LCC Design Criteria in Electrical PlantsOriented to The Energy SavingA. Canova, F. Profumo, M. TartagliaDipartimento di Ingegneria Elettrica Industriale, Politecnico di Torino, ItalyCorso Duca degli Abruzzi, 24 – 10129 Torino (Italy)profumo@polito.itAbstract - In this paper a Life Cycle Cost (LCC) approach is proposed to design electric installations suitable to industrial and civil applications. The structure of the electric system under study is supposed radial, as in most cases, and composed by transformers and lines, while users are simply represented by means of their load diagrams. The LCC procedure allows to evaluate the main characteristics of transformers (rated power)and lines (rated current) and it can be adapted to choose also particular loads like induction motors. The paper shows an improvement of the standard procedures to design electric lines constituted by cables including also the case of their parallel connections and the most convenient types of bus bars. A similar concept is also applied to transformers taking into account their thermal behaviour to establish their limit performance, starting from the supplied system load characteristics. In the case of induction motors, the mechanical load is considered in the evaluation of the most convenient solution. The procedure has been applied to the case of an real industrial plant and the results reported in the paper are related to it.I. I NTRODUCTIONThe design of electrical installations requires to satisfy many technical constraints like electrical, thermal and mechanicals taking into particular consideration the system operating costs. It is well known that usually the solution corresponding to the lower initial cost could be quite different from the solutions which optimise both the initial cost and the energy saving. Because of the relatively long “life” duration in time of electrical installations (more than 20-30 years), the design procedure can be conveniently based on the choice of design parameter values which satisfy the technical limits and minimise the life cycle cost (LCC), that is defined as follows:()()()n n n p p p OC p p p IC p p p LCC ...,,...,,...,,212121+=(1)where:IC is the initial cost of investment (c.u.*),OC is the operating cost (c.u.*),p 1, p 2,…., p n is the set of design parameters.* c.u.current unitThe first term IC is the cost necessary to build the electric installations and it depends on chosen materials,manufacturing costs, used technologies, etc. The second term OC can be split as the sum of different terms like:ECC : energy consumption cost (c.u.),MC : maintenance cost (c.u.),NTC : non operating time cost (c.u.).As a first simplification, the term due to the cost of the energy consumption can be considered as the prevalent, thus the Eq. (1) can be written in the form:()()()n n n p p p ECC p p p IC p p p LCC ,...,,,...,,,...,,212121+==(2)The design criteria summarised by Eq. (2) can be applied tothe choice of the main components of an a. c. electric plant as lines and transformers for a defined system structure and for fixed system rated voltages. In such a way, the design of main components of the electrical system mainly depends on load waveforms.The cost of the complete system is the sum of costs of each branch and, under the frequent hypothesis of a radial network structure, the LCC of each component mainly depends on its own design parameters and is weekly influenced by the design parameters of other branches. Therefore the optimum solution for the complete system is found when the best solution of each component is got. In fact one can start from the terminal branches nearest to the users and he can design them independently one from other the others and goes back up to the generator side. A simple power summation can be performed according to the Boucherot rules, neglecting the voltage variations in the network depending on each branch electrical parameters and usually are lower than 4%.In the present paper, the well-known criterion standardised in [1] is used to choose electric cables having negligible dielectric losses. This criterion has been improved to consider also the case of power lines with multiple conductors connected in parallel, as proposed in [2] and it has been extended also to the case of bus bars too. Similar considerations have been applied to the choice of induction motors [3] and power transformers according to [4].In the case of the induction motors the main electric parameter is the rated power: the optimal solution can be found starting from prospective mechanical load and considering the mechanical, the Joule, the iron and the excess losses of these systems, comparing machines having different rated power and comparing also traditional motors and high efficiency machines. A similar method can be applied to the choice of transformers. As a first step, the network power flows is evaluated to select the minimum size of transformer (rated power) and thus evaluating the most convenient solution according to equation (2).The application of the LCC procedure requires:0-7803-7116-X/01/$10.00 (C) 2001 IEEE1. The knowledge of the initial cost2. The evaluation of the consumption cost due to the energylosses.The computation of the initial cost mainly depends on manufacturer factors, as: the frame size, the enclosure type,the temperature rating, the service factor, etc.. Moreover the market price for a component can be lower than the list price and the discount level often depends by the dimension of the buyer.The evaluation of the consumption cost requires the computation of the following term:()()∫=Te L dt t C t P ECC (3)where T [h]is the expected operational life, P L are the total component losses (depending