1、382 2009 ASHRAEThis paper is based on findings resulting from ASHRAE Research Project RP-1395.ABSTRACTUtility power sub-stations, industrial plants, and build-ings use electrical power equipment in low voltage (LV) and medium voltage (MV) levels. A large number of electrical brands and products toge
2、ther with a lack of power efficiency information provide complications for HVAC engineers and designers in predicting power equipment heat loss. This is very true for LV and MV switchgear. Environmental heat gain infor-mation is very important for sizing HVAC equipment. The objectives of this paper
3、are to update information on heat loss by LV and MV switchgear and to show these losses can be predicted. Two spreadsheet models are presented that calcu-late the power loss for low and medium voltage switchgear and practical examples are shown for each case using realistic information. The spreadsh
4、eets provide an approximation of the dissipated power losses if actual current loadings are used. However, if breaker and bus amp ratings (unknown loading case) are used in place of the actual loadings, the dissipated power loss of the LV and MV switchgear will be overestimated. This emphasizes the
5、need of using the actual loadings for each bus and circuit breaker to obtain realistic results.INTRODUCTIONGeneral description, classification, and construction details are explained for low and medium voltage switchgear. Common construction techniques and components are detailed and described, espe
6、cially those components giving rise to environmental heat gain. In RP 1104, rudimentary spreadsheet models for LV and MV switchgear were presented. The low and medium voltage switchgear spread-sheets have been greatly improved and use updated compo-nent power losses. These switchgear components incl
7、ude circuit breakers, transformers, bus bars, enclosures, space heaters, in addition to relaying and control systems. These updated power losses were obtained from manufacturers (brochures and data sheets), previous research papers, and the bus model of White and Piesciorovsky (2009). In this paper,
8、 efficiency is defined asEfficiency = Switchgear Output Power 100 /Switchgear Input Power % (1)while the power loss of the equipment is defined asPower Loss = Switchgear Input Power Switchgear Output Power watts, Btu/h (2)The low and medium voltage switchgear spreadsheets are used to estimate the po
9、wer losses of two practical problems considering actual loadings. However, the power losses for the same problems are also estimated using the rated current loads for the bus and breakers showing a considerable increase in the estimated dissipation. Comparing the estimated power losses for these two
10、 situations, an important conclusion is presented at the end of this paper. SWITCHGEAR CLASSIFICATION AND GENERAL COMPONENTSBefore classifying the switchgear, it is important to understand the difference between low voltage switchboards and low voltage switchgear, equipment that sometimes look simil
11、ar but have different power distribution applications and characteristics. These differences are demonstrated in Table 1.Switchgear are classified as either outdoor or indoor equipment. However, this paper only treats indoor switchgear. Building Heat Load Contributions from Medium and Low Voltage Sw
12、itchgear Part II: Component and Overall Switchgear Heat GainsEmilio C. Piesciorovsky Warren N. White, PhDE.C. Piesciorovsky is a graduate student in the Department of Electrical and Computer Engineering, and W.N. White is an associate professor in the Department of Mechanical and Nuclear Engineering
13、, Kansas State University, Manhattan, KS.LO-09-035 (RP-1395) 2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2009, vol. 115, part 2. For personal use only. Additional reproduction, distribution, or transmission
14、in either print or digital form is not permitted without ASHRAEs prior written permission.ASHRAE Transactions 383Switchgear are also segregated according to the voltage level as illustrated in Figure 1.The total power loss of the LV or MV switchgear can be calculated by the sum of all the partial po
15、wer losses produced by each switchgear component and device. The LV and MV switchgear components and devices include the enclosures, horizontal and vertical bus bars, circuit breakers, current trans-formers, power transformers, control power transformers, relaying and control systems (electromechani
16、cal and micro-processor type), and space heaters.LOW VOLTAGE AND MEDIUM VOLTAGE SWITCHGEAR CHARACTERISTICSCircuit BreakersThe circuit breakers used in low voltage switchgear are given the designation of “Low Voltage Power Circuit Breaker” (LVPCB) and these breakers are rated from 800 to 5000 amp at
17、600 volt. The LVPCB has an interrupting current capability, up to 200 kA. This type of breaker can be used as a fused circuit breaker (non automatic) and a non-fused circuit breaker (automatic). The type of mechanism used to open the breaker is a stored energy spring system while the trip sensor typ
18、e is a microprocessor based RMS (root mean square) sensor. The LVPCB are drawn out mounted (can be slid out from the switchgear) allowing easy inspection operations. The power circuit breaker losses follow the IR calcula-tion. Table 2 shows a range of power circuit breaker frame sizes. For each fram
19、e size power loss figures are provided for the cases of fused and non-fused application. These power loss figures represent the power losses that occur when rated frame current flows in the breaker. For smaller currents, the resis-tance values shown in Table 2 are used to predict the power loss thro
20、ugh an IR calculation.The most common medium voltage switchgear circuit breaker found in buildings and factories (5/15 kV) is the vacuum circuit breaker type. It is classified as a vacuum circuit breaker with either a magnetic or spring actuator mechanism. Figure 1 Low voltage and medium voltage swi
21、tchgear classification.Table 1. Low Voltage Switchboard versus Low Voltage SwitchgearCharacteristics Low Voltage Switchboard Low Voltage SwitchgearApplication Load distribution before the panelboards Substation application before the switchboardsDesignStand-alone enclosure mounted away from a wall.
