This paper presents a thorough control structure of the distributed generators inside the microgrid during both grid-connected and islanded operation modes. These control structures of the DGs voltage source inverters are implemented in synchronous reference frame (SRF) and controlled using linear PI controllers. By implementing the control structures, the desired real and reactive power can be efficiently transferred to the local loads and the utility load by the microgrid generating units. A modified droop control technique is introduced to facilitate the microgrid performance during both modes of operation. The active and reactive power sharing of the load demand between the utility grid and the microgrid can be performed by this drop control technique during the islanded mode. The system performance during intentional islanding event and utility load increase is investigated. The effectiveness of the offered control structures is confirmed through simulation results during both modes of operation.
Reactive power sharing in microgrid using virtual voltage IJECEIAES
The traditional droop control strategy has been applied previously in microgrids (MGs) to share accurately the active power. However, in some cases the result obtained when sharing reactive power is not the best, because of the parameters related to the distances from distributed generators (DGs) to the loads and the power variations. Therefore, this paper proposes a reactive power control strategy for a low voltage MG, where the unequal impedance related to the distances between generators and loads requires adjustments to work with the conventional frequency and voltage droop methods. Thus, an additional coefficient is calculated from parameters of the network that relate the location of elements. The test is perfomed by simulations in the MATLAB-Simulink software, considering a three-node MG with three DGs and a load that can change power at different periods of time. The results show that it is possible to improve reactive power sharing between the DGs located in the MG according to the load changes simulated and to improve voltages with this method.
An Adaptive Virtual Impedance Based Droop Control Scheme for Parallel Inverte...IAES-IJPEDS
This document presents an adaptive virtual impedance based droop control scheme for parallel inverters in a microgrid. The scheme uses an impedance estimator to monitor changes in line impedances between inverters and the point of common coupling. It estimates the line impedance in real time using output voltages and currents of the inverters as well as voltages at the point of common coupling. The estimated line impedance is then fed into a virtual impedance loop to adjust the virtual impedance value and compensate for reactive power mismatches due to changing line impedances, improving power sharing performance. Simulation results show the effectiveness of the proposed adaptive scheme compared to conventional control methods.
This paper presents a droop control technique for equal power sharing in islanded microgrid. In this study, the proposed controller is based on the frequency droop method, is applied to a robust droop controller in parallel connected inverters. The previous robust droop controller deals with voltage droop method. A modification has been formed against this controller by adding a fuzzy logic controller with the frequency droop method. The only sharing error which is concentrated in this paper is the error in sharing the rated frequency among the inverters. By adapting fuzzy in the robust droop, it tries to eliminate the frequency error, hence that the frequency reference of the inverters keeps maintain at 50Hz. A derivation of generalized models of a single-phase parallel-connected inverter system is shown. The simulation results show that the proposed controller with FLC is able to improve the stability of frequency reference and the performance of power sharing between the inverters under the inductive line impedance.
This work includes the establishment of a Photovoltaic system connected to the grid by means of an inverter. The fundamental goal of the work is to incorporate an advanced active power flow management scheme in order to adopt load at any weather condition along with the advantage of maximum active power flow and zero harmonics from PV inverter to the grid. The outcome of analysis and control design of grid connected PV inverter using a Proportional-Integral (PI) control technique is based on synchronous dq rotating reference frame so as to achieve maximum output voltage and record the active power. It has been observed that the model provides a better rate of stability as compared to the existing topology.
Power Quality Improvement with Multilevel Inverter Based IPQC for MicrogridIJMTST Journal
A micro grid is a hybrid power system consists of several distributed resources and local loads .Now a
days with increasing on a day to day life micro grid plays a vital role in power generation using Renewable
Energy Sources. Usage of power electronic devices in a micro grid results in harmonic generation and leads to
various power quality issues. Inorder to overcome voltage fluctuations and over current a magnetic flux
control based variable reactor is proposed. The performance of IPQC can be verified by using
MATLAB/SIMULINK`
This document discusses predictive direct power control (PDPC) of a grid-connected dual-active bridge multilevel inverter (DABMI) for renewable energy integration. A DABMI topology is proposed that uses two cascaded inverters to generate multilevel output voltages. A PDPC control strategy is adopted to control the real and reactive power injected into the grid. Simulation results show that the proposed DABMI produces lower power ripple and achieves currents with low total harmonic distortion within IEEE standards using PDPC control.
A NOVEL CONTROL STRATEGY FOR POWER QUALITY IMPROVEMENT USING ANN TECHNIQUE FO...IJERD Editor
The proposed system presents power-control strategies of a Micro grid-connected hybrid generation
system with versatile power transfer. This hybrid system allows maximum utilization of freely available
renewable energy sources like wind and photovoltaic energies. For this, an adaptive MPPT algorithm along with
standard perturbs and observes method will be used for the system.
The inverter converts the DC output from non-conventional energy into useful AC power for the
connected load. This hybrid system operates under normal conditions which include normal room temperature
in the case of solar energy and normal wind speed at plain area in the case of wind energy. However, designing
an optimal micro grid is not an easy task, due to the fact that primary energy carriers are changeable and
uncontrollable, as is the demand. Traditional design and optimization tools, developed for controlled power
sources, cannot be employed here. Simulation methods seem to be the best solution.
The dynamic model of the proposed system is first elaborated in the stationary reference frame and
then transformed into the synchronous orthogonal reference frame. The transformed variables are used in
control of the voltage source converter as the heart of the interfacing system between DG resources and utility
grid. By setting an appropriate compensation current references from the sensed load currents in control circuit
loop of DG, the active, reactive, and harmonic load current components will be compensated with fast dynamic
response, thereby achieving sinusoidal grid currents in phase with load voltages, while required power of the
load is more than the maximum injected power of the DG to the grid. In addition, the proposed control method
of this paper does not need a phase-locked loop in control circuit and has fast dynamic response in providing
active and reactive power components of the grid-connected loads.
This paper presents a thorough control structure of the distributed generators inside the microgrid during both grid-connected and islanded operation modes. These control structures of the DGs voltage source inverters are implemented in synchronous reference frame (SRF) and controlled using linear PI controllers. By implementing the control structures, the desired real and reactive power can be efficiently transferred to the local loads and the utility load by the microgrid generating units. A modified droop control technique is introduced to facilitate the microgrid performance during both modes of operation. The active and reactive power sharing of the load demand between the utility grid and the microgrid can be performed by this drop control technique during the islanded mode. The system performance during intentional islanding event and utility load increase is investigated. The effectiveness of the offered control structures is confirmed through simulation results during both modes of operation.
Reactive power sharing in microgrid using virtual voltage IJECEIAES
The traditional droop control strategy has been applied previously in microgrids (MGs) to share accurately the active power. However, in some cases the result obtained when sharing reactive power is not the best, because of the parameters related to the distances from distributed generators (DGs) to the loads and the power variations. Therefore, this paper proposes a reactive power control strategy for a low voltage MG, where the unequal impedance related to the distances between generators and loads requires adjustments to work with the conventional frequency and voltage droop methods. Thus, an additional coefficient is calculated from parameters of the network that relate the location of elements. The test is perfomed by simulations in the MATLAB-Simulink software, considering a three-node MG with three DGs and a load that can change power at different periods of time. The results show that it is possible to improve reactive power sharing between the DGs located in the MG according to the load changes simulated and to improve voltages with this method.
