Robust Design of a New Control Method for a Grid-Connected Micro-Grid (original) (raw)
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The ever increasing energy demand, the necessity of a reduction in costs and higher reliability requirements are driving the present scenario towards Distributed Generation (DG). The DG has been considered as a promising alternative for the coordinated and flexible expansion of the present energy distribution system with reduced cost and improved reliability . In particular, the small DG systems, typically from 1KW to 10 MW and located near to the loads, are gaining popularity due to their higher operating efficiencies and lower emission levels as provider of electrical energy to the consumers. These DG systems are powered by one or more microsources such as: fuel cells, photovoltaic cells, batteries, wind-turbine, micro-turbines etc. A recent evolution resulting by the diffusion of the DG systems is emerged with the concept of Microgrid, which consists in a cluster of loads and paralleled DG systems operating as a single power system that provides power to its local area . A Microgrid is a systematic organization of DG systems and therefore it has larger capacity and more control flexibility to fulfil system reliability and power quality requirements, in addition to all the inherent advantages of a single DG system. The above characteristics can be obtained thanks to the use of grid-connected inverters able to quickly manage power generated by the microsources and to generate reactive power near loads, allowing losses reduction. Therefore, high performance control algorithms for power flows control and voltage regulation are required . These algorithms should preferably have no communication links between the paralleled DG systems, which can be located far apart; thus, the control algorithms of each individual DG system should be based on feedback variables that can be measured locally and moreover, they have to ensure a safety operation of the Microgrid avoiding instability problems, which can occur especially when many DG systems are located in a same area. A good solution for the aforementioned problems can be obtained by the application proposed in this chapter. It is based on the use of a control strategy for grid-connected inverters able to dynamically change the energetic contribution of the microsources, that so adapts oneself to variations of the grid characteristics and contributes to the power management of the Microgrid. The control strategy is developed so as to combine the
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International Journal of Advanced Research in Electrical, Electronics and Instrumentation Energy, 2013
In this paper, a high performance inverter, including the functions of stand-alone and grid connected power supplies, is developed so that distributed generation units can operate individually or in a microgrid mode. Off-grid islanding describes the condition in which a microgrid or a portion of the power grid, which consists of a load and a distributed generation (DG) system, is isolated from the remainder of the utility system. In this situation, it is important for the microgrid to continue to provide adequate power to the load. Under normal operation, each DG inverter system in the microgrid usually works in constant current control mode in order to provide a preset power to the main grid. When the microgrid is cut off from the main grid, each DG inverter system must detect this islanding situation and must switch to a voltage control mode. In this mode, the microgrid will provide a constant voltage to the local load. This paper describes an Adaptive Total Sliding Mode Control (...
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Distribution Generators(DGs) are the renewable energy resource which can be connected to the grid. When it is connected to the grid it should be operated with controlled voltage and reactive power control. And in autonomous mode(i.e disconnected mode) it should operate in backup generation mode. These DGs are connected towards the micro grid operation. The proposed control system facilitates flexible and robust DG operational characteristics such as active/reactive power (PQ) or active power/voltage (PV) bus operation in the grid- connected mode, regulated power control in autonomous micro-grid mode, smooth transition between autonomous mode and PV or PQ grid connected modes and vice versa, reduced voltage distortion under heavily nonlinear loading conditions, and robust control performance under islanding detection delays. Evaluation results are presented to demonstrate the flexibility and effectiveness of the proposed controller.
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In this paper, a new control method for the parallel operation of inverters operating based on LV micro grids is proposed. This new approach can be applied to the inverter-based micro grids using renewable energy sources where communication wires are not reliable due to the remote locations. The proposed strategy is based on the advanced droop control technique where only the locally measured values are used as feedback. Unfortunately, the trade-off has to be made between the transient response and the power sharing accuracy for the conventional voltage and frequency droop control method. Moreover, the output and line impedance of the inverter presents a great impact on the power sharing accuracy. This paper explores the resistive output impedance and line impedance of the parallel connected inverters in island micro grid. The active and reactive output current is used as the control variables so as to limit the current spikes during the initial and transient states. Additionally, t...
