ADVANCES IN INDUSTRIAL SUBSTATION DESIGN USING THREE WINDING POWER TRANSFORMERS

Transcription

1 ADVANCES IN INDUSTRIAL SUBSTATION DESIGN USING TREE WINDING POWER TRANSFORMERS Copyright Material IEEE Paper No. PCIC-2008-XX Doug Brooks P.Eng Don Morency P.Eng. Pascal Tang P.Eng Senior Member, IEEE Suncor Energy Northern Transformers Bantrel Co. Calgary, Alberta Concord, Ontario Calgary, Alberta Canada Canada Canada Abstract Industrial substations for petro-chemical facilities must be reliable, easy to operate and easy to maintain, as well as have enough spare electrical capacity and spare space to handle all anticipated future growth needs. In many cases three winding power transformers offer an attractive alternative to the traditional two winding transformers without sacrificing reliability, operability, maintainability or future growth capability. Index Terms Two-winding power transformer, Threewinding power transformer. I. INTRODUCTION The traditional approach to industrial substation design is based on specifying 2 x 100% redundant power transformers connected to secondary selective double-ended switchgear line-ups for each main utilization voltage. These power transformers are typically two winding designs which step down the voltages from distribution levels such as 34.5 kv, 25 kv or 13.8 kv to utilization voltages levels such as 4,160 V, 600 V or 480 V for supplying plant motor and other loads. The quantities and sizes of these two winding power transformers are based on the number of required voltage transformations, and the downstream switchgear load carrying and interrupting capabilities. In recent years there has been an increase in the use of three winding liquid filled power transformers to replace the traditional two winding liquid filled power transformers. This paper will take a close look at the advantages and disadvantages of three winding vs. two winding power transformers in industrial substation design applications to gain a better understanding why some industrial users are specifying three winding power transformers. The two and three winding power transformers discussed in this paper are oil filled sealed type, designed and tested in accordance with Canadian CSA C88 standard which is very similar to the American ANSI C57.12 standard. The term two or three winding transformer can be misleading as transformers may incorporate additional windings that are internal to the tank of the transformer. For our purposes two or three winding means a transformer with 2 or 3 sets of bushings labeled for the primary, X for the secondary, and Y for the tertiary. The transformers are all configured with Delta primaries and grounded Wye secondaries and in the case of three winding transformers, the tertiaries will also be configured as grounded Wye designs. Transformers will all incorporate one stage of fan cooling to provide both ONAN and ONAF ratings. The transformers are arranged as 100% redundant pairs supplying secondary selective double ended switchgear lineups. The transformers incorporate similar monitoring and protective devices including oil level and temperature gauges and alarm contacts, pressure gauge, pressure relief vents and sudden pressure relay. A. Two Winding Power Transformer Core and Coil Design The design of the windings for Two Winding Power Transformers is based on high side primary coils are wound around the low side X secondary coils and steel core. This paper will look into the cost, reliability and operability of industrial substations based on multiple different configurations of secondary selective double ended designs using two winding and three winding power transformers. II. TWO AND TREE WINDING TRANSFORMER DESIGNS Fig. 1 - Two Winding Power Transformer Design [1] 1

2 B. Three Winding Power Transformers Core and Coil Design There are two configurations of three winding power transformers that will be considered in this paper: Type 1 (Symmetrical) Both the secondary and tertiary windings are wound and sized the same to produce the same secondary and tertiary voltages with the same MVA capacity. A standard design for Type 1 transformers is shown in Fig. 2. This design is intended to serve load equally and continuously through the secondary windings. Fig Core & Coil Assembly of a 3 2/1 MVA, /.48 KV Type 2 Three Winding Power Transformer Fig. 2 Type 1 (Symmetrical) Three Winding Transformer Design [1] The electrical connection for the V Winding is typically center fed so in effect there are two V windings connected in parallel. The amp turns in each V parallel section balance with the respective secondary or tertiary winding X and Y. This will ensure that there is no undue local heating or gassing caused by flux unbalance due to unequal loading on the secondary and tertiary windings. Type 2 (Asymmetrical) The secondary and tertiary windings are sized differently to produce different voltages with different MVA capacities. III. TWO & TREE WINDING TRANSFORMER CONFIGURATIONS Two winding and three winding transformers are compared in equivalent double ended substation configurations as noted in Figures 5, 6, 7 & 8. Three sets of load configuration cases are compared as follows: 1) Case 1A & 1B 4 x 2.5 MVA of 480 V induction motor loads. 2) Case 2A & 2B - 2 x 4 MVA of 4.16 kv induction motor loads & 2 x 2.5 MVA of 480 V induction motor loads 3) Case 3A & 3B 2 x 9 MVA of 4.16 kv induction motor loads & 2 x 2.5 MVA of 480 V induction motor loads A standard design for Type 2 transformers is shown in Fig. 3. In this design the tertiary windings are closest to the core followed by the secondary and then the primary. T1 T2 T3 T4 Y X X Y Fig. 5 Case 1A Fig. 3 Type 2 (Asymmetrical) Three Winding Transformer Design [1] 2

