing that the damping control strategy provides voltage support by injecting higher currents in the phase(s) where the fault is presented. Second, the positive effect on the residual grid voltage is validated by means of simulation and experimental results.

Electric Power Systems Research, 2010

A new simple analytical method for the calculation of the optimum inverter size in grid-connected... more A new simple analytical method for the calculation of the optimum inverter size in grid-connected PV plants in any location is presented. The derived analytical expressions contain only four unknown parameters, three of which are related to the inverter and one ...

Over the last decade, environmental concerns and the liberalization of the electric and gas marke... more Over the last decade, environmental concerns and the liberalization of the electric and gas markets have led to a reform of the power distribution networks towards the increasing use of distributed generation units. These units use power electronics to interface with the network, thus causing distortion of the network current and voltage shapes. This affects the power quality and may cause damage to electric equipment. This paper aims to identify the levels of the harmonic content and distortion caused in a distribution network due to the connection of distributed generators with power electronics interface at the low-voltage side of the grid. More specific, the level of harmonic distortion in networks with distributed generators using PWM is examined and the impact of installing harmonic filters at the output of power electronic devices is evaluated. The network under study is an underground medium voltage distribution line of an urban area in Northern Greece. All the simulations as well as the evaluation of the results have been conducted using the NEPLAN© software.

2015 IEEE Eindhoven PowerTech, 2015

ABSTRACT In this paper, a new software tool is presented for the simulation of electrical network... more ABSTRACT In this paper, a new software tool is presented for the simulation of electrical networks under steady-state conditions. Its distinct advantage is the robust integration of distributed generation droop controls, while offering the ability to simulate extended networks fast and reliably. The proposed simulation tool is based on the combination of two well-known software products, namely MATLAB and OpenDSS. The latter is employed as an unbalanced power flow solver, whereas the former implements the droop control of DG units. Simulation results for a simple and extended low-voltage network show the effectiveness of the proposed tool and mainly the reduction in the execution times over other conventional time-domain-based software products.

2011 IEEE EUROCON - International Conference on Computer as a Tool, 2011

This paper deals with the modeling of a single-phase PV inverter using current control. Two contr... more This paper deals with the modeling of a single-phase PV inverter using current control. Two control methods are described; the hysteresis band and the zero-tolerance control. A study for the optimization of the Total Harmonic Distortion (THD) of the output current and for the minimization of the switching losses is being conducted. A new modulation method is also presented, defined

2011 IEEE EUROCON - International Conference on Computer as a Tool, 2011

ABSTRACT

International Journal of Electrical Power & Energy Systems, 2015

ABSTRACT In order to utilize Doubly Fed Induction Generators (DFIGs) as primary power source in a... more ABSTRACT In order to utilize Doubly Fed Induction Generators (DFIGs) as primary power source in an isolated system, they should be able to regulate the voltage and frequency of the system as well as ride-through faults. This paper proposes a new control strategy for a DFIG operating in an isolated power system, accomplished by a Dynamic Voltage Restorer (DVR) and a Supercapacitor Energy Storage System (SCESS), in order to ride through symmetrical and asymmetrical faults. During faults, the DFIG continues to operate normally, while the active power mismatch is handled by the SCESS. In particular, during asymmetrical faults, the DFIG and the DVR are properly controlled in order to feed the non-faulty phases uninterruptedly. When integrated in a power system with conventional synchronous generators, the proposed control strategy improves the Fault Ride-Through (FRT) capability of a DFIG, while providing frequency and voltage support to the system throughout the fault duration. Thus, the transient stability of the power system is significantly improved. The effectiveness of the proposed control method under different fault conditions is verified by detailed simulation results.