An Adaptive Virtual Impedance Based Droop Control Scheme for Parallel Inverte...IAES-IJPEDS
This document presents an adaptive virtual impedance based droop control scheme for parallel inverters in a microgrid. The scheme uses an impedance estimator to monitor changes in line impedances between inverters and the point of common coupling. It estimates the line impedance in real time using output voltages and currents of the inverters as well as voltages at the point of common coupling. The estimated line impedance is then fed into a virtual impedance loop to adjust the virtual impedance value and compensate for reactive power mismatches due to changing line impedances, improving power sharing performance. Simulation results show the effectiveness of the proposed adaptive scheme compared to conventional control methods.
This paper presents a droop control technique for equal power sharing in islanded microgrid. In this study, the proposed controller is based on the frequency droop method, is applied to a robust droop controller in parallel connected inverters. The previous robust droop controller deals with voltage droop method. A modification has been formed against this controller by adding a fuzzy logic controller with the frequency droop method. The only sharing error which is concentrated in this paper is the error in sharing the rated frequency among the inverters. By adapting fuzzy in the robust droop, it tries to eliminate the frequency error, hence that the frequency reference of the inverters keeps maintain at 50Hz. A derivation of generalized models of a single-phase parallel-connected inverter system is shown. The simulation results show that the proposed controller with FLC is able to improve the stability of frequency reference and the performance of power sharing between the inverters under the inductive line impedance.
This work includes the establishment of a Photovoltaic system connected to the grid by means of an inverter. The fundamental goal of the work is to incorporate an advanced active power flow management scheme in order to adopt load at any weather condition along with the advantage of maximum active power flow and zero harmonics from PV inverter to the grid. The outcome of analysis and control design of grid connected PV inverter using a Proportional-Integral (PI) control technique is based on synchronous dq rotating reference frame so as to achieve maximum output voltage and record the active power. It has been observed that the model provides a better rate of stability as compared to the existing topology.
Power Quality Improvement with Multilevel Inverter Based IPQC for MicrogridIJMTST Journal
A micro grid is a hybrid power system consists of several distributed resources and local loads .Now a
days with increasing on a day to day life micro grid plays a vital role in power generation using Renewable
Energy Sources. Usage of power electronic devices in a micro grid results in harmonic generation and leads to
various power quality issues. Inorder to overcome voltage fluctuations and over current a magnetic flux
control based variable reactor is proposed. The performance of IPQC can be verified by using
MATLAB/SIMULINK`
This document discusses predictive direct power control (PDPC) of a grid-connected dual-active bridge multilevel inverter (DABMI) for renewable energy integration. A DABMI topology is proposed that uses two cascaded inverters to generate multilevel output voltages. A PDPC control strategy is adopted to control the real and reactive power injected into the grid. Simulation results show that the proposed DABMI produces lower power ripple and achieves currents with low total harmonic distortion within IEEE standards using PDPC control.
A NOVEL CONTROL STRATEGY FOR POWER QUALITY IMPROVEMENT USING ANN TECHNIQUE FO...IJERD Editor
The proposed system presents power-control strategies of a Micro grid-connected hybrid generation
system with versatile power transfer. This hybrid system allows maximum utilization of freely available
renewable energy sources like wind and photovoltaic energies. For this, an adaptive MPPT algorithm along with
standard perturbs and observes method will be used for the system.
The inverter converts the DC output from non-conventional energy into useful AC power for the
connected load. This hybrid system operates under normal conditions which include normal room temperature
in the case of solar energy and normal wind speed at plain area in the case of wind energy. However, designing
an optimal micro grid is not an easy task, due to the fact that primary energy carriers are changeable and
uncontrollable, as is the demand. Traditional design and optimization tools, developed for controlled power
sources, cannot be employed here. Simulation methods seem to be the best solution.
The dynamic model of the proposed system is first elaborated in the stationary reference frame and
then transformed into the synchronous orthogonal reference frame. The transformed variables are used in
control of the voltage source converter as the heart of the interfacing system between DG resources and utility
grid. By setting an appropriate compensation current references from the sensed load currents in control circuit
loop of DG, the active, reactive, and harmonic load current components will be compensated with fast dynamic
response, thereby achieving sinusoidal grid currents in phase with load voltages, while required power of the
load is more than the maximum injected power of the DG to the grid. In addition, the proposed control method
of this paper does not need a phase-locked loop in control circuit and has fast dynamic response in providing
active and reactive power components of the grid-connected loads.
In a distributed generation system, divers renewable agents are connected to the low voltage 3 phase utility grid by an inverter which is used as power condition and must assurance the higher efficiency of the renewable agent. To achieve this level of efficiency, a unitary power factor between the utility grid voltages and the inverter currents is necessary, and a synchronization algorithm is required for the perfect synchronization between the 3-phase utility grid and the renewable agent. The aim of this paper is to present the optimization of the performance of a Synchronization controller for a 3-phase photovoltaic grid-connected system, assessing its accuracy under different conditions and studying their drawbacks and advantages. A grid connected photovoltaic system with a nominal power of 5 kW is used so as to assess the behavior of the synchronization algorithm when the 3 phase utility grid is affected by some disturbances such as voltage unbalances.
Autonomous microgrid based parallel inverters using droop controller for impr...journalBEEI
The existing microgrid has become a challenge to the sustainable energy source to provide a better quality of power to the consumer. To build a reliable and efficient microgrid, designing a droop controller for the microgrid is of utmost importance. In this paper, multiple voltage source inverters connected in parallel using an active power-frequency/reactive power-voltage droop scheme. The proposed method connected to two distributed generators local controllers, where each unit consists of a droop controller with an inner voltage-current controller and a virtual droop controller. By adding this controller to the microgrid reliability and load adaptability of an islanded system can be improved. This concept applied without any real-time communication to the microgrid. Thus, simulated using MATLAB/Simulink, the obtained results prove the effectiveness of the autonomous operation's microgrid model.
Hybrid bypass technique to mitigate leakage current in the grid-tied inverterIJECEIAES
The extensive use of fossil fuel is destroying the balance of nature that could lead to many problems in the forthcoming era. Renewable energy resources are a ray of hope to avoid possible destruction. Smart grid and distributed power generation systems are now mainly built with the help of renewable energy resources. The integration of renewable energy production system with the smart grid and distributed power generation is facing many challenges that include addressing the issue of isolation and power quality. This paper presents a new approach to address the aforementioned issues by proposing a hybrid bypass technique concept to improve the overall performance of the grid-tied inverter in solar power generation. The topology with the proposed technique is presented using traditional H5, oH5 and H6 inverter. Comparison of topologies with literature is carried out to check the feasibility of the method proposed. It is found that the leakage current of all the proposed inverters is 9 mA and total harmonic distortion is almost about 2%. The proposed topology has good efficiency, common mode and differential mode characteristics.
Droop control method for parallel dc converters used in standalone pv wind po...eSAT Journals
Abstract The rising rate of consumption and price of fossil fuel along with environmental pollution by conventional power generation draw global attention to renewable energy sources and technology. Paper gives analysis study on current sharing issues of parallel DC converters in standalone photovoltaic (PV) WIND system. Solar wind power generating system with maximum power point tracking (MPPT) technique – incremental conductance method is used for the simulation analysis. The main drawbacks of parallel converters used in system are poor power sharing and voltage drop. The paper describes about instantaneous droop calculation considering effect of cable resistance using droop index to improve the power sharing performance. The control technique is simulated using MATLAB/SIMULINK in PV- wind power generating system with MPPT and case study has been done on the control strategy and verifies the effectiveness of adaptive droop control on output converter voltage. Key Words: Microgrid; droop method; incremental conductance (Incond); maximum power point tracking (MPPT).