on time) [kW]and C e [c.u./kW]is the cost of electricity (usually depending on time).II. M ODEL PROBLEMAs stated above, the electric plant structure of electric plant is assumed to be radial and the voltages on each node are considered equal to their rated values, thus so that they are known before computing the electric circuit parameters. Thesteady state a. c. conditions are considered at the rated frequency, @ 50 Hz. The starting points to design the network are the waveforms of loads i.e. the active and the reactive power values versus time. The above load powers allow to compute the power flowing in a branch, by simply summating the powers of all branches supplied by the considered one.The following economic evaluations neglect the operating cost due to maintenance and repair. This assumption is realistic in the cases of cables and bus bars which are usually free of faults during their operating life. This point could be analysed in more details for transformers and motors but one can simplify this problem assuming a low influence of these costs (maintenance and repair) when a comparison must be performed and this point is often confirmed in many industrial plants. In such a way, one can consider the life cycle duration shorter than the mean time between failures or,more simply, shorter than the average operating life of any component. Moreover in the Eq. (2) no residual value is given to each component at the end of the considered period. In other words, one can assume that the expected life of the electric installation is practically equal to the useful life of many components, also by the point of view of expected technological evolution of materials and installationstandards. In other words, we can assume to analyse a timeduration equal to 25-30 years which agrees with the operatinglife of cables, transformers and motors. More accurate modelshave been suggested for cables [5], but this point of viewseems more interesting in the case of distribution powersystems. Finally it is worthwhile to note that the most convenient components work at the operating conditions more favourable than the rated one so that their materialsundergo lower stresses and their expected life become longer as discussed in [6], [9], [10]. If one neglects these benefits, a further security margin on the obtained result is found when comparing the life cycle costs with the same time duration.As a final consideration no attention is given in the paper to the optimal choice of the reactive power compensation and the capacitor systems are considered like known loads.A. Electric LinesIn electric installations, lines allow the connection to the source of any load and cables are the most frequent and convenient solution. A growing diffusion of bus bars is foundat a the rated voltage lower than 1.000 V (low voltage); theselines are more expensive than cables but they allow to modifyeasily line system configuration. The determination of most convenient line is a well-known problem and the IECStandard [1] suggests the procedure to minimise the total cost in the case of cables interested only by joule losses. This standard procedure has been generalised to more cables connected in parallel and to bus bars. Considerations onpossible future developments of examined electrical installation can suggest solutions when one finds different possibilities having more or less the same life cycle cost.Cables For a cable at the rated voltage, frequency and the layout condition, the main parameters are the cross section and the insulation material. The analysis of cables according to IEC criterion has been detailed in [2] and the most interesting results can be here recalled as follows:• the most convenient cross section of the cable (economic section) is usually larger than the thermal value when the cable duration is longer than a few years;• the cable total cost decreases when energy cost increases,thus the economic section strongly depends on load factors and on the tariff parameters;• the decreasing discount rating increases the economical advantage in using economic section;• when the cable cross section becomes too large it is necessary to subdivide it into more subconductors connected in parallel: the optimal solution can be found also in this case by comparing different solutions employing different number of subconductors having different cross sections. Particular care in the analysis is necessary to avoid increased losses depending on geometrical disposition of conductors and on their thermal interaction;• in the low voltage applications economic cable sectionsuggests to use poorer insulating material (see Fig. 1) toget a more convenient solution;• when using economic section one obtains lower voltagedrops and increased fault currents which are more simpleto be detected.50100150200250Current size [A]C o s t [E u r o /m ]20040060080010001200Current size [A]C o s t [E u r o /m ]Fig. 1. Cost vs. current size for cables Fig. 2. Cost vs. current size for busbar[Working current= 100 A, Cable size (thermal)=50 mm 2, Expected operational life=25 years,Equivalent operating load hours per year=6,000 h, Energy cost=0.0865 Euro/kWh, Discount rate=2.5 %]Bus barsThe rated current is the main characteristic of this line and also in this case the total cost has a minimum value as it is shown in Fig. 2 (where the constant cost versus time of the electric energy ha been assumed like in Fig. 1). The general considerations are quite similar to the case of cables and they are weakly influenced by the bus bar manufacturer. In this case, the rated currents up to thousand of amperes are available and usually it is not necessary to use more lines connected in parallel.B. Electric