Egwaile J. O; Onohaebi S.O; Ike S. A; Department of Electrical/Electronic Engineering, University of Benin, Benin City, Nigeria.

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1 Evaluation Of Distribution System Losses Due To Unbalanced Load In Transformers A Case Study Of Guinness 15MVA, 33/11KV, Injection Substation And Its Associated 11/0.415kv Transformers In Benin City, Nigeria Egwaile J. O; Onohaebi S.O; Ike S. A; Department of Electrical/Electronic Engineering, University of Benin, Benin City, Nigeria. Abstract Distribution network losses can vary significantly depending on the load unbalance. Here, an analysis of distribution system losses is presented that considers load unbalance and the effect on the copper losses of a power distribution transformer. The study was carried by analyzing the load readings taken from all the public 11/0.415kv transformers fed from the Guinness injection substation. Comparison was made between the transformer copper losses calculated from the existing unbalanced load condition and the losses that would have resulted if the loads on the transformer were equally distributed among the phases The result shows that high levels of load unbalance produced greater losses in the transformers, and the total transformer copper losses on both feeders considered can be reduced by about 6% if steps are taken to balance the loads on the phases of the transformer. This will ultimately lead to economic savings, and increase in the systems availability and reliability. Keywords: Distribution networks, transformer, losses, load unbalance, three-phase load.substation, phase currents 1.0 Introduction Energy efficiency, in limited energy resources scenario, is considered as a source of energy in a distribution system. [1]. This is particularly important in a country like Nigeria whose distribution System is faced with low voltage and high loss, these two problems of high voltage drop and losses in the distribution network varies with the pattern of loading on the distribution network. [2]. Since system losses represent a considerable cost for utilities and energy consumers, its evaluation and reduction have been recognized as of interest by researchers. There are many distribution network devices responsible for energy loss, these includes losses along distribution feeders, losses in transformer windings and losses associated with unbalanced loads connected to transformers. Unbalanced load is a common occurrence in threephase distribution systems. However, it can be harmful to the operation of the network components, its reliability and safety. Thus, a distribution system unbalance phenomenon has been the focus of research in recent decades [3]-[5]. Therefore, considering the importance of loss analysis, the objectives of this work is to evaluate losses due to unbalanced loading in a transformer. Transformers are the link between the generators of the power system and the transmission lines and between lines of different voltage levels.[13]. Transformers power losses can be divided into two main components: no-load losses (hysteresis and eddy current losses) and load losses (ohmic heat losses and conductor eddy current losses). There are, however, other two types of losses namely extra losses created by harmonic and unbalanced currents flowing in the transformer winding, respectively. 2.1 Transformer Losses Three-phase power transformers are invariably used in transmission, sub transmission and distribution substations for essentially voltage transformation. The power transformer is a complex static electromagnetic machine with windings and a non-linear iron core.[6]. We will first present the transformer equivalent circuit in actual physical units then relate the losses in a transformer to these units. Figure 1.0: Basic transformer equivalent circuit An ideal transformer would have no energy losses, and would be 100% efficient. In practical transformers, energy is dissipated in the windings, core, and surrounding structures. Larger transformers are generally more efficient, and those 1