To eliminate the adverse effect from parameter variations as well as distorted grid conditions, a current control scheme of an LCL-filtered grid-connected inverter using a discrete integral sliding mode control (ISMC) and resonant compensation is presented. The proposed scheme is constructed based on the cascaded multiloop structure, in which three control loops are composed of grid-side current control, capacitor voltage control, and inverter-side current control. An active damping to suppress the resonance caused by LCL filter can be effectively realized by means of the inverter-side feedback control loop. Furthermore, the seamless transfer operation between the grid-connected mode and islanded mode is achieved by the capacitor voltage control loop. To retain a high tracking performance and robustness of the ISMC as well as an excellent harmonic compensation capability of the resonant control (RC) scheme at the same time, two control methods are combined in the proposed current controller. As a result, the proposed scheme yields a high quality of the injected grid currents and fast dynamic response even under distorted grid conditions. Furthermore, to reduce the number of sensors, a discrete-time reduced-order state observer is introduced. Simulation and experimental results are presented to demonstrate the effectiveness of the proposed scheme.
IRJET- Power Quality Improvement in Solar by using Fuzzy Logic ControllerIRJET Journal
This document describes a proposed system for improving power quality in solar photovoltaic systems using a fuzzy logic controller. The system uses a single-phase inverter controlled by a predictive control algorithm to perform maximum power point tracking from the PV array and deliver power to the grid, while also compensating for current harmonics and reactive power from nonlinear loads. A fuzzy logic control method is applied for maximum power point tracking to handle model uncertainties and nonlinearity. The performance of the proposed system is evaluated using MATLAB simulation.
International Journal of Engineering Research and Applications (IJERA) is an open access online peer reviewed international journal that publishes research and review articles in the fields of Computer Science, Neural Networks, Electrical Engineering, Software Engineering, Information Technology, Mechanical Engineering, Chemical Engineering, Plastic Engineering, Food Technology, Textile Engineering, Nano Technology & science, Power Electronics, Electronics & Communication Engineering, Computational mathematics, Image processing, Civil Engineering, Structural Engineering, Environmental Engineering, VLSI Testing & Low Power VLSI Design etc.
International Journal of Engineering Research and Applications (IJERA) is an open access online peer reviewed international journal that publishes research and review articles in the fields of Computer Science, Neural Networks, Electrical Engineering, Software Engineering, Information Technology, Mechanical Engineering, Chemical Engineering, Plastic Engineering, Food Technology, Textile Engineering, Nano Technology & science, Power Electronics, Electronics & Communication Engineering, Computational mathematics, Image processing, Civil Engineering, Structural Engineering, Environmental Engineering, VLSI Testing & Low Power VLSI Design etc.
IJERA (International journal of Engineering Research and Applications) is International online, ... peer reviewed journal. For more detail or submit your article, please visit www.ijera.com
Voltage and frequency control of microgrid in presence of micro-turbine inter...IJECEIAES
The active and reactive load changes have a significant impact on voltage
and frequency. In this paper, in order to stabilize the microgrid (MG) against
load variations in islanding mode, the active and reactive power of all
distributed generators (DGs), including energy storage (battery), diesel
generator, and micro-turbine, are controlled. The micro-turbine generator is
connected to MG through a three-phase to three-phase matrix converter, and
the droop control method is applied for controlling the voltage and
frequency of MG. In addition, a method is introduced for voltage and
frequency control of micro-turbines in the transition state from gridconnected mode to islanding mode. A novel switching strategy of the matrix
converter is used for converting the high-frequency output voltage of the
micro-turbine to the grid-side frequency of the utility system. Moreover,
using the switching strategy, the low-order harmonics in the output current
and voltage are not produced, and consequently, the size of the output filter
would be reduced. In fact, the suggested control strategy is load-independent
and has no frequency conversion restrictions. The proposed approach for
voltage and frequency regulation demonstrates exceptional performance and
favorable response across various load alteration scenarios. The suggested
strategy is examined in several scenarios in the MG test systems, and the
simulation results are addressed.
IJERA (International journal of Engineering Research and Applications) is International online, ... peer reviewed journal. For more detail or submit your article, please visit www.ijera.com
International Journal of Engineering Research and Applications (IJERA) is an open access online peer reviewed international journal that publishes research and review articles in the fields of Computer Science, Neural Networks, Electrical Engineering, Software Engineering, Information Technology, Mechanical Engineering, Chemical Engineering, Plastic Engineering, Food Technology, Textile Engineering, Nano Technology & science, Power Electronics, Electronics & Communication Engineering, Computational mathematics, Image processing, Civil Engineering, Structural Engineering, Environmental Engineering, VLSI Testing & Low Power VLSI Design etc.
A Technique for Shunt Active Filter meld micro grid SystemIJERA Editor
The proposed system presents a control technique for a micro grid connected hybrid generation system ith case study interfaced with a three phase shunt active filter to suppress the current harmonics and reactive power present in the load using PQ Theory with ANN controller. This Hybrid Micro Grid is developed using freely renewable energy resources like Solar Photovoltaic (SPV) and Wind Energy (WE). To extract the maximum available power from PV panels and wind turbines, Maximum power point Tracker (MPPT) has been included. This MPPT uses the “Standard Perturbs and Observe” technique. By using PQ Theory with ANN Controller, the Reference currents are generated which are to be injected by Shunt active power filter (SAPF)to compensate the current harmonics in the non linear load. Simulation studies shows that the proposed control technique performs non-linear load current harmonic compensation maintaining the load current in phase with the source voltage.
A Review on Power Quality Issues and their Mitigation Techniques in Microgrid...ijtsrd
Power Quality is playing an increasingly significant role both at supply and demand sides. With the advent of participation of private players in distribution systems, the power quality is expected to be the pivotal decisive factor before the consumers. Due to ever growing application of switching devices, the power quality is bound to get deteriorated, at the same time such devices are also prone to malfunction due to poor power quality. The world is driven by the carbon emission to replace the conventional generation by as much renewable generation as possible. The above situation has attracted the attention of researchers to identify and suggest the mitigation techniques of power quality issue’s for improving the performance of microgrid containing renewable energy resources. An attempt has been made to comprehensively present a review of the research carried out thusfar. Anita Chaudhery | Pramod Kumar Rathore "A Review on Power Quality Issues and their Mitigation Techniques in Microgrid System" Published in International Journal of Trend in Scientific Research and Development (ijtsrd), ISSN: 2456-6470, Volume-6 | Issue-2 , February 2022, URL: https://www.ijtsrd.com/papers/ijtsrd49299.pdf Paper URL: https://www.ijtsrd.com/engineering/electrical-engineering/49299/a-review-on-power-quality-issues-and-their-mitigation-techniques-in-microgrid-system/anita-chaudhery
A Grid-Tied Solar Power System with Harmonic Filter to Enhance Power QualityIRJET Journal
This document proposes a grid-tied solar power system integrated with an active shunt harmonic filter to improve power quality. The system uses a photovoltaic array and maximum power point tracking algorithm to power a three-leg voltage source converter acting as the active harmonic filter. The filter extracts a reference current using instantaneous power theory and generates compensating current through PWM to cancel harmonics in the source current. Simulation results show the source current is cleaned of harmonics when the filter is integrated into the system.
Frequency control in a microgrid including controllable loadIAEME Publication
This document summarizes a research paper that proposes a method for frequency control in microgrids that includes renewable energy sources, energy storage devices, and controllable loads. The microgrid model analyzed includes solar power, wind power, batteries, supercapacitors, and electric water heaters. The document describes the components of the microgrid, simulation parameters and assumptions. Frequency control is achieved by coordinating the energy storage devices and generators using optimized proportional-integral controllers. Electric water heaters can also help control frequency by adjusting their operating temperature setpoints in response to frequency deviations.
