Stand-alone Mode Of Operation Research Articles

Protection Method for Radial Distribution Systems With DG Using Local Voltage Measurements

This paper presents a voltage-based protection method for distribution systems with distributed generation (DG). Considering the fact that any system fault is associated with a voltage dip at the faulted line ends, a novel protection relay characteristic is formulated from an extensive analysis of voltage behavior during fault conditions. The proposed method is independent of type, size, and location of DG units, as well as grid-connected or islanded (stand-alone) mode of operation of the distribution system. Moreover, the method is communication-less and uses only the local voltage magnitude to determine the relay operating time, hence, a low-cost protection method is provided. A threshold voltage level is defined outside the nominal voltage range. However, if under a fault condition the voltage dip is still within the nominal range, a current starter is used to trigger the relay. The effectiveness of the proposed method is verified by several simulation tests carried out on low voltage (LV) and medium voltage (MV) distribution systems under different fault condition and DG size and location for grid-connected and islanded modes of operation. The simulation results show the protection method is selective with appropriate speed, and the protection settings are independent of the distribution system modes of operation.

Seamless Transfer Algorithm of AC Microgrid Inverter Compensating Load Current for Weak Grid

In this paper, we propose a voltage control technique for the seamless transfer algorithm of a microgrid inverter. When the microgrid inverter is switched from grid-connected mode to standalone operation mode, an output voltage transient may occur, and adversely affect the load. To solve this, seamless transfer algorithms that complete mode switching within 4 ms have been studied. However, previous control techniques have very complex structures. In this study, we find the cause of the voltage transient occurring in the mode conversion, through state average modeling of the inverter. Subsequently, we propose a feed-forward compensator for seamless transfer. The cause of the voltage transient is removed by the compensation, and smooth mode switching is achieved. The proposed controller is verified by Powersim (PSIM) simulations.

An Optimal Energy Management Technique Using the $\epsilon$ -Constraint Method for Grid-Tied and Stand-Alone Battery-Based Microgrids

The intermittent characteristics of microgrids (MGs) have motivated the development of energy management systems (EMSs) in order to optimize the use of distributed energy resources. In current studies, the implementation of an EMS followed by experimental-based analyses for both grid-tied and stand-alone MG operation modes is often neglected. Additionally, the design of a management strategy that is capable of preserving the storage device lifetime in battery-based MGs using a power gradient approach is hardly seen in the literature. In this context, this work presents the application of an EMS for battery-based MGs which is suitable for both grid-tied and stand-alone operation modes. The proposed EMS is formulated as an optimal power flow (OPF) problem using the $\boldsymbol {\epsilon }$ -constraint method which is responsible for computing the current references used by the EMS to control the MG sources. In the optimization problem, the total generation cost is minimized such that the active power losses are kept within pre-established boundaries, and a battery management strategy based on power gradient limitation is included. Finally, the effectiveness of the proposed EMS is evaluated by two scenarios which enable detailed analyses and validation. The first considers a dispatchable and a non-dispatchable source, whereas the second a dispatchable source and a storage device. The experimental results showed that the proposed EMS is efficient in both operation modes and is also capable of smoothing the state of charge ( SoC ) behavior of the storage device.

Modeling of Energy Performance of Stand-Alone SPV System Using HOMER Pro

Stand-alone solar photovoltaic (SA-SPV) systems are attractive option for rural electrification programs in many countries. These systems are invariably provided with battery energy storage (BES) for using stored electricity during nighttime. Availability of bi-directional power electronic converter promises to enhance application of these SA-SPV systems for generating, storing and feeding electricity to local micro-grids. Further, the advantages of such systems can be maximized by minimizing energy losses at different sub-system levels in the system. In the present work, we demonstrate use of HOMER Pro simulation software for simulation of energy performance of SA-SPV (6.75 kWp) system installed in Renewable Energy Systems Laboratory in our Institute, aiming at quantitative estimation of energy losses due to stand-alone mode of operation. The system is provided with battery energy storage (800 Ah) that is used for supplying electricity for night time street lighting on campus up to eleven hours per day. The results of simulation show that when the existing SA-SPV system is upgraded to grid-tied SPV system, by incorporating bi-directional converter, the system will produce total 11086 kWh annually at the site, out of which 4536 kWh will be fed to the local single-phase micro-grid, which accounts for energy lost if the system continues to operate as SA-SPV system.

