Stand-alone Mode Of Operation Research Articles

Power Conditioning for a Small-Scale PV System with Charge-Balancing Integrated Micro-Inverter

The photovoltaic (PV) power conditioning system for small-scale applications has gained significant interest in the past few decades. However, the standalone mode of operation has been rarely approached. This paper presents a two-stage multi-level micro-inverter topology that considers the different operation modes. A multi-output flyback converter provides both the DC-Link voltage balancing for the multi-level inverter side and maximum power point tracking control in grid connection mode in the PV stage. A modified H-bridge multi-level inverter topology is included for the AC output stage. The multi-level inverter lowers the total harmonic distortion and overall ratings of the power semiconductor switches. The proposed micro-inverter topology can help to decrease the size and cost of the PV system. Transient analysis and controller design of this micro-inverter have been proposed for stand-alone and grid-connected modes. Finally, the system performance was verified using a 120 W hardware prototype.

Design, Operation, and Control of S3 Inverter for Single-Phase Microgrid Applications

A single-phase voltage source inverter with a front-end dc–dc conversion stage followed by a synchronized push–pull configuration operating at a desired fundamental frequency (FF) is presented. The duty cycle of the dc–dc conversion stage is varied in the form of a unidirectional sine wave to produce a similar output voltage across the dc-link capacitor. The unidirectional voltage is made into an alternating voltage by the synchronized push–pull configuration. This inverter employs three semiconductor switches, in which one is operating at a high frequency and the rest are operating at an FF. Hence, it is named as the S3 inverter. Furthermore, simple and cost-effective analog circuits are presented for the generation of switching pulses and the control of the amount of power fed to the grid. The hardware prototype of the S3 inverter has been built in a laboratory, and its performance during the stand-alone and grid-connected modes of operation is validated.

A protection strategy for micro‐grids based on positive‐sequence component

In recent years, the concept of micro-grid has appeared as an appropriate way for the integration of distributed energy resources (DERs) in the distribution networks. However, micro-grids have encountered a number of challenges from control and protection aspects. One of the main issues relevant to the protection of micro-grids is to develop a suitable protection technique which is effective in both grid-connected and stand-alone operation modes. This study presents a micro-grid protection scheme based on positive-sequence component using phasor measurement units and designed microprocessor-based relays (MBRs) along with a digital communication system. The proposed scheme has the ability to protect radial and looped micro-grids against different types of faults with the capability of single-phase tripping. Furthermore, since the MBRs are capable of updating their pickup values (upstream and downstream equivalent positive-sequence impedances of each line) after the first change in the micro-grid configuration (such as transferring from grid-connected to islanded mode and or disconnection of a line, bus or DER either in grid-connected mode or in islanded mode), they can protect micro-grid lines and buses against subsequent faults. Finally, in order to verify the effectiveness of the suggested scheme and the designed MBR, several simulations have been undertaken using DIgSILENT PowerFactory and MATLAB software packages.

Stand Alone Performance of Permanent Magnet Synchronous Wind Power Generator with Current Source Matrix Converter

—A matrix converter is a voltage/current source AC/AC frequency converter. In grid-connected operation of a variable-speed permanent magnet synchronous wind power generator, the matrix converter is normally controlled as a voltage source converter. In this control method, the generator-side voltage is synthesized from the grid-side voltage source. However, in the stand-alone mode of operation, the grid-side stiff voltage source is not available, and the input filter of the matrix converter is unstable. In this article, a new control method is presented that controls a permanent magnet synchronous wind generator in a stand-alone mode with a matrix converter as a current source converter. The proposed method is so fast that it can control the matrix converter small capacitor voltage despite the generator large inductance and without any input filter instability. A multi-pole permanent magnet synchronous wind generator is simulated to prove the performance of the proposed control strategy.

An Effective Smooth Transition Control Strategy Using Droop-Based Synchronization for Parallel Inverters

This paper proposes a smooth transition control strategy for voltage-source inverters between stand-alone (SA) and grid-connected (GC) modes of operation. In both GC and SA modes of operation, the inverters use the droop control method to regulate the real power flow without the need of any external communication between them. One of the distributed energy sources working in the microgrid system is designated as a dispatch unit (DU) to facilitate interconnection with utility grid and achieve smooth transition between the modes of operations. During transition modes, this selected unit takes extra responsibility to ensure the continuous power delivery to the load. For smooth transition from GC to SA mode of operation, the DU compensates for the grid current immediately after the grid disconnection, and then, other units pick up this responsibility gradually through their individual droop controllers. The DU also adjusts its output power to synchronize the inverters with the grid during the transition from SA to GC mode. The simulation and experimental results, as well as the stability analysis, are provided to verify the effectiveness of the proposed control technique.