2 rated for electricity distribution usually perform better than smaller ones.[7] Transformer losses are produced by the electrical current flowing through the coils and the magnetic field alternating in the core. The losses associated with the coils are called load losses, while the losses produced in the core are called no-load losses. The no-load losses are basically the power required to keep the core energized. These are commonly referred to as core losses, and they exist whenever the unit is energized. No-load losses depend primarily upon the voltage and frequency, so under operational conditions they vary only slightly with system variations. Load losses, as the terminology might suggest, result from load currents flowing through the transformer. The two components of the load losses are the I 2 R losses and the stray losses. I 2 R losses are based on the measured dc (direct current) resistance, the value of which is due to the winding conductors and the current at a given load. The stray losses is a term given to the accumulation of the additional losses experienced by the transformer, which includes winding eddy losses and losses due to the effects of leakage flux entering the internal metallic structures. Auxiliary losses refer to the power required to run auxiliary cooling equipment, such as fans and pumps; they are not normally included in the total losses as defined below. losses in a transformer can be summed up as shown in equation 1.1 P TL = P NL + P L 1.1 Where P TL = Losses P NL = No load Losses P L = Load Losses No Load Losses Early transformer developers realized that cores constructed from solid iron resulted in prohibitive eddy current and hysteresis loss (as high as 99% of the no-load losses), and their design mitigated this effect with cores consisting of stack layers of laminations, a principle that has remained in use.[8][9]. The no-load loss can be significant, so that even an idle transformer constitutes a drain on the electrical supply and a running cost. (a) Hysteresis losses: Each time the magnetic field is reversed, a small amount of energy is lost due to hysteresis within the core. According to Steinmetz's formula [11][12], the heat energy due to hysteresis is given by, and, 1.2 hysteresis loss is thus given by 1.3 where, f is the frequency, η is the hysteresis coefficient and β max is the maximum flux density, the empirical exponent of which varies from about 1.4 to 1.8 but is often given as 1.6 for iron.[11] (b) Eddy current losses: Ferromagnetic materials are also good conductors and a core made from such a material also constitutes a single shortcircuited turn throughout its entire length. Eddy currents therefore circulate within the core in a plane normal to the flux, and are responsible for resistive heating of the core material. The eddy current loss is a complex function of the square of supply frequency and inverse Square of the material thickness. Eddy current losses can be reduced by making the core of a stack of plates electrically insulated from each other, rather than a solid block; all transformers operating at low frequencies use laminated or similar cores[10][12]. Referring to equivalent circuit of figure 1.0, Core loss and reactance is represented by the following shunt leg impedances of the model: Core or iron losses: R C Magnetizing reactance: X M. R C and X M are sometimes collectively termed the magnetizing branch of the model Transformer load losses [7] These losses are commonly called copper losses or short circuit losses. Load losses vary according to the transformer loading; they are composed of Ohmic heat losses, sometimes referred to as copper losses, since this resistive component of load losses dominates. These losses occur in transformer windings and are caused by the resistance of the conductors. The magnitude of these losses increases with the square of the load current and are proportional to the resistance of the windings. They can be reduced by increasing the cross section of conductor or by reducing the winding length. Using copper as the conductor maintains the balance between weight, size, cost and resistance; adding an additional amount to increase conductor diameter, consistent with other design constraints, reduces losses. Referring to equivalent circuit of figure 1.0 above, Winding joule losses and leakage reactance are 2

3 represented by the following series loop impedances of the model: Primary winding: R P, X P Secondary winding: R S, X S. Mathematically copper loss in a transformer is given by: Copper losses = I 2 R 1.4 Where I is the load current and R is resistance of the transformer winding. There is, however, another type copper loss created as a result unbalanced currents flowing in the phases of the transformer. For a three phase transformer, let the secondary load currents flowing in each of the phases be I R, I Y, and I B respectively. Thus total load current = and its associated 11/0.415kV transformers in Benin City, Nigeria 2.3 Network Overview Guinness Injection substation is located along the Benin Agbor road, immediately after Guinness Nigeria Limited Brewery premises. The substation is fed by the Ikpoba Dam 33kv feeder which radiates from the Benin 132/33kv transmission station along Benin/Sapele road, Benin City. The one line diagram showing the source of power for the Guinness injection substation is shown in figure KV BUS 60MVA, 132/33KV 33KV BUS I T = I R + I Y + I B 1.5 Copper losses in each phase = R (Red ), R (Yellow ), R, (Blue ). Where R is the winding resistance of the transformer per phase. Therefore copper loss = R + R + R = R( + + ) 1.6 If the load on the transformer is balanced, then I R = I Y = I B = I Therefore equation 1.6 becomes: copper loss = R( I 2 + I 2 + I 2 )=3 I 2 R 1.7 Equation 1.7 gives the total copper losses in a transformer under balanced load condition, while equation 1.6 gives the total copper losses for unbalanced load. Subtracting equation 1.7 from 1.6 yields: BDPA FEEDER IKPOBA DAM FEEDER 33KV BUS GUINNESS 15MVA, 33/11KV INJECTION SUBSTATION 11KV BUS Figure 1.0: One line Diagram Showing Power Source for the Injection Substation. As shown in figure 1.1, from the Guinness injection substation radiates two (2) feeders. Asaba Road feeder and BDPA feeder. The Asaba Road feeder has a total of fifty seven (57) 11/0.415KV distribution transformers of various ratings connected to it at various points along its length, while the BDPA feeder has a total thirty-seven (37) 11/0.415KV distribution transformers connected to it at various points along its length. ASABA ROAD FEEDER R( + + ) - 3 I 2 R = P loss unbalanced load R(( + + ) - 3 I 2 ) 1.8 Equation 1.8 shows that the total losses in a transformer would be higher as a result of unequal current flowing through the different phases of the transformer under unbalanced load condition. In the section that follows we will evaluate the total losses due to unbalanced transformer loading in the Guinness 15MVA, 33/11KV, Injection Substation 2.4 Collection of Data The injection substation under review was visited and the following data was collected: (1)Single lines diagram of the Guinness 15MVA, 33/11/0.415KV injection substation and its associated feeders. (2) Document containing list of all 11/0.415kv transformers connected to the substation and their ratings. 3

