Comparison of Wheeling Cost using Power Flow Tracing Methods in Deregulated Electric Power Industry.

Transcription

1 Comparison of Wheeling Cost using Power Flow Tracing Methods in Deregulated Electric Power Industry. K.Hema Lalitha Student M.Tech MVGR College of Engineering Vizianagaram I.Kranthi Kiran Associate Professor MVGR College of Engineering Vizianagaram ABSTRACT: The power industry across the World has experienced a lot of restructuring from regulated to deregulated environment to improve the technical and commercial efficiency. The purpose of deregulation is to restructure the power industry so that power production and retail sales will be competitive, leading to Independent power producers (IPPs).In deregulated power sector, the overall power system is restructured in three main parts - generation, transmission and distribution of electricity. Electricity transmission and wheeling service pricing has become a more complex and more important task. Pricing of transmission services plays a vital role in determining whether the providing transmission services are economically beneficial to both wheelers and customers. The wheeling cost depends on how much power is being moved through a particular line. To know the power flow in this case, Bialek s method and Kirschen s method has been considered. There are several methods to determine the wheeling cost. In this paper, the power flow based MW-KM method is used to calculate the wheeling cost of IEEE 14 bus system and the results are compared in both tracing methods. Keywords:Deregulation, Power tracing, Transmission pricing, Wheeling cost. 1. INTRODUCTION: The electric power industries over the years are vertically integrated utilities which are the only electricity providers in the region. It was difficult to segregate the costs incurred in generation, transmission or distribution. So, the utilities often charged their customers on an average tariff rate depending on their aggregated cost during a period. Therefore, deregulation occurred. Electric deregulation is the process of changing rules and regulations that control the electric industry for providing customers the choice of electricity suppliers by allowing competition leading to Independent Power Producers (IPPs). In this process, some new entities are expected to appear and there are: Generation Companies (GenCos.), Transmission Companies (TransCos.), Distribution Companies (DisCos.), Independent Power producer (IPP), Independent System Operator (ISO), Power Exchange (PX), Retail Energy Service Companies (RESCos.). Pricing of transmission services plays a crucial role in determining whether the providing transmission services are economically beneficial to both the wheelers and the customers. Wheeling can be defined as the transfer of electrical power from one network to another to deliver power of and for another entity or entities. To evaluate the costs of wheeling transaction, various salient methods are there like Postage Stamp method, Contract Path method, Boundary flow method and Flow based methods. In those methods, Power Flow based MW-km method is used here which is accurate and reliable.to know the power transfer between individual generators to lines and generators to loads, there are different power flow tracing methods, but in this paper we use Bialek s Power Flow Tracing method (Node method) and Kirchen s power flow tracing method(common method) to determine the power flows in an IEEE 14 bus system. The wheeling cost is then calculated by using Power flow based MW-km method for this case. 861

2 2. BIALEK S POWER FLOW TRACING METHOD: Bialek s power tracing technique describes that which generators are supplying a particular load, how much use each generator is making use of a transmission line and what is each generator s contribution to the system losses. It is possible to calculate how much of a particular generators output supplies a particular load and how much of a particular load is supplied by particular generator. The main principle used in this power flow tracing method is Proportionate Sharing principle. In Bialek s method, there are two algorithms for power tracing: Upstream tracing algorithm and Downstream tracing algorithm. In this paper, only Upstream tracing is used so downstream tracing is not discussed. 2.1 Proportionate Sharing Principle: The proportional sharing principle is based on Kirchhoff s current law and is topological in nature. It assumes that the network node is a perfect mixer of incoming flows. Practically the only requirement for the input data is that Kirchhoff s current law must be satisfied for all the nodes. Figure 1: Example of Proportionate Sharing Principle Here, four lines are connected to node i, two inflows and two outflows. The total power flow through the node is P i=40+60= 100 MW of which 40% is supplied by line j-i and 60% by line k-i. Hence, the 70 MW out flowing in line i-m consist of 70 x 40/100=28 MW supplied by line j-i and 70 x 60/100=42 MW supplied by line k-i. In the same way, 30MW out flowing in line i-l 30 x 40/100=12 MW consists of supplied by line j-i and 30 x 60/100 = 18 MW supplied by line k-i. 2.2 Upstream tracing algorithm: The total flow and the inflow to the i th bus, is the sum of all the power inflows through the lines connected to that bus and the bus power injection at that bus by generators. It is given by equation 1. P = P + P for i=1,2,.n; j=1,2, n (1) where i= Sending end bus number, j= Receiving end bus number, n=number of nodes/buses, α =Set of nodes supplying directly node i (i.e. power must flow towards node i in the relevant lines), P =Line flow into node i in line j-i, P = Generation at node i If the line losses are neglected, P = P Equation 1 can further be expanded to the following given by equation 2 P = P + P (2) 862