An integrated control and protection system for photovoltaic microgrids

The microgrid has shown to be a promising solution for the integration and management of intermittent renewable energy generation. This paper looks at critical issues surrounding microgrid control and protection. It proposes an integrated control and protection system with a hierarchical coordination control strategy consisting of a stand-alone operation mode, a grid-connected operation mode, and transitions between these two modes for a microgrid. To enhance the fault ride-through capability of the system, a comprehensive three-layer hierarchical protection system is also proposed, which fully adopts different protection schemes, such as relay protection, a hybrid energy storage system (HESS) regulation, and an emergency control. The effectiveness, feasibility, and practicality of the proposed systems are validated on a practical photovoltaic (PV) microgrid. This study is expected to provide some theoretical guidance and engineering construction experience for microgrids in general.

IJIREEICE - Electrical, Electronics, Instrumentation and Control

2 Abstract : This paper presents the dynamic analysis of load following performances of split-shaft microturbine generator (MTG) and Diesel Generator systems considering P-I type load following and P-I type speed controllers under stand-alone mode of operation. In this paper microturbine system coupled with a synchronous generator (SG) is considered. The dynamic performances of split-shaft MTG and Diesel Generator system are also examined when they are connected together to satisfy the load demand. Finally, the MTG and diesel generator systems are connected to an 11 kV rural distribution network to examine the transient behaviour during active power injection. Simulation result for all the cases are analyzed and presented in this paper.

A Method of Seamless Transitions Between Grid-Tied and Stand-Alone Modes of Operation for Utility-Interactive Three-Phase Inverters

A method for the seamless transition of three-phase inverters switched between grid-tied and stand-alone modes of operation is presented in this paper. In this method, only the inverter current and voltage sensors are utilized, and no control over the grid-side static transfer switch is needed. The presented method contains two strategies for grid-tied-to-stand-alone and stand-alone-to-grid-tied transitions. In the stand-alone-to-grid-tied transition strategy, a novel algorithm is presented for estimating the grid angle nearly instantaneously, which allows the three-phase inverter to respond very quickly if the grid and point-of-common-coupling voltages are out of phase. This fast response allows the inverter to effectively eliminate the transient overcurrent that would normally occur if it was connected to the grid without first being synchronized. The fast response also allows the inverter to return to normal operation very quickly after such an event. The strategy for the seamless transition from grid-tied to stand-alone mode is also presented. These strategies have been verified through experiments, and the results are presented in this paper.

Hierarchical Control of Parallel AC-DC Converter Interfaces for Hybrid Microgrids

In this paper, a hierarchical control system for parallel power electronics interfaces between ac bus and dc bus in a hybrid microgrid is presented. Both standalone and grid-connected operation modes in the dc side of the microgrid are analyzed. Concretely, a three-level hierarchical control system is implemented. In the primary control level, the decentralized control is realized by using the droop method. Local ac current proportional-resonant controller and dc voltage proportional-integral controller are employed. When the local load is connected to the dc bus, dc droop control is applied to obtain equal or proportional dc load current sharing. The common secondary control level is designed to eliminate the dc bus voltage deviation produced by the droop control, with dc bus voltage in the hybrid microgrid boosted to an acceptable range. After guaranteeing the performance of the dc side standalone operation by means of the primary and secondary control levels, the tertiary control level is thereafter employed to perform the connection to an external dc system. Meanwhile, the impact of the bandwidth of the secondary and tertiary control levels is discussed. The closed-loop model including all the three control levels is developed in order to adjust the main control parameters and study the system stability. Experimental results of a 2 ×2.2 kW parallel ac-dc converter system have shown satisfactory realization of the designed system.

