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Improved Control of Isolated Power System by the Use of Feeding Technique

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Improved Control of Isolated Power System by the Use of Feeding Technique
  IMPROVED  ONTROL OF ISOLATED POWER SYSTEM  Y THE USE OF FEEDING TECHNIQUE Amit Soni, C.S. Ozveren University of Abertay. UK ABSTRACT The lack of energy supplied inrural areas is a chronic problem In many developing countries less than 1 of the rural population has access to electricity. Rural electrification by conventional means such as grid extension or the use of diesel generator is very expensive. Becauseof this obstacle standing in front of conventional grid extension to reach the non-electrified regions, reliable and affordablesolution is the development ofan Isolated Power System  IPS that include wind power solar power batteries,fly wheel etcin addition to conventional sources. Forany reliable electrical system, it is necessary to keep the power balance according to load demand This paperreviews an efficient solution to the problem of control strategy in IPS. Here It will be shown that how the combinationof renewable and conventional energyconversionsystems in addition to batterysupplies are connected into isolated grid and operated in grid-forming mode Keywords: rural electrification, isolated power system,control strategy, Design approach, 1. INTRODUCTION 2. CATEGORIESOF POWER UNITS The integration of renewable energies in toelectrical To alleviate this review, the power units-generator and power supply is of growing relevance [1]  Hybrid loads of an electrical system will be classified by their energy system which is mainly basedon thefunction, according to their role in keeping its power contribution of thelocallyavailable renewable energy balance. This classification will bebased on thecontrolsources, represent an innovation and sustainable method of power pro u tion or consumption which is solution for decentralized and remote power supply. as below [2] Solar and wind energy can be efficiently integrated in Grid forming unit: The grid-forming unit is liable for off-gridregions. In order to offer an uninterruptible controlling voltage and the frequency of grid and power supply hybrid system are additionally equipped balancing them This can be performed by increasing with batteries or combustion engines [1]  or decreasing power production. A standard electrical power system must contain one grid-forming unit as a Especially in thecase of renewable energysystems master which should be a diesel generator set a battery fluctuations are high due to the intermittent and inverter ora hydrogen cell fluctuating nature of their resources. Thus, an intermediate D C stage between the energyconversionGrid supporting unit: A term grid supporting unit is system and isolated grid is suggested here. The defined by the production and consumption of function of this stagewill be in such a way that it predefined amount of power The management unit decouples the frequency and voltage of the grid from usually considers the value of this power So if there is those of energy conversion unit. Also, thecontrol and a power imbalance in gridthen power production or management function will bedeveloped in such a way consumption can not be changed. However that the require amount of power and energy for the management unit according to requirements and the satisfaction of the load requirements are always specifications of machines can change the reference fulfilled. value of system. Therefore the grid supporting unit can support the stability ofsystem and raise thecontrol for Figure 1 shows a diagramof an isolated grid supplied grid forming unit. mainly with renewable energy sources like PV generator and wind energy converters. Grid parallel unit: A Grid parallel unit is designed to feed as much power into thegrid as possible. So power production or consumption in this unit is not controllable. These unit containsloads and 974  uncontrollable generators like wind energy converters to supply the load.If the total PV and wind turbines with outcontrolor PV invertersfor grid connection. power is greater than the required load, then they charge the battery.If thebattery bank is completelycharged then the excess power from the PV array and 3. DESIGN OF IPS MODEL WEC sent to dump load, which canbe utilized for heatingpurpose. If the PV and WEC are incapable of 3.1 General Design supplying the load, thebattery bank will supply the The hybrid system model will be designedsuch that load. When thebattery KW-hr levels falls below 20 PV array and wind turbine will have ahighest priority of therated value the diesel generator is turned on. [4]  Ip  1 w~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ INIA dI Bg ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 1-_ 6M/ 6// _   Ir   - G  ~~~~~~~~to i __ ots ~~~~~~~~~~~~~~sse Thi unit use thactual an the reference e I ss D   c d   d p r s   D   v , t ldI X1-4 +666666M-JfOh   I p i   Figure 1 Isoae Po e Syte ingi origm d 18 ~~~~~~~~ytem Thi uIt use th acua an th reernc Figue 1 how D.Ccouledisoltedowe syte D otg theste of th batr adthI upto  IPS) that combines four different power conversion the MS Which decide whether to charge or discharge sources; tworenewable  PV, wind turbine) and two the battery; to switch thediesel generator ON or OFF; conventional  Diesel and Battery), in whichpower to reduce the power production of the renewable output of all A C sources is converted in to D C This energyvsources or not. [3]1 [5]. conet to th gri thogivre. Heeteidetf th thr coto tg. Cotolr r maaeet syte  MS pefom th frt stag operatedb th inu curet to th D /DC Inomto aboutload an Metoooia codto covrestruhseIaluis hc r tahdi coe t hiS,fro whc tcluae h aon ovre tef In tIstd tevlaecnrle of currents to be supp~~~~~~~~~~~~lidb he poesureandh ontrosteP syte the WECis cnrlle by1 pic exescrrn ta t esplidb h baku coto or spe coto  th baer iscnold ite14 1 t   ~97  through state of charge controller and the speedmaintain the VG constant under variation of IG, Eq. (1) governor controlsthe dieselgenerator. requires VINV, AC to be changed accordingly. The voltage phasor diagramofVG, VINV AC, and VL, is shown in Figure 2. The figure 2a shows a reference 3.3 Developed control strategy in Grid- forming case. If only the magnitude of IG changes with out