Modeling of a hybrid marine current-hydrogen active power generation system

This paper presents the modeling and the simulation of a hybrid marine current-hydrogen power generation system. The marine current power generation system consists of a fixed pitch marine current turbine directly coupled to a permanent magnet synchronous generator (PMSG). The generator is connected to a DC link capacitor via a controlled rectifier, which has two modes of operation. The first mode is the maximum power point tracking (MPPT) by using torque control when the generator runs below the rated speed. The second mode is the power limitation (at the rated value) when the generator runs above the nominal speed. The generated power is transferred from the DC-link to the load via an inverter to run the system in a stand-alone operation mode. An energy storage system must cover the difference between the generation and the consumption for this scheme. The hydrogen, compared with the different energy storage systems, exhibits characteristics more applicable for marine current power generation systems. When the generated power is higher than the load requirements, a Megawatt-scale proton exchange membrane (PEM) electrolyzer consumes the surplus energy for hydrogen generation. The generated hydrogen is stored in tanks to feed a PEM fuel cell system to generate power in case of shortage. Based on this topology and operation procedure, the overall system is called an active power generation system. The MW scale PEM electrolyzer model is presented based on state of the art and the literature of different scales PEM electrolyzer system modeling.

Finite state machine control and power quality enhancement using Goertzel filter for multifunctional battery energy storage system

This study presents the design and control of a multifunctional battery energy storage system (BESS) to provide reactive power and harmonics compensation in grid-connected and uninterrupted supply modes to loads during grid non-availability. The BESS comprises of a bidirectional DC–DC converter and a grid-connected voltage source converter (VSC). The bidirectional converter is controlled to provide the battery charging and to maintain constant DC link voltage. The VSC uses a multimode controller and the finite state machine (FSM) based logic for mode transitions. The multimode controller uses a sliding Goertzel transform based filter for grid-connected mode and voltage controller for standalone mode of operation. The FSM logic monitors the system parameters using a voltage and the rate of change of frequency for deciding the mode of operation and for smooth transition between these modes. Simulated and test results validate the effectiveness of the proposed control in achieving the set objectives.

Techno-economic and environmental analysis of hybrid energy systems for a university in Australia

ABSTRACTIntegrating renewable energies into the grid has proven to be a promising solution to address clean environment. In this aspect, there have been several studies conducted on distributed energy resources to deliver power efficiently with more reliance at many places including isolated villages, resorts, etc. In this paper, we address the optimum sizing of renewable energy source like photovoltaics connected to the grid and stand-alone mode of operation for Victoria University in Footscray, Australia, after analysing the electricity load. These models were compared with optimised model of diesel generator as the source of energy. Economic feasibility and environmental impact analyses have been conducted for these optimised models using Hybrid Optimization Model for Multiple Energy Resources software.

Active Load Simulation Study of Solar and Wind Hybrid Energy System in Stand-alone Mode of Operation

Background: This work presents results based on realistic data as inputs obtained from the NASA surface metrology consisting a PV-Wind hybrid system. Methods: This system includes PV and wind subsystems, load and a hybrid system. The system included a 3.6kW PV array and a 3.2kW wind turbine and collected realistic data from January to December 2015 from the NASA website at Manipal location (INDIA) to calculate the solar and wind power. The study simulated daily and monthly data, and validated the simulation results for the physical system. Two Boost converters transfer a maximum power of Wind Energy Conversion System (WECS) and Photo Voltaic (PV) system to DC link. These converters are operated based on Perturb and Observed (P & O) MPPT algorithm and with a change in duty cycle. Based on PQ control strategy, the single-phase voltage source inverter supplies power to the grid and maintains the DC link voltage 400V

A Voltage-Independent Islanding Detection Method and Low-Voltage Ride Through of a Two-Stage PV Inverter