IRJET- A Review on Solar based Multilevel Inverter with Three Phase Grid SupplyIRJET Journal
- The document discusses solar-powered multilevel inverters that can supply three-phase grid power. Multilevel inverters have advantages over single-level inverters like lower harmonic distortion, reduced electromagnetic interference, and the ability to operate at several voltage levels.
- The literature review covers prior research on different multilevel inverter topologies for photovoltaic systems, including the flying capacitor, neutral point clamped, and cascaded H-bridge inverters. It also discusses control methods like maximum power point tracking and modulation techniques.
- The goal is to develop a multilevel inverter powered by PV panels that can supply three-phase grid power with minimum harmonic distortion and reduced component requirements compared to
Emc model for modern power electronic systems for harmonics, losses & emi...eSAT Journals
Abstract
Electromagnetic compatibility of power electronic systems becomes an engineering discipline and it should be considered at the
beginning stage of a design. Thus, a power electronics design becomes more complex and challenging and it requires a good
communication between EMI and Power electronics experts. Three major issues in designing a power electronic system are Losses,
EMI and Harmonics. These issues affect system cost, size, efficiency and quality and it is a tradeoff between these factors when we
design a power converter, filter. In this paper the EMC model is discussed which should be considered while designing the power
electronics systems. The design considerations in this paper help us to remove losses, harmonics & EMI elimination and power
quality improvement of Power systems.
Index Terms: Converter, EMI, EMC, Filter, Harmonics
IJRET : International Journal of Research in Engineering and Technology is an international peer reviewed, online journal published by eSAT Publishing House for the enhancement of research in various disciplines of Engineering and Technology. The aim and scope of the journal is to provide an academic medium and an important reference for the advancement and dissemination of research results that support high-level learning, teaching and research in the fields of Engineering and Technology. We bring together Scientists, Academician, Field Engineers, Scholars and Students of related fields of Engineering and Technology.
Control strategies for seamless transfer between the grid-connected and isla...IJECEIAES
This document describes two control strategies for seamless transfer between grid-connected and islanded modes of operation in a microgrid system: inverter output current control and indirect grid current control. It presents the modeling of a three-phase inverter power stage and discusses the control schemes for each strategy in detail. Simulation results are presented to validate the design methodology and compare the performance of the two control strategies under different operating modes including grid-connected, islanded, and during transition between modes.
This document summarizes a research paper on microgrid digital twins. It introduces the concept of a microgrid digital twin, which creates a virtual representation of a microgrid that mirrors the behavior of the physical system using real-time data exchange. The paper discusses how microgrid digital twins can be established through modeling physical components and processes, connecting real-time data sources, and continuously updating models. A variety of applications of microgrid digital twins are explored, including design, control, forecasting, fault diagnosis, and resilient operation management. Future trends around increasing data sources, analytics capabilities, and integration of artificial intelligence are also discussed.
Virtual power lines (VPLs) use energy storage systems, such as large batteries, to integrate renewable energy into electric grids and avoid or delay costly infrastructure upgrades. VPLs connect storage at two points - one near renewable generators to store excess power that cannot be transmitted, and another near demand centers to discharge stored power when transmission capacity is limited. This allows renewable power to be transmitted virtually along battery-supported pathways rather than building new wires. VPLs can integrate higher shares of renewable energy faster and more cost-effectively than traditional grid expansions.
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In a distributed generation system, divers renewable agents are connected to the low voltage 3 phase utility grid by an inverter which is used as power condition and must assurance the higher efficiency of the renewable agent. To achieve this level of efficiency, a unitary power factor between the utility grid voltages and the inverter currents is necessary, and a synchronization algorithm is required for the perfect synchronization between the 3-phase utility grid and the renewable agent. The aim of this paper is to present the optimization of the performance of a Synchronization controller for a 3-phase photovoltaic grid-connected system, assessing its accuracy under different conditions and studying their drawbacks and advantages. A grid connected photovoltaic system with a nominal power of 5 kW is used so as to assess the behavior of the synchronization algorithm when the 3 phase utility grid is affected by some disturbances such as voltage unbalances.
Autonomous microgrid based parallel inverters using droop controller for impr...journalBEEI
The existing microgrid has become a challenge to the sustainable energy source to provide a better quality of power to the consumer. To build a reliable and efficient microgrid, designing a droop controller for the microgrid is of utmost importance. In this paper, multiple voltage source inverters connected in parallel using an active power-frequency/reactive power-voltage droop scheme. The proposed method connected to two distributed generators local controllers, where each unit consists of a droop controller with an inner voltage-current controller and a virtual droop controller. By adding this controller to the microgrid reliability and load adaptability of an islanded system can be improved. This concept applied without any real-time communication to the microgrid. Thus, simulated using MATLAB/Simulink, the obtained results prove the effectiveness of the autonomous operation's microgrid model.
Hybrid bypass technique to mitigate leakage current in the grid-tied inverterIJECEIAES
The extensive use of fossil fuel is destroying the balance of nature that could lead to many problems in the forthcoming era. Renewable energy resources are a ray of hope to avoid possible destruction. Smart grid and distributed power generation systems are now mainly built with the help of renewable energy resources. The integration of renewable energy production system with the smart grid and distributed power generation is facing many challenges that include addressing the issue of isolation and power quality. This paper presents a new approach to address the aforementioned issues by proposing a hybrid bypass technique concept to improve the overall performance of the grid-tied inverter in solar power generation. The topology with the proposed technique is presented using traditional H5, oH5 and H6 inverter. Comparison of topologies with literature is carried out to check the feasibility of the method proposed. It is found that the leakage current of all the proposed inverters is 9 mA and total harmonic distortion is almost about 2%. The proposed topology has good efficiency, common mode and differential mode characteristics.
Droop control method for parallel dc converters used in standalone pv wind po...eSAT Journals
Abstract The rising rate of consumption and price of fossil fuel along with environmental pollution by conventional power generation draw global attention to renewable energy sources and technology. Paper gives analysis study on current sharing issues of parallel DC converters in standalone photovoltaic (PV) WIND system. Solar wind power generating system with maximum power point tracking (MPPT) technique – incremental conductance method is used for the simulation analysis. The main drawbacks of parallel converters used in system are poor power sharing and voltage drop. The paper describes about instantaneous droop calculation considering effect of cable resistance using droop index to improve the power sharing performance. The control technique is simulated using MATLAB/SIMULINK in PV- wind power generating system with MPPT and case study has been done on the control strategy and verifies the effectiveness of adaptive droop control on output converter voltage. Key Words: Microgrid; droop method; incremental conductance (Incond); maximum power point tracking (MPPT).
To eliminate the adverse effect from parameter variations as well as distorted grid conditions, a current control scheme of an LCL-filtered grid-connected inverter using a discrete integral sliding mode control (ISMC) and resonant compensation is presented. The proposed scheme is constructed based on the cascaded multiloop structure, in which three control loops are composed of grid-side current control, capacitor voltage control, and inverter-side current control. An active damping to suppress the resonance caused by LCL filter can be effectively realized by means of the inverter-side feedback control loop. Furthermore, the seamless transfer operation between the grid-connected mode and islanded mode is achieved by the capacitor voltage control loop. To retain a high tracking performance and robustness of the ISMC as well as an excellent harmonic compensation capability of the resonant control (RC) scheme at the same time, two control methods are combined in the proposed current controller. As a result, the proposed scheme yields a high quality of the injected grid currents and fast dynamic response even under distorted grid conditions. Furthermore, to reduce the number of sensors, a discrete-time reduced-order state observer is introduced. Simulation and experimental results are presented to demonstrate the effectiveness of the proposed scheme.