3 Let C = to express the relationship between line flow and the nodal flow at the j th node, using proportional sharing principle. P = C P. Substituting this in 2 which gives equation 3 P = C P + P (3) Which on arrangement becomes P C P = P or A u P=P G P is the vector of gross nodal flows; P G is the vector of nodal generations, A u is the Upstream matrix of order (n n). The (i, j) element of A u is given by the following equation 4 1 for i = j [A ] = C = for j α 0 otherwise -1 If A exists, then P=A u P G and its i th element is given by equation 5 P = [A ] P for i=1,2,.. n (5) for i=1,2,.n; j=1,2, n (4) This equation shows the contribution of k th system generator to i th nodal power flow. A line out flow in line j-i from node i can be therefore calculated using proportional sharing principle, as given by equation 6 P = [A ] P for i=1,2,.n (6) This equation gives the contribution of k th generator to power flow in each line j-i. Finally, load demand at the i th bus, applying the proportional method, is given by equation 7 as P = [A ] P (7) P = P P P This equation shows that the contribution of the k th generator to the i th load demand and can be used to trace where the power of a particular load comes from. 3. KIRSCHEN POWER TRACING METHOD: This method is proposed by Daniel Kirschen, Ron Allan and Goran Strbac. Firstly, the active power flows are calculated using a solved power flow computation. This method arranges the buses and branches of the network into homogeneous groups. Once this organization is over, it is then possible to represent the state of the system by a directed, acyclic graph. Based on the graph and the assumption, the contribution of each generator to the branches and the loads can then be computed. From the starting of a power flow solution, first identify the busses and then find the set of busses supplied to the same generators. The method involves some new concepts as: Domains, Commons, Links. Domain of a generator is defined as set of buses that are reached by the power produced by the generator. A common is defined as a group of neighboring buses supplied by the same generators. One bus belongs to one common only. Unconnected sets of buses supplied by same generators are treated as separate commons. The number of generators that supply a common is defined as the rank of the common, and its 863

4 range will be between one and the total number of generators in the system. The lines that connect two different commons are defined as links. The power flows in the lines of a particular link are in the same direction, always from a common of lesser rank to common of higher rank. In this method, the assumption is that the proportion of inflow from generators to common is equal to the proportion of outflows to that common. The contribution of each generator to each load in the common can be calculated recursively by using the following relations below: F = C. F (8) I = F (9) C = (10) Where, C ij: contribution by generator i to the load and external flow of common j C ik: contribution by generator i to the load and external flow of common k F jk: flow from common j to common k through the link F ijk: flow from common j to common k through the link, coming from common i I k : internal flow of common k. 4. WHEELING COST CALCULATION USING POWER FLOW BASED MW-KM METHOD: Wheeling can be defined as the transfer of electrical power through transmission and distribution lines from one utility service area to other. Wheeling charge is the price per unit (MW-hour) that a transmission owner receives for using its transmission lines to transfer power from seller to buyer i.e., from generating stations or IPPs to consumers. Wheeling is more important for Independent Power Producers as they do not own transmission lines. They need some path to transfer the generated power. So, wheeling charge computation become more prominent in deregulated power sector. Among all wheeling cost computation methods, Power flow based MW-KM method is selected, because the actual power flows in the lines are taken into consideration which gives the actual transmission charges for every network. The step by step procedure to calculate the transmission charges is described below. The transmission charges for power flow in each line is calculated by multiplying the magnitude of power flow in each line due to each transaction (MW t,l in MW) with the length of each line (L l in km) and a predetermined weighing factor which reflects the unit rate (W l in Rs./MW/km) and summed over for all lines in the network. It is given as: MWkm = W MW, L (11) Then determine the total cost for each generator in the network. The cost allocated for each generator for power transaction i.e., the share of the total transmission network capacity cost, TC, allocated to transaction t is calculated using the formula below: Total transmission of each generator Transaction cost = total line cost Total transmission cost of all generators TC = TC,, (12) Where, TC t =cost allocated to transaction t TC = total cost of all lines in Rs. L k = length of line k in km W k =cost per MW per unit length of line k 864

