Load Flow Analysis for Radial Distribution Networks Using Backward/Forward Sweep Method

Transcription

1 Open Access Journal Journal of Sustainable Research in Engineering Vol. 3 (3) 2016, Journal homepage: Load Flow Analysis for Radial Distribution Networks Using Backward/Forward Sweep Method Nanghoguina Madjissembaye 1*, Christopher M. Muriithi 2, C. W. Wekesa 3 1 Department of Electrical Engineering, power option, Pan African University, Institute for Basic Sciences, Technology and Innovations, Kenya 2 Department of Electrical & Power Engineering, Technical University of Kenya, Nairobi, Kenya 3 Department of Electrical and Information Engineering, University of Nairobi, Nairobi, Kenya. * Corresponding Author - smadjissembaye@gmail.com Abstract Power flow analysis is important for the performance s evaluation of operational conditions of electric power system and the planning of its future upgrading. The radial distribution networks RDN are characterized by load unbalances and high R/X ratios making the Newton Raphson and Fast Decoupled methods to fail in the analysis. In this work, the backward and forward sweep method was proposed as an efficient and easy tool for Radial Distribution Network analysis. The sweep path is based on Kirchhoff s voltage and current laws. The method was tested on the IEEE 33 bus radial test systems with different load scenarios under MATLAB and the results comparison has shown better voltage profile and accurate line power losses. Keywords Backward/Forward Sweep Method, Power Flow Analysis, Radial Distribution Network 1. Introduction The power flow or load flow analysis is essential for the planning, operation, optimization, and control of the power systems. It is called the heart of decision making in the electric power systems [1]. Information provided by the load flow analysis consists of active and reactive power flow in each branch and the associated line power losses, voltage magnitude and phase angles at each bus and the current magnitude in various branches under steady state condition. The load flow analysis is highly complex and it involves hundreds of buses and several distribution links, therefore resulting in extensive calculations [2], [3]. The load flow analysis in electric power systems consists of primary evaluating the voltage magnitudes and phases at each node, then using the obtained values, to compute the current at each branch and also the power flowing in various branches along with the power system losses associated with the current flowing in these lines [4]. The same principle is applied in the single wire earth return analysis, underground cables or overhead distribution 82 systems for the optimization, upgrading the existing system equipment or installing a new distribution system. There are mainly two topologies of electric power distribution systems namely the ring loop and the radial distribution systems. The ring distribution systems are more reliable and robust but very expensive. Therefore, many existing electric grids adopted the radial distribution systems which have the following characteristics [5]: Radial or weakly meshed networks (power sources supplied at one side only), Low X/R ratios, due to high resistance and low reactance of the line, Unbalanced operation, Unbalanced distributed load and multi-phase, Distributed generation is not easily dispatchable, Less expensive but least reliable network configuration. These features make the radial distribution networks to be known as ill-conditioned systems and they become challenging for the conventional, Gauss-Seidel method,

2 JSRE S. Madjissembaye et. al., Load Flow Analysis for Radial Distribution Networks Newton-Raphson, Fast Decoupled and their variants to effectively analyze them [2]. The Newton-Raphson method is one of the most widely used algorithms in the electric power industries for the strongly meshed transmission lines with several redundant paths and parallel lines but it failed in the radial distribution network due to the above-mentioned reasons and the convergence challenges though it could be effective and robust in the voltage convergence [2], it could not be effective for the optimal power flow computation due to the time consumption and large storage memory required [3], [5]. Authors in [6] proposed a Newton-Raphson method for solving ill-conditioned power systems. Their work demonstrated voltage convergence but could not be effectively applied for large power flow calculations. In this paper, the proposed backward and forward sweep method was used to calculate branch currents and nodal voltages using the Kirchhoff s law [7]-[9]. The validity of the method was tested on the IEEE 33-bus radial distribution system with different load scenarios. The rest of this paper is organized as follow, the section 2 will discuss the formulation of the problem of the radial distribution system followed by the backward and forward sweep method concept then the analysis of the results in section 4 before concluding in section 5 of this paper. 2. Problem Formulation The distribution systems are closed to the consumers and they are subjected to the abnormal connection of the loads which might be three or single phases. In the rural area with scattered consumers, the radial systems which are mainly fed on one side only are used and the voltage at the receiving end should not exceed ±5% of the nominal value. Therefore, the evaluation of the power losses in the lines becomes very important for the study of behavior of the power system and balance of the system [8]. The objective function of this study is to calculate the total lines losses formulated as follow: (1) (2) Where PTL and QTL are the total active and reactive power losses in the system respectively, I y is the current magnitude flowing from node y to node y+1. R y and X y are the resistance and reactance of the line respectively with N number of branches. The value of the current can be calculated using the two bus network shown in Figure Vy PDy + jqdy I Ry+ jxy Vy+1 Fig.1. Two buses network configuration PDy+1 +jqdy+1 From Figure 1, the current between the sending and receiving node of the network is given by: ) (3) (4) The voltage at receiving end can be calculated as follow: Where P y and Q y are the active and reactive power flowing from bus y to bus y+1 respectively, V y and V y+1 are voltage magnitude at sending and receiving node respectively. 3. Backward and Forward Sweep Method The backward and forward sweep method is used to solve the power flow analysis of radial distribution systems with recursive equations. The method known as the modified Ladder iterative technique was proposed by W.H. Kersting et al [10], [11] and R. Berg et al [12], the convergence of the method was explained in [9], [13]. The backward and forward sweep method is based on the Kirchhoff s voltage and current law and in each iteration, two computation stages occur, the forward path and the backward walk as described in the next section Backward Path The purpose is to find the current flowing in each branch in the tree by considering the constant value of voltages found in the previous iteration while a flat voltage value is assumed in the initial iteration. The backward path starts from the last node to the source node. (5)