22、Construction with internal barriers between devices and busses is optional.Stand-alone enclosure mounted away from a wall.Construction with internal barriers between devices and busses. Breakers fully compartmentalized with barriers.Bus BarsHorizontal and vertical bus bars (3 phases and ground)Horiz
23、ontal and vertical bus bars (3 Phases)Breaker AmpereRatings150 /5000 amperes (Minimum/Maximum) 800 /5000 amperes (Minimum/Maximum)Access Front and rear access Front and rear accessDisconnect DevicesFusible switches, molded case circuit breakers,insulated case circuit breakers, and fused and non-fuse
24、d low voltage power circuit breakersFused and non-fused low voltage power circuit breakersStandardsUL 891(dead front switchboards), NEMA PB-2 (switchboards), UL 489 (circuit breakers)ANSI C37.20.1, UL 1558, ANSI C37.13, UL 1066(circuit breakers)384 ASHRAE TransactionsThe vacuum circuit breaker is ra
25、ted from 1200 to 4000 amp. This medium voltage circuit breaker is drawn out mounted and it is a non-fused breaker. The power losses of medium voltage circuit breakers can be calculated as previously reported in White, Pahwa and Cruz (2004). Construction DetailsThe compartments, cabinets, and the hor
26、izontal and verti-cal bus dimensions are different for low and medium voltage switchgear. They are designed by manufacturers according to industrial standards. The differences between low and medium voltage switchgear influence the dissipated heat loss. The bus heat loss is given by the proximity an
27、d skin effect while the enclosure heat loss is given by the stray loss. Skin effect causes the current in an AC conductor to crowd to the outer edges creating an increase in electrical resistance and correspondingly higher heat loses than those caused by the same size DC current. The proximity effec
28、t occurs when the magnetic field of one conductor induces a magnetic field in an adjacent conductor that creates losses through a process that is similar to induction heating. The magnetic fields produced by the phase conductors interact, altering the current distribution in one another. The stray l
29、oss of the enclosure involves losses induced in surrounding conducting structures. Using the described analytical models for the skin, prox-imity and the stray loss effect (White and Piesciorovsky 2009), several Visual Basic programs and spreadsheets were created. Bus bar and enclosure power losses
30、were calculated for low and medium voltage switchgear, considering the switchgear dimensions from several manufacturers. Tables 3 and 4 show the horizontal & vertical bus bar, enclosure, and total heat loss resistances for low voltage (600V) and medium voltage (5-15Kv) switchgear considering copper
31、bus bars, galvanized sheet steel enclosures, 60 Hz frequency, 40C (104F) ambient temperature, and 65C (149F) bus bar temperature rise.Special Equipment LossesThe LV and MV switchgear are composed of different special equipment belonging to the control, protection and metering electrical circuits of
32、the system. This equipment consists of transformers, usually protected by fuses at both sides (primary/secondary coil) together with relaying and control systems. Transformers can be classified, according to their use, as potential, control power, and current transformers. Potential transformers (PT
33、s) are also called voltage transformers (VTs). They are designed to offer a negligible load to the secondary through a voltage ratio, converting high voltage into low volt-age and feeding input signals for metering, control, and protective devices. Control power transformers (CPT) are larger than po
34、tential transformers, because these transformers are used to provide the necessary input energy to feed all the metering, control, and protective circuits in the MV and LV switchgear. The CPT primary coils are connected to the LV or MV supply (600V or 5/15 kV). They can have one phase (1, 3, 5, 7.5,
35、 10, 15, 25, or 50 kVA) and three phases (45 or 75 kVA). Current transformers (CTs) are designed to provide a current in its secondary coil proportional to the current flow-ing in its primary. The CT primary is connected to breakers, buses and other power devices, transforming the primary high curre
36、nt into a secondary low current. They are used to offer a Table 2. Power Circuit Breaker Heat Losses for Low Voltage SwitchgearFrame Current, ampsFused Low Voltage Power Circuit Breaker Non-Fused Low Voltage Power Circuit BreakerPower Loss,watts, (Btu/h)Heat LossResistance, ohmsPower Loss, watts, (B
37、tu/h)Heat Loss Resistance, ohms800 600 (2047) 0.0009375 95 (324) 0.00014841200 1050 (3582) 0.0008681 212 (723) 0.00014721600 1500 (5118) 0.0005859 378 (1289) 0.00014772000 2250 (7677) 0.0005625 500 (1706) 0.00012503000 3375 (11529) 0.0003750 1042 (3559) 0.00011573200 3600 (12283) 0.0003516 1150 (392
38、3) 0.00011234000 4500 (15354) 0.0002813 1372 (4681) 0.00008585000 4700 (16036) 0.0001880 1650 (5629) 0.0000660ASHRAE Transactions 385negligible current to the secondary through the current ratio and this low current is used to feed the input signals for meter-ing, control, and protective devices.Rel
39、aying and control systems can be classified according to their technology into either the electromechanical or micro-processor type. The electromechanical type (EM) is charac-terized by electromechanical relays (switches). Each relay can have several auxiliary contacts that have different character-
40、istics (normally closed, normally open, single pole, double pole, etc). The EM relay is operated by an energized coil and provides different outputs using the auxiliary contacts charac-terized by the electromechanical relay construction. Each EM relay has moving components and an electrical coil tha
41、t dissi-pates heat. Depending on the complexity of the relaying and control circuit used (number of relays involved in the system), the electromechanical type relaying and control system can be classified into either electromechanical simple type or the electromechanical complex type. The microproce
42、ssor type is characterized by solid state switches so that these relays do not have any moving components, increasing their lifetime. Microprocessor switches are more efficient and dissipate less heat loss than EM relays. For this reason, new relaying and control systems use the microprocessor type.
