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Improvement of Dynamic Voltage Stability in a Microgrid by Voltage Stabilizer

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http://www.ijmer.com/papers/Vol2_Issue6/DC2644244428.pdf

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  • 1. International Journal of Modern Engineering Research (IJMER) www.ijmer.com Vol.2, Issue.6, Nov-Dec. 2012 pp-4424-4428 ISSN: 2249-6645 Improvement of Dynamic Voltage Stability in a Microgrid by Voltage Stabilizer J. Eswaraiah, 1 P. Bhaskara Prasad, 2 M. Tech student Department of EEE Annamacharya Institute of Technology and sciences, Rajempeta, India Assistant professor Department of EEE Annamacharya Institute of Technology and sciences, Rajempeta, IndiaAbstract: The microgrid concept has the potential to solve Are long, a coordinated effort may not be suitable due to themajor problems arising from large penetration of distributed excessive voltage drop resulting from the transfer of reactivegeneration in distribution systems. A proper control strategy power within long distances. That is why, in practice,should be implemented for a successful operation of a Reactive power compensation is usually comingmicrogrid. This paper proposes the use of a coordinated from local sources. In micro-grids, the electrical distancescontrol of reactive sources for the improvement of the between the sources of the Reactive power and the loads,dynamic voltage stability in a microgrid. The associated which need the reactive power compensation, is not much;controller is termed as a Micro Grid Voltage Stabilizer thus a coordinated compensation of reactive power sources(MGVS). The MGVS is a secondary level voltage controller for dynamic voltage stability should be desirable. Severalwhich generates a control signal. This control signal is lackouts have been associated with voltage stabilitydivided among the reactive power sources in the microgrid problems in a power system [2] [3]. The presence of weakin proportion to their available capacities; thus each source microgrids with insufficient amount of dynamic reactivewill be required to generate certain amount of reactive power capabilities can also cause blackouts In microgridspower. The MGVS is implemented in a micro grid test system and consequently the main power system. In this paper, thein MATLAB environment. A dynamic simulation of the test modeling of a microgrid is presented and a novelsystem is carried out for the cases of with and without the coordinated control method for dynamic reactive powerMGVS for various disturbances. Both grid-connected and sources is proposed. The associated controller is termed as aislanded modes of operation are considered. Results show Micro Grid Voltage Stabilizer (MGVS). MGVS is used tothat, with the addition of MGVS, the dynamic voltage profile improve the dynamic voltage stability of the microgrid andof the microgrid system, especially at the load Buses, to prevent voltage collapses. The input to the MGVS is aimprove drastically. voltage deficiency of the microgrid in dynamic state and the output of the MGVS is divided between the DistributedKeywords: MGVS-(Microgrid Voltage Stabilizer) Generators (DGs) depending on the nature of DG and its proximity to the voltage sensitive loads. A 21-Bus microgrid I. INTRODUCTION test system is used to verify the performance of the proposed The increase in power demand is stressing the controller. The dynamic modeling of the microgrid and thetransmission and generation system capabilities, leading to proposed controller has been done using MATLABfrequent power outages. In USA alone, these frequent power programming. Simulations are run for various dynamicoutages due to overloaded grid costs the economy $ 104 to $ events and the voltages of the load Buses are compared with164 billion dollars per year. The central plants are at best and without the Presence of MGVS.35% efficient due to generation and transmission losses. The The effectiveness of MGVS is studied in both grid-greenhouse gas emissions have risen owing to the less connected and islanded modes of microgrid.efficient power system [1]. This led to increased research In the following section, the modeling of microgridaiming to meet the growing energy demand without adding and its components is discussed. In section III the proposedthe transmission system capabilities. The use of distributed MGVS is explained. Simulation results and conclusion aregeneration (wind turbines, PV arrays, etc) at the distribution explained in the section IV and