BASIC ARCHITECTURE OF MICRO GRID • Group of radial feeders • Point of coupling(pcc) • Micro Grid central controller (MGCC) • Power electronic-interface- e.g. rectifier,Inverters. • Micro sources • storage • Critical and non critical loads
Storage• It is very much a key to unlock the doors of renewable energy.• Needed because of inherent intermittency of supply from renewable sources.• Needed to ensure secure and continuous supply during off grid and on grid condition.
TYPES OF STORAGE• Electrochemical energy storage – Battery and super capacitors.• Super conducting magnetic storage.• Kinetic energy (inertia) storage.• Thermal storage.
ADVANTAGES OF MICROGRID• Energy efficiency• Minimization of overall energy consumption• Improved environmental impact• Improvement of energy system reliability• Network benefits• Cost efficient electricity infrastructure replacement
POWER OUTPUT CONTROL OF MICRO SOURCES• Resembles with electrical power system – speed governors and voltage regulators replaced by a central supervisory controller• To keep real and reactive power output constant : P-Q control
MODE OF OPERATION• Single master operation (SMO).• Multi master operation (MMO).• Grid connected operation• Stand alone or Islanded mode of operation.
SMO• Most of the micro sources connected to the network with a P-Q control type and a single VSI inverter is associated with the storage device- the master which provides frequency reference.
MMO• There are several inverters operating as VSI with a predefined frequency/active power and voltage/reactive power droop. Other P- Q controlled inverters may also co- exist.
GRID CONNECTED• Micro grid(MG) connected parallel to main grid – to either draw or supply power to grid.• Should not actively regulate voltage at PCC.• Harmonics and DC current injected to grid should be below required levels.• Should satisfy all of its load requirements and contractual obligations with grid.
ISLANDED OPERATION• Disconnected from main grid due to abnormal conditions in grid or due to planned switching.• Problem of voltage and frequency management.• Problem of balance between supply and demand• Problem of power quality management• Micro source issues• Communication among MG components.• For multi customer system have to be well organized concerning contractual issues.
Cooperation Of Micro Source And Storage Device During Islanding Operation . Pure droop control. • Inverter mode control. • Primary energy source control. • Autonomous control. • Reverse droop control. • Multi agent based PQ control.
Comparison of above strategies• Inverter mode ,primary energy source and multi agent based control approaches guarantee that voltage and frequency will not vary much from nominal value hence suitable to drive quality sensitive loads• Pure droop reverse droop and autonomous control are suitable for loads which can tolerate fluctuations• Choice also depend on micro source,ownership and storage
Literature Review(1) S. Chakraborty, Manoja D. Weiss and Simoes, “Distributed Intelligent Energy Management System for a Single Phase , High frequency AC micro grid”, IEEE transaction on industrial electronics, vol. 54, no.1,Feb.2007 In this paper a single phase high frequency micro grid is shown as a solution for better utilization of micro grid. 1-Φ 500 Hz buses are used to combine renewable sources with loads and grid.Proposed system utilizes an active filter called unified power quality conditioner .Also a unified power flow conditioner is used to control the power flow from/to grid.
… ContinuedBoth unified power quality conditioner and unified power flow conditioner consist of series and shunt active filters that use instantaneous ‘p-q’ theory to calculate compensating voltage and current, which then are synthesized using pulse width modulation in voltage source inverter(VSI).
Optimization of Operating Cost• A distributed integrated energy management system (DIEMS) has been implemented for optimization of operating cost. A neural network is incorporated in DIEMS.• Function of DIEMS is to generate set points for the sources and storage in such a way that economically optimized power dispatch will be maintained to fulfill certain load demands.
Salient features of HFAC. Advantages-easy filtering of harmonics,improved luminous efficiency,increased flexibility and reliability of storage,reduction in size of passive circuit components .• Power transmission and distribution- two significant factors affect the suitability of transmission is increased voltage drop and Power loss along the line.• Applications of HFAC-military, aircraft industry,iron steel industry,light and air conditioning.
(2)Hannu Laaksonen and Kimmo Kauhaniemi, “Voltage and Current THDin micro grid With Different DG Unit and Load Configurations”, CIRED Seminar on Smart Grids for Distribution, Paper No. 0057 , 2008 This paper studies the voltage and current total harmonic distortion (THD) before and after islanding in the micro grid with different micro grid configurations. • Causes – caused by nonlinear devices. System impedance at each harmonic frequency determines impact of nonlinear load on voltage distortion, higher order harmonics caused by converters may resonate with system impedance.
continued• Standards -Various organizations on national and international level have made standards both for individual and total harmonic distortion of voltage, for distributed generator units more tight limits are defined.• When the micro grid transfers from normal operation to islanded operation, micro grid is more sensitive to disturbances, therefore the effect of different filter types, sizes and switching frequencies must be carefully examined.
Solutions suggested—(1) L-C-L filters should be used(2) Amount of thyristor connected rectifier loads directly connected to micro grid should be low (15-20% of total load)(3) IGBT based converters must be preferred(4) SVM should be preferred over PWM(5) Use of negative sequence filtering in control system of converters is beneficial
References John Baker, “New technology and possible advances in energy storage,” Energy Policy 36, 4368-4373, 2008. Peter J. Hall,Euan J. Bain,” Energy-storage technologies and electricity generation,” Energy Policy 36, 4352- 4355,2008. M. Uzunoglu, O.C. Onar and M.S. Alam,”modelling control and simulation of a PV/FC/UC based hybrid power generation system for stand alone applications”,Renewable energy,doi:10.1016/j.renene,2008. J. Pecas Lopes, etal,”Defining Control Strategies for Micro Grids islanded operation,” IEEE Trans. On Power Systems, vol.21,PP 616924, 2006
continued J. Pecas Lopes, etal,”Micro Grid Black start andIslanded Operation,” 15th Power SystemsComputation Conference, Belgium,2005.H. Laaksonen, P. Saari and R.Komulainen,”Voltageand Frequency Control of Inverter Based Weak LVNetwork Micro grid,”International Conference onFuture Power Systems, 2005.. J. Oyarzabal, etal,”Agent-based Micro GridManagement System,”International Conference onFuture Power Systems, 2005
...continue d D. Georgakis, etal,”Operation of a Prototype Micro Grid System Based on Micro sources Equipped with Fast-acting Power Electronics Interfaces,” 35th annual IEEE Power Electronics Specialists Conference, Vol.4,2521-2526, 2004. P. Chiradeja and R. Ram Kumar, “An approach to quantify the technical benefits of distributed generation,”IEEE trans on Energy Conversion, Vol. 19, No. 4, 764-773, December 2004. Ph. Strauss and A. Engler,”AC coupled pv hybrid systems and micro grid state of art and future trends,” third world conference on photovoltaic energy conversion,Japan, May 2003.