4 Since the load reading for each of the 11/0.415 transformer in the network was not available, we embarked on taking load readings at each transformer (excluding privately owned transformers because these are not severely affected by unbalanced loads) at different times of the day, this we did between October 2011 September 2012) with the help of Mastech Digital Power Clamp; Model MS2203 capable of measuring power (real and reactive), voltage, power factor and phase current. The average load reading for the period under review and other data collected for the two feeders is presented in tables 1.0 and 1.1. Table 1.0: Substation Parameters; (Asaba Road feeder) Average Load Current (Amps) S/N NAME OF SUBSTATION Transformer Rating (KVA) Red Yellow Blue TOTAL Load Current 1 ADUWAKA OWIE ODIONVBA OGBESON PALACE IYOBOSA ADAZE EBIKADE OHOVBE PALACE IGABOR UNITY UGOKPOLOR LIBERTY IGBINIDU AZAGBA SONOWE PHILOVE JUNCTION DAO PIPELINE UWAIMA II UWAIMA II UYIGUE AMUFI UGBOZIGUE JEHOVAH AGBOWO AGBOWO IGUOMON IGUOMON

5 29 IKHUENIRO IKHUENIRO NEPASCO NEPASCO ST MICHEAL AFENGE EHIKHIANMWEN BULLSEYE GODIAC Table 1.1: Substation Parameters (BDPA feeder) Average Load Current(Amps) S/N NAME OF SUBSTATION Transformer Rating (KVA) Red Yellow Blue TOTAL Load Current 1 ARUNDE EVBADOLOYI IHASE ASOWATA OSASUMWEN ALAGHODARO IFASUYI OTABOR OWANAZE SUNNY FOAM POGAH POGAH RELIEF UGBOKODU ARMY SIGNAL OBASUYI OBASUYI BDPA BDPA BDPA OBANOSA Data Analysis From tables 1.0 and 1.1, the copper losses for each transformer in the network was calculated using equation 1.6; i.e. copper loss (unbalanced load condition) = R + R + R = R ( + + ). In doing this the winding resistance per phase is assumed to be unity since this value is the same and constant for all phases of the transformer irrespective of loading. Next balanced load condition was considered, which implies that the total load current will be shared equally among the phases of the transformer. Under this condition equation 1.7 holds. i.e. 5

6 copper loss (balance load condition) = R ( I 2 + I 2 + I 2 ) = 3 I 2 R. Thus the total copper losses under balanced load condition were also calculated. The results for the two feeders are presented in table 1.3 and 1.4 respectively. Table 1.3: Copper Losses for BDPA feeder Unbalanced Load Condition Balanced Load Condition S/N NAME OF SUBSTATION Transfor mer Rating (KVA) CU Loss (Red ) CU Loss (blue ) CU Loss (yellow ) loss in the Three s CU Loss (Per ) loss in the three s 1 ARUNDE EVBADOLOYI IHASE ASOWATA OSASUMWEN ALAGHODARO IFASUYI OTABOR OWANAZE SUNNY FOAM POGAH POGAH RELIEF UGBOKODU ARMY SIGNAL OBASUYI OBASUYI BDPA BDPA BDPA OBANOSA