This paper presents an islanding detection method for a two-stage photovoltaic (PV) inverter. The novelty in this method is that it does not rely on instantaneous value of the grid voltage for identifying the islanded condition. Instead, the islanding condition is detected by the saturation of the proportional integral (PI) controller of the outer voltage control loop. This makes the proposed detection algorithm more reliable compared with other voltage-based detection algorithms, as it is immune to malfunctions caused by the sudden load changes or other transients. In addition, a simple but accurate implementation strategy for low-voltage ride through (LVRT) operation is also proposed and integrated with the anti-islanding operation. Both these methods are placed inside the overall control structure that can perform seamless transition between grid-connected and standalone modes of operation. The simulation results demonstrate the effectiveness of the proposed anti-islanding and LVRT techniques. Finally, hardware results on a 2 KW laboratory prototype are presented for experimental verification of the proposed anti-islanding and LVRT schemes.

КОМПЛЕКСНОЕ МОДЕЛИРОВАНИЕ ЭЛЕКТРИЧЕСКИХ РЕЖИМОВ В СЕТЯХ ВНЕШНЕГО И ВНУТРЕННЕГО ЭЛЕКТРОСНАБЖЕНИЯ ПРЕДПРИЯТИЙ С СОБСТВЕННОЙ ГЕНЕРАЦИЕЙ

When integrating own generation including emergency power supply sources it is necessary to perform an atypical at the present time complex of calculations of electrical regimes in internal power supply schemes of enterprises including the calculations of steady and optimized regimes, electromechanical transients and power quality indicators. The PURPOSE of the study is to identify the potential risks of accidents resulting in the interruption of power supply of particularly responsible electric receivers when connecting to internal power supply systems of own generation facilities including emergency power supply sources both in parallel and stand-alone operation mode with the power system. The METHODS used include conducting specialized studies of the parameters of electrical modes and a complex of calculations of steady and optimized modes, electromechanical transients and power quality indicators in the DIgSILENT PowerFactory software package. RESULTS. The articles presents the problematic aspects revealed in the specialized researches and integrated modeling of electrical modes in the networks of external and internal power supply of enterprises with own generation and emergency power sources, as well as the measures recommended in order to improve the reliability of consumer power supply. CONCLUSIONS. It is necessary to raise the requirements for the volume of electric mode modeling in the networks of external and internal power supply of enterprises with own generation facilities and emergency power supply sources both at the design stage and during energy inspections in order to to verify the validity of the adopted design decisions.

Self-Excitation Criteria of the Synchronous Reluctance Generator in Stand-Alone Mode of Operation

The self-excited synchronous reluctance generator (SynRG) represents an alternative to supply electric power to remote communities. It is robust, simple, and less expensive compared with other types of brushless generators. However, for reliable operation in stand-alone mode, the self-excitation of the machine must be assured, which demands that certain starting conditions are met. It is observed through experiments that the existence of a minimum residual flux in the core, though necessary, is not sufficient to trigger self-excitation. The start-up acceleration also plays an important role since it may create transients in the q- axis flux linkage that can lead to demagnetization of the core and consequent failure in the terminal voltage build-up. This paper studies the effects of these starting conditions on the self-excitation phenomenon and presents criteria in terms of minimum residual flux and maximum start-up acceleration to trigger self-excitation without the risk of core demagnetization. The criteria to ensure the self-excitation for synchronous reluctance machines is developed using the dq -model of the machine and the energetic model. The developed concept is also applicable to other SynRGs operating in stand-alone mode.

Operation Modes and Combination Control for Urban Multivoltage-Level DC Grid

With the development of urban economy and the growing integration of renewable energy, there are increasing challenges on the urban power grid. Due to the transmission stress and short-circuit current limitation, the traditional urban power grid cannot satisfy the new requirements. Multivoltage-level dc grid (MDCG) is an alternative approach to the urban power grid upgrade. This paper presents an MDCG topology, which can be used in a future urban power grid. Three operation modes of an urban MDCG are proposed: auto operation mode, power-limited operation mode, and stand-alone operation mode. The auto operation mode is the normal one for urban MDCG. The power-limited operation mode is designed to keep the capability to support urban high voltage ac transmission grid in emergencies. The stand-alone operation mode is used to keep stability in case the dc–dc converter system outage occurs. The combination control of three operation modes is put forward to enhance the reliability of urban MDCG in various operating conditions. The simulation results show that the proposed operation modes and combination control are effective and have good performance.

