Grid-connected Operation Modes Research Articles

A Smooth Transition Control Strategy for Microgrid Operation Modes

According to the characteristics of microgrid in both grid-connected and islanding operation modes, control strategies are proposed to achieve smooth transition between these two modes. Under grid-connected mode, the fluctuation of power in microgrid is balanced by utility power grid. while DGs are adopting the PQ control to ensure the balance of power and realize energy management when the energy storage is in the state of charge standby; When microgrid transits to islanding mode, the droop V/f control strategy is employed to energy storage to support the voltage and frequency of the microgrid system and the power shortage of the whole system will be compensated. When microgrid reconnect to utility power grid, a three phase software phase-locked loop (SPLL) based pre-synchronizing unit is designed to track the utility grid voltage, which makes the process of reconnecting to grid stable and safe. In the whole process, the optimal distribution and reliable protection of the energy storage is well considered to meet control targets of smaller capacity and high efficiency, which can reduce the pollution and emissions levels. Simulations are carried out by MATLAB/SIMULINK. Simulation results confirm that the smooth switch of the control system is effective and stable.

Power Management Based Current Control Technique for Photovoltaic-Battery Assisted Wind–Hydro Hybrid System

Abstract This article proposes new power management based current control strategy for integrated wind–solar–hydro system equipped with battery storage mechanism. In this control technique, an indirect estimation of load current is done, through energy balance model, DC-link voltage control and droop control. This system features simpler energy management strategy and necessitates few power electronic converters, thereby minimizing the cost of the system. The generation–demand (G–D) management diagram is formulated based on the stochastic weather conditions and demand, which would likely moderate the gap between both. The features of management strategy deploying energy balance model include (1) regulating DC-link voltage within specified tolerances, (2) isolated operation without relying on external electric power transmission network, (3) indirect current control of hydro turbine driven induction generator and (4) seamless transition between grid-connected and off-grid operation modes. Furthermore, structuring of the hybrid system with appropriate selection of control variables enables power sharing among each energy conversion systems and battery storage mechanism. By addressing these intricacies, it is viable to regulate the frequency and voltage of the remote network at load end. The performance of the proposed composite scheme is demonstrated through time-domain simulation in MATLAB/Simulink environment.

Line-Interactive UPS for Microgrids

Line-interactive uninterruptible power supply (UPS) systems are good candidates for providing energy storage within a microgrid to help improve its reliability, economy, and efficiency. In grid-connected mode, power can be imported from the grid by the UPS to charge its battery. Power can be also exported when required, e.g., when the tariffs are advantageous. In stand-alone mode, the UPS supplies local distributed loads in parallel with other sources. In this paper, a line-interactive UPS and its control system are presented and discussed. Power flow is controlled using the frequency and voltage drooping technique to ensure seamless transfer between grid-connected and stand-alone parallel modes of operation. The drooping coefficients are chosen to limit the energy imported by the UPS when reconnecting to the grid and to give good transient response. Experimental results of a microgrid consisting of two 60-kW line-interactive UPS systems are provided to validate the design.

Coordination of Overcurrent, Directional and Differential Relays for the Protection of Microgrid System

Microgrid implementation draws new challenges especially in the coordination of protective devices that are not likely to appear in a traditional distribution network. Variation of fault current levels, bi-directional fault current and changing of microgrid operation modes make protection of the microgrids more difficult. In this work, a systematic procedure for coordination of different protection schemes is presented to secure the operation of a microgrid system. The procedure initially starts with coordination of overcurrent (OC) relays and followed by directional overcurrent (DOC) relays in the system. If the coordination of these two types of relays is unsatisfactory, then differential (DIF) relays are used to replace the OC relay in the faulty section. The proposed method takes into account device selectivity, sensitivity and proper protection function simultaneously. Simulation results show that the proposed coordination method safely protect a microgrid system during both grid-connected and islanded operation modes.

Loading constraints for first swing stability margin enhancement of distributed generation

This study extracts effective loading constraints for synchronous machine-based distributed generators (SMDGs) in a micro-grid to ensure their post-fault first swing stability. The notion of trajectory sensitivity is used to determine the sensitivity of the SMDGs rotor angles and velocities with respect to the mechanical input power. Then, appropriate loading constraints for SMDGs are calculated for a set of pre-specified faults. The proposed methodology can be used for a micro-grid with an arbitrary number of SMDGs, under various fault scenarios, as well as both grid-connected and autonomous (islanded) modes of operation. The proposed methodology is verified using time-domain simulation of a modified benchmark system of the IEEE standard 399-1997.