IRJET- Power Quality Improvement in Solar by using Fuzzy Logic ControllerIRJET Journal
This document describes a proposed system for improving power quality in solar photovoltaic systems using a fuzzy logic controller. The system uses a single-phase inverter controlled by a predictive control algorithm to perform maximum power point tracking from the PV array and deliver power to the grid, while also compensating for current harmonics and reactive power from nonlinear loads. A fuzzy logic control method is applied for maximum power point tracking to handle model uncertainties and nonlinearity. The performance of the proposed system is evaluated using MATLAB simulation.
International Journal of Engineering Research and Applications (IJERA) is an open access online peer reviewed international journal that publishes research and review articles in the fields of Computer Science, Neural Networks, Electrical Engineering, Software Engineering, Information Technology, Mechanical Engineering, Chemical Engineering, Plastic Engineering, Food Technology, Textile Engineering, Nano Technology & science, Power Electronics, Electronics & Communication Engineering, Computational mathematics, Image processing, Civil Engineering, Structural Engineering, Environmental Engineering, VLSI Testing & Low Power VLSI Design etc.
International Journal of Engineering Research and Applications (IJERA) is an open access online peer reviewed international journal that publishes research and review articles in the fields of Computer Science, Neural Networks, Electrical Engineering, Software Engineering, Information Technology, Mechanical Engineering, Chemical Engineering, Plastic Engineering, Food Technology, Textile Engineering, Nano Technology & science, Power Electronics, Electronics & Communication Engineering, Computational mathematics, Image processing, Civil Engineering, Structural Engineering, Environmental Engineering, VLSI Testing & Low Power VLSI Design etc.
IJERA (International journal of Engineering Research and Applications) is International online, ... peer reviewed journal. For more detail or submit your article, please visit www.ijera.com
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An_Improved_Droop_Control_Strategy_for_R.pdf
1. Aalborg Universitet
An Improved Droop Control Strategy for Reactive Power Sharing in Islanded Microgrid
Han, Hua; Liu, Yao; Sun, Yao; Su, Mei; Guerrero, Josep M.
Published in:
I E E E Transactions on Power Electronics
DOI (link to publication from Publisher):
10.1109/TPEL.2014.2332181
Publication date:
2015
Link to publication from Aalborg University
Citation for published version (APA):
Han, H., Liu, Y., Sun, Y., Su, M., & Guerrero, J. M. (2015). An Improved Droop Control Strategy for Reactive
Power Sharing in Islanded Microgrid. I E E E Transactions on Power Electronics, 30(6), 3133 - 3141 .
10.1109/TPEL.2014.2332181
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H. Han, Y. Liu, Y. Sun, M. Su, and J. M. Guerrero, “An Improved Droop Control Strategy for Reactive Power Sharing in
Islanded Microgrid,” IEEE Transactions on Power Electronics, 2014
An Improved Droop Control Strategy for Reactive Power
Sharing in Islanded Microgrid
Hua Han, Yao Liu, Yao Sun, Mei Su, and Josep M. Guerrero
School of Information Science and Engineering, Central South University, Changsha 410083,
Hunan Province, China
J. M. Guerrero is with the Department of Energy Technology, Aalborg University, 9220 Aalborg East, Denmark
(Tel: +45 2037 8262; Fax: +45 9815 1411; e-mail: joz@et.aau.dk).
Abstract – For microgrid in islanded operation, due to the effects of
mismatched line impedance, the reactive power could not be shared
accurately with the conventional droop method. To improve the
reactive power sharing accuracy, this paper proposes an improved
droop control method. The proposed method mainly includes two
important operations: error reduction operation and voltage recovery
operation. The sharing accuracy is improved by the sharing error
reduction operation, which is activated by the low-bandwidth
synchronization signals. However, the error reduction operation will
result in a decrease in output voltage amplitude. Therefore, the voltage
recovery operation is proposed to compensate the decrease. The needed
communication in this method is very simple, and the plug-and-play is
reserved. Simulations and experimental results show that the improved
droop controller can share load active and reactive power, improve the
power quality of the microgrid, and also have a good dynamic
performance.
KEY WORDS:Microgrid; droop control; reactive power sharing;
low-bandwidth synchronization signals; voltage recovery mechanism
I INTRODUCTION
The application of distributed generation (DG) has
been increasing rapidly in the past decades. Compared to
the conventional centralized power generation, DG units
have advantages of less pollution, higher efficiency of
energy utilization, flexible installation location, and less
power transmission losses. Most of the DG units are
connected to the grid via power electronic converters,
which introduces system resonance, protection
interference, etc. To overcome these problems a
microgrid concept was first proposed in the US by the
consortium for electrical reliability technology solutions
[1]. Compared to using a single DG unit, microgrid could
offer superior power management within the distribution
networks. Moreover, the microgrid can operate both in
grid-connected mode and islanding mode and benefit
both the utility and customers in economy [2-7].
In islanding mode, the load power in the microgrid
should be properly shared by multiple DG units. Usually,
the droop control method which mimics the behavior of a
synchronous generator in traditional power system is
adopted, which does not need the use of critical
communications [8-14, 21-22]. The active power sharing
is always achieved by the droop control method easily.
However, due to effects of mismatched feeder impedance
between the DGs and loads, the reactive power will not
be shared accurately. In extreme situations, it can even
result in severe circulating reactive power and stability
problems [11].
To overcome the reactive power sharing issue, a few
improved methods have been proposed. Specifically,
there are manly three approaches to address the effect of
the interconnecting line impedance on droop-based
control. The first approach is to introduce the virtual
output impedance by modifying the output voltage
reference based on output current feedback [11,13-14,23].
This method can reduce the reactive power sharing error
by reducing the relative error of the output impedances.
However, the introduction of the virtual impedance may
lead to degradation of the system voltage quality. The
second approach is based on signal injection technique.
In [15], a certain harmonic signal containing reactive
power information is injected into the output voltage
reference of each DG unit, and the output reactive power
is regulated according to the harmonic power to improve
the accuracy of the reactive power sharing. However, this
method results in output voltage distortion. In [16], in
order to reduce the reactive power sharing errors, the
method injects some small disturbance signal containing
3. 2
2
reactive power information into the frequency reference
of each DG unit. By using the active power error before
and after the injecting signal, this method can eliminate
the reactive power error. However, this method is a
classic event-triggered control and its stability is not easy
to be guaranteed. Additionally, the third approach is
usually based on constructed and compensated method.
In [17], the method constructs an integral control
concerning the common bus voltage to ensure the
reactive power sharing. However, in practical situation,
the common bus voltage information is difficult to get.
In this paper, a new reactive power sharing method is
proposed. The method improves the reactive power
sharing by changing the voltage bias on the basis of the
conventional droop control, which is activated by a
sequence of synchronizationn event through the low
bandwidth communication network. It is a cost-effective
and practical an approach since only a low bandwidth
communication network is required. Simulation and
experimental results are provided to verify the
effectiveness and feasibility of the proposed reactive
power sharing method.
The paper is organized as follows. Section II gives the
system configuration and the reactive power sharing
errors analysis with conventional droop control. Section
III proposes an improved reactive power sharing control
strategy, and the convergence and robustness is analyzed.
Simulation and experimental results are given in Section
IV. Section V gives the conclusion.
II ANALYSIS OF THE CONVENTIONAL DROOP
CONTROL METHOD
A. Configuration and operation of AC Microgrid
A classic configuration of a microgrid which
consists of multiple distributed generation (DG) units
and dispersed loads is shown in Fig.1. The microgrid
is connected to the utility through a static transfer
switch at the PCC. Each DG unit is connected to the
microgrid through power electronic converter and its
respective feeder.
This paper aims to solving the fundamental active
and reactive power sharing in islanding mode, and the
power sharing issues on harmonic currents is out of
the scope of the paper.
