LOSS MINIMISATION IN TRANSMISSION AND DISTRIBUTION NETWORKS S. Musa and A. G. Ellams Department of Electrical Engineering, Kaduna Polytechnic, Kaduna Abstract One of the outstanding and major problems in the transmission and distribution system is the losses. This paper highlighted various ways in which energy is lost between the point of generation and consumption and how these losses can be minimized. Matlab was used for the simulation(power system blockset). Some of these losses are technical while others are non technical. Technical losses are inherent in the system and are due to energy dissipated in the conductors and equipment used for transformation, transmission, sub-transmission and distribution of power. Electrical energy is almost exclusively generated transmitted and distributed in the form of alternating current. Thus power factor come into play. Low power factor is highly undesirable as it causes an increase in current, resulting in additional losses of active power in all elements of power system. In order to ensure most favourable condition for a supply system from engineering and economical stand point, it is important to have power factor as closed to unity as possible These losses can be reduced to an optimum level with used of devices like OLTC, FACTS and switched capacitor banks.thus improving the power factor to a value closed to unity as possible. Introduction In any network, energy losses arise as power flows through the network to supply the loads or consumers, with the new prepaid system of the Power Holding Company of Nigeria the question is who pay s for the losses. The losses generate a substantial cost and are a significant issue in power system management. Independent economic studies have shown that the additional capital expenditure in reducing losses on systems that have become inefficient can be more cost effective than installing additional generation (Bayliss, 2001). It is therefore, important for system planners to show interest in loss reduction on system. Reduction in losses will bring about maximization in returns and that will amount to a significant financial saving to utilities as well as customers (Dardson et al., 2002; Salawu and Achife, 2001). Losses can be minimized by reducing either the resistance or impedance of the transmission medium. This is possible by selecting a conductor with small resistivity. Another way of reducing losses in a system is by decreasing the current and maximizing voltage (Low power factor is highly undesirable as it causes an increase in current, resulting in additional losses of active power in all elements of power system). These can be achieved through the use of tap changer transformers and regulating transformers. Voltage control in transformers is required to compensate for varying voltage drops in the distribution system and to control reactive power flow over transmission lines. Practically many distribution transformers have taps in one or more windings for changing the turns ratio, to improve the voltage profile in the system. Tap changer control is the most popular method because it is used for controlling voltages at all levels, (i.e at both transmission and distribution voltage levels). (Nedic, 2002), reported that the off-nominal tap ratio determines the additional transformation relative to the nominal transformation. The value normally ranges from 0.9-1.1 (1.0 corresponds to no additional transformation or the nominal value). The tapping can be automatic or manually operated via a switch. Alternatively, the change involves physically and manually changing tapping connections. Such arrangements are found on smaller distribution transformers. Tap changing can be off circuit, offload and on- load depending on the system (Zhu and Tomsoric, 2001; Ochoa,). Changing of tap setting or turns ratio will change the system impedance matrix. Therefore the Ybus admittance matrix is changed after each tap ratio adjustment and that affects the power flow solution Components of losses transmission and distribution Energy losses arise due to technical and non technical losses as power flows through the network. These 79
technical losses are inherent in the system and can be reduced to an optimum level. Technical losses are due to the current flowing in the electrical network and include line losses, copper resistance and iron losses of transformers (iron losses in transformer include both hysteresis and eddy current loss, these losses are minimized by using steel of high silicon content for the core and by using very thin laminations).it was reported that, (Bhalla, ; Lukman and Blackburn, 2004; Lukman et al, 2000), Non- technical losses are more dominant in the lower levels of distribution networks, they include unauthorized line tapping, equipment vandalization, and inaccuracies of meter reading which will lead to inaccurate customer billing e.t.c. Factors influencing system losses Following are some of the factors that influence system losses (Dardson et al., 2002): Circulating current: In modern highly interconnected networks, failure to maintain a flat voltage profile across networks, will result in the flow of circulating currents. It is therefore important for a power system to maintain stringent voltage limits to minimize losses. Phase balancing: This is of significance when dealing with heavily loaded lines, the objective is to balance the phase load, so that the maximum deviation from the average is below 10%. Power factor: At unity power factor the current is minimum and any reactive component will cause an increase in current with a resultant increase in real power losses. For large inductive loads losses due to volt ampere reactive (var) become significant and demand side compensation become necessary (i.e. by installation of shunt capacitors). Furthermore as a result of increase in current in the system the voltage drop due to line resistance is greater than it would be at unity power factor. Voltage regulation; since line losses increase with the square of load current either maintaining and or increasing the normal operating voltage of the system, can reduce both maximum demand and energy losses. Power flow simulation using power system blockset Using drag-and-drop operations, a power system blockset model of the three distribution transformers was configured as shown in fig. 1. The peak load for each transformer will be used for further analysis. Recorded in table1 is the real