PO Box 14460, St. Louis, MO. 63178-4460. Abstract. Modeling the electric system "architecture" consists in arranging the basis general scheme as a solution of ...
Natural and prospective parameters modeling architecture of electric power systems Giuseppe Parise (M. IEEE ) Luigi Martirano (St. M. IEEE)
Peter E. Sutherland (SM. IEEE)
Electrical Engineering Dept., University of Rome "La Sapienza" Via Eudossiana, 18 - 00184 - Rome, Italy
IGC SuperPower 450 Duane Avenue Schenectady, NY 12304 USA
Abstract. Modeling the electric system "architecture" consists in arranging the basis general scheme as a solution of the correlations system determined by assembling all the prospective components, sources and protection devices. The basis scheme will start and assist the iterative system designing process. The aim of the system modeling "architecture" is to achieve the objectives, in order to safety, maintenance, operation and reliability, without utilizing special components and complex design. Within an IEC modeling approach, the paper points out considerations assistant for structuring the architecture of the electric power system versus the status variation. The criterion suggested is to take care and/or to control the variation range around the natural values of real parameters, defined for each node of the electric power system assumed in the default status (normal supply and basic configuration). The goal of the “natural” modeling is to create for the system an "architecture" adapted, in components and arrangements, to guarantee a controlled range of its parameters for all the prospected configurations of installation, expansion and restoration. SYMBOLS The symbols in the Figures and in the text are : I” t I”2 t S IZ
prospective maximum value (rms) of the initial short circuit current total fault clearing time of the selected protection device let-through energy cross-sectional area for the cables cable current-carrying capacity. It can be rated empirically by the CENELCOM formula IZ=a Sb [6], where: - a is the current-carrying capacity for a same cable of 1 mm2 cross-sectional area (17 A for EPR, based on Ta = 30C and TZ = 90°C) to correct by derating factors; - b is a parameter equal to 0.625 In ANSI calculations, the cable size should then be converted to kcmil or AWG, using the formula 1.0 mm2 = 1.97 kcmil, in conjunction with a standard wire table correlating kcmil to AWG, such as in the National Electrical Code (NEC) [8, Ch. 9, Table 8],
Vincent Saporita (M. IEEE) Dan Neeser (M. IEEE) Cooper Bussmann PO Box 14460, St. Louis, MO 63178-4460
and checked with an ampacity table, such as NEC Tables 310-16 and 17 IB supplied load current Un nominal voltage, a.c. (rms) Ir, P rated current and power ukr% transformer rated short-circuit voltage Ta ambient temperature, °C. 30°C for the IEC example, 40°C for the ANSI example T0 initial temperature of the cable insulation prior to a current change. For THWN-2 thermoplastic insulation (ANSI example), T0=90°C. For Ethylene – Propylene EP insulated cables (IEC example), T0=90°C Tf final temperature of the cable insulation after a current change. For THWN-2 thermoplastic insulation (ANSI example), Tf=250°C. For Ethylene – Propylene EP insulated cables (IEC example), Tf=250°C TZ operating temperature of the cable insulation for current carrying capacity calculations, typically equal to T0 TZR zero resistance temperature value (234 for copper, 228 for aluminum) h constant value depending on the material conductor (0.0297 for copper, 0.0125 for aluminum). The conversion of h constant from CM unit-system to mm2 unit-system is the following: h [A2s/ (mm2)2]= 3.96 ⋅106 ⋅h [A2s/CM2] K IEC constant value equal to: √ {h log10 [(Tf + TZR)/(To + TZR)]} dependent on the kind of cable and its operating temperature; for an IEC example, copper conductors, Ethylene – Propylene EP insulated, K is equal to 143 As1/2/ mm2. K2 is used in ANSI calculations [7, pp 244-245]. For example, with THWN-2 thermoplastic insulated copper conductors, K2 is equal to 0.00518 [A2s/CM2]. This is equivalent to K = 143 As1/2/ mm2 2 2 K S admissible let-through energy for the cable x”% a.c. generator subtransient reactance LAS line of alternate source LAS LSS line of static switch subscripts and exponents: i bus or node considered * parameter correlated to an other one assumed as reference T transformer
0-7803-7420-7/02/$17.00 © 2002 IEEE
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G UPS
a.c. generator uninterruptible power system Natural natural value correlated to the worst default status Emergency emergency status
