13-05-2014, 11:29 AM
Microgrids and Distributed Generation
Microgrids and Distributed .doc (Size: 52 KB / Downloads: 15)
Abstract:
In this paper, The increase in the level of global warming renewable energy based distributed generators will increasingly play a dominant role in electricity production. Distributed generation based on solar energy, wind, biomass, mini hydro along with use of fuel cells and micro turbines will gain considerable momentum in the real future. A micro grid consists of clusters of load and distributed generators that operate as a single control system. The inter connection of the DG to the utility or grid through power electronic converters has raised concern about safe operation and protection of the equipment.
INTRODUCTION
The concern for climate change is driving major changes in electricity generation and consumption patterns. Various countries have set a target of 20 % greenhouse gas reduction by the year 2020 Large scale changes in both transmission and distribution levels are expected to occur in the near future. Transmission systems will be bolstered to transmit power generated from large wind farm, geothermal and solar thermal generations.
CONTROLLING ACTION OF A GRID
A micro grid is a cluster of loads and micro sources operating as a single controllable system that provides power to its local area. To the utility, the microgrid can be thought of as a single controllable load that can respond in seconds to meet the needs of the transmission system. To the customer, the microgrid can meet their special needs; such as, enhancing local reliability, reducing feeder losses, supporting local voltages, providing increased efficiency through the use of waste heat, voltage sag correction or providing uninterruptible power supply functions to name a few . In ref , the focus on systems of distributed resources that can switch from grid connection to island operation without causing problems for critical loads.
POWER SHARING IN DISTRIBUTED GENERATION
Parallel converters have been controlled so as to deliver desired power (and reactive power) to the system. Local signals are used as feedback to control converters, since in a real system, the distance between the converters may make the communication impractical. A common approach for real and reactive power sharing is droop control of two independent quantities – the frequency and the fundamental voltage magnitude .
In this, the real power controls the system frequency, while the reactive power controls the voltage magnitude. In real and reactive power management strategies of electronically interfaced distributed generation (DG) units in the context of a multiple- DG microgrid system are addressed, where emphasis is primarily on electronically interfaced DG (EI DG) units. Robust voltage regulation with harmonic elimination under island and decoupled active and reactive power flow control under grid-connected mode is proposed.
CONTROLS FOR GRID AND ISLAND OPERATION
Power electronic interfaces introduce new control issues and possibilities. It is necessary to create a power electronic interface, which allows large clusters of micro generators to operate in both an island mode and as a satellite to the power grid while providing a high quality of power at a minimum equipment cost. Basic requirements of the power electronic interface are:
To provide fixed power and local voltage regulation To facilitate DG fast load tracking using storage To incorporate “frequency droop” methods to insure load sharing between micro-sources in islanded operation without communications propose the use of a low-bandwidth data communication system along with locally measurable feedback signal for each DG.
This is achieved by combining two control methods: droop control method and average power control. The average power method with a slow update rate is used in order to overcome sensitivity voltage and current measurement errors. In addition, a harmonic droop scheme for sharing harmonic content of the load currents is also proposed. But the communication between DGs may not be always possible in reality due to the physical distance between them. The application of adaptive control or robust control in distributed generation .A strategic analysis and optimal voltage control technique for distributed generation .
SYSTEM STABILITY
The system stability during load sharing has been explored by many researchers The Transient stability of the power system with high penetration level of power electronics interfaced (converter connected) distributed generation is explored .But the study is based on presence of an infinite bus. The other important issue, with isolated operation of the power system network has been overlooked in the study. A scheme for controlling parallel connected converter in a standalone ac system is presented .
A modular structure of the controller is presented. The structure can be modified to meet the control requirement for any other ac system. The scheme proposed a P-I regulator to determine the set points for generator angle and flux. The dynamic performance of the system can be substantially improved by using other advanced control technique. Similar to the small- signal stability of conventional power system, To identify the possible feedback signals for controllers, a sensitivity analysis is carried out. It is shown that with the change in power demand, the movement of the low frequency oscillations to new location affects the relative stability of the system.
CONCLUSION
The work on the microgrid has progressed well. The
work at Wisconsin demonstrates the effectiveness of
local control of distributed generation thereby reducing
or eliminating the need for central dispatch. During
disturbances, the generation and corresponding loads can
separate from the distribution system to isolate the
microgrid’s load from the disturbance (and thereby
maintaining high level of service) without harming the
transmission grid’s integrity. Intentional islanding of
generation and loads provide