DC
Side
Local
controller
Synchron
-ization
signal
Low-bandwidth
communication
Local
controller
Synchron
-ization
signal
DG1
DGn
Main Grid
Common load
Local load
Feeder 1
Feeder n
Common
Bus
Static
switch
Low bandwidth
synchronization
signal
LC filter
Feeder
DC
Side
LC filter
Fig.1. Illustration of the AC microgrid configuration.
B. The conventional Droop Control
Fig. 2 shows the equivalent model of a DG unit,
which is interfaced to the common bus of the AC
microgrid through a power inverter with a output LCL
filter. As shown in Fig.2, Ei∠δi is the voltage across
the filter capacitor, Vpcc∠0°
is the common AC bus
voltage. Compared with the inductance of the LCL
filter, the line resistance can be ignored. Then the
impedance between inverter and the common bus can
be described as Xi (Xi=ωLi).
f
L
f
C
dc
V
i i
P jQ
Power line
0
pcc
V
i
L
i i
E
Fig.2 Model of a DG unit.
According to the equivalent circuit in Fig. 2, the
inverter output apparent power is Si, and it can be
given by
2
cos
sin
i pcc i pcc i pcc
i i i i
i i
EV EV V
S P jQ j
X X
(1)
From equation (1), the output active and reactive
power of the DG units are shown as
2
sin
cos
i pcc
i i
i
i pcc i pcc
i
i
EV
P
X
EV V
Q
X
(2)
4. 3
3
Usually, the phase shift angle δi is small. Therefore,
the real power Pi and reactive power Qi of each DG
can be regulated by δi and the output voltage
amplitude Ei, respectively [24]. Then the conventional
droop control is given by
*
*
=
=
i i i
i i i
m P
E E n Q
(3)
Where ω* and E* are the nominal values of DG
angular frequency and DG output voltage amplitude,
mi and ni are the active and reactive droop slopes,
respectively. i
P and i
Q are the measured averaged
real and reactive power values through a low pass
filter, respectively.
C. Reactive Power Sharing Errors Analysis
For simplicity, a simplified microgrid with two
DG units is considered in this section.
According to equations (2) and (3), the reactive
power of the i-th DG unit is obtained
*
( cos )
cos
pcc i pcc
i
i pcc i i
V E V
Q
X V n
(4)
Assume the i-th and j-th DG unit are working in
parallel with the same nominal capacity and droop
slope. Note that shift angle δi is usually vary small
(sinδi≈δi, cosδi≈1), then the reactive power sharing
relative error with respect to i
Q can be expressed as
follows
i j j i
err
i j pcc j
Q Q X X
Q
Q X V n
(5)
It is shown that, the reactive power sharing relative
error is related to some factors, which include the
impedance Xj, the impedance difference (Xj−Xi), the
voltage amplitude Vpcc of PCC and the droop slope nj.
According to (5), there are two main approaches to
improve the reactive power sharing accuracy:
Increasing impedance Xj and the droop gain nj. Usually,
increasing impedance is achieved by the virtual
impedance [11,13-14], which requires a
high-bandwidth control for inverters. Increasing the
droop gain nj is a simpler way to reduce the sharing
error. However, it may degrade the quality of the
microgrid bus voltage, and even affects the stability of
the microgrid system [18-20].
III PROPOSED REACTIVE POWER SHARING
ERROR COMPENSATION METHOD
A. Proposed Droop Controller
The proposed droop control method is given as
follows:
*
=
i i i
m P
(6)
1
*
1 1
( ) ( )
k k
n n
i i i i i
n n
E t E n Q t K Q G E
(7)
where k denotes the times of synchronization event
until time t. According to (7), the control is a hybrid
system with continuous and discrete traits. In the
digital implementation of the proposed method, the
continuous variables ( )
i
E t and ( )
i
Q t are discretized
with sampling period s
T , and s
T is greatly less than
the time interval between two consecutive
synchronization events. Therefore, the droop equation
(7) at the k-th synchronization interval could be
expressed as
1
*
1 1
k k
k k n n
i i i i i
n n
E E nQ K Q G E
(8)
where ω* and E* are the values of DG angular
frequency and output voltage amplitude at no-load
condition; mi and ni are the droop gain of frequency
and voltage of DG-i unit; Gn
is the voltage recovery
operation signal at the n-th synchronization interval,
Gn
has two possible values: 1 or 0. If Gn
=1, it means
the voltage recovery operation is performed. Qi
n
represents the output reactive power of DG-i unit at
the n-th synchronization interval. Ki is a compensation
coefficient for the DG-i unit, ΔE is a constant value
for voltage recovery. For simplicity of description, the
third term of (8) is referred to the sharing error
reduction operation, and the last term is called the
voltage recovery operation. For simplicity, the output
voltage for the DG-i unit in (8) is written as follows in
iterative method.
1 1 1
( )
k k k k k k
i i i i i i i
E E n Q Q K Q G E
(9)
Therefore, in its implementation, only
1
k
i
E
and
1
k
i
Q
should be stored in DSP. To better understand
5. 4
4
the proposed method, a specific example is given. If
there are two DG units with the same capacity
working in parallel, and only the conventional droop
is used. There will be exists some reactive power
sharing error due to some factors. If the sharing error
reduction operation for each DG unit is performed at
the time, the resulting reactive power sharing error
will decrease. The principle behind the sharing error
reduction operation can be understood with the aid of
Fig. 3. If the aforementioned operation is repeated
with time, the reactive power sharing error will
converge. However, the associated operations will
result in a decrease in PCC voltage. To cope with the
problem, the voltage recovery operation will be
performed. That is to say if the output voltage of one
DG unit is less than its allowed lower limit, then the
DG unit will trigger the voltage recovery operation
until its output voltage is restored to rating value. The
output voltage of all the DG units will be added an
identical value ΔE to increase the PCC voltage. The
idea for the voltage recovery operation can be
comprehended by the aid of Fig. 4.
1 1 1
pcc pcc
E E Q X E
i
E
*
=
i q i
E E n Q
1
1 1
k
K Q
i
Q
1
Q 2
Q
1new
Q 2new
Q
1
2 2
k
K Q
pcc
V
new
pcc
V
1 1 1
new new new new
pcc pcc
E E Q X E
err
Q
new
err
Q
Fig. 3 Schematic diagram of the shaing error reduction operation
i
E
i
Q
1
Q 2
Q
new
err
Q
pcc
V
new
pcc
V
0
E
1 1 1
= +( / )
new new new new
pcc pcc
E V X V Q
q
n
err
Q
Fig. 4 Schematic diagram of voltage recovery mechanism
B. Communication setup
A DG unit can communicate with other DG units by
RS232 serial communication. Each DG unit has the
opportunity to trigger a synchronization event on the
condition that the time interval between two
consecutive synchronization events is greater than a
permissible minimum value and the output voltage of
each DG unit is in the reasonable range. If the output
voltage of one DG unit is less than its allowed lower
limit, it will ask for having the priority to trigger a
synchronization event at once. Until the constraint
which two consecutive synchronization events is
greater than a permissible minimum value is satisfied,
the DG unit with the priority will trigger a
synchronization event, and in this event, the command
for voltage recovery operation will be sent to other
DG units. If the communication fails (the time interval
between two consecutive synchronization events is
greater than a permissible maximum value), all the
error reduction operations and voltage recovery
operations should be disabled and the proposed control
method is revert back to the conventional one.
According to the analysis above, such a microgrid
system only needs a low-bandwidth communication.