Sustainable Energy, Grids and Networks, 2015

ABSTRACT In order to widely integrate high power microgrids within the distribution networks, the... more ABSTRACT In order to widely integrate high power microgrids within the distribution networks, they should possess Fault Ride-Through (FRT) capabilities, as well as provide ancillary services during abnormal grid operation. This paper proposes a new control strategy for improving the ability of a microgrid to ride though balanced and unbalanced grid voltage sags. The microgrid consists of several inverter-interfaced Distributed Energy Resources (DERs), powered by Wind Turbine Generators (WTGs), each combined with a Supercapacitor Energy Storage System (SCESS). During balanced and unbalanced grid voltage sags, the proposed control strategy maintains the microgrid grid-connected, while provides the local loads with a high quality voltage profile. The developed control method is complemented by properly sized controlled series inductances, placed at the point of common coupling (PCC) with the main grid. Under fault conditions, the DERs operate collectively in order to support the voltage within the microgrid, by injecting additional reactive power, without the necessity of any physical communication among them. During unbalanced grid voltage conditions, the DERs are controlled to compensate the unwanted negative and zero sequence voltage components. Thus, the microgrid loads are supplied with a set of balanced three-phase voltages. The proposed control strategy is verified by detailed simulation results.

2014 IEEE 5th International Symposium on Power Electronics for Distributed Generation Systems (PEDG), 2014

ABSTRACT The increased penetration of converter-interfaced distributed generation in microgrids h... more ABSTRACT The increased penetration of converter-interfaced distributed generation in microgrids has emerged the serious problem of protection during faults, due to the lack of large current injection. This paper proposes a fault detection and clearing control method for three-phase symmetrical faults in a microgrid with looped topology. The fault detection method is based on measuring the microgrid impedance variation during the fault, through the injection of a slightly distorted current. When the fault is identified, the distributed energy sources (DERs) switch their control strategy from droop control to current source mode, in order to inject a current proportional to the measured microgrid impedance. In this way, the DER closer to the fault injects a relatively larger current. The fault clearing process is carried out with simple overcurrent relays, which have the same settings. The coordination of the protection means is implemented from the discrete current injection of the DERs. Furthermore, in order to ensure the current injection even in the case of lack of power from the primary renewable source, a supercapacitor energy storage system (SESS) is added on the DC-link. A significant contribution is the voltage recovery after the fault clearance with a seamless transient effect. The effectiveness of the proposed control strategy is evaluated through simulation tests, conducted in PSIM software environment.

2013 International Conference on Renewable Energy Research and Applications (ICRERA), 2013

ABSTRACT The concept of microgrids is recently attracting considerable interest. However, in orde... more ABSTRACT The concept of microgrids is recently attracting considerable interest. However, in order to widely integrate microgrids within the distribution networks, a shift in the philosophy of interconnecting them with the utility grid seems necessary. A grid-connected microgrid is required to possess Faults Ride-Through (FRT) capabilities, as well as provide ancillary services during abnormal grid operation. In this paper, a control strategy for improving the ability of an inverter-based microgrid to ride though symmetrical and asymmetrical grid faults is proposed. The microgrid is formed of several Distributed Energy Resources (DERs), which utilize Wind Turbine Generators (WTGs) as primary renewable energy source, each combined with a Supercapacitor Energy Storage System (SCESS). During balanced and unbalanced grid voltage sags, aim of the proposed control strategy is to keep the microgrid connected to the grid, according to the FRT requirements, while maintaining an acceptable voltage profile within the common ac bus. Each DER is controlled to support the voltage within the microgrid by injecting reactive power, without any physical communication. During unbalanced utility voltage conditions, the DERs operate collectively in order to compensate the undesirable negative and zero sequence voltage components. Thus, a set of balanced three-phase voltages is provided within the common ac bus. Simulation results demonstrate that the microgrid can ride through heavily balanced and unbalanced utility voltage sags, while supplying its loads with a high quality voltage profile.