5 MW t,k = flow in line k, due to transaction t T - set of transactions K - set of lines Total transmission network capacity cost is calculated as follows: TC = W P, L (13) Finally, calculate the per unit cost for each generation using equation below: per unit cost Rs. MW = (14) 4. CASE STUDY: IEEE 14 bus system consists of 14 buses,20 lines,2 generator buses and 11 load buses. Bus 1 is considered as reference bus. Figure-2 shows the line diagram of IEEE 14 bus system. Firstly, the load flow is done for the base case IEEE 14 bus system to know the power flows in each line and load flows through Matpower 4.1 software in MATLAB. Then, by using the Bialek s Upstream looking algorithm, the contributions of each generator in the system to power flow through each line in the network and loss in each line, contribution of each generator to each load in the system are determined using equations (1) to (7). By using equations (8) to (10 ), the contributions of each generator to each line and each load in the system are calculated in Kirschen s method. The wheeling cost is computed using equations (11) to (14). Figure 2: Line Diagram of IEEE 14 bus system 865

6 5. RESULTS: Table 1: Generator contribution for power flow in each line S.no. Line (MW) (MW) Table 2: Total transmission network capacity cost Maximum power flow in the line (MW) TC = W P, L ( Rupees ) S.no Line P max W l =0.5 Rs./MW/km W l =1 Rs./MW/km

7 E Total Table 3: Transmission cost of each generator for power flow in each linefor W k=1rs./mw/km Kirschen method Bialek method S.no Line Total

8 Table 4: Transmission cost of each generator for power flow in each linefor W k=0.5 Rs./MW/km Kirschen method Bialek method S.no Line Total Table 5: Transaction cost of each generator for different values of W k Kirschen method Bialek method W k=0.5 Rs./MW/km W k=1 Rs./MW/km

9 (Rupees/MW) Table 6: Per unit cost of each generator Kirschen method (Rupees/MW) (Rupees/MW) Bialek method (Rupees/MW) W k=0.5 Rs./MW/km W k=1 Rs./MW/km DISCUSSION ON RESULTS: Table-1 shows the contribution of each generator for power flow through each line in IEEE 14 bus system. Table-2 shows the total transmission network capacity cost for two different values of W l. The value of weighing factor depends on the type of the line (short, medium or long), rating of the line. So, it is assumed as 0.5 and 1 and the results in both cases are observed. Table-3 and table-4 gives the line-wise transmission cost of each generator in both Kirschen and Bialek method. The values clearly says that the contribution of particular generator for power flow in a particular line may vary in both methods but the total power flow in the line is equal to the sum of the contributions from and. Table-5 and Table-6 shows the Transaction cost and Per unit cost of each generator in both power flow tracing methods for different values of W l..the transaction cost of each generator in both the methods is compared. 7. CONCLUSIONS: As the electric power industry is deregulated, generation, transmission and distribution are separated. The main objective is to bring fair and open access in the system. Power tracing became important in the process of unbundling. For power tracing, two efficient methods Bialek s method and Kirschen s method are considered in this paper. These two methods are based on proportionate sharing principle. The contributions of each generator to each line and each load are calculated by Bialek s Upstream looking algorithm and Kirschen s method and the results are compared. For a system with less number of buses, both methods give similar results but as the number of buses increases the difference will be more. Power flow based MW-km method is used in this paper which gives accurate pricing. 8. REFERENCES: [1] POWER SECTOR REFORMS AND RESTRUCTURING IN INDIA -S.A.Khaparde Department of Electrical Engineering Indian Institute of Technology - Bombay, Mumbai, India. [2] M. A. Pai. Computer Techniques in Power System Analysis, Tata McGraw-Hill, New Delhi, [3] Electric Power Industry Restructuring in India: Present Scenario and Future Prospect,S.N. Singh, Senior Member, IEEE and S.C. Srivastava, Senior Member, IEEE,2004 IEEE International Conference on Electric Utility Deregulation, Restructuring and Power Technologies (DRPT2004) April 2004 Hong Kong. [4] H.H. Happ, Cost of Wheeling Methodologies, IEEE Trans. On Power System, Vol.9, No.1, Feb. 1994, pp [5] Lee, W.J., Lin, C.H., and Swift, L.D. (2001) Wheeling charge under a deregulated environment, IEEE Trans. Ind. Appl., vol. 37, 1, pp [6] J. Park, J. Lim and J. Won, An analytical approach for transmission costs allocation in transmission system, IEEE Trans. on Power Systems, vol. 13, no. 4, pp , November,

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