Motors (Three Phase Induction Machines)According to a survey in the European Union in 1992, the motor energy consumption is about the 69 % of the total energy consumption in the industrial sector and the 36% in the tertiary sector. Moreover the 90 % of the consumption is due to induction motors in the range of power 0.75 kW to 750kW. In this context, the use of Energy Efficient Motors (EEM), instead of standard motors, or the application of choice criteria, as the LCC, represent important tools to obtain energy and money savings. Following the LCC procedure, the knowledge of the initial cost of the motor and the evaluation of the energy losses consumption cost are required. The computation item of the first cost largely depends on many factors, such as: the frame size, the type of rotor (squirrel or wound-rotor cage), the enclosure type, the temperature rating, the service factor, motor manufacturer,etc. The calculation of the consumption cost needs the evaluation of total motor losses, which include the following terms:1. Stator winding loss P JS [kW]2. Rotor winding loss P JR [kW]3. Magnetic core loss P µ [kW]4. No-load friction and windage loss P fw [kW]5. Full-load stray load loss P s [kW]All these terms are related to the per unit load defined as the ratio between the output power P out and the rated output power:P outR : outRoutR P P L =.Working in the stable part of the motor speed-torque characteristic (where the speed is practically constant) and assuming a constant voltage supply, the above losses can be easily related to their rated values through the following equations:Stator Joule losses:()αR JSR JS L P P ≅ (4)Rotor Joule losses:()αR JRR JR L P P ≅ (5)Magnetic core losses:RP P µµ≅(6)(because the supply voltage is constant)Mechanical losses:fwRfw P P ≅(7)(because the speed is practically constant)Excess losses:()βR sR s L P P ≅ (8)In the range from 1 up to 200 Hp, the exponents α and βare: 2≅≅βα.As we can see, one of the main difficulties encountered in the evaluation of the motor losses is the knowledge of each term. The motor manufacturers usually provide the motor efficiency versus the load percentage load but do not give any other information about the behaviour of motor loss components. Luckily, in the last few years, thanks to the growing interest for the rational use of electric energy, many studies have been carried out about losses distribution for a large range of motors power [4,7].0100002000030000255075100125150175200Motor Size [hp]C o s t [E u r o ]Fig. 3. Costs versus motor size[Expected operational life=10 years, Mechanical load=50 Hp, Operating no-load hours per year=4,000 h, Operating load hours per year =3,000 h, energycost=0.0865 Euro/kWh, discount rate=5.5%]It is important to point out that the consumption costs have to be computed in different time intervals. In fact, the Joule and the excess losses occur only under load conditions, while the iron and the mechanical losses exist also under no-load conditions. As an example, behaviour of initial cost and consumption cost versus motor size are reported in Fig. 3.The total cost curve (LCC) shows that a minimum is reached for the motor size of 50 hp, corresponding to the mechanical load.Technical constrainsThe limitation of load is due to the maximum temperature in different parts of the motor. These temperatures depend on the type of insulating material (insulation class) and on other parameters, such as the cooling air temperature or the altitude where the motor is installed. These effects are taken into account by suitable derating power factors, as prescribed in international standards [8], producing, for a stated output power, an increment of the load factor L R .ApplicationThe procedure has been applied to a motor-pump installation for a fluid cooling system employed in a car manufacture industry. As reported in Fig. 4, the system is divided into two groupa of motor-pumps system. The first ones moves the fluid from the fluid reclamation tank to the store tank and the second one pumps the fluid from the store tank to the piping network; the use of the system is continuous (8760 h/yr). Today the system uses standard induction motors of 30 Hp and of 75 Hp, respectively for the first and the second sub-system.The application of the LCC criteria coupled to the use of EEM shows interested savings, in terms of energy and money. As an example, in Fig 5, the total and the energy consumption costs, for the second sub-system, are represented versus motor size. From the Fig.5 we can see that total cost of standard motors, in the range of rated power 75-125 hp, ispractically the same, but the 125 hp motor size allows a significant energy savings if compared with smaller sizes.C. Three Phase TransformersIn many occasions and, recently in a few a IEEE Winter Meeting several experts, in the field of transmission and distribution of electric energy, have outlined the importance of transformer efficiency [4]. Three factors must be considered to understand the importance of energy saving design criteria for transformers even if their efficiency is already greater than that of many other devices:1. the total amount of energy absorbed by an electric plantpasses through the distribution transformer;2. transformers are energised 24 ours per day;3. the expected operation lifespan is commonly quite long(more than 30 years)Similarly to electric motors, the use of Energy Efficient Transformers (EET), instead of standard transformers, and the application of a choice criteria, as the LCC, allow significant energy savings. To apply the LCC procedure, the initial cost of the transformer and the energy loss consumption cost are required. The computation of the first cost