3 JSRE Journal of Sustainable Research in Engineering Vol. 3 (3), Forward Path This starts from the source node to the far end node aims to calculate the voltages at each bus while keeping the current obtained from previous walk constant meaning that the current obtained in the backward walk will be held constant during the forward propagation. The calculated voltages are compared with the specified voltage and if the error is within tolerance limits, then the process is stopped and the power line losses are computed otherwise, the process is repeated until criteria conditions are met Backward/forward sweep algorithm The pseudo code of the backward /forward sweep method is illustrated in the flowchart shown in Fig. 2 start Read data Evaluate the per unit values Backward path Find current at each branch Forward path Find voltage at each node Convergence? YES Calculate the active and reactive power losses Print all results stop Fig. 2. Flowchart of Backward and forward method NO Step 1: Read Bus data and line resistance and reactance data, Step 2: Read base MVA and base KV and calculate the per unit values of the data loaded. Step 3: Backward walk from end node to source node to find all branch currents by using (3) and (4) while keeping constant flat initial voltages, Step 4: Forward walk from the source node to the far end node, to find all voltages using (5) while updating the constant current values obtained in the previous iteration and check for convergence criterion. Step 5: Check if the mismatch of the specified and calculated voltages at the substation is less than the convergence tolerance. If yes, go to next step. Otherwise, repeat step 3 and step 4. Step 6: calculate the total active and reactive power losses using (1) and (2) with the currents and voltages obtained from the backward and forward sweep method. Step 7: Print the result of all bus voltage, branch currents and Total loss in the system, Step 8: Stop. 4. Results and Discussion The effectiveness of the proposed method was tested on the IEEE 33 bus radial distribution systems with different load scenarios, the minimum, medium and high loading conditions are considered [14], [15]. The characteristics of the system is given below: Total number of nodes: 33, Total number of branches: 32, Kilovolt base: KV, MVA base is 100MVA, In this simulation, three scenarios with different power demands corresponding to different peak loads are considered in this study, namely: Case 1 Minimum Loading: The total minimum active and reactive power loadings are KW and 2070 KVAr. Case 2 Medium loading: The total medium active and reactive power loadings are 3715 KW and 2300 KVAr respectively. Case 3 Maximum loading: The total maximum active and reactive power loadings are KW and 2990 KVAr respectively. The backward and forward was run under MATLAB R2013a and the voltage magnitudes for different loadings at each node and total system losses are presented in Table 1. The voltage profile calculated and the total power losses in the systems were compared with the existing methods [16], [17]. The method has shown better results than the existing ones. Figure 3 shows the voltage profile at different loading; for all the cases, the minimum voltage magnitude occurs at node 18 but the magnitudes of the voltage differ p.u. for [16] and p.u. for the proposed method. Figures 4 and 5 show the compilation of 84

4 JSRE S. Madjissembaye et. al., Load Flow Analysis for Radial Distribution Networks active and reactive power losses in the system respectively. The total active and reactive power loss for the medium loading are KW and KVAr for reference [16] while for the proposed method, total active and reactive power losses are respectively KW and KVAr. The maximum active loss occurs at branch 2 whereas the reactive loss occurs at branch 5. Table 2 gives the results of the branch losses for the radial distribution system simulation. From the results we noticed that there was voltage profile changes and power losses increment with the increase in the loading of the system, therefore, further increase in loading without compensation may lead to a voltage collapse or blackout. Therefore, the proposed method, provides an accurate the status of the power losses and the voltage magnitude which could be used for further studies of the power system Minimum voltage bus PTL(KW) QTL (KVAr) Table 1: Voltage profile (p.u.) for IEEE 33 Bus Radial Distribution system and the total system power losses. Medium Loading Min L Max L proposed proposed method nodes ref [16] method Fig. 3. Voltage profile for different loading of 33 buses. Fig. 4. Branch active power losses for the 3 cases 85