I. INTRODUCTION The designing goals of industrial and commercial electric power systems should guarantee operational performances as: -safety of personnel and preservation of property; -reliability and easier maintainability of the system on the whole and of its parts; -insensitivity to faults between the various load groups and between the different areas of the power system; -optimal ratio between the initial cost and operating as well as maintenance costs; - flexibility and expandability; - conformance with regulatory requirements. The high availability requirement of all the loads or part of them is satisfied utilizing more power sources and modeling suitably the distribution system. Generally, in relevant, strategic or high quality buildings, as hospitals [1], the normal power source is furnished by the electric utility and the required alternate power source by an on-site power source such as a generator set, uninterruptible power supply (UPS), or battery/inverter system. The distribution systems are basically divided into two categories: the normal electrical system (non-essential) and the essential electrical system. To increase the reliability of the system, the circuits of these categories require to be installed separately from
each other and from all other types of circuits. The system modeling pursues its goals on safety and on operational performances, arranging and sizing the electrical system components, such as sources, generators set, cables, fuses or breakers or switches, transformers, connected either in series or parallel between system nodes. A basic modeling guideline is the same goal of complying with the prospective parameters, (rated current, voltage drop, short-circuit current, let-through energy, etc), recommended for components and system arrangements as admissible or prescribed by the rules or the standards. Sometimes, the arrangement of the system "architecture" rises also by an intuitive process guided by the designer's professional skills. The natural modeling criterion is suggested to the designer in addition to his experience, his good engineering judgment and his knowledge of many similar systems, in arranging the basis scheme of the power system and assisting the iterative system designing process. II. MOSAIC SYSTEM MODELING AND CORRELATED PARAMETERS OF COMPONENTS. Considering that all the system components are generally available in series standardized, in an IEC approach, the system modeling logic proceeds as a “mosaic” assembling, where each component has to be coordinated with other ones. A correlation among the electrical parameters of the system components as short-circuit current, let-through
Fig. 1 Case study of supplies status variation
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energy, protective device characteristics, cable rate current, exists owing to: - the electrical laws, such as power losses and voltage drop; - the rules and safety conditions, such as the actual letthrough energy no higher than admissible value for all components, the prospective supplied load no higher than the rated current of the components; - the usual standardized sizes of components. So, chosen a reference parameter, prospective correlated values remain defined for the others ones (* defines correlated parameters). In the figure 1 an example is shown as case study. Adopting IEC symbols [2], let’s consider the node bus 1 in the system, the maximum short circuit I”1 is adopted as reference parameter. Its correlated parameters remain defined as: - (I”21 t)* is the correlated let-through energy; - S1*=[√ (I”21 t)* ] / K is the minimum admissible cross-sectional area for the cables; - IZ1*=a⋅(S1*)b is the correlated cable current-carrying capacity; - IB1*< IZ1* is the maximum prospective correlated supplied load; generally IB=0,8 IZ . The correlation function in other words is: IZ / a⋅[(√ I”2 t) / K]b = 1 It is easy to verify that the prospective maximum value (rms) of the initial short circuit current on the bus 1, for the main supply, is about equal to I”1≈50 IrT, assuming the rated short-circuit voltage ukr%= 4% and the arrangement of 2 transformers in parallel with the nominal current value IrT, considering a negligible contribution of motors, to simplify. For example, in the IEC case, if for each transformer PT =630 kVA, Un= 400V, IrT=0.91kA, I”1 is equal to ≈45.5kA. In the ANSI case, if for each transformer PT =750 kVA, Un= 480V, IrT=0.90kA, I”1 is equal to ≈45.1kA. The prospective value I”1 characterizes the short-circuit breaking capacity of the tapped protective devices PD. The correlated let-through energy remains assigned considering the total fault clearing time of each selected PD. For example assuming t= 0.02s, that is an usual value, the available conservative value (I”21 t)* results equal to 41,4 106 A2s in the IEC case. For the ANSI case, (I”21 t)* results equal to 40.7 x 106 A2s, assuming the same 0.02 s clearing time. Better, it is considering the real let-through energy of the selected PD, value given by the PD manufacturer.
