Implementation of grid interface for generation side power management

Implementation of grid interface for generation side power management

Roopa Ravindran

Student,

Amrita Viswa Vidyapeetham

Dept. Of Electrical and Electronics Engg

R. R. Lekshmi

Assistant Professor, Amrita Viswa Vidyapeetham

Dept. Of Electrical and Electronics Engg

Abstract

Increasing electrification of daily life causes growing electricity consumption and the rising number of sensitive or critical loads demand for high quality electricity. One of the main problems facing today is that related with the transmission and distribution of electricity. Due to the rapid increase in global energy consumption and the diminishing of fossil fuels, the customer demand for new generation capacities and efficient energy production, delivery and utilization k eeps rising. Utilizing distributed generation, renewable energy and energy storage can potentially solve problems as energy shortage. A promising structure to interconnect these distributed energy resources(DER) is the microgrid paradigm. Since microgrid has the ability to generate, distribute and regulate the flow of electricity, this can be thought of as an effective solution to this problem. In this paper, considering the different cases of generation and demand, a Matlab/Simulink model of microgrid is developed, incorporating an energy storage system, such that when the demand is less, the charging of battery takes place and vice versa. Also, depending on the frequency variations, the ON and OFF of the non – critical loads were done automatically and the status of the battery and the different loads are decided by the control unit.

1. Introduction

   When referring to the power industry, Grid is the term used for an electric ity network. In other words, electrica l grid is an interconnected network for delivering electric ity fro m suppliers to the consumers. The ma in functions of the Grid can be classified into Electricity generation, power transmission, distribution

   and control. Thus the grid is having a Hiera rchal structure with high voltage transmission network at the top level, then comes the mediu m voltage sub transmission network and finally the low voltage distribution network which deliver power to consumers. In the traditional method of electricity generation and distribution, one of the main limitations is that, with some minor e xceptions that the electrical energy cannot be stored and electricity should be generated as and when required in order to meet the demand. So, some effective measures should be taken to ensure that the electricity generation closely matches the demand; otherwise the system will not be an efficient one. The centralised grid system is definitely the backbone of the electricity distribution system, but it has some drawbacks like less reliability, less energy utilizat ion, pollution etc. Now a days the demand for energy has increased in such a way that the centralised grid system is insuffic ient to meet the demand and also the conventional power system uses the non renewable resources which are fast depleting. As such an effective measure that can be imple mented is the Microgrid system in the power sector, which concentrates on the distributed generation technique.

   A mic rogrid can be defined as an electrica l powe r distribution network that can operate in isolation (islanded mode) and in the grid connected mode. In Islanded mode, the microgrid continues to supply power to the consumers by its own, without having any connection with the central grid, with the help of various distributed energy resources (DER). DERs are the sources of electric energy in a microgrid, and are also known as micro sources. The installed distributed energy sources include biomass, fuel cells, geotherma l, solar panels, wind turbines, small steam turbines, micro turbines etc. In the grid connected mode, the mic rogrid itself will be having a connection with the central grid, and the power demand is met by the power generated

   by the central grid and the mic rogrid together. Thus by incorporating the microgrid technology into the centralised grid system, the impacts of the increasing energy stress can be reduced.

   This paper investigates about the detailed structure of the mic rogrid and also focuses on the automatic controlling of the battery storage system as well as the various non critical loads associated with the microgrid. The entire simulation is done, considering the different cases of generation and demand of power, based on the frequency variations.

2. Structure of microg rid

   The microgrid structure includes an aggregation of diffe rent loads and different micro sources, along with energy storage units, operating together as a single unit providing both power and heat. The mic ro sources (DERs) are the prima ry source of energy within the microgrid. The major issues related with the mic rogrid structure include the interface, control and protection require ments for each DER as well as the mic rogrid voltage control, power flow control etc during islanding, and overall protection. The installed DERs include bio mass, fuel cells, geotherma l, solar, wind, small steam turbines etc. The most important function of the microgrid is its ability to operate in the islanded mode as well as in the grid connected mode. The simp le block d iagra m representation of a mic rogrid is shown in fig. 1.

   Fig. 1. Microgrid structure

   Another important potential benefit of microgrids is its e xpanded opportunity to utilize the waste heat fro m the conversion of the primary fuel to e lectricity. Th is

   Generation	Demand	Switch(S1)	Switch(S 2)
   0	0	0	0
   1	0	0	1
   0	1	0	1
   		1	0

   feature is very much significant, because in most cases, half to three- quarters of the primary energy consumed in power generation is ultimate ly released unutilized to the environment.

3. Microgrid operatio n

   The ob jectives of this project work were to imple ment a mic rogrid structure with a provision to operate it in the islanded mode, to have an interface with the central grid, and to use a storage system whenever the generation is more than the demand. The loads are classified as necessary (critical) loads and unnecessary (non- critica l) loads. Necessary loads are that loads which needs a continuous supply of power and cannot be turned off frequently once it is started operating. Unnecessary loads are that which can be turned off as and when required.

   The frequency is taken as the measure of demand of the system. Whenever the generation of power beco mes more than the demand, the battery storage system is automatically connected for charging, so as to avoid the wastage. When the demand is more than the total power generated from the microgrid, a signal is send through wire less to the smart sensors placed at the load side. On receiving this signal, all the unnecessary loads are switched off. This takes a small t ime , during which the battery system can be used to feed back the stored energy. The mic rogrid also has a provision to get connected with the central grid permanently. The entire operation can be represented in the form of a b lock diagra m as shown in fig. 2.

   Fig. 2. Microgrid structure with interfacing switches.