and reactive power on the three load buses at 0.8 power factor. Normally, the power factor of the whole load on the supply system is lower than 0.8, as reported by (Lemos et al; Del Rosso et al, & Sa adat). Table 1 bus data Bus data Bus no. Voltage Power Remark V(P.U) Phase angle Real (kw) Reactive in degree (kvar) 1 1.01 0 0 0 Slack bus 2 1.00 0 228 171 Load bus 3 1.00 0 232 174 Load bus 4 1.00 0 214 160 Load bus The power system network of fig. 1 was simulated under different power factor as per the loads recorded for each bus on table 1. From simulation results at a power factor of 0.8lag, the losses were found to be 21,45kW with voltage of 349V at the bus terminal as can be seen from fig. 3, 4 and 5 for the three transformers respectively.: Again at a power factor of 0.95 lag the losses where found to be 18.75kW, with bus voltage of 410V as can be seen from fig. 7, 8 and 9 for T 1, T 2 and T 3 respectively. The losses were earlier determined by this relationship Losses = input power output power. Table 2 provides bus voltages for nominal transformer taps position at different power factor. 80
Table 2a. Bus voltage for nominal transformer taps at 0.8p.f. lagging Bus no Network bus 1 2 3 4 Magnitude (P.U) 1.01 0.84 0.84 0.84 Phase angle degree 0.00 0.80 0.80 0.80 Table 2b. Bus voltage for nominal transformer taps at 0.95p.f. lagging Bus no Network bus 1 2 3 4 Magnitude (P.U) 1.01 0.98 0.98 0.98 Phase angle degree 0.00 0.95 0.95 0.95 It can be seen that at 0.8 lag power factor, voltage are found to be outside acceptable limits because they fall below 0.95 p.u. But at 0.95lag p.f voltage was found to be within acceptable limits of 0.98 which is above.95p.u. 81
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From the analysis it is clear that if a transformer is operating at a poor power factor, that goes to show that bus voltages are lower than expected value. This condition will make the transformer to rise to full load quickly even if the load in circuit is not up to the full load condition. Under this situation more losses are incurred in the lines and customer will pay more. If on the other hand power factor is improved upon the reverse condition is seen and losses in this case are minimal. From simulation result at a power factor of 0.95, losses were found to be 18.746kw again at 0.8 power factor, were found to be 21.45kW. This explains need to have supply voltage at the desired level. The following graphs are obtained from the simulation work of the three distribution transformer network model. Fig 3.2 Real and reactive power for the system. Fig. 3.3. Voltage Level T 1 at p.f 0.8 At 0.8 power factor at 0.8 power Fig. 3.4 Voltage Level T 2 at p.f 0.8 Fig. 3.5 Voltage Level at T 3 p.f 0.8 83
Fig. 3.6 Real and reactive power of the system at 0.95 power factor Fig.3.7 Voltage of T 1 at 0.95 power factor Real and Fig. 3.8 Voltage of T 2 at 0.95 power factor Fig. 3.9 Voltage of T 3 at 0.95 power factor Conclusion The need to reduce electricity bills cannot be overemphasized. One way to go about it is to reduce the technical losses in the distribution network. This is expected to save the consumers a substantial amount of money annually. Losses can be minimized by reducing either the resistance or impedance of the transmission medium. This is possible by selecting a conductor with small resistivity. Another way of reducing losses in a system is by decreasing the current and maximizing voltage. These can be achieved through the use of tap changer transformers and regulating transformers switched capacitor banks. Base on the power system block sets model developed for the three distribution transformer 11kV network a power flow analysis was conducted. It was found that at a p.f of 0.8lag the bus voltage in the network is 0.84 p.u for all the transformer and total loss for the system was found to be 21.45kW. On the other hand at a power factor of 0.95lag the bus voltage increase to 0.98, p.u while the total system loss for the whole network was found to reduce to 18.75kW. It is therefore important to consider the option of installing devices for power factor improvement, this will go a long way reducing electricity bills. References Bayliss C.R (2001), Transmission and Distribution Electrical Engineering (2 nd ed) London: Newness. Bhalla M.S Transmission and Distribution losses (Power),Retrieved 19 th may, 2009. www.sbisratweb.uqac.ca/archivage/17833301.pdf. Dardson I.E, Odubiyi A, and Kacheinga M.O (2002), Technical losses computation and economic dispatch model for transmission and distribution in deregulated electricity supply Industry, Power Engineering journal April, 2002. PP 55-60. Del Rosso, A.D Canizares C.A & Dona V.M A study of TCSC controller design for power system stability improvement, Retrieved 20 th November, 2006 htt/thunderbox.waterloo.ca/claudio/papers/alberto.p df, pp 1-6. 84
Lemos F.A.B, Feijo Fr, W.L, Werberich L.C, & Rosa M.A (2002), Assessment of a sub-transmission and distribution system under coordinated secondary voltage control, 14 th PSCC, Serilla, 24 28 June,2002, pp. 1-7. Retrieved 11 th April, 2005. http://www.pscccentral.org/uploads/tx_ethpublications/s21p01.pdf Lukman D & Blackburn T.R (2004), Modified algorithm of load flow simulation for loss minimization in power systems, Proceedings of the Australian Universities Power Engineering Conference (AUPEC 94), 2004. Lukman D, Walshe K & Blackburn T.R (2000), Loss minimization in industrial power system operation, AUPEC 94 Brisbane, Australia, 24-27 September, 2000. Nedic D (2002), Tap adjustment in A.C loadflow, UNIST, Sept. 2002 Retrieved 6 th may 2009. www.unist.ac.wc/departments/mcee/reseach/publicate ion/tap.adjustments.pdf Ochoa l.f, Ciric R.M, Feltrin A.P, & Harrison G.P, Evaluation of Distribution System losses due to load unbalance, Retrieved 19 th may, 2009. www.see.ed.ac.uk/~gph/publications/pscco5.pdf Sa adat H (2002) Power System Analysis, Tata McGraw-Hill Publishing Company ltd, New Delhi. Salawu R.I. and Achife J.K, (2003) Technical losses in the transmission and distribution of PHCN:. A Case study of the Ibadan transmission and distribution Network, Proceedings of the Colloquium on Current Trends in High Voltage Engineering Focusing on the Nigerian Power Sector, Abuja, Nigeria, July 2003, pp 41-47 Zhu Y & Tomsoric K (2001) Adaptive power flow method for distribution system with dispersed Generation. http//www.pser.wisc.edul/publication/2001 public/finalk.pdf, pp. 1-7. 85