Interlink Converter with Linear Quadratic Regulator Based Current Control for Hybrid AC/DC Microgrid

A hybrid alternate current/direct current (AC/DC) microgrid consists of an AC subgrid and a DC subgrid, and the subgrids are connected through the interlink bidirectional AC/DC converter. In the stand-alone operation mode, it is desirable that the interlink bidirectional AC/DC converter manages proportional power sharing between the subgrids by transferring power from the under-loaded subgrid to the over-loaded one. In terms of system security, the interlink bidirectional AC/DC converter takes an important role, so proper control strategies need to be established. In addition, it is assumed that a battery energy storage system is installed in one subgrid, and the coordinated control of interlink bidirectional AC/DC converter and battery energy storage system converter is required so that the power sharing scheme between subgrids becomes more efficient. For the purpose of designing a tracking controller for the power sharing by interlink bidirectional AC/DC converter in a hybrid AC/DC microgrid, a droop control method generates a power reference for interlink bidirectional AC/DC converter based on the deviation of the system frequency and voltages first and then interlink bidirectional AC/DC converter needs to transfer the power reference to the over-loaded subgrid. For efficiency of this power transferring, a linear quadratic regulator with exponential weighting for the current regulation of interlink bidirectional AC/DC converter is designed in such a way that the resulting microgrid can operate robustly against various uncertainties and the power sharing is carried out quickly. Simulation results show that the proposed interlink bidirectional AC/DC converter control strategy provides robust and efficient power sharing scheme between the subgrids without deteriorating the secure system operation.

Design of Ćuk-Derived Transformerless Common-Grounded PV Microinverter in CCM

Photovoltaic (PV) microinverters dispense with the line frequency transformer, however at the cost of system grounding and ground leakage current problems. These have been erstwhile resolved by the topologies derived from buck, boost, buck–boost, Zeta, Watkins–Johnson, and Cuk converters, or combinations of these. The Cuk derived inverters, employing second-order input and output filters, offer the most efficient, lightweight, and economical solution in the class. This paper presents design and detailed operation of a Cuk derived, common-ground PV microinverter in continuous conduction mode operation. The inverter is shown to be compatible with both linear and nonlinear loads, in stand-alone and grid-connected modes of operation. Optimal design rules of passive components are rigorously derived to ensure attenuation of input voltage ripples arising from the twin effects of switching and double-frequency output power oscillation. Additionally, the design rules also incorporate considerations of efficiency maximization and some aspects of easing control complexity. Inverter performance is experimentally validated with a 300 VA, 110 V, and 50/60 Hz laboratory prototype.

Multimode Controller Design for DSTATCOM integrated with Battery Energy Storage for Smart Grid Applications

A Distribution Static Compensator (DSTATCOM) can provide reactive power support, power factor correction and voltage regulation to a distribution system feeder. In this paper, a multimode multifunctional control algorithm is proposed for the controller of the Voltage Source Converter (VSC) to provide DSTATCOM and load leveling functionalities for smart grid applications through a Battery Energy Storage System (BESS) connected to the grid through the VSC. The designed controller enables the VSC to function in both grid-connected and standalone modes of operation. In grid-connected mode, the VSC provides load leveling, dynamic reactive power exchange and unity power factor functionalities based on consumer's choice. When a fault occurs on the grid, the controller opens the circuit breaker and isolates the load from the grid and then switches to the voltage controller for the standalone mode of operation. The novelty of the proposed controller is, the inverter structure which provides load leveling and DSTATCOM functionalities during grid-connected mode will also serve the load during a grid failure, thereby providing uninterrupted power supply to the load. The simulation results show the ability of the proposed multimode controller to successfully operate the multifunctional DSTATCOM-BESS system in various modes.