Capacitor Design Considering First Swing Stability of Distributed Generations

This paper proposes an algorithm for designing a fixed shunt capacitive compensator to improve first swing stability margin of a micro-grid including synchronous machine-based distributed generators (SMDGs). The notion of trajectory sensitivity analysis is used to determine the sensitivity of the SMDGs' relative rotor angles and velocities with respect to the compensator susceptance. For a given fault contingency, the minimum required susceptance and compensator location are determined to maintain the first swing stability of SMDGs. The proposed methodology can be used for a micro-grid with an arbitrary number of SMDGs, various fault scenarios, and both grid-connected and autonomous (islanded) modes of operation. The proposed algorithm is verified using time-domain simulation of a modified benchmark system of the IEEE standard 399-1997.

Control of a Multiple Source Microgrid With Built-in Islanding Detection and Current Limiting

An approach for the control of a voltage-sourced converter-interfaced distributed energy resource microgrid environment with multiple energy sources is analyzed and experimentally validated. The control approach is designed to operate in grid-connected and islanded modes of operation, as well as provide a smooth transition between the two modes. Additional features including islanding detection with positive feedback and dynamic overcurrent limiting are also evaluated. Validation is achieved through the results obtained from a scaled down prototype system with further results from the time-domain simulation of a medium-voltage microgrid.

Transition Control Strategy Between Standalone and Grid-Connected Operations of Voltage-Source Inverters

This paper proposes a smooth transition control strategy for voltage-source inverters between standalone (SA) and grid-connected (GC) modes of operation. In the GC mode, the amount of power exchanged with the utility grid is controlled by regulating the phase currents. In the SA mode, the load voltage is regulated by the inverter with its phase dictated by the inverter control. The transition between SA and GC operations that will ensure continuous power delivery to the load requires continuation in the phase of the system voltage. The proportional-integral, trapezoidal, sinusoidal, and staircase frequency variation techniques have been analyzed to find the best approach for minimizing the total harmonic distortion (THD). A smooth frequency variation technique has been developed, which provides lower THD on the voltage waveforms compared with the other techniques. The new algorithm has been implemented on a 5-kW single-phase utility interactive inverter having the SA operation capability. The simulation and experimental results show that the proposed technique provides seamless transition between the inverter modes of operation with minimal distortion at the inverter output voltage.

A Flexible Control Strategy for Grid-Connected and Islanded Microgrids With Enhanced Stability Using Nonlinear Microgrid Stabilizer

The energy sector is moving into the era of distributed generation (DG) and microgrids (MGs). The stability and operation aspects of converter-dominated DG MGs, however, are faced by many challenges. Important among these, are: 1) the absence of physical inertia; 2) comparable size of power converters; 3) mutual interactions among generators; 4) islanding detection delays; and 5) large sudden disturbances associated with transition to islanded mode, grid restoration, and load power changes. To overcome these difficulties, this paper presents a new large-signal-based control topology for DG power converters that is suitable for both grid-connected and islanding modes of operation without any need to reconfigure the control system and without islanding detection. To improve MG stability, the proposed control structure is realized via two steps. First, an emulated inertia and damping functions are adopted. Second, to guarantee stability and high performance of the MG system during sudden harsh transients such as islanding, grid reconnection, and large load power changes, a nonlinear MG stabilizer is proposed. An augmented converter model is developed and used to design the MG stabilizer via the adaptive backstepping (AB) technique to guarantee large-angle stability and robustness against unmodeled dynamics. Theoretical analysis and evaluation results are presented to show the effectiveness of the proposed control scheme in achieving stable and smooth operation of a MG system in grid-connected, islanding, and transition modes.

Universal wind turbine working in grid-connected and island operating modes

In the future power system based on active distribution grid, allowing some portion to be disconnected, the concept of universal wind turbine systems may arise. This concept covers both grid-connected and island operations. This may happen if the voltage goes outside the range recommended by the standards. In such conditions, an island is constituted where the universal wind turbine is capable to manage the local voltage and frequency assuring the continuous supply to local loads. In this paper, a control loop based on droop and voltage controllers disposed in cascade is employed in both operation modes. Droop control offers power load sharing between the distributed generation sources connected in parallel in the island. The voltage controller is designed to provide smooth transitions between modes and power quality. A synchronization system is also introduced for assuring safe reconnections with the main utility. Simulation and experimental results are presented to verify the performance of the complete system.