   S2 is the switch wich connects the battery storage system with the microgrid and S1 is the switch which connects the microgrid itself to the central grid. Based on the demand generation status, the following switch positions can be selected.

   Table 1. status of different switche s.

   The single line diagra m of the microgrid stru cture considered in this project work including the control unit is shown in fig. 3.

   Fig. 3. Single line diagram of the microgrid structure.

   The installed DER considered for simulat ion is a small steam turbine and the energy storage device is the inverter interfaced battery bank. They ensure the balance between the energy generation and consumption during sudden changes in the load or in the power generation. The loads are classified as critica l and non- critical loads. The critical loads need reliable source of energy where as the non- critical loads may be shed during emergency situations.

4. Flowcharts

   The status of the battery as well as the non- critical loads is decided taking frequency as a measure of demand. Every instant the frequency is measured by the processor in the control unit, and the measured frequency value is compared with the normal frequency value of 50Hz. Depending on the difference between these two values, a signal is generated, which acts as the control signals to the different switches in the circuit. Three cases of generation and demand are considered in this project work. i.e., the three cases are- when generation is greater than demand (G>D), when generation is equal to demand (G=D) and when generation is less than demand (G<D). Fig.4. shows the decision ma king, based on the three conditions of generation and demand. Fig.5. shows the decision ma king for turning ON and OFF of non-critica l loads. Fig.6. shows the decision making on charging and discharging of the battery. One manual switch is also provided along with the non- critical loads so that these loads can be turned off even if the generation is more than demand.

   Fig. 4. Decision making, based on the three conditions on generation and

   demand.

   Fig. 4. Deci sion making for turning on and off of non-critical loads.

   Fig. 6. Decision making on charging and discharging of the battery.

5. Simulation and results

   The simulink model for the complete mic rogrid structure along with the battery storage unit and the control unit is shown in the fig. 7. The total generation capacity of the steam turbine generator set is 1.3M VA . the generated current and voltage waveforms are shown in fig. 8. Whenever the power generated becomes more

   than the power demand, battery charging takes place as shown in fig. 9. Whenever the power demand becomes more than the power generated, the extra power needed for meeting the demand is supplied by the battery storage system and the state of charge (SOC) o f the battery will be as shown in fig . 10. The switch used in this project work is the SCR based switch which is a combination of t wo anti- paralle l thyristors.

   Fig. 7. Complete microgrid model in simulink.

   Fig. 8. Generated current and voltage waveforms

   Fig. 9. Plot showing charging of the battery

   Fig. 10.Plot showing discharging of the battery

6. Conclusion

   The mic rogrid technology using the distributed generation resources is now eme rging as a new technology for economica l and uninterrupted power supply, and is being used almost all over the world. Also, since they have lower carbon emissions, it is environmental friendly and is one of the main reasons why mic rogrids are pre ferred. It is obvious in speculating that by the onset of the year 2020, energy crunch would be the burning issue before us if not resorted to alternative methods of power generation and invisioned planning of energy management. In vie w of the lucid merits, the mic rogrid system can contribute a lot in the sustainity of mankind in the future.

7. References

1. B. Kroposki, C. Pin k, J. Lynch, V.John, S. Meor Danie l, E. Benedict and I.Vihinen, Develop ment of a high speed Static switch for Distributed Energy and Microgrid Applicat ions, IEEE, Powe r Conversion Conference, pp. 1418- 1423,April 2007.

2. Bo Zhao, Xuesong Zhang, Hangwei Tong, Li Guo, Yanbo Che and Bin Li, Design and imple mentation of an Integrated Microgrid System, China International Conference on Electricity Distribution, pp.1- 9, September 2010.

3. D.P Kothari and I J Nagrath, Modern Power System Analysis,Tata McGra w-Hill Publishing Co mpany Limited.,2007

4. Fei Wang, Jorge L. Duarte and Marcel A. M. Hendrix, Grid- Interfac ing Converter systems with Enhanced Voltage quality for M icrogrid Applicat ion concept and Imp le mentation, IEEE Transactions on Power Electronics, vol 99, 2011.

5. J. Driesen and F. Katirae i, Design for distributed energy resources, IEEE Po wer and Energy Magazine, vol. 6, pp. 30 40, May/June 2008.

6. M. A. Laughton, "Analysis of unbalanced poly phase networks by the method of phase coordinates. Part I System representation in phase frame of reference, Proc. IEE, 115 (8),1163-1172, Aug. 1968.

7. MICROGRIDS Large Scale Integration of Micro-Generation to Lo wVo ltage Grids, EU contract ENK5-CT-2002-00610, http://microgrids.power.ece.ntua.gr

8. Ned Mohan, Tore M. Undeland, Willia ms P. Robbins, Power Electronics Converters, Applications and Design, pp 475- 480, John Wiley edition 2009

9. R. Lasseter ,The CERTS M icrogrid Concept, White paper on Integration of Distributed Energy Resources, April 2002

10. Sandeep Bala and Giri Venkatara manan, Autonomous power electronic interfaces between Microgrids, IEEE, Energy Conversion Conference and Exposition, pp. 3006- 3013, september 2009.

11. S. Bala , Integration of Single- phase Microgrids,PhD thesis, University of Wisconsin- Madison, 2008.

12. S. M. Halp in, L. L. Grigsby, C. A. Gross, R. M. Nelms ,"An Improved method for including detailed synchronous machine representations in large power system models for fault ana lysis, IEEE Trans. Energy Conversion, Vol. 8, No 4, December 1993, pp 719 -725.

13. www.howstuffworks.com

14. www.wikipedia.co m

International Journal of Engineering Research & Technology (IJERT)

ISSN: 2278-0181

Vol. 1 Issue 3, May – 2012