For the tapped circuits the minimum admissible crosssectional area, S1 (mm2), of the cable remains defined as higher or equal to: S1*>[√ (I”21 t)* ] / K = 45 mm2 (1) and the standardized value for IEC is S1* = 50 mm2, equivalent to ANSI AWG No. 1 (area = 42.4 mm2). The cable will be sized obviously also from voltage drop and current carrying considerations. Obviously, corresponding to S1*, the correlated cable current-carrying capacity IZ1* remains assigned: considering for example in the IEC case single conductors in free air, , based on ambient temperature Ta of 30 °C, with an operating temperature TZ of 90 °C , IZ1* is equal to: IZ1* = 17 x (50)0.625 = 196A. The prospective correlated supplied load results IB1* = 196 x 0.8 = 156A, independently by the real load. It represents the correlated prospected load for the node bus 1. If the ambient temperature is assumed equal to 40°C, for comparison with the ANSI example, the value of I’z is rated by:
I 'Z IZ
=
Tz − Ta 2 90 − 40 = = 0,913 Tz − Ta1 90 − 30
and IZ1* is equal to: IZ1* = 0,913 x 17 x (50)0.625 = 179A. The prospective correlated supplied load results IB1* = 0,913 x 196 x 0.8 = 143A. In the ANSI case, the rating of three single currentcarrying conductors in free air is obtained not by calculation, but from the National Electrical Code [8, Table 310.20]. Based on ambient temperature Ta of 40 °C, with an operating temperature TZ of 90 °C , the ampacity of AWG No. 1 copper cable is 185 A. This only differs by 3% from the value for the 50 mm2 cable. Considering the load carrying capability, assuming that this cable continuously loaded, the 80% factor also applies [8, Sec. 215.2 (1)]. A prospective correlated load of 148 A results. Simplifying this example does not continue to show an other correlation to model the subsequent nodes, in reference to the minimum worst configuration. The minimum short-circuit value of the node bus 1 defines the following correlated parameters [4]: - the maximum admissible length of the circuit, assigned a value for the voltage drop, - the rating of the protective device.
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In the example of the figure 1 three different solutions are shown to feed four loads, IBi (i= 1,2,3,4), supplied directly from the main switchboard, (bus 1, solution a), from a distribution switchboard (bus 2, solution b), and from local switchboard (bus 3, solution c). For the solution a, considering, in the IEC example, the case of low values of the real loads IBi=38A if there are not installed protective device limiting the let-through energy, the ratio IBi/IB1*=38/156 and consequently the efficiency IBi/IZ1 =38/196 of the circuits is low. In the ANSI example, the ratio IBi/IB1*=38/148, and IBi/IZ1 =38/185. For dealing the modeling approach from the component side, let’s consider the real case of a load of IB =38A such as reference value and its correlated parameters. First, the IEC example: adopting single copper conductors in free air, Ethylene – Propylene EP insulated, the supplying suitable cable presents a correlated current-carrying capacity IZ(B)* of 38/0.8 = 48A and a commercial cross-sectional area S(B)*=6mm2 (S calculated =(Iz/a)1/b=(48/17)1/0.625 =5.26). Secondly, the ANSI example: adopting three single copper conductors in free air, thermoplastic THWN-2 insulated, the supplying suitable cable presents a correlated current-carrying capacity IZ(B)* of 38/0.8 = 48A and a standard size of AWG No. 8 (area = 16.51 kcmil = 8.367 mm2) may be used. The ampacity of this cable by NEC Table 310.20 is 66 A. In the IEC example, using the (1) the correlated maximum admissible short circuit current is I” (B)= 6 kA. In the ANSI example, using K2 = (I/S)2t = 0.00518, with t = 0.02 s, and S = 16,510 CM, we have I” (B)= 8.4 kA. Changing from the solution a to the solution c, the reliability, the cost and the IZ value have a reducing trend. The efficiency IB/IZ has an increasing trend, becoming the load current IB in general more influent on the sizing of the conductors. In conclusion, the modeling designer has to take care that configuring a system arrangement, every node offers correlated prospective size parameters for the component setting in connection and, vice versa, every chosen standardized component offers its correlated parameters to be set in the system, such as in a mosaic assembling. III. NATURAL MODELING PARAMETERS OF THE DEFAULT STATUS Most electric power systems during the operation shall vary configuration and/or supply (by utility or local