And it is robust to the delay of communication. To
illustrate this point, the control timing diagram shown
in Fig.5 is used. The sharing error operation and the
voltage recovery operation are performed in update
interval. Sampling operation occurs in sampling
6. 5
5
interval. There is a time interval , which is long
enough to guarantee the system having been in steady
state. It is obvious that proposed method is robust to
the time delay because all the necessary operations
only need to be completed in an interval, not a critical
point.
Update
interval
k-1-th synchronization
Sampling
interval
1
k
i
Q 1
k
i
E k
i
Q k
i
E
k-th synchronization k+1-th synchronization
Update
interval
Sampling
interval
w
t w
t
t
Fig. 5 Control timing diagram of one DG with the two consecutive
synchronization events.
C. Convergence Analysis
In this subsection, the convergence of the proposed
method will be proved. Without loss of generality, the
sharing reactive power error between DG-i and DG-j
with the same capacity will be analyzed. According to
(8), the reactive droop equation for DG-j can be
expressed as
1
*
1 1
k k
k k n n
j j j j j
n n
E E n Q K Q G E
(10)
Subtracting (10) from (8), then
1
1
=
k
k k n
ij ij ij
n
E n Q K Q
(11)
where n=nj=ni, K=Kj=Ki., and ΔEk
ij is the voltage
magnitude derivation of DG i and j in the k-th control
period; ΔQk
ij is the reactive power sharing errors.
Similarly, we can get equation (11) in the k+1-th
interval.
1 1
1
=
k
k k n
ij ij ij
n
E n Q K Q
(12)
Combining (11) and (12), it yields:
+1 1
k k k k k
ij ij ij ij ij
E E n Q n Q K Q
(13)
According to the feeder characteristic, as shown in
(2), the following expressions can be obtained.
1
+1 1 1
1
= ( )
k
pcc
k k k
ij i i j j
V
E Q X Q X
(14)
1
= ( )
k
pcc
k k k
ij i i j j
V
E Q X Q X
(15)
Assume the PCC voltage value satisfy the
following
1
1
k k
pcc pcc
V V V
(16)
Subtracting (13) from (14), it yields
+1 1 1
= ( )+ ( )
j
k k k k k k
ij ij ij ij i i
X X
E E Q Q Q Q
V V
(17)
where i j
X X X
.
Combining the expression (13) and (17), then
+1 +1
( ) [ ]
j
k k k k
X
ij ij i i
V n X V
Q r Q Q Q
(18)
where 1
X j
V
X j
V
n K
n
r
. According to the contraction
mapping theorem, if 1
r and 0
X
, then reactive
power sharing error will converge to zero. However,
0
X
, we should also consider the effect of the
second term of (18).
According to the feeder characteristic, as shown
in (1), we have
1
+1 ( )
k k
k k i i
i i
i
E E V
Q Q
X
(19)
Because of the voltage recovery mechanism, we can
ensure min max
k
i
E E E
for all k.
+1
max min
( )
k k
i i
i
V
Q Q E E
X
(20)
Therefore, the second term of (18) is bounded.
According to analysis above, it can be concluded that
the reactive power sharing error is also bounded.
IV SIMULATION AND EXPERIMENTS RESULTS
A. Simulation Results
The proposed improved reactive power sharing
strategy has been verified in MATLAB/Simulink and
experimentally. In the simulations and experiments, a
microgrid with two DG systems, as shown in Fig. 1, is
employed. The associated parameters for Power stage
and control of the DG unit are listed in Table I. Also in
the simulations and experiments, in order to facilitate
the observation of the reactive power sharing, the two
DG units are designed with same power rating and
different line impedances. The detailed configuration
7. 6
6
of the single DG unit is depicted in Fig. 6, where an
LCL filter is placed between the IGBT bridge output
and the DG feeder. The DG line current and filter
capacitor voltage are measured to calculate the real
and reactive powers. In addition, the commonly used
double closed-loop control is employed to track the
reference voltage [5], [7], [12].
line
L
Improved droop
control (Eq.8)
SPWM
sin ( )
ref ref
E dt
Double loop
control
ref
E
ref
*
ref
E
c
u
Line
i
L
i
c
u
P Q
f
L
f
C
dc
V
Common
Bus
The proposed
Controller
Main circuit
Power
calculation
Fig. 6 Configuration of one single-phase DG unit.
Tab. I
Associated parameters for Power stage and control of the DG unit
Parameters Values Parameters Values
urate (V) 220 kpu 0.05
Lf (mH) 1.5e-3 kpi 50
rf (Ω) 0.25 Kip 0.2
Cf (μF) 20 wc(rad/s) 31.4
LLine1 (mH) 0.6e-3 m(rad/sec·w) 5e-5
LLine2 (mH) 0.3e-3 n(v/var) 5e-3
fs (KHz) 12.8 Ke(v/var) 0.001
frate (Hz) 50 ΔE0(V) 5
Ts (s) 1/12.8e3 Tsyn(min) (s) 0.1
1) Case 1: power sharing accuracy improvement
Two identical DG units operate in parallel with the
proposed voltage droop control. Fig.7 illustrates the
reactive power sharing performance of the two DGs.
Before t=0.5s, the sharing error reduction operation
and voltage recovery operation are disabled, which is
equivalent to the conventional droop control being in
effect. There exists an obvious reactive power sharing
error due to the unequal voltage drops on the feeders.
After t=0.5s, the reactive power sharing error
reduction operation is performed, it is clear that the
reactive power sharing error converges to zero
gradually. After t=1s, the voltage recovery operation is
performed. It can be observed that the output reactive
power increases but the reactive power sharing
performance does not degrade. Fig.8 shows the
corresponding output voltages. It can be observed that
the output voltages decrease during the sharing error
reduction operation, while the voltage recovery
operation ensures that DG output voltage amplitude
can restore back nearby to the rated value. The whole
process of adjustment can be done steadily in a
relatively short period of time. Fig.9 illustrates active
power sharing performance of the two DG units. It is
obvious that the proposed improved reactive power
sharing strategy does not affect active power sharing
performance.
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
0
100
200
300
400
500
600
t/s
Q/Var
DG2
DG1
stage2 stage3
stage1
Fig. 7 Output reactive powers of two inverters with the improved droop
control.
0 0.5 1 1.5 2
290
295
300
305
310
315
t/s
Uo/V
DG1
DG2
stage1 stage2 stage3
syn
T
Limited voltage
Fig. 8 Output voltage amplitude of two inverters with the improved
droop control.
0 0.5 1 1.5 2
0
500
1000
1500
2000
2500
3000
t/s
P/w
DG1
DG2
stage1 stage2 stage3
Fig. 9 Output active powers of two inverters with the improved droop
control.
2) Case 2: Effect of the communication delay
To test the sensitivity of the proposed improved
droop control to the synchronized signal accuracy, a
0.02s delay is intentionally added to the signal
8. 7
7
received by DG1 unit at t=0.5s as shown in Fig.11,
and the simulation results are shown in Fig.10, 11 and
12. Compared to the case 1 in Fig.7 and 9, a small
disturbance appears in both the reactive and active
power, while the voltage recovery operations are still
able to ensure that the DG unit can deliver the
expected reactive power. After t=2.0s, the active and
reactive power sharing errors are almost zero.
Therefore, the proposed reactive power sharing
strategy is not sensitive to the communication delay.
Then it is illustrated that it is robust to some small
communication delays.