3 T5 T6 Fig. 6 Case 1B T7 T8 T9 T10 Fig. 9 Assembled 3 2/1 MVA, /.48 KV Type 2 Three Winding Power Transformer Fig. 7 Case 2A & Case 3A T11 T12 Fig. 8 Case 2B & Case 3B The three winding transformer ratings and impedances referenced in this paper are based on actual three winding transformers. The equivalent two winding transformer ratings and impedances were then chosen to match the three winding transformer ratings and impedances. It is important to note that the impedance tolerance when specifying impedances is +/- 7.5% for two winding transformers and +/- 10% for three winding transformers. Fig. 10 Transformer Secondary and Tertiary Bushings for a Type 2 Three Winding Power Transformer IV. EVALUATION CRITERIA The two winding and three winding transformers described in the various cases noted in Section III are evaluated on the following basis: 1) Total Installed Cost 2) Space Requirements 3) Bolted Three Phase Fault Levels 4) Arc Flash azard 5) Transformer Losses 6) Voltage Regulation 7) Reliability, Operability and Maintainability All results are given in Attachment A. 3

4 A) Total Installed Cost The Total Installed Costs (TIC) were evaluated for the various cases based on recent tar sands projects in northern Alberta. The cost analysis include transformer costs, primary & secondary switchgear costs, cable / bus duct costs, transformer yard grounding costs and civil costs. In every case the Three Winding Transformer option offers the lowest total installed cost with savings varying from 17% to 26%. B) Space Requirements The space saving afforded by using 3-winding transformers allows for more flexibility and reduced area requirements for the Process Plant which can result in significant savings. By reducing the size required for substations and transformers, the process unit plot space can also be reduced. This reduction in process unit size translates directly into reduced quantities of pipe, cable, cable tray and supporting steel as well as reduced pump & pump motor sizes. By specifying three winding transformers the number of transformers and the associated transformer yard sizes can theoretically be reduced by half compared to a transformer yard consisting of two winding transformers. Space saving two storey substations can then be designed to take full advantage of the reduction in transformer yard space. These cost savings are in addition to the cost savings shown in Attachment A and in many cases are more significant than the cost savings shown in Attachment A. C) Bolted Three Phase Fault Levels Simplified methodologies have been used to calculate the bolted three phase fault levels based on system inductive reactance only. The faults will be assumed to occur at the transformer secondary or tertiary terminals. Both utility contribution and motor contribution are included. Motors fault contribution reactance (Z M) is assumed to be the motor sub transient reactance (X D ) plus some cable reactance. For 4160 V motors loads a combined motor and cable reactance of.19 is uses. For 480 V motors a combined motor and cable reactance of.22 is used. The lumped motor MVA rating (U M) will be assumed to be equal to the transformer secondary or tertiary MVA rating. Two Winding Power Transformer Fault Calculation Methodology The two winding transformer simplified impedance diagram is shown in Fig. 11. Fig. 11 Simplified Impedance Model for Two Winding Transformers Z U = Utility Impedance Z PS = Transformer Impedance Z MS = Motor Impedance The fault calculation is straight forward and is based on the following equations:. ZU p.u. = UBASE / UUTIL = 10/10,000 =.001 (1) ZPS p.u. = ZPS x (UBASE / UPS) (2) ZMS p.u. = ZMS x (UBASE/UM) (3) UBASE = Base MVA = 10 MVA UUTIL = Utility MVA = 10,000 MVA UPS = Transformer MVA UPS = Transformer MVA UM = Running Motor MVA Three Winding Power Transformer Fault Calculation Methodology Z MS The methodology for calculating bolted three phase fault levels for three winding transformers is more complex than for two winding transformers due to the magnetic coupling effect of the three windings. Impedance values for transformers are specified from terminal to terminal in percent on the base MVA rating of the transformer. For 3-Winding transformers three impedance values are required Z PS %, Z PT%, and Z ST% where: [2] Z PS % = Leakage impedance between the P and S winding, with the T winding open-circuited, expressed in percent on the kva and voltage of the P winding. Z PT % = Leakage impedance between the P and T winding, with the S winding open-circuited, expressed in percent on the kva and voltage of the P winding. 4