source). Varying the status, in each system point generally the real “modeling” parameters shall assume different values, which must comply with the prospective ones. The modeling designer has to take care that a fundamental goal is defining, generally by iterating steps, the arrangement of the system in the worst configuration of the maximum short circuit level. This system arrangement has to be assumed as default status, reference for the correlated values of all the characteristic parameters of electric power system. It is ideally the solution of convergence for the sizing correlation of all the system components. In reference to the defaults status, the correlated parameters are defined as the natural parameters for designing the components and its protective devices of the power system. In figure 1 the default status can consider the operation of the normal condition of supply, as two transformers in parallel, and, in each point of the system the reference values of the prospective parameters define the natural values of point or node. The criterion of the “natural” modeling aims to contribute for a global efficiency of the system for all changes of the supply status, in coordination with the other performances, as cost, reliability safety etcetera. The modeling criterion is to design the system arrangement optimizing the real values of the modeling parameters in order to present a reduced or controlled variation around the natural values versus all prospective status changing for all the prospected configurations of installation, expansion and restoration. The activity of the designer is controlling in each system point the influence of the variations of the short circuit current and so of the correlated parameters. The suggested optimizing criteria, as much as possible and practical to pursue ("what doing if possible"), are the following means: I. adapting the sizes of components to be set in the system with the best coordination of the parameters, II. vice versa adapting the system arrangement to set the real component; III. modeling the system arrangements, adopting a separate essential distribution from the alternate source up to overlap to main essential system (fed from the utility), where the parameters variation, in this common part of the system, is
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guaranteed reduced in a specified range, varying the supply and configuration status. Let’s consider that, changing the supply status in the distribution power system, as changing of the parallel transformers number, the variations of the short circuit parameters are more sensitive on main levels (lower cosφi) than on the end levels of the branch circuits (peripheral effect of insensitivity) [5]. Whenever the normal power source experiences a power failure, the essential electrical power system is transferred to the alternate power supply. For a fault occurring within the essential system zone, the fault current magnitude from the alternate power source would be relatively small and have a fast decay rate. So that the essential system is generally the system part which needs controlling the parameters variations . Really, on the basis of peripheral effect, the zone of the essential system influenced by status variations is generally limited to main levels. In the figure 1 the natural value of the maximum short circuit I”4 on the bus 4 is practically equal to that I”1Natural = I”1 =45.5 kA in the IEC example, and 45.1 kA in the ANSI example, of the bus 1, if the line of alternate source LAS is very short. In the example, the real value of the maximum short circuit on the bus 4 is equal to about I”4Emergency≈10 IrG, assuming for the a.c. generator the subtransient reactance equal to x”%= 10% and the nominal current value IrG, which can be a reduced rate of the nominal current of the transformer. For the IEC example, if PG =150 kVA, Un= 400V, IrG= 216A, I”4Emergency results relatively small and equal to ≈2.16kA, (I”4Emergency /I”1Natural =100 x 2.16 / 45.5 = 4.7%). For the ANSI example, if PG =150 kVA, Un= 480V, IrG= 180A, I”4Emergency results relatively small and equal to ≈1.80kA, (I”4Emergency /I”1Natural =100x1.80/45.1= 4.0%). Analogously, the natural value of the maximum short circuit on the bus 5 is practically equal to that of the bus 1, if both line of alternate source LAS and line of static switch LSS are very short. When the supply of the sensitive loads provided by the UPS the real value of the maximum short circuit on the bus 5 is equal to about 2 IrUPS, considering that the nominal current value IrUPS, is generally a reduced rate of the nominal current of the transformer and of the generator. These factors must be taken into account when designing distribution and rating and setting protective