7 Table 1.4: Copper Losses for Asaba Road feeder Unbalanced Load Condition Balanced Load Condition S/N NAME OF SUBSTATION Transformer Rating (KVA) CU Loss (Red ) CU Loss (blue ) CU Loss (yellow ) loss in the Three s CU Loss (Per ) loss in the three s 1 ADUWAKA OWIE ODIONVBA OGBESON PALACE IYOBOSA ADAZE EBIKADE OHOVBE PALACE IGABOR UNITY UGOKPOLOR LIBERTY IGBINIDU AZAGBA SONOWE PHILOVE JUNCTION DAO PIPELINE UWAIMA II UWAIMA II UYIGUE AMUFI UGBOZIGUE JEHOVAH AGBOWO AGBOWO IGUOMON IGUOMON IKHUENIRO IKHUENIRO NEPASCO NEPASCO ST MICHEAL AFENGE

8 35 EHIKHIANMWEN BULLSEYE GODIAC Discussion The results from the copper losses calculations for both balanced and unbalanced load conditions and the resultant tables (table 1.3 & 1.4 above) show that: (a) The copper losses of transformer varies considerably with the degree of load unbalance. (b)the total transformer copper loss in the Asaba road feeder is units and units for balance and unbalanced load conditions respectively. (c) The total transformer copper losses in the BDPA feeder is units and units for balance and unbalanced load conditions respectively (d) The total transformer copper losses on both feeders can be reduced by about 6% if steps are taken to balance the loads on the phases of the transformer. (e) Unbalanced loading will reduce the capacity of the transformers since the protective devices of the overloaded phase will operate even before other phases senses overload. 2.6 Conclusion and Recommendations. In this paper loss evaluation in distribution systems considering both unbalanced load and balanced load Scenarios in the transformers is presented. The study shows that high levels of load unbalance produced greater losses in the transformers. This means that network reconfiguration considering load balancing is highly recommend in order to diminish overall system losses. It is recommended that balanced repartition of single-phase loads between the phases of the three-phase network should be vigorously pursued by the authorities concerned. The authorities concerned should also ensure that all the phases are always available to discourage consumers from shifting their loads when a phase fails. To this end, it is recommended that automatic phase monitors that would promptly report an open phase be installed in all the transformers in the network. This will not only help to reduce losses but it will also enhance the systems availability and reliability. References. [1]. Ikbal A, Mini S. T, Pawan K; Optimal capacitor placement in smart distribution systems to improve its maximum loadability and energy efficiency International Journal of Engineering, Science and Technology Vol. 3, No. 8, [2]. Oodo O S; Liu Y; Sun H; Application of Switched Capacitor banks for Power Factor Improvement and Harmonics Reduction on the Nigerian Distribution Electric Network International Journal of Electrical & Computer Sciences IJECS-IJENS Vol: 11 No: 06 [3] Meliopoulos A. P; Kennedy J, C; Nucci, C.A; Borghetti A;and Contaxies, G Power distribution practices in USA and Europe: Impact on power quality, 8th International Conference on Harmonics and Quality of Power Proceeding, pp 24-29, October 1998 [4] Balda J.C; Oliva A.R; McNabb D.W; and Richardson R.D; Measurements of neutral currents and voltages on a distribution feeder, IEEE Trans. Power Delivery, 12(4), pp , October 1997 [5] Chen T.H; and Yang W.C;, Analysis of Multigrounded Four-Wire Distribution Systems Considering the neutral grounding, IEEE Trans. Power Delivery, 16(4), pp , October 2001 [6]. Nasser D. Tleis; 2008 Power Systems Modelling and Fault Analysis Published by Elsevier Ltd [7]Kubo, T; Sachs, H.; Nadel, S. (2001). Opportunities for New Appliance and Equipment Efficiency Standards. American Council for an Energy-Efficient Economy. Visited November 16th, 2012 [8] Allan, D.J. (Jan. 1991). "Power Transformers The Second Century". Power Engineering Journal 5 (1): [9] Kulkarni, S. V.; Khaparde, S. A. (May 24, 2004). Transformer Engineering: Design and Practice. CRC. [10]Riemersma, H.; Eckels, P.; Barton, M.; Murphy, J.; Litz, D.; Roach, J. (1981). "Application of Superconducting Technology to Power Transformers". IEEE Transactions on Power Apparatus and Systems PAS-100 (7): [11] Steinmetz's Formula for Magnetic Hysteresis". Retrieved 7 February [12] Say M.G 2005, The Performance and Design of Alternating Current Machines CBS publishers and distributors. [13]Grainger, J.; Stevenson, W. (1994). Power System Analysis. New York: McGraw Hill. ISBN