2013 International Conference on Renewable Energy Research and Applications (ICRERA), 2013

ABSTRACT The increase in the renewable energy penetration level imposes the microgrid concept, wh... more ABSTRACT The increase in the renewable energy penetration level imposes the microgrid concept, which can be consisted of several inverter-interfaced distributed resources (DERs) and loads, operating in dual state; either connected with the utility grid or isolated in island mode. The power sharing among the connected DERs is carried out by the combination of the droop characteristics of each DER according to the active and reactive power demand of the loads. When a fault occurs within the microgrid operating in island mode, it is very difficult to be detected due to the lack of large current production capacity. The fault situation can be further complicated, if the fault takes place between two phases or between a single phase and the earth. This paper proposes a fault detection method for the symmetrical and asymmetrical faults, which is independent of any further communication means. The three-phase faults can be detected through the impedance variation of the islanded microgrid, while the asymmetrical ones by the negative sequence components of the output voltage of each DER. A significant contribution is the voltage recovery after the fault clearance with a seamless transient effect. After the fault clearance, the microgrid will continue feeding its loads, through the implementation of a positive- and negative-sequence control strategy. The effectiveness of the proposed control strategy is evaluated through a set of simulation tests, conducted in PSIM software environment.

2012 47th International Universities Power Engineering Conference (UPEC), 2012

2012 47th International Universities Power Engineering Conference (UPEC), 2012

ABSTRACT The penetration of renewable energy sources in small-scale power production gives the op... more ABSTRACT The penetration of renewable energy sources in small-scale power production gives the opportunity parts of the grid to work as microgrids. The microgrid should be able to work both in grid-connected and island mode, while its voltage and frequency deviations follow the EN 50160 standard. The use of energy storage system is generally recommended in order to absorb the mismatches between the demand and the generation side and to preserve the quality of the microgrid voltage. While the up to day research is mainly concentrated on energy management based on communication, this paper proposes a wireless method for keeping the voltage and the frequency within the limits, using a battery as an energy storage system (ESS). An analytical expression for calculating the battery capacity is also proposed. The active and reactive power sharing among the parallel resources is achieved using the droop control method and an algorithm proportional to droop characteristic and the rated apparent power of each resource. According to the values of frequency and voltage and the State of Charge (SoC), the battery is connected in the microgrid, working in charging or discharging mode. A microgrid consisting of two inverter-interfaced power resources, a battery and a constant power load is investigated. Simulation results demonstrate that the proposed wireless control method provides the load with a high quality voltage in both grid-connected and islanded mode under several load scenarios.

The Journal of Engineering, 2014

ABSTRACT An enhanced role for the energy storage system (ESS), strategically placed at the point ... more ABSTRACT An enhanced role for the energy storage system (ESS), strategically placed at the point of common coupling (PCC) of the microgrid with the utility grid, is proposed. During island operation, the ESS ensures that the frequency and magnitude of the voltage will remain within the limits specified by the Standard EN 50160. By implementing an adjustable droop control method, the distributed energy resources (DERs) adjust their active and reactive powers in order to fulfil the load demand. When the grid is recovered, the ESS detects its presence and achieves a seamless synchronisation of the microgrid with the main grid, without any kind of communication. In grid-connected mode, the DERs deliver their available active power, whereas their reactive power is determined by a zero-sequence voltage. This voltage is injected by the ESS and aims to the zeroing of the amount of reactive power at the PCC. In this way, a reduction of power losses in the distribution lines of the microgrid is achieved. The effectiveness of the proposed control method in all operation modes, without any physical communication means, is demonstrated through detailed simulation in a representative microgrid with DERs fed by photovoltaics.

Since the penetration of photovoltaic (PV) systems in the Low Voltage (LV) distribution network i... more Since the penetration of photovoltaic (PV) systems in the Low Voltage (LV) distribution network is increasing, the need to register and model the contribution of these systems to the harmonic distortion of current and voltage waveforms is becoming an up-to-date issue. As PV systems incorporate power conditioning units, which are harmonic generating devices, they will have an influence on quality of supply, reliable operation of system equipment as well as component life expectancy. This paper investigates the harmonic impact of a 20 kWp PV system connected to the LV distribution network in Greece. The harmonic behavior of the PV plant as a function of the solar radiation under several weather conditions is analyzed. Measurements results are compared to those obtained from the power simulator packages PSIM© and Harmoniques©. The level of penetration of PV systems in the LV distribution network without harmonic limits been exceeded is investigated.