Coordination of energy storage systems and DR resources for optimal scheduling of microgrids under uncertainties

Wind energy integration into microgrids, has introduced new challenges to energy management systems because of its intermittent behaviour. Owing to continuous changes in generation and consumption, the energy price is volatile. This study proposes a new two-stage stochastic framework for day-ahead scheduling of microgrids. Uncertainties associated to wind power generation, real-time electricity price and load demand are considered. Different scenarios are generated using autoregressive moving-average method and then are reduced using fast-forward technique. On the first stage, the microgrid master controller determines the procured energy from the day-ahead market and commitment states of distributed energy resources (DERs). In the second stage, the purchased energy from the real-time market and schedules of committed DERs are obtained. The problem is modelled using mixed-integer linear programming approach and is solved via CPLEX® optimizer. Both grid-connected and stand-alone modes of operation are investigated. Despite of high operation cost in island mode, coordination of energy storage systems, incentive-based and price-based demand response (DR) programmes affect economy of microgrids. The framework is examined on a test microgrid. Results show that both of the releasing the microgrid master controller authority and DR resources result in significant saving in operating; especially in emergency conditions such as islanded mode.

Coordination Control Strategy for AC/DC Hybrid Microgrids in Stand-Alone Mode

Interest in DC microgrids is rapidly increasing along with the improvement of DC power technology because of its advantages. To support the integration process of DC microgrids with the existing AC utility grids, the form of hybrid AC/DC microgrids is considered for higher power conversion efficiency, lower component cost and better power quality. In the system, AC and DC portions are connected through interlink bidirectional AC/DC converters (IC) with a proper control system and power management. In the stand-alone operation mode of AC/DC hybrid microgrids, the control of power injection through the IC is crucial in order to maintain the system security. This paper mainly deals with a coordination control strategy of IC and a battery energy storage system (BESS) converter under stand-alone operation. A coordinated control strategy for the IC, which considers the state of charge (SOC) level of BESS and the load shedding scheme as the last resort, is proposed to obtain better power sharing between AC and DC subgrids. The scheme will be tested with a hybrid AC/DC microgrid, using the tool of the PSCAD/EMTDC software.

A Protection Strategy for Micro-Grids Based on Positive-Sequence Impedance

ABSTRACTIn 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 article presents a new micro-grid protection strategy based on positive-sequence impedance using Pha-sor Measurement Units (PMUs) and a designed Microprocessor-Based Relay (MBR) along with a narrowband communication system (only to exchange digital information, not electrical power). The main attribution of the proposed scheme is that it has the ability to protect micro-grids containing either radial or looped feeders; additionally, the capabilities of automatic updating of pick-up threshold value and single-phase tripping have been considered in the...

Super-Twisting Sliding Control Design of Three-Phase Inverter for Stand-Alone Distributed Generation Systems

This paper presents a super-twisting algorithm control strategy for a three-phase voltage source inverter of a distributed generation system in the stand-alone operation mode to guarantee good performance of the load output voltage under a sudden load, unbalanced load, and nonlinear load. The proposed control is established for an inner-loop current controller and an outer-loop voltage controller in a dual-control scheme. The proposed controller is very simple and robust to the parameter uncertainties. The stability and robustness of the proposed controller are proven based on the Lyapunov theory. The simulation results are illustrated by MATLAB/Simulink software and are compared to the performance of the traditional sliding controller to validate the effectiveness of the proposed controller.

Line-Interactive UPS for Low-Voltage Microgrids

Line-interactive uninterruptible power supply (UPS) systems are good candidates for providing energy storage within a microgrid. In this paper, a control scheme for a line-interactive UPS system applied in a low-voltage microgrid is presented. It is based on the Q-w and P-E droop control to achieve a seamless transition between grid-connected and stand-alone operation modes. Moreover, a new model for designing the controllers is built in the dq-frame based on the instantaneous power definition. The new-built model takes into account the dynamic performance of the output impedance of the inverter in the dq-frame and can be evaluated in the time domain. Compared to the traditional model based on the instantaneous power definition, the new-built model is more accurate to describe the dynamic performance of the system. Simulation and experimental results obtained with a microgrid consisting of two 40-kW line-interactive UPS systems are given to validate the control strategy of the line-active UPS system and the accuracy of the new-built model.