largely depends on many factors, such as: the frame size, the type of transformer (liquid filled or dry type), the core material and the geometry,the conductor material, the tank type, the transformer manufacture, etc. The calculation of consumption cost needs the evaluation of the following terms:1. Winding losses P J2. Magnetic core loss P µUnder the hypothesis that no power is needed to cool the transformers. All these terms are related to the per unit load L R (defined as the ratio between the apparent power S and the rated power S R ):Joule losses:()R JR J L P P ⋅=2(9)Magnetic core losses:RP P µµ=(10)(because the supply voltage is constant)The main difficulty encountered in the evaluation of transformer losses is the knowledge of the electric load and its time behaviour. A first rough evaluation of total average electric load P [kW] is obtained through statistical formula suggested in literature as:()()∑=ii R i UC P K K P (11)where:K C is a contemporary coefficient [p.u.] i indicate the generic load,()i UK is the load factor of the i-th load [p.u.]()i RPis the rated power of the i-th load. [p.u.]TABLE 1.Finally, to get the apparent power an average power factor has to be considered.A second and more accurate method is based on the knowledge of time waveform of active and reactive power of any load of the plant. Using this method it is also possible to take into account the different energy cost during the day.These two methods have been implemented, but only the second one is used in the following application.It is important to point out that the consumption costs have to be computed at different time intervals. In fact, the Joule losses occur only under load conditions, while the iron losses 0200004000060000800001000002004006008001000120014001600Transformer size [kVA]C o s t [E u r o ]Fig. 6. Costs versus transformer size[Expected operational life= 35 years, Electric load=500 kVA, Operating no-load hours per year=8,760 h, Operating load hours per year =2,000 h, energycost=0.0865 Euro/kWh, discount rate=2.5 %]Technical constrainsThe transformer load limitation depends on the maximum temperature arising in the different parts of the transformer.The evaluation of the maximum temperature in the so called “hot spot” is defined by technical standard [9,10]. When maximum temperature is overcome it is possible to predict the reduction of machine operating life. According to this standard procedure it is possible to look for transformer rated power in two different ways:1. to choose transformer rated power according to anaverage power, accepting over-temperature for a reduced working life;2. to find the transformer size which always satisfytemperature limits, keeping unchanged the rated working life.ApplicationThe procedure has been applied to a transformer installed in a car industry. In this case the electric load behaviour is available (see Fig. 7). The other technical parameters are:• type: oil transformer,• frequency: 50 Hz• voltage ratio: 21.5kV/525V • connection: delta/star.The energy cost in not constant during the day and its behaviour is reported in Fig. 8.In Fig. 9, initial, consumption and total costs for dry and oil transformer are reported; the minimum total cost is found different for the two type of transformers (1,250 kVA for dry transformer and 1,600 kVA for oil transformer). Finally in Fig. 10 the minimum cost and their relative transformer size versus the expected working life are shown.100200300400500600123456789101112131415161718192021222324Time [h]P o w e r [k W -k V A r ]P o w e r f a c t o r00.010.020.030.040.050.060.070.08TimeE n e r g y c o s t [E u r o /k W h ]Fig. 7. Electric load: active and reactive power versus timeFig. 8. Energy cost versus time100002000030000400005000060000700008000090000100000400630800100012501600200025003000Transformer size [kVA]C o s t [E u r o ]20000250003000035000400004500050000550006000010152025303540Working life [year]T o t a l c o s t [E u r o ]Fig. 9. Total and consumption cost vs. transformer size [Operational life= 35 years, discount rate=2.5 %]Fig. 10. Minimum total cost and relative transformer size vs. workinglife [Discount rate=2.5 %]A KNOWLEDGEMENTSThe authors wish to thank FIAT AUTO Spa for technical co-operation and for the useful discussions.R EFERENCE[1] IEC Standard 60287-3-2 (1995-07) (1995). Electric Cables –Calculation of the Current Rating – Part. 3: Sections on Operating Conditions-Section2: Economic Optimization of Power Cable size.[2] Canova, A., Longhi, I., Tartaglia, M. (1997). OttimizzazioneEconomica della Sezione dei Cavi Elettrici. 97a Riunione annuale AEI ,Baveno Italy.[3] Nadel, S., Shepard, M., Greenberg, S., Katz, G., de Almeida, A.T.(1992). Energy Efficient Motor Systems: A Handbook on Technology,Program and Policy Opportunities . American Council for Energy Efficient Economy.[4] Hammons, T.J., Kennedy, B., Lorand, R., Thigpen, S., McConnel,B.W., Rouse, S., Prevost, T.A., Prues,C., Dale, S.J., Ramana, V.R.,Baldwin, T.L. (1998). Future Trends in Energy-Efficient Transformers.IEEE Power Engineering Review.[5] Rudasil, C.L., Ward, D.J. (1997). Distribution Underground CableEvaluation . IEEE Trans. On Power Delivery , vol. 12, No. 3.[6] Salgò, C. (1993). Caratteristiche dei motori elettrici ad alto rendimentoe valutazione del risparmio energetico conseguente. Risparmio energetico , N. 41.[7] Andreas J.C. (1982). Energy-Efficient Electric Motors: Selection andApplication . Marcel Dekker, Inc. New York and Basel[8] IEC 60034 (1994-03) Rotating electrical machines: Rating andperformance.[9] IEC 60354 (1991-10) (1991). Loading Guide for Oil-Immersed powerTransformers .[10] IEC 60905 (1987-12) (1989). Loading Guide for Dry-Type PowerTransformers .。