A novel unsymmetrical faults analysis for microgrid distribution systems

A novel unsymmetrical faults analysis method with hybrid compensation for microgrid (MG) distribution systems is proposed. The method employs the actual three-phase models to handle unsymmetrical faults. Two matrices, which are built from the topological characteristics of MG distribution networks, and then combined with the proposed hybrid compensation method for injecting branch mismatch currents caused by the fault are used to analyze the branch mismatch currents and bus mismatch voltages directly. An unsymmetrical fault, with appropriate fault boundary conditions can be obtained. The proposed method can then be developed from these two matrices and be used to solve the various types of single or simultaneous unsymmetrical faults. This paper also discusses modeling of a microturbine generation (MTG) as a distributed generator (DG), which delivered for both islanded and grid-connected modes of operation. Test results show that the proposed method is efficient, accurate and easy to program for unsymmetrical faults analysis.

Control Strategy of Microgrid Based on Distributed Generation

Upon the traditional wind-solar hybrid distributed generation system,a single-phase microgrid experimental system platform has been built based on the DC bus,Z topology was used to increase the energy utilization ratio. On the basic of realizing the maximum power point tracker of the new energy, different microgrid controlling strategy has been designed to apply to the island and grid connected operation mode: in the cases of island running, master slave control mode will be utilized in the system and the battery will be used as the main control unit to provide the reference voltage; on the other hand, a control method which can merge the generation into the grid as much as possible will be adopted in the case of grid connected. By the use of system model, the simulation of the two operation mode switch state and power flow has been done and the feasibility and effectiveness of the control strategy can be guaranteed,it also has been verified by the microgrid experimental platform.

RTDS를 이용한 신재생에너지 기반 마이크로그리드 시뮬레이션 해석

Due to enhanced demands on quality, security and reliability of the electric power energy system, a microgrid has become a subject of special interest. In this paper, output characteristics of energy storage system (ESS) with an electric double layer capacitor (EDLC) and battery energy storage system (BESS) of a renewable energy based microgrid were analyzed under grid-connected and islanded operation modes. The microgrid which consists of photovoltaic and wind power turbine generators, diesel generator, ESS with an EDLC, BESS and loads was modeled using real time digital simulator. The results present the effective control patterns of the microgrid system.

A Control Strategy for a Distributed Power Generation Microgrid Application With Voltage- and Current-Controlled Source Converter

This paper presents a pseudodroop control structure integrated within a microgrid system through distributed power generation (DPG) modules capable to function in off-grid islanded, genset-connected, and grid-connected modes of operation. System efficiency has an important role in order to harvest the maximum available renewable energy from dc or ac sources while providing power backup capability. A control strategy is proposed in off-grid islanded mode method based on the microgrid line-frequency control as agent of communication for energy control between the DPG modules. A critical case is where the ac load demand could be lower than the available power from the photovoltaic solar array, where the battery bank can be overcharged with unrecoverable damage consequences. The DPG voltage-forming module controls the battery charge algorithm with a frequency-generator function, and the DPG current source module controls its output current through a frequency-detection function. The physical installation between DPG modules is independent, since no additional communication wiring is needed between power modules, which represent another integration advantage within the microgrid-type application.

Investigation on Electromagnetic Transients of Distributed Generation Systems in the Microgrid

The increasing interconnection of distributed generation sources of diverse technologies to low-voltage grids introduces considerable complexity in its operation and control. The concept of the microgrid is emerging as a solution to this and also to take full advantage of the potential offered by distributed generation. In this article, the performance of a typical microgrid with multiple distributed generation systems in grid-connected and autonomous modes of operation has been investigated through simulation. The developed model of the microgrid consists of a converter-interfaced microturbine generation system, a synchronous-generator-based distributed generation system, and a wind power generation system with an asynchronous generator. Investigation has been carried out to study the typical electromagnetic transients of a microgrid, due to preplanned and unplanned switching events. The performance of the bidirectional grid interface topology for a microturbine generation system in a microgrid is evaluated in this work. It has been observed from the simulation results that the motoring mode operation of the microturbine generation system during starting does not cause any disturbances in the microgrid. The study also indicates that the microgrid can maintain the desired power quality at the point of common coupling.