mode change in angle 0, as shown in figure 2b, then the magnitudeof VL changes accordingly, resulting in There are different ways of controllingthegrid from change ofboth the magnitudeof VINV AC and its angle grid side or energyconversion system side. As we 6 with respect to VG. A change of the angle 0 with out discussed earlier, standard electrical system contains at a change of the magnitude IG, as shown in figure 2c, least one grid-forming unitas a master that canbe a result in change in thedirection of VL without change diesel generator set, a battery bank or a hydrogen cells. in its magnitude.Consequently,both the magnitude of VINV, AC and its angle are influenced. As shown in Before identifyingthecontrol function in grid forming figure 1 control unit produces two signals. One of them mode we need to consider some definitions [6]. is magnitude Vx (outputvoltage of inverter) and the Feeding the electrical grid based on the grid-driven other is its angle p. That signals goes in to a space feeding mode is called  griddrivenfeeding and the v tor modulator and that produces asignal to the point on which this feeding takes place is called grid- inverter. With this signal, the operation of theinverter drivenfeeding point . The whole electrical system is controlled in such a way that it produces the required starting from the conversion unit up to the point to the voltage VINV, AC. grid is called  grid-driven feeding unit . The system management is responsible for preparing As a grid-forming unit, the feeding unitcontrolsthethe signals, which are foundthrough the voltage magnitudeof the voltage  V) at the connection pointof variationacrossthe intermediate capacitance. These grid and the frequency  f) of the system. Its function is signals are transferred to the ECS.As shown in figure to feed as much as current into the grid as necessary. 1 the voltage Vc across the intermediate capacitor C is Figure 1 shows the grid drivenfeeding mode in a grid- given by forming case. V0 S I 4da   C LW v L LVN Where C is the capacitance of the intermediate  V INV I ACV\ EV\ NV\ /  : V   capacitor, Ic is the current flowingthrough capacitor AC v JV 3> . ; and Vo is initial voltage. > Now the current Ic is the difference between the  a) b)  c) current fed by D.C. to D.C. converter and the current withdrawn by inverter linv Fig. 2 Voltage-phasor diagrams [6] Grid-driven feeding point herehas to formand I:=DC   liDC  4) maintain the frequency and voltage of the electrical system. As shown in figure 1, the function of inverter e Under steady state condition; is equal to  c So no on the grid side is to maintain the frequency. The value itT of frequency equal to the reference value fREF supplied chang in flo d c tiongo energ p unk ditrbs to thecontrol unit. The voltage on the grid side VG iS the stead condition It energy production sr related to the inverter s A.C. side voltage V.n through tesed tt odto I h nrypouto rlettinrrs..i g vhr increases or the loaddecreases then current flows the voltage across the inductor VL is given by through capacitors and so Vc will increases, according to Eq.  3). On the other hand increase in load or a V L dI, Gdecrease in energy production causes Ilvto exceed lDC, : -   t which result in a current flow out fthe capacitorIC, (1) will be negative inthis case. So Vc willdecrease. So ~~~~~~~~~~~~~Original value of Vc is determined by comparison to So ~~~~~~~~~~~~a eferencevalue Vref. The reference value is   ~~~~~~~proportion l to the required grid voltage (the voltage of TZ =V r VL   ± VG T thegrid at the connection point. The difference is   h~~~~ (2) transmitted in to a P1 controller, which in turn produce If theloads of grid change while VG is constant, this current signal  ref~ change appears only as a change in current IG. To 976  The management system processes this signal with Soest Campus Division of Electrical Engineering, otherinput and output signals to the different ECSs. Soest, Germany. Jan.2005. The signal to each ECS is received first by its pulse generator  PG . By the use of acontrolcurrent Ico from the D.C. to D.C. converter, PG1 4 processes its reference signals  refl4 and produces a pulsed voltage7. AUTHOR SADDRESS that regulates the operation of the all the D.C. to D.C. converter, which is attached to energy conversion unit. The first author can be contacted at Amit Soni With this proposed reviewsystem we can controls University Of Abertay Dundee magnitude of the voltage of the connection point to the School of Computing and Creative Technologies grid V as well as the frequencyof the system f and Kydd Building hence maintain the power balance in the electrical Bell Street system. Dundee DD HG Scotland 4. CONCLUSION U.K. E-mail: OOO ~b ycu According to the structure reviewedaboveanddependingon the number of feeding units connected to IPS,the developed control and management functions of new system can be specified. This system is quite expensive  butaffordable becauseof involving amount of power electronics controlled devicesbut after all it gives a security of supply and independent controlled ofeach generator. Also, the proposed system is totallyflexible and the developed control and management functions do not depend on the number types and sizes of energyconversion unit. 5. REFERENCES   MLandau C.Bendel, A Engler, P.StrauB, M.Viotto, Renewable Energies in Distributed GenerationSystem. VDI GET Tagung Entwicklungslinien der Energietechnik, 4 5 Sept. 2002. 2. Ph Strauss, A.Engler. A.C. Coupled PV Hybrid Systems andMicro Grids-State of Art and Future Trends. 3rd World Conferenceon Photovoltaic Energy Conversion, Osaka Japan, 11 18 May 2003. 3. Osama Omari A Simulation Model For ExpandableHybrid Systems. 2nd European PV Hybrid and Mini-grid Conference.25 26 Sept.2003. Kassel, Germany. File no. 46. 4. M.Vadenbergh S.Beverungen B.Buchholz, ExpandableHybrid System for Multi-user Mini Grids,  7th European Photovoltaic Solar Energy Conference Proceedings. Munich Germany 2001 Volume 2, Page 2330 2335. 5 Tim Meyer Jochen Benz Hens George Puls, Dirk Uwe Sawer. Integrated Design Approach for PV Hybrid System 2nd European PV Hybrid andMini Grid Conference. Kassel, Germany 25 26 Sept. 2003. File no. 13. 6. Osama  dam Omani,   Conceptual Development of a General Supply Philosophy for IsolatedElectrical Power System thesis. South Westphalia 977
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