Nano Magnetic STT-Logic Partitioning for Optimum Performance

Magnetoresistive RAMs (MRAMs) are the new generation of nonvolatile memories that use magnetic tunnel junctions (MTJs) to store bit information. Horizontal (bit and source) and vertical (word) lines partition the MRAM into a 2-D grid similar to a conventional memory. This paper relies on a logic-in-memory architecture where MRAM cells are placed in close proximity so that they can behave both as logic elements and as memory bits. When the clock signal is active the cells compute logic, while at other times the cells store the data in them and behave as memory. Using these MRAM cells, in this paper, we have designed two fundamental components in datapath and logic circuits, the XOR and majority. By transferring logic responsibilities between the metal lines, CMOS peripherals and the MTJs, we have achieved a significant reduction in MTJ cell count, energy, and delay over previous designs. For example, an energy savings of more than 75% and a cell reduction of more than 81.25% are obtained for a 2-input XOR in standalone mode of operation. Though this hybrid sharing of responsibilities between the MTJs and CMOS has apparently increased the overhead on CMOS, it has however reduced the net power consumption in the CMOS peripherals. This happens because the CMOS now has a fewer number of cells to control. The reduction in the CMOS power varies from close to 50% for a 2-input XOR to greater than 10% for a majority. In addition, the designs also obey the rules of hierarchical modeling which helped us to use the novel 2-input XORs as bricks for designing the novel 3-input XOR. Again a 3-input XOR and majority are used to custom develop a one-bit full adder. Together with an inter-cell spacing of 20 nm and low power spin transfer torque current-driven write and clock operations, the novel designs presented in this paper has the potential to target the low energy and high density logic-in-memory applications.

분산모듈형 태양광 전력조절기의 독립운전 제어

This paper proposes a control method under stand-alone operating mode for distributed-module type photovoltaic power conditioners. In conventional schemes, there are some problems of a controller saturation in the DC-link (or load) voltage controller when overly-heavy load, light load, and the generated PV power reduction occurs, as well as when a transition occurs from an overly-loaded to normal conditions. To overcome these problems, the proposed controller method switches the main control target from DC-link voltage to the maximum power point, which is closer to the stable operating point when it returns to normal operating conditions. For the analysis, a state-plane trajectory was given and the circuit analysis by PSIM simulation was done. For the verification, a prototype hardware with 110[W] and 50[W] dual photovoltaic modules has been implemented. From the results, it can be seen that PV power tracking is successfully done with the proposed method even under a stand-alone operation mode.

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.

Extension of the pulsed power supply network of ASDEX Upgrade by a set of compact modular generators

Some years ago, ASDEX Upgrade (AUG) examined its future power supply needs. The experimental program could make use of an extension of the IPP pulsed energy storage, both to allow new scenarios at higher plasma current to be investigated, as well as to allow longer plasma flat-top time. Studies performed in 2001 and 2002 by IPP and external collaborators showed that an attractive solution for this extension is a parallel connection of commercially available compact flywheel generators. Especially the main challenges of this system will be explained in the paper. Further on, the paper will present the parallel and stand-alone mode of operation, analyse the results of measurements obtained during commissioning, compare them to the calculated design values and report on the performance achieved during AUG plasma experiments and Additional Heating operation.

電圧形に切替え可能な系統連系用電流形インバータ

We propose a novel inverter that can be operated either as a Current Source Inverter (CSI) or as a Voltage Source Inverter (VSI) by changing only the control signals. It is proper to apply it to the interconnecting system with renewal energy, such as photovoltaic cells or wind generation systems, to a grid. This inverter is usually operated as the CSI connected to the grid. Even if the energy source has a lower voltage than the grid, the energy can be supplied to the grid through the proposed inverter. The power factor can be briefly maintained at almost unity. When power supply from the grid is interrupted, the proposed circuit should be operated as the VSI in the stand-alone operation mode. In this way, the circuit can maintain a constant output voltage to the loads. In this paper, the proposed circuit configuration and the control schemes for both the CSI and the VSI are described. Further, the circuit characteristics for both are discussed experimentally.

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.

마이크로터빈발전시스템 독립운전을 위한 동적 모델링

Distributed Generation (DG) is predicted to play a important role in electric power system in the near future. insertion of DG system into existing distribution network has great impact on real-time system operation and planning. It is widely accepted that micro turbine generation (MTG) systems are currently attracting lot of attention to meet customers need in the distributed power generation market. In order to investigate the performance of MT generation systems, their efficient modeling is required. This paper presents the modeling and simulation of a MT generation system suitable for stand-alone operation. The system comprises of a permanent magnet synchronous generator driven by a MT. A brief description of the overall system is given, and mathematical models for the MT and permanent magnet synchronous generator are presented. Also, the use of power electronics in conditioning the power output of the generating system is demonstrated. Simulation studies with MATLAB/Simulink have been carried out in stand-alone operation mode of a DG system.