5 JSRE Journal of Sustainable Research in Engineering Vol. 3 (3), Fig. 5. Branch reactive power losses for the 3 scenarios. Table 2. Numerical results of the active power losses in the IEEE 33 bus RDN. Active power loss in KW Branch Min L Med L Max L Conclusions In this paper, the backward/forward sweep method was used for the analysis of IEEE 33 bus radial distribution systems and the results have shown superiority on the existing methods. This method is a bridge over the high R/X ratios and the formulation of Jacobian matrix. The different loadings have resulted in different voltage profiles and different power losses in the system. It is obvious that the proposed method is suitable for the fast convergence for the power flow analysis of radial or weakly meshed structure of the distribution systems compared to Newton Raphson method. Acknowledgement This research is fully supported by the African Union through the Pan African University, Institute of Basic Sciences, Technology and Innovation PAUISTI/JKUAT. I am grateful to the institution and the discussion with Dr. C. M. Muriithi was very productive. References [1] Kothari, Dwarkadas Pralhaddas, and I. J. Nagrath. Modern power system analysis. Tata McGraw-Hill Education, Pp [2] S. C. Tripathy, G. D. Prasad, O. P. Malik and G. S. Hope, "Late discussion and closure to "Load-Flow Solutions for Ill- Conditioned Power Systems by a Newton-Like Method"," in IEEE Transactions on Power Apparatus and Systems, vol. PAS-103, no. 8, pp , Aug [3] Rupa, JA Michline, and S. Ganesh. "Power Flow Analysis for Radial Distribution System Using Backward/Forward Sweep Method." World Academy of Science, Engineering and Technology, International Journal of Electrical, Computer, Energetic, Electronic and Communication Engineering8.10 (2014): [4] Kamaruzzaman, Z. A., Mohamed, A. & Shareef, H. Effect of grid-connected photovoltaic systems on static and dynamic voltage stability with analysis techniques a review, University Kebangsaan Malaysia, July [5] Balamurugan, K., and Dipti Srinivasan. "Review of power flow studies on distribution network with distributed generation." Power Electronics and Drive Systems (PEDS), 2011 IEEE Ninth International Conference on. IEEE,

6 JSRE S. Madjissembaye et. al., Load Flow Analysis for Radial Distribution Networks [6] S.C.Tripathy, G.Durga Prasad, O.P.Malik and G.S.Hope, Load Flow for Ill-ConditionedPower Systems by a Newton like Method, IEEE Trans., PAS-101, October 1982, pp [7] Bhutad, A. G., S. V. Kulkarni, and S. A. Khaparde. "Three-phase load flow methods for radial distribution networks." TENCON Conference on Convergent Technologies for the Asia- Pacific Region. Vol. 2. IEEE, [8] N. Madjissembaye, et al., Optimal siting and sizing of single SPV system in radial distribution network for loss reduction based on maximum power saving technique. International Journal of Engineering Modern Research, IJMER Vol.6, No. 7, pp , July [9] Bompard, E., et al. "Convergence of the backward/forward sweep method for the load-flow analysis of radial distribution systems." International journal of electrical power & energy systems 22.7 (2000): [10] W.H. Kersting, W.H: Distribution System Modeling and Analysis,CRC Press,2002 [11] Kersting, W. H., Mendive, D. L.: An application of Ladder Network theory to the solution of three phase Radial Load-Flow Problem, IEEE PAS Winter meeting, New York, 1976, IEEE Paper No. A [12] Berg, R., Hawkins,E.S., and Plelines, W.W. Mechanized Calculation of Unbalanced Load Flow on Radial Distribution Circuits, IEEE Trans. on Power Apparatus and Systems,1967, 86, (4), pp [13] Michael McAsey and Libi Mou, Convergence of the Forward Backward Sweep Method in Optimal Control IL [14] K. Nadhir et al, Contribution à la gestion des réseaux de distribution en présence de génération d énergie dispersee, Thèse de Doctorat en Génie Electrique de l université de Batna, December, 2014 [15] E. Afzalan, M. A. Taghikhani and M. Sedighizadeh, "Optimal Placement and Sizing of DG in Radial Distribution Networks Using SFLA," International Journal of Energy Engineering, Vol. 2, pp , [16] Teja, B. Ravi, V. V. S. N. Murty, and Ashwani Kumar. "An Efficient and Simple Load Flow Approach for Radial and Meshed Distribution Networks."International Journal of Grid and Distributed Computing 9.2 (2016): [17] Teng, Jen-Hao. "A direct approach for distribution system load flow solutions." IEEE Transactions on Power Delivery 18.3 (2003):