Voltage Stability Enhancement of Radial Distribution System Using Distributed Generators

Transcription

1 16th NATIONA POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, Voltage Stability Enhancement of Radial Distribution System Using Distributed Generators K. Vinothkumar #, B. Santosh Kumar # and M.P.Selan * * Assistant Professor, #, * Department of EEE, National institute of Technology, Tiruchirappalli, Tamilnadu, India ( selanmp@nitt.edu) Abstract - This paper presents an approach for enhancement of oltage stability of radial distribution system employing distributed generators (DG). Among arious indices reported in the past, a superlatie index is identified and utilized for determination of distribution system oltage stability by reducing the radial distribution system into two bus uialent. Thereafter, suitable locations for DG placement are identified to enhance the oltage stability and reduce system power loss. The simulation study is carried out on 31 bus and 33 bus radial distribution systems using the software program deeloped in MATAB enironment. Key words Voltage Stability Index, Radial Distribution System, Distributed Generation I. INTRODUCTION An electric power system can be classified in accordance with the operating oltage leels as generation, transmission and distribution systems. The distribution system widely aries from the transmission system in respect of its operation and characteristics and is generally radial in nature. The power distribution network is constantly experiencing an eer growing load demand. The load on the system is not constant and aries oer a wide range within the same day. Hence, the system stability is of great concern under seere loading conditions in the distribution system. System stability is characterized by an initial slow ariation in system operating point until a sharp accelerated change occurs with load increase. If the system loading increases beyond this point then the distribution system will experience a oltage collapse. Research efforts hae yielded different tools and methodologies for the prediction of system oltage collapse or instability [1] [3]. These indices are the measure of distribution system oltage stability following any ariations in the connected loads, which can hae a maximum alue of one at the erge of point of oltage collapse and a minimum alue of zero when there is no load present in the system. Owing to fast depleting conentional energy sources and the need for achieing reduced emission leels, the penetration of renewable energy sources as distributed generation (DG) into distribution grids is gaining significant importance. Moreoer, the distribution system losses are also reduced with DG placement [4]. In this paper, the radial distribution system (RDS) is represented by its two bus uialent system and a superlatie index is identified for the determination of its oltage stability. With the help of this superlatie index and employing DG placement, an attempt to enhance the oltage stability of the considered distribution system is proposed in this paper. Finally, the suitable location for DG placement is identified based on achieed leels of oltage stability index and system power loss reduction. II. TWO BUS EQUIVAENT OF RDS The two bus uialent of radial distribution system is deried in [3]. The reduced single line network is shown in Fig. 1. P + jq Pr + jqr Vs In Fig.1, r + jx z θ Fig. 1 Single line diagram of two bus uialent of RDS x r R 2 2 ( P + Q ) X 2 2 ( P + Q ) and Vr (1) Department of Electrical Engineering, Uni. College of Engg., Osmania Uniersity, Hyderabad, A.P, INDIA.

2 16th NATIONA POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, where r and x are the uialent resistance and reactance of the line respectiely, R and X are the total real and reactie power loss, P and Q are the total real and reactie powers injected at the first node. III. IDENTIFICATION OF SUPERATIVE VOTAGE STABIITY INDEX In the past, seeral indices were proposed to ealuate the oltage stability of RDS. Among all, the indices gien below [1-3] are considered in the present analysis based on the reduced two bus uialent of RDS: 4( r + Pr ) p (2) V cos θ δ ( ( )) 2 s 22 ( θ + φ ) [ V cosδ ] 4Sr cos (3) Y s (( ) ( ) ) r r r r x P r Q + x Q + r P (4) Fig. 3. Voltage stability index of bus-33 in 33 bus system It can be inferred from Fig. 2 that the index performs better compared to the other two in indicting the oltage collapse point. This inference is confirmed by performing similar study on 33 bus system [5] and the results are presented in Fig. 3. Based on the aboe study, the index is identified as a superlatie index for determination of oltage stability of the distribution system. IV. VOTAGE STABIITY ANAYSIS A. Voltage Stability under Base Case oading where θ, φ and δ are the impedance angle, power factor angle and angle difference between the oltages V and V s r The aboe indices are computed for 31 bus system (single line diagram and data are gien in the Appendix) with different load leels at bus-20, which is shown in Fig. 2. The index alues are zero when there is no load aailable in the system and as the system load is increased (at bus-20), the indices alue increases until it approaches unity at the point of oltage collapse or oltage instability. Fig. 2. Voltage stability index of bus-20 in 31 bus system Fig. 4 indices with loading at all buses in 31bus system The index cures for 31 bus system with arying loading conditions at each bus are shown in Fig. 4. It can be obsered that the buses nearer to substation can be loaded slightly more compared to the nodes farther from the substation. This is attributed to the line loss and subsuent reduced oltages at these nodes. This inference can also be taken from index shown in Fig. 4. From the aboe analysis, three locations are identified as discussed below for DG placement in the distribution system in order to enhance the oltage stability of the distribution system. Department of Electrical Engineering, Uni. College of Engg., Osmania Uniersity, Hyderabad, A.P, INDIA.