devices for the essential electrical system. For a near short circuit, downstream the bus 4, the main protective device of the generator, especially if of relative low power, could anticipate the tripping of the proper protective device of the faulted tapped circuit. Analogously for a short circuit, downstream the bus 5, the main protective device of the UPS could have an untimely tripping. The optimal “natural” criterion is achieved if the system design allows obtaining a fault current magnitude from the utility supply on the essential panel boards of the same order as that of the alternate power supply. In other words, when the system changes its supply status, alternate sources have to clasp the system, in a node suitable to its parameters. Each case needs a proper solution, confirming the exigency of a modeling engineering professionalism. In the arrangement of the figure 1 as corrective measures, if necessary on the basis of the natural criterion, could be the following. As example of application of the mean I), a corrective natural measure is to analyze the size of the two sources: the generator and the transformer. More solutions exist; for example, the transformers could be three, equal, with size a third of total and with an independent operation. As example of application of the mean II), a first way is to operate on modeling suitably the line LAS, if the real case of the power system offers the possibility of this solution. Let’s consider that, in the IEC example, 30 m of a power cable with cross section 240 mm2 reduces I”4 to the value ≈28.3 kA from 45.5kA (peripheral effect). In the ANSI example, 100 ft. of a 500 kcmil power cable reduces I”4 to the value ≈26.5 kA from 45.1kA. A corrective natural measure derives by comparing the parallel or independent operation of the two transformers. The criterion would suggest the independent operation. The advantages of using the two transformers with mains and tie normally-closed are better voltage regulation, and flicker less transfer upon loss of one power source. The disadvantages are greater complexity, greater fault current, greater cost, and loss of isolation between sensitive loads and high inrush loads. Considering also the greater variations of real short circuit values around the natural parameters, the complexity requires that the design engineer carefully
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coordinates and specifies the required additional protection to assure proper operation. As example of application of the mean III), another radical way to follow is to install an essential alternate source distribution, downstream side of the generator main protective device bus 6, independent by the correspondent normal distribution, downstream bus 7 of figure 1. At the level of the essential loads group or of secondary switchboard the two distribution could be overlapped in a common end part of the system, downstream an automatic switching. In conclusion, an interesting discussion could be introduced on the three solutions a, b, c to feed four loads of the figure 1, in the case of a low value. The results could demonstrate that the solution a is more suitable for the essential zone with the connected problems, instead the solution c is more suitable for the normal zone. IV. CONCLUSIONS. In an IEC approach, the designer has to take care that, modeling the power system, every component has an array of parameters, correlated owing to the electrical laws, the rules and the usual standardized sizes of the same components. An ancient statement declares “ Est modus in rebus” in the meaning that “take care all things have its proper sizes” and so the logic of a mosaic assembling. The fundamental goal of the designer is "founding" the components arrangement and defining the default status for the system configuration as the natural solution for the arrays of the correlated parameters, also by iterating steps. The natural modeling criterion is maintaining for the real parameters “low-variant” values in the range of the natural ones, versus the status variations. As arranging a mechanical structure looking to the weight moments, the designers as much as possible pursuing optimal costs, practical ness, reliability et cetera , has to model the “weight”-size of components and to arrange the correct “fulcrum” point for setting them. In this way is centered also the classic statement “Natura non facit saltus” in the meaning that “the nature agrees to follow the continuity: not variations, not-problems”.