43、 SWITCHGEAR SPREADSHEET MODELSThe following sections present two spreadsheet models that can calculate the power loss for low and medium voltage switchgear. Practical examples are shown for each case. Before the spreadsheets are presented some considerations about the spreadsheets will be covered.Sp
44、readsheet ConsiderationsIf the loading is unknown, 0.8 (expected load equal to 80% of the full load) is a reasonable value to use. Table 3. Horizontal and Vertical Bus Bar, Enclosure, and Total Heat Loss Resistance for Low Voltage SwitchgearAmpereRating,ampsHorizontal Bus Bar and Enclosure Heat Loss
45、 Resistance per Phase, /m (/ft)Vertical Bus Bar and Enclosure Heat Loss Resistance per Phase, /m (/ft)Bus BarResistanceEnclosureResistanceTotalResistanceBus BarResistanceEnclosureResistanceTotalResistance800 51.6 (15.7) 0.35 (0.11) 51.95 (15.81) 50.0 (15.2) 5.31 (1.62) 55.31 (16.82)1600 28.9 (8.8) 0
46、.76 (0.23) 29.66 (9.03) 28.5 (8.7) 5.48 (1.67) 33.98 (10.37)2000 22.5 (6.8) 1.33 (0.40) 23.83 (7.20) 22.0 (6.7) 5.66 (1.72) 27.66 (8.42)3200 17.9 (5.4) 1.49 (0.45) 19.39 (5.85) 17.5 (5.3) 7.96 (2.42) 25.46 (7.72)4000 14.8 (4.5) 2.20 (0.67) 17.00 (5.17) 14.5 (4.4) 9.64 (2.94) 24.14 (7.34)5000 12.0 (3
47、.7) 3.22 (0.98) 15.22 (4.68) 11.8 (3.6) 11.61 (3.54) 23.41 (7.14)Table 4. Horizontal and Vertical Bus Bar, Enclosure, and Total Heat Loss Resistance for Medium Voltage SwitchgearAmpere Rating,ampsHorizontal Bus Bar and Enclosure Heat Loss Resistance per Phase,/m (/ft)Vertical Bus Bar and Enclosure H
48、eat Loss Resistance per Phase,/m (/ft)Riser Bus Bar and Enclosure Heat Loss Resistance per Phase,/m (/ft)Bus Bar ResistanceEnclosure ResistanceTotalResistanceBus Bar ResistanceEnclosure ResistanceTotalResistanceBus Bar ResistanceEnclosure ResistanceTotalResistance120021.97(6.69)12.07(3.67)34.04 (10.
49、36)21.97(6.69)8.35(2.54)30.32(9.23)21.97(6.69)23.20(7.07)45.17 (13.76)200017.43(5.31)12.07(3.67)29.50(8.98)17.43(5.31)8.36(2.54)25.79(7.85)17.43(5.31)22.91(6.98)40.34 (12.29)30008.73(2.66)12.09(3.68)20.82(6.34)8.73(2.66)8.39(2.55)17.12(5.21)8.73(2.66)21.34(6.50)30.07(9.16)40006.84(2.08)12.11(3.69)18.95(5.77)6.84(2.08)8.41(2.56)15.25(4.64)6.84(2.08)20.65(6.29)27.49(8.37)386 ASHRAE TransactionsFor each circuit breaker, one current transformer of 3 coils has to be included, the size of which depends upon the rating of the circuit breaker. The