section V.system seems to be a viable solution. But, unplannedapplication of individual distributed generators, while II. MODELING OF MICROGRIDsolving some problems, can cause additional problems [1]. In this paper, the modeling of the microgridThe microgrid concept has the potential to solve major includes the modeling of the Diesel Engine Generators,problems arising from large penetration of distributed system loads and the transmission systemgeneration in distribution systems. Microgrids are almost85% efficient as they have very little transmission losses and A. Modeling of Diesel Engine Generatoruse the surplus heat to warm or cool buildings [1]. During A diesel engine generator (DEG) is widely used inpower outage or disturbance, microgrids can island remote locations, household, commercial and industrialthemselves and retain power availability, avoiding blackouts applications. The prime-mover is an internal combustionand lost productivity. Sufficient amount of dynamic reactive engine which is coupled to a synchronous generator withpower capabilities are needed to avoid a fast voltage exciter and a governor. The generator and the prime-movercollapse. In principle, a coordinated effort among the are mechanically coupled.reactive sources could result in better effectiveness of these The differential equations for the machine with theresources. However, in typical power systems, IEEE-Type I exciter and a turbine governor are given below Where the electrical distances between the reactive insources and where these reactive powers are needed (1)- (7) [5]. www.ijmer.com 4424 | Page
  • 2. International Journal of Modern Engineering Research (IJMER) www.ijmer.com Vol.2, Issue.6, Nov-Dec. 2012 pp-4424-4428 ISSN: 2249-6645 III. MICROGRID VOLTAGE STABILIZER The MGVS gives an input to the excitation systems or reactive power loops of DGs, which acts to kick in more reactive power into the microgrid to prevent any voltage collapse. Any small increase in reactive load can be met by the DGs, avoiding the use of expensive dynamic reactive sources, Such as STATCOM, SVC or capacitor banks. The microgrid voltage stabilizer model and its simplified version are shown in Figure 3 and Figure 4 [6]. The stator algebraic equations are derived from thedynamic equivalent circuit as shown in Figure 1[5] and aregiven below in (8)-(9).B. Modeling of DEGs and loads Connected to the MicrogridThe loads and DEGS are connected to the distributionnetwork with a known Y-matrix is shown in Figure 2. Theoverall microgrid system is modeled by writing the powerflows equations for all Buses [5] as shown below. The power flow equations for generator buses are givenbelow in [10]-[11] VMGVS represents the total MGVS correcting signal, which is divided between the DGs depending on the generation reactive reserve, proximity to inductive loads, etc. www.ijmer.com 4425 | Page
  • 3. International Journal of Modern Engineering Research (IJMER) www.ijmer.com Vol.2, Issue.6, Nov-Dec. 2012 pp-4424-4428 ISSN: 2249-6645The weighting factors for the generator Buses (1 to g) are β1, A. Voltage Stability Analysis of the Microgrid System inβ2…. βg. VMGVSi is the input to the ith generator’s Grid-Connected Modeexcitation system as shown in Figure 5. In the grid connected mode, the microgrid test system is connected to the main grid. The load at Bus 15 is IV. SIMULATION RESULTS the largest load in the microgrid. So, the load Bus weighing A 21-Bus microgrid test system is used to verify the factor is highest for Bus 15. The MGVS data table consistingPerformance of the proposed controller [7]. The modeling of load Bus weighting factors, generator Bus weighingand the simulation of the microgrid system and the MGVS is factors and MGVS control parameters are given as follows.done in MATLAB environment. In this section, simulation Table I. Load Bus Weighing Factorresults are presented for dynamic voltage analysis for variousdynamic events under both, grid-connected and islandedmicrogrid modes of operation and also during the islandingprocess. The dynamic events include line outage, three-phaseshort circuit fault, and load switching. The results arecompared with and without the presence of the proposedMGVS in each case. This will show the effectiveness of theMGVS to use reactive power compensation to improve thevoltage profile of the microgrid in case of such differentdisturbances. The microgrid test case, as shown in Figure 6,have three distributed generators and six constant powerloads. The microgrid is connected to the main grid,represented by large synchronous generator. The basicsimulation includes, calculating the steady state power flowof the system and the Initial value calculation of the statevariables of the system. Then, the dynamic case is initializedwith the initial values and run for the simulation time. Thedisturbances are included by changing the correspondingvalues of the system during simulation period. A three phase short circuit fault is applied at Bus15. The disturbance starts at 5% of the total time (i.e. 10sec), and ends after 2 secs. The results show that the loadbus voltages improve by more than 10 % in the presenceof a MGVS as shown in Figure 7 at Bus 19. www.ijmer.com 4426 | Page
