Post on 03-Nov-2014
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ABSTRACTInnovations in technologies, economic benefits, quality requirements and environmental concerns are changing the face of the existing power system. Centralized generating facilities are giving way to smaller, more distributed generation partially due to the loss of traditional economies of scale. The need of an intelligent grid, better known as Micro grid at the distribution end, has been recognized to accommodate distributed energy resources (DERs) and renewable energy technologies on large scale. Micro grids can provide improved electric service reliability and better power quality to end customers and can also benefit local utilities by providing dispatch able load for use during peak power conditions or allowing system repairs without effecting customer loads. This report highlights the concept, benefits and features of Micro Grids.
Introduction to Micro Grid. Microgrid Operating Modes. The Need of Microgrid. Utility grid shortcomings and microgrid value propositions Microgrid market segments Interconnected Microgrids Power Parks. Environmental Aspects. Conventional Grid versus Microgrid. Advantages & Disadvantages of Microgrid. Future Directions on Microgrid Research.
1. INTRODUCTIONUp till now small generation units have been dispersed throughout power systems basically as uninterruptible power supplies. Generally these sources are not synchronized with the grid power supply though, but rather cut in when the primary supply is interrupted. Distributed generation located close to demand delivers electricity with minimal losses. This power may therefore have a higher value than power coming from large, central conventional generators through the traditional utility transmission and distribution infrastructure. With the use of renewable distributed generation, the dependency on fossil fuels and on their price can be minimized. This step will also lead to a significant reduction of carbon dioxide emissions, which is required in several government programs. If, in addition, distributed generation and consumption in a certain area are integrated into one system, reliability of the power supply may be increased significantly, as shown in figure 1. The importance and quantification of these benefits has been recognized, although these are yet to be incorporated within the technical, commercial, and regulatory framework. However, under todays grid codes, all distributed generation, whether renewable or fossil-fueled, must shut down during times of utility grid power outages. This is precisely when these on-site sources could offer the greatest value to both generation owners and society. A microgrid is a regionally limited energy system of distributed energy resources, consumers and optionally storage. It optimizes one or many of the following: Power quality and reliability, sustainability and economic benefits and it may continuously run in off-grid- or on-grid mode, as well as in dual mode by changing the grid connection status. With the role of distributed generation changing from backup to primary energy supply, more flexible connection strategies are required. To realize the emerging potential of distributed generation a system approach is to be taken which views generation and associated loads as a subsystem or a microgrid. The concept of Micro Grid has grown out of this desire for truly interconnected operation of distributed generation. It is envisioned that this microgrid concept will prove to be an ideal solution to rural electrification besides its very well use in industrial parks, commercial and institutional campuses and many other situations requiring improved reliability and power quality. A micro grid enables small communities to take control of their energy use and reduce their carbon footprint through a new and innovative way of generating and managing electricity.
2. THE MICRO-GRID CONCEPTA microgrid can be simply defined as an aggregation of electrical generation, storages and loads. The generators in the microgrid may be microturbines, fuel cells, reciprocating engines, or any of a number of alternate power sources. A microgrid may take the form of shopping center, industrial park or college campus. To the utility, a microgrid is an electrical load that can be controlled in magnitude. The load could be constant, or the load could increase at night when electricity is cheaper, or the load could be held at zero during times of system stress
The Micro Grid assumes three critical functions that are unique to this architecture:
1. Microsource Controller Regulate power flow on a feeder as loads on that feeder change their operating points Regulate the voltage at the interface of each microsource as loads on the system change Insure that each microsource rapidly picks up its share of the load when the system islands. It responds in milliseconds and uses locally measured voltages and currents to control the microsource during all system or grid events. 2. Energy Manager The Energy Manager provides for system operation of the MicroGrid through dispatch of power And voltage set points to each Microsource Controller. Insure that the necessary heat and electrical loads are met by the microsources Insure that the Microgrid satisfies operational contracts with the bulk power provider Minimize emissions and/or system losses
Maximize the operational efficiency of the microsources 3. Protection The protection coordinator must respond to both system and MicroGrid faults. For a fault on the grid, the desired response may be to isolate the critical load portion of the MicroGrid from the grid as rapidly as is necessary to protect these loads.
2.1 OPERATING MODES OF MICROGRIDOperating modes of Microgrid are: 1. Grid connected 2. Island connected Basic Microgrid architecture is shown below. This consists of a group of radial feeders, which could be part of a distribution system or a buildings electrical system. There is single Point of connection to the utility called as point of common coupling. Some feeders (feeders A-C) have sensitive loads, which require common generation. The non-critical load feeders do not have any local generation. In our example this is feeder Feeders A-C can island from the grid using static switch which can separate in less the cycle. In this case, there are four micro sources at nodes 8, 11, 16 and 22 which control the operation using only local voltages and currents measurements. There is a problem with utility supply. The static switch will open, isolating the sensitive loads from the power grid. If it is assumed that there is sufficient generation to meet the loads demands. When the micro grids are grid connected power from the local generation can be directed to feeder D.Static switch is closed and utility grid is active.
In case of island mode utility grid is not supplying power. Static switch is open. Feeder A, B, C is being supplied by micro sources and feeder D is dead. 2.1.1 Grid connected: It is often a challenge to control the voltage profile in distributed grid due to the low X/R ratio. The voltage profile is not mainly influenced by the reactive power as the case in high voltage transmission power grid. The active power flow is found to be critical in controlling the voltage profile in
distribution power grid. Thus the maximum active power that can be transmitted from or to the power grid through the distribution line is limited, in order to keep the voltage level ofthe hybrid power system in the allowable range. In grid connected operation, DG units work in current controlled mode, assuming the grid voltage is more or less constant. However, in a very weak power grid, the terminal voltages of the generator and VSC will fluctuate. The fluctuation of voltage can cause coupling effect between active and reactive power. Phase locked loop is thus necessary to measure the accurate phase angle. With accurate phase angle of the voltage, active and reactive power can be efficiently decoupled. 2.1.2 Principles of grid connected operation: The principles for grid connected operation are: DFIG wind farm operates under maximum power production mode; Solar power system operates under maximum power production mode; Pumped storage operates under motor mode, provides water for drinking or irrigation and keep water reservoir at a certain level. Grid side converter controls the DC voltage, generator side converter controls motor power; Residential and industrial loads work under maximum; The distribution line is in service, transmits power from or to the power grid depending on the balance between power production and consumption in this system; Frequency is controlled by the main power grid, while wind farm and solar power can participate in the primary frequency control by using frequency droop control; Pumped storage, wind farm and solar power system all participate in the voltage control of the local power grid. Islanded operation In islanded operation, without a constant voltage source, the current control mode of DG unit of grid-connected operation is not appropriate. Without a reference voltage source, the DG units have to control the voltage and frequency in the power grid by themselves. This is not an easy control task, especially when several generators and VSCs are operated in parallel. The islanding operation of VSCs can be found in literatures and . The control method of single DFIG for stand-alone operation is found in literatures but this method is not appropriate for controlling multiple DFIGs in one islanded system.
2.2 PRINCIPLES OF ISLANDED OPERATIONThe principles for islanding operation are: Pumped storage station works as a virtual power grid which determines the voltage and frequency of the hybrid power system. Grid side converter controls the AC voltage, generator side converter controls DC voltage; DFIG wind farm