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This paper describes a control technique for enhancing the stable operation of distributed generation (DG) units based on renewable energy sources, during islanding and grid-connected modes. The Passivity-based control technique is considered to analyse the dynamic and steady-state behaviours of DG units during integration and power sharing with loads and/or power grid, which is an appropriate tool to analyse and define a stable operating condition for DG units in microgrid technology. The compensation of instantaneous variations in the reference current components of DG units in ac-side, and dc-link voltage variations in dc-side of interfaced converters, are considered properly in the control loop of DG units, which is the main contribution and novelty of this control technique over other control strategies. By using the proposed control technique, DG units can provide the continuous injection of active power from DG sources to the local loads and/or utility grid. Moreover, by setting appropriate reference current components in the control loop of DG units, reactive power and harmonic current components of loads can be supplied during the islanding and grid-connected modes with a fast dynamic response. Simulation results confirm the performance of the control scheme within the microgrid during dynamic and steadystate operating conditions.
A New Decentralized Control Scheme for Improving Frequency Stability in Islanded Micro-grids
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Usually Micro grids (MGs) are portion of low voltage distribution feeder including two type sources: slow response for frequency control such as Micro turbine (MT), diesel generator, Fuel cell (FC) and fast dynamic response source such as Battery storage systems (BSs) that can play an important role in restoring balance between supply and demand. In this paper, a new decentralized method for enhancement power sharing between distributed generation resources (DGs) is presented for transient stats in the islanded MGs. This method by changing the conventional droop characteristic of DGs in load variation times improves the MG stability with no communication link. The Overload (OL) ability of the BS is employed for fast handling of the frequency control at transient times of loads variation. To achieve this purpose, by considering the batteries OL characteristic and slow dynamic of some sources, a new power control scheme is developed for improving power sharing. Using the introduced me...
Suggested New Voltage and Frequency Control Framework for Autonomous Operation of Microgrids
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Decentralized control strategies are popular candidates in microgrids control because of their reliability and performance. Conventionally, droop control (as a main decentralized strategy) is been utilized in order to prevent permanent droop of voltage and frequency after change in loads and also to share generated power between distributed generation units. In this paper, a new droop control strategy was introduced to control the voltage and frequency of autonomous microgrids. Following a review of the basic droop equations, it was concluded that the new form of droop equations enhanced the voltage and frequency control performance better than conventional droop equations. The voltage control behavior in the proposed method was within the acceptable range, and the frequency also returned to the nominal value after a change in loads. The simplicity and accurateness of the proposed method is a unique characteristic compared with the other recent control methods. Simulation studies sh...
MODELING, STABILITY ANALYSIS AND CONTROL OF MICROGRID
With the increase in the level of global warming, renewable energy based distributed generators (DGs) will increasingly play a dominant role in electricity production. Distributed generation based on solar energy (photovoltaic and solar thermal), wind, biomass, mini-hydro along with use of fuel cells and micro turbines will gain considerable momentum in the near future. A microgrid consists of clusters of load and distributed generators that operate as a single controllable system. The interconnection of the DG to the utility/grid through power electronic converters has raised concern about safe operation and protection of the equipments.
Power Sharing Method for a Grid connected Microgrid with Multiple Distributed Generators
Journal of Electrical Engineering and Technology, 2012
In this paper, a grid connected microgrid with multiple inverter-based distributed generators (DGs) is considered. DG in FFC mode regulates the microgrid as a controllable load from the utility point of view as long as its output is within the capacity limit. The transition mode causes a change in frequency of microgrid due to the loss of power transferred between main grid and microgrid. Frequency deviation from the nominal value can exceed the limit if the loss of power is large enough. This paper presents a coordinated control method for inverter-based DGs so that the microgrid is always regulated as a constant load from the utility viewpoint during grid connected mode, and the frequency deviation in the transition mode is minimized. DGs can share the load by changing their control modes between UPC and FFC and stabilize microgrid during transition.