REFERENCES [1]. 602-1996 IEEE Recommended Practice for Electric Systems in Health Care Facilities (IEEE White Book). [2]. J.E.Propst,D.R.Doan, “Improvement in modeling and evaluation of electrical power system reliability” 2000 PCIC Conference. Sept.11-13, St. Antonio, Texas USA [3]. IEC Standard 60364, "Electrical installations of Buildings," International Electrotechnical Commission Geneva, Switzerland [4]. U. Grasselli, G. Parise: "Design criteria for selectivity and reliability of building power systems", IEEE/IAS I&CPS Technical Conference 1994, Irvine- California 1-5 may. [5]. G. Parise, M. Massimiano, M. Halpin: «Short circuit analysis on a simple power system network: the "characteristic " currents method", 27th IEEE Southeastern Symposium on System Theory March 12-14, 1995 Starkville – Mississippi. [6]. IEC 287 Calculation of the continuous current rating of cables. Part I:100% load factor. [7]. 141-1993 IEEE Recommended Practice for Electric Power Distribution in Industrial Plants (IEEE Red Book). [8]. NFPA 70, National Electrical Code 2002 Edition Giuseppe Parise IEEE M.1982. In 1972 he received his degree in Electrical Engineering from the University of Rome. He has been at this university ever since 1973 and is currently a Full Professor of Electrical Power Systems. His research, professional and consulting activities cover power systems design, planning, safety, security, electro-forensic engineering and energy management. Since 1983, he has been a member of Superior Council of Ministry of Public Works as expert of power systems. He is member of the ltalian Electrical Commission (CEI) CT/SC 11A "Generation, transmission and distribution systems of electric power" and of the IEEE\IAS Power Systems Grounding Subcommittee. Since 1975 he has been Registered Professional Engineer, he is President of the Electrical Commission of Engineers Association of Rome's Province CEOIR and of AEEE. Luigi Martirano Student IEEE M. He was born in Cosenza, Italy, on August 24, 1973. In 1998 he received his degree in Electrical Engineering from the University of Rome, discussing a thesis on “ Life Loss Of Insulated Power Cables”. He is a Ph.D. student in the Department of Electrical Engineering of the University of Rome "La Sapienza". His research activities cover power systems protection and coordination, safety. He has been Registered Professional Engineer. Peter E. Sutherland (M '83, SM ‘97) received the A.S. Degree in Electrical Engineering Technology ('79) and the B.S. degree in Electrical Engineering ('83) from the University of Maine at Orono. In 1986 he received the M.Sc.E degree in Electrical Engineering from the University of New Brunswick (Canada). He has worked as a Test Engineer and a Design Engineer for Accutest Corp. of Chelmsford, Mass., a manufacturer of automatic test equipment for the semiconductor industry. For a short time, he worked as a Planning Engineer for an Electric Utility Company. In 1987 he joined General Electric Company, and has been employed as an Engineer in the GE Industrial Power Systems Engineering Operation in Schenectady, N.Y., as an Instructor in the GE Training and Development Center, as a Power Systems Engineer in Albany, N.Y and as Senior Engineer in the GE Power Systems Energy Consulting Department. His current position is Advisory Applications Engineer
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at IGC SuperPower in Schenectady, NY. Mr. Sutherland is currently enrolled as a part-time student in the Ph.D. program in Electric Power Engineering at Rensselaer Polytechnic Institute, Troy, N.Y. Mr. Sutherland is the author of numerous technical papers and is a member Tau Beta Pi, Eta Kappa Nu and Phi Kappa Phi. He is a Registered Professional Engineer in Maine and New York, a CEng in the UK, and is a member of IEE (London). Vincent Saporita (S’71-M’74) received the B.S.E.E. degree from the University of Missouri, Rolla, and the M.B.A. degree from Lindenwood College, St. Charles, MO. He has been with Cooper Bussmann Inc., St. Louis, MO, since graduation in various roles, and is currently Vice-President, Technical Sales and Services. Standards experience includes membership on National Electrical Code Panels 10 and 11, NFPA 70E (Standard for Electrical Safety Requirements for Employee Workplaces), and IEC TC32B (Low Voltage Fuses). Mr. Saporita is a member of Petroleum and Chemical Industry and Commercial Power Systems Committes of the IEEE Industry Applications Society. He is also a member of the National Fire Protection Association, International Association of Electrical Inspectors, and National Electrical Manifacturers Association (Switch, Fuse, and Industrial Control Sections). He is a Registered Professional Engineer in the State of Illinois. Dan Neeser He’s with Cooper Bussmann Inc., St. Louis, MO, as manager of technical sales. Mr. Neeser is a member of Industry and Commercial Power Systems Committee of the IEEE Industry Applications Society. He works actively in the IEEE/IAS and actually he’s chair of the Codes and Standard Committee.
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