  • 4. International Journal of Modern Engineering Research (IJMER) www.ijmer.com Vol.2, Issue.6, Nov-Dec. 2012 pp-4424-4428 ISSN: 2249-6645 C. Voltage Stability Analysis of the Microgrid System in In other case, the load at Bus 15 has been Islanded Modeincreased by 30%. The disturbance starts at 5% of the In the islanded mode of operation, the main gridtotal time (i.e. 10 sec), and ends after 5 seconds. The is removed and the total load is supported by DGs.results show that the voltage at Bus 15 improves in thepresence of a MGVS as shown in Figure 8. A three phase short circuit fault is applied at Bus 15. The disturbance starts at 5% of the total time (i.e. 5 sec), and ends after 70 cycles. The results show that the load bus voltages improve by more than 15 % in the presence of a MGVS as shown in Figure 10 at Bus 19. V. CONCLUSION In this paper, the concept of the MGVS isB. Voltage Stability Analysis of the Microgrid System introduced. Then, the modeling of a microgrid with DGsduring Islanding Operation and MGVS for voltage stability analysis was studied. The During the islanding operation, the line between differential algebraic equations (DAEs) related to theBus 2 and Bus 3 is removed, effectively disconnecting the microgrid system were derived. Various disturbances andmicrogrid from the main grid. Now the total load is faults like three-phase short circuit fault, load switching,supported by DGs at Buses 5, 9 and 17. The load and line outage were simulated with and without MGVSparameters remain unchanged, but the power generation in both grid connected and islanded modes of operationat the generators changes during the dynamic events. The and also during the islanding operation. Simulationdisturbance starts at 5% of the total time (i.e. 10 sec), and results show that the MGVS can Significantly improveends after 3 seconds. The results show that the voltage at the voltage profile of the system for various disturbances.Bus 19 improves by more than 7 % in the presence of a During three phase short circuit fault the MGVSMGVS as shown in Figure 9. improves the voltages at all the un-faulted load Buses. During load switching disturbance, MGVS reduces the reactive power dependence on the main grid by sharing most of the reactive load between the DGs. During line outage and islanding operation, MGVS is effective to coordinate the DGs reactive power generation and supply the lost generation from the grid. www.ijmer.com 4427 | Page
  • 5. International Journal of Modern Engineering Research (IJMER) www.ijmer.com Vol.2, Issue.6, Nov-Dec. 2012 pp-4424-4428 ISSN: 2249-6645 REFERENCES [4] L. Robertr, A. Akhil, M. Chris, S. John, D. Jeff, G.[1] M.U. Zahnd, "Control Strategies for Load- Ross, A. S. Sakis, Y. Robert and E. Joe, "Integration Following Unbalanced MicroGrids Islanded of Distributed Energy Resources: The CERTS Operation," Faculté Sciences ET Techniques de Micro Grid Concept," U.S. Department of Energy, lIngénieur, Lausanne, MS Thesis 2007. White Paper LBNL-50829, 2002.[2] A. Kurita and T. Sakurai, "The power system failure [5] P.W. Sauer and M.A. Pai, Power System Dynamics on July 23, 1987 in Tokyo," in Proceedings of the and Stability, 2nd ed. Champaign, USA/Illinois: 27th IEEE Conference on Decision and Control, Stipes Publishing, 2006. Austin, 1988, pp. 2093 - 2097. [6] P. L. Jeffrey, "Modeling of Dynamic Loads for[3] US-Canada power system outage task force, "Final Voltage Stability Studies," Tennessee report on the august 14,2003 Blackout in the United Technological University, Cookeville, MS Thesis States and Canada: Causes and Recommendations," 2007. Nautral resources Canada and U.S. department of [7] F. Katiraei, M.R. Iravani and P.W. Lehn, "Micro- energy, 18 Pages Report. grid autonomous operation during and subsequent to islanding process," IEEE Transactions on Power Delivery, vol. 20, no. 1, pp. 248-257, Jan 2005.373 www.ijmer.com 4428 | Page