28-09-2013, 04:53 PM
A SEMINAR REPORT ON MICROGRID
MICROGRID.doc (Size: 288 KB / Downloads: 238)
INTRODUCTION
Up 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.
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.
THE MICRO-GRID CONCEPT
A 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
A microgrid combined with power electronic interface is a completely self-sufficient network, with preferably autonomous control, communication and protection. It is capable of providing capacity support to the transmission grid while in grid-connected mode, and with capacity in excess of coincident peak demand. So, the Micro grids comprise low voltage LV distribution systems with integration of Diverse Energy Resources DER such as photovoltaic, wind, bio-mass, bio fuel and fuel cell together with Distributed storage DS like flywheels, energy capacitors and batteries and Controllable Loads that behave as a coordinated entity networked by employing advanced power electronic conversion and control capabilities
Operating modes of Microgrid
Operating 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.
Expected Microgrid features
Autonomy: Microgrids include generation, storage, and loads, and can operate autonomously in grid-connected and islanded mode. In the first case, a Microgrid can independently optimize its own power production and consumption under the consideration of system economics such as buy or sell decisions. In islanded mode the system is capable of balancing generation and load and can keep system voltage and frequency in defined limits with adequate controls.
Stability: Independent local control of generators, batteries, and loads of Microgrids are based on frequency droops and voltage levels at the terminal of each device. This means that a Microgrid can operate in a stable manner during nominal operating conditions and during transient events, no matter whether the larger grid is up or down.
Scalability: Microgrids can simply grow through the additional installation of generators, storage, and loads. Such an extension usually requires an incremental new planning of the Microgrid and can be performed in a parallel and modular manner in order to scale up to higher power production and consumption levels.
Efficiency: Centralized as well as distributed Microgrid supervisory controller structures can optimize the utilization of generators, manages charging and discharging energy storage units.
Interconnected Microgrids
Local interconnection standards vary considerably from one bulk power provider to the next. A national standard, ANSI standard P1547 (Draft) Standard for Distributed Resources
Interconnected with Electric Power Systems is being drafted by the IEEE working group. This standard rests on certain assumptions about the contribution of DER to power quality and
System reliability. The standard applies at the point where a Microgrid connects to the grid and is related to the aggregate DER rating within the Microgrid. In other words, the rules applied
to a Microgrid containing many small DER devices would be the same as for one large DER.However, the applicability of P1547 is limited to a DER rating of 10 MVA, which is larger than
The ratings expected for MicroGrids.
Because a Microgrid exploits low voltage, use of waste heat, and the flexibility of power electronics, its practical size may be limited to a few MVA (even
Though IEEE draft standard P1547 specifies an upper limit of 10MVA). In a large complex, loads could be divided into many controllable units e.g., among buildings or industrial sites.
Each unit
Could be supplied by one or more Microgrids connected through a distribution system. The advantages of this system are that the Microgrid structure insures greater stability and controllability,
Allows for a distributed command and control system, and provides redundancy to insure greater power supply reliability for the power park.