Steady-State and Transient Performance of Biodiesel-Fueled Compression-Ignition-Based Electrical Generation

This paper investigates the performance characteristics of rapeseed methyl ester, EN 14214 biodiesel, when used for electrical generation in compression ignition engines. The work was inspired by the need to replace fossil diesel fuel with a sustainable low carbon alternative while maintaining generator performance, power quality, and compliance with ISO 8528-5. A 50-kVA Perkins diesel engine generator was used to assess the impact of biodiesel with particular regard to gen-set fuel consumption, load acceptance, and associated standards. Tests were performed on the diesel gen-set for islanded and grid-connected modes of operation, hence both steady-state and transient performance were fully explored. Performance comparisons were made with conventional fossil diesel fuel, revealing minimal technical barriers for electrical generation from this sustainable, carbon benign fuel. Recommendations for improved transient performance are proposed and validated through tests.

Control System for a Diesel Generator and UPS Based Microgrid

Control System for a Diesel Generator and UPS Based MicrogridIn this paper a microgrid composed of a diesel generator and two uninterruptible power supply systems with separate battery banks is introduced. The microgrid located in three academic buildings of Tallinn University of Technology. A three-level control and monitoring system for the microgrid based on the EtherNet/IP communication network is developed. In addition, a control strategy of the microgrid in the grid-connected and stand-alone mode of operation is proposed.

Seamless Transfer of Microturbine Generation System Operation Between Grid-connected and Islanding Modes

The intentional islanding operation of grid-connected distributed generation systems can greatly improve the reliability and quality of the power supply. The existing control techniques for distributed generation systems are designed to operate either in the grid-connected or islanding modes of operation, thus, not allowing for both modes to be implemented and transitioned between. In this article, a novel scheme for automatic mode switching of a microturbine-based distributed generation system between the grid-connected and islanding modes of operation is proposed. The presented scheme is based on the phase angle estimated by the phase-locked loop. The developed phase-locked loop provides an accurate estimation of the phase angle even under unbalanced conditions. The presented scheme does not negatively affect the distributed generation system or utility operations and can work even under matching distributed generation and load power ratings. In this work, back-to-back converters are used to interface the microturbine-based distributed generation system to the grid. Converter control strategies developed for both modes of distributed generation operation is also presented. The simulation results show good accuracy of the proposed scheme.

A Control Strategy for a Distributed Generation Unit in Grid-Connected and Autonomous Modes of Operation

This paper presents a new voltage control strategy for an electronically-interfaced distribution generation (DG) unit that utilizes a voltage-sourced converter (VSC) as the interface medium. The control strategy is based on the concept of voltage-controlled VSC (VC-VSC) rather than the conventional current-controlled VSC (CC-VSC). The proposed VC-VSC 1. enables operation of a DG unit in both grid-connected and islanded (autonomous) modes, 2. provides current-limit capability for the VSC during faults, 3. inherently provides an islanding detection method without non-detection zone, 4. provides smooth transition capability between grid-connected and autonomous modes, and 5. can accommodate ride-through capability requirements under a grid-connected mode. This paper also investigates performance of the proposed VC-VSC strategy based on an eigenanalysis in MATLAB, and time-domain simulations in the PSCAD/EMTDC environment.

Stability of Voltage and Frequency Control in Distributed Generation Based on Parallel-Connected Converters Feeding Constant Power Loads

SummaryIn this paper stability and dynamic properties of voltage and frequency droop control of power electronic converters are investigated for a distributed generation system. Droop control is utilized to share active and reactive power among the source converters. The voltage and frequency controllers are designed so that stand-alone converter operation feeding constant power loads performs satisfactory. These controllers are adapted to mimic the behaviour of present rotating generators connected directly to the power system, for seam-less transfer between island and grid-connected modes of operation. The target system for the analysis is a stand-alone system to which three power electronic converters and one rotating generator could be connected (Fig. 1). Small-signal and switch-mode, time-domain simulation results of a three-converter distributed generation system facilitating stand-alone operation verify the operation. Simulation and experimental results of a three-converter stand-alone system, with and without a rotating generator present, are also included for verification.