3 16th NATIONA POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, ocation 1: Bus with lowest slope for index cure - bus 29 ocation 2: Bus with medium slope for index cure - bus 27 ocation 3: Bus with highest slope for index cure - bus 11 The size of DG unit to be connected is considered to be ual to 25% of the total load in the network and for simplicity of the analysis, DG is modeled as negatie constant power load. B. Voltage Stability Analysis with DG at ocation 1(Bus 29) Fig. 6. index with DG at bus 27 in 31 bus system D. Voltage Stability Analysis with DG at ocation 3(Bus-11) Fig. 5. index with DG at bus - 29 in 31 bus system The Index cures for 31 bus system with DG at bus 29 are shown in Fig. 5. The slopes of the cures are similar to that of base case. The oltage stability index has the lowest slope at the considered location, which indicates that the considered location is highly stable. No significant changes in the index alues are obsered after placement of DG units also. C. Voltage Stability Analysis with DG at ocation 2 (Bus-27) The index cures for 31 bus system with DG at bus-27 are shown in Fig. 6. This location is geographically at the middle of the network and hence power fed by DG flows in both the directions and thereby reduces the line losses. It is obsered from Fig. 6 that index shows a slightly reduced slope compared to the base case. This indicates that an increased loading with enhanced stability can be achieed using DG placement at this location. Fig. 7 index with DG at bus-11 in 31 bus system The index cures for 31 bus system with DG at bus-11 are shown in Fig. 7. Substantial reductions in the steepness of the cures compared to the base case are obsered. This shows that bus-11 is ideally suitable for placement of DG units as increased loading without losing stability of the system can be achieed. E. Voltage Stability Analysis with ual distribution of DG at all three locations The total capacity of the DG considered in the preious cases is ually distributed at all the three locations and the corresponding index cures are shown in Fig. 8. The results are similar to that obtained by placing DG at bus-27. Department of Electrical Engineering, Uni. College of Engg., Osmania Uniersity, Hyderabad, A.P, INDIA.

4 16th NATIONA POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, Fig. 8 index with DG at all three locations in 31 bus system Fig. 11 index with DG at bus-6 in 33 bus system Fig. 9. index with loading at all buses in 33 bus system Fig. 12 index with DG at bus-17 in 33 bus system Fig. 10 index with DG at bus-22 in 33 bus system Fig. 13 index with DG at all three locations in 33 bus system Department of Electrical Engineering, Uni. College of Engg., Osmania Uniersity, Hyderabad, A.P, INDIA.

5 16th NATIONA POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, Thus it is understood that the suitable location for DG placement is the node with large slope for cure. In order to ensure the alidity of this obseration, similar analysis is carried out with 33 bus system. The index cures for the base case are shown in Fig. 9. Based on this, the locations identified for DG placement are: ocation 1: lowest slope - bus 22 ocation 2: medium slope - bus 6 ocation 3: highest slope - bus 17 Considering DG at the aboe identified locations, the index cures with DG placement at the identified locations are gien in Figs V. POWER OSS ANAYSIS The total power loss without and with DG are furnished in Tables. I and II for 31 and 33 bus systems respectiely. TABE I POWER OSS IN 31 BUS SYSTEM Real Power oss (MW) Reactie Power oss (Mar) Base Case (Without DG) DG at bus DG at bus DG at bus DG of ual size at all three bus TABE II POWER OSS IN 33 BUS SYSTEM Real Power oss (MW) Reactie Power oss (Mar) Base Case (Without DG) DG at bus DG at bus DG at bus DG of ual size at all three bus From Tables I and II, it can be obsered that the power loss reduction is more, when DG is placed at a location identified in the index with highest slope, i.e. bus-11 and bus-17 in 31 and 33 bus systems respectiely. Hence, it can be concluded that suitable location for DG placement can be identified with the index so as to enhance oltage stability and reduce power loss in the distribution system. VI. CONCUSION A method for placement of DG units in the distribution system is proposed in this paper, in order to enhance the oltage stability and reduce power loss. A superlatie index is identified and DG is placed at different locations during the analysis. Based on the results of the analysis, it can be concluded that the location with large slope for the index cure will be the suitable location for DG placement. DG placement at such selected location enhances the oltage stability and reduces system power loss to a great extent. REFERENCES [1] M. Moghaemi, M.O.Farurue, Technique for assessment of oltage stability in ii-conditioned radial distribution network, IEEE Power Engineering Reiew, January, 2001, pp [2] G.B.Jasmon,.H.C.C. ee, Stability of load flow techniques for distribution system oltage stability analysis, IEE Proceedings-C, ol. 138, No. 6, Noember, 1991, pp [3] Mohamed M. Hamada, Mohamed.A.A. Wahab, Nasser. G.A. Hemdan, Simple and efficient method for steadystate oltage stability assessment of radial distribution systems, Electric power and energy systems, 2009, pp [4] W.El-Khattam, M.M.A.Salama, Distributed generation technologies, definitions and benefits, Electric Power Systems Research, 71, 2004, pp [5] Selan M P, and Swarup K S., Distribution system load flow using object oriented methodology, International Conference on Power System Technology, POWERCON 2004, Singapore, No.21 24, 2004, Department of Electrical Engineering, Uni. College of Engg., Osmania Uniersity, Hyderabad, A.P, INDIA.

6 16th NATIONA POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, APPENDIX Fig. 14 Single line diagram of 31 bus system TABE III OAD DATA OF 31-BUS SYSTEM Bus j Maximum load at bus j P(kW) Q(kVAR) Branch Number TABE IV BRANCH DATA OF 31-BUS SYSTEM Bus i Bus j Branch impedance Rij(Ω) Xij(Ω) Department of Electrical Engineering, Uni. College of Engg., Osmania Uniersity, Hyderabad, A.P, INDIA.