0 0.5 1 1.5 2
0
100
200
300
400
500
600
t/s
Q/Var
DG2
DG1
stage1 stage2 stage3
Communication
delay
Fig. 10 Output reactive powers of the two inverters when 0.02s time
delay occurs in synchronization signal of DG1 unit
t/s
Uo/V
0.02s delay
DG2
DG1
Fig. 11 DG output voltage of the inverters when 0.02s time delay
occurs in synchronization signal of DG1 unit
0 0.5 1 1.5 2
0
500
1000
1500
2000
2500
3000
t/s
P/w
DG1
DG2
stage1 stage3
stage2
Communication
delay
Fig. 12 Output active powers of the two inverters when 0.02s time
delay occurs in synchronization signal of DG1 unit
3) Case 3: Effect of load change
In order to test the effect of load change with the
proposed method, the active load increases about
1.6kW and the reactive load increases about 0.4kVar
at t=2.5s, and at t=4.5s the active load decreases about
3.0kW and the reactive load decreases about 0.8kVar.
The corresponding simulation results are shown in
Fig.13 and 14. As can be seen, a large reactive power
sharing deviation appears at t=2.5s and t=4.5s.
However, the deviation becomes almost zero after a
while. Fig.15 illustrates the corresponding output
voltage waveforms. It can be found that there exists a
obvious output voltage decrease and output voltage
increase process during each reactive power sharing
error reduction process.
t/s
Q/Var
Load change
DG2
DG1
Fig. 13 Reactive power sharing performance of the improved droop
control (with load varying)
t/s
P/W
Load change
Fig. 14 Active power sharing performance of the improved droop
control (with load changing)
t/s
Uo/V
DG2
DG1
Load change
Fig. 15 DG output voltage of the improved droop control (with load
changing)
B. Experimental Results
A microgrid prototype is built in lab as shown in
Fig.16. The microgrid consists of two micro-sources
based on the single-phase inverter. The parameters for
output filter are the same as those in simulation. The
load consists of a resistor of 16Ω and a inductor of
3mH. The sample frequency is 12.8 kHz. A
permissible minimum time interval between two
9. 8
8
consecutive synchronization events is 0.5s. The
permissible minimum output voltage does not less
than the rated voltage by 90%.
Fig.16 Prototype of parallel inverters system setup
Fig. 17 and Fig.18 shows the measured waveforms
with the conventional and improved droop control
methods, respectively. The waveforms from top to
down are circular current (i0H=i01-i02), the output
current of inverter 1 (i01), the output current (i02) of
inverter 2 and PCC voltage (UL), respectively. As can
be seen from Fig. 17, there is a quite large phase
difference between two output currents when the
conventional droop control is applied. As a result, the
circular current is pretty high and the peak value of
circular current is up to 1.80 A. The main reason for it
is the impedance difference in DG feeders. Compared
with the circular current in Fig.17, the circular current
in Fig.18 is very small, which indicates that the
improved method is efficient in reducing the circular
current mainly caused by the output reactive power
difference between the inverters.
UL
I01
I0H
I02
(2A/div)
(2A/div)
(2A/div)
(50v/div)
Fig.17 Steady state experimental waveforms with the conventional
droop control.
UL
I01
I0H
I02
(2A/div)
(2A/div)
(2A/div)
(50v/div)
Fig.18 Steady state experimental waveforms with the improved droop
control.
Fig.19 shows the steady-state output active and
reactive power of each inverter with the conventional
and the improved droop control. Fig.19 (a) shows the
results with the conventional droop. The steady-state
output active powers of the inverters are 31.4 W and
30 W, and the output reactive powers are 21.2 Var and
-10.4 Var. When using conventional P-f droop control,
no active power divergence appear since frequency is
a global variable, i.e. same frequency can be measured
along the microgrid; however, voltage may drop along
the microgrid power lines, which produces the well
know reactive power divergence. Fig. 19(b) shows the
results with the improved droop. As can be seen, the
output active powers of the inverters are 30.6 W and
31.1 W, and the reactive powers are 3.9 Var and 4.4
Var. These results indicates that the proposed
improved droop control has no effect on the active
power sharing performance, but makes reactive power
be shared precisely .
0 0.5 1 1.5
0
10
20
30
40
Time(s)
P(W)
/
Q(Var)
Q1 and Q2
P1 and P2
0
0
20
40
P(w)
/
Q(Var)
t(s)
0 0.5 1 1.5
-20
0
20
40
Time(s)
P(W)/Q(Var)
Q1
Q2
P1 and P2
-20
20
40
0
0.5 1 1.5 2 0 0.5 1 1.5 2
(a) (b)
Fig. 19 Steady-state active power and reactive power a) with the
conventional droop; b) with the improved droop control.
To verify the effectiveness of the sharing error
reduction operation and voltage recovery operation of
the proposed method, the experiments with only one
operation being continuously used are performed. As
can be seen from Fig.20, the circular current
converges to a small value gradually when only the
reactive power sharing error reduction operation is
performed. In the meanwhile, a continuous decrease in
10. 9
9
PCC voltage could be found. Fig.21 shows the results
when only the voltage recovery operation is
performed. It can be seen that the PCC voltage
increases linearly during this time, and the circular
current is always small and be almost kept constant.
Fig.22 shows the results when the two operations are
combined. i.e. the proposed method is applied. The
circular current is controlled to be small value, and the
quality of the PCC voltage is guaranteed successfully.
I0H
UL m
U
(2A/div)
(25v/div)
m
U
Fig. 20 Circulating current and PCC voltage waveforms of DGs with
only sharing error reduction operation performed.
E
I0H
UL
(2A/div)
(25v/div)
Fig. 21 Circulating current and PCC voltage waveforms of DGs with
only voltage recovery operation performed.
UL
I0H (2A/div)
(25v/div)
Fig.22 Circulating current and PCC voltage waveforms of DGs with
the improved droop.
To test the sensitivity of the proposed method to
synchronization signal, a 0.2 s delay is intentionally
added to the synchronization signal received by DG1
unit every time. The associated experimental results
are shown in Fig. 23. Compared to the normal case,
there is no obvious difference between the two cases,
and the reactive power sharing error can still reduce to
a small value. Therefore, the proposed method is
robust to the communication delay because all the
necessary operations only need to be completed in an
interval, not a critical point.
Fig.24 shows the experimental results when the
synchronization signal of DG1 unit fails, which is
equivalent to the time delay is infinity. It is obvious
that, before t=t1, the circulating current is kept to be a
small value because the improved droop control is in
effect. After t=t1, the sharing error reduction operation
and voltage recovery operation are disabled due to the
lost of the synchronization signal of DG1 unit. As a
result, the peak value of the circulating current
increases to about 2.8A from a small value. In
conclusion, the results in Fig.23 and Fig.24 indicate
that the proposed method only needs a low-bandwidth
requirement, and it is robust to a small time delay of
communication. However, once communication fails
completely, the reactive power sharing accuracy
performance may be worse.
I01
I0H
I02
Communication
delay
t1
(2A/div)
(2A/div)
(2A/div)
t1
Fig.23 Output current and circulating current waveforms when 0.2 s
time delay occurs in synchronization signal of DG1 unit.
11. 10
10
(2A/div)
(2A/div)
(2A/div)
I01
I0H
I02
Communication
failure
t1
t1
Fig.24 Output current and circulating current waveforms when the
synchronization signal is lost in DG1 unit.
V CONCLUSIONS
In this paper, a new reactive power control for
improving the reactive sharing was proposed for
power electronics interfaced DG units in AC
micro-grids. The proposed control strategy is realized
through the following two operations: sharing error
reduction operation and voltage recovery operation.
The first operation changes the voltage bias of the
conventional droop characteristic curve periodically,
which is activated by the low-bandwidth
synchronization signals. The second operation is
performed to restore the output voltage to its rated
value. The improved power sharing can be achieved
with very simple communications among DG units.
Furthermore, the plug-and-play feature of each DG
unit will not be affected. Both simulation and
experimental results are provided to verify the
effectiveness of the proposed control strategy.
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