5 Z ST % = Leakage impedance between the S and T winding, with the P winding open-circuited, expressed in percent on the kva and voltage of the P winding. The equivalent wye impedance network (Fig.12) is derived from Z PS, Z PT, and Z ST using the following equations: [2] Z P% = ½ (Z PS% + Z PT% -(U P/U S) x Z ST%) (4) Z S% = ½((U P/U S) x Z ST% + Z PS % Z PT %) (5) Z T% = ½( Z PT% +(UP/US) x Z ST% Z PS %) (6) U P = MVA rating of the primary winding U S = MVA rating of the secondary winding Z S p.u. = Z S x (U BASE / U P) (9) Z T p.u. = Z T x (U BASE/U P) (10) Z MS p.u. = Z MS x (U BASE/U MS) (11) Z MT p.u. = Z MT x (U BASE/U MT) (12) U BASE = Base MVA = 10 MVA U UTIL = Utility MVA = 10,000 MVA U MS = Running Secondary Motor MVA U MT = Running Tertiary Motor MVA Secondary and Tertiary Thévenin equivalents are calculated as follows: Z S Thévenin = (((Z U p.u. + Z P p.u.) II (Z T p.u. + Z MT p.u.)) + Z S p.u.) II Z MS p.u. (13) Z T Thévenin = (((Z U p.u. + Z P p.u.) II (Z S p.u. + Z MS p.u.)) + Z T p.u.) II Z MT p.u. (14) Bolted three phase short circuit fault levels in MVA and ka are calculated as follows: F2 MVA = U BASE / Z S Thévenin (15) F2 ISC = F2 MVA / (S VOLTS X 3 ) (16) Fig. 12 Three Winding Transformer Equivalent Wye Impedance Circuit The simplified impedance model for the three winding power transformer including the utility supply and motor loads is shown in Fig. 13. F2 And; F3 MVA = U BASE / Z T Thévenin (17) F3 ISC = F3 MVA / (T VOLTS X 3 ) (18) S VOLTS = Transformer Phase to Phase Secondary Voltage T VOLTS = Transformer Phase to Phase Tertiary Voltage Bolted Three Phase Fault Levels Fig. 13 Simplified Impedance Model for Three Winding Transformers All impedances are converted to per unit values using a common base of 10 MVA. For short circuit calculations the internal voltage is shorted and the positive sequence network is driven from the voltage appearing at the fault. Z U p.u. = U BASE / U UTIL = 10/10,000 =.001 (7) Z P p.u. = Z P x (U BASE /U P) (8) F3 Fault Levels and Transformer impedance values are given in Attachment A. The fault level on two winding transformers is 30 percent higher when maximum motor contribution is considered. For three winding transformers the fault current considering maximum motor contribution from the secondary and tertiary windings is typically 40 percent higher (on the 480V Bus ) than the fault level without motor contribution. The reason for the higher fault levels in three winding transformers is that the secondary and tertiary windings are magnetically coupled. As a result there will be a fault contribution between the secondary and tertiary windings depending on the running motor load on the un-faulted bus and the equivalent impedance network. The motor contribution to the fault current is greater with Type 5

6 2 transformers, from the secondary (4160V) winding to the tertiary (480V) winding. Fault current from the tertiary (480V) winding to the secondary (4160V) is generally low. The motor contribution for Type 1 transformers from secondary to tertiary or tertiary to secondary is identical assuming the same running motor load on the un-faulted bus. In general the motor contribution to fault current for a fault on the 480V bus from the un-faulted bus is lower with Type 1 transformers than for Type 2 transformers. This is because the fault impedance in the equivalent network is lower and the running motor load on the un-faulted bus is higher for Type 2 transformers. Motor contribution to fault current from the un-faulted winding for three winding Type 1 transformers is low, in the order of 8 percent. For three winding Type 2 transformers motor contribution from the secondary (4160V) un-faulted winding to the tertiary (480V) is in the order of 10 to 12 percent. D) Arc Flash Incident Levels Calculated Arc Flash Incident Levels are given in Attachment A. The following calculations are used to determine the approximate arc flash incident levels. [3] E 480 = 3.11(Ibf)(t) (19) E 4160 = 5.1(Ibf)(t) (20) E 480 = Arc flash incident energy (cal/cm 2 ) for 480 V systems E 4160 = Arc flash incident energy (cal/cm 2 ) for 4160 V systems Ibf = Three phase symmetrical bolted fault current (ka) t = Arc flash fault clearing time (sec) Arc Flash incident energy levels at both 4160 V and 480V are compared for two winding and two winding transformers and the results are tabulated in Attachment A. A trip time of.07 sec was used in the calculation on the assumption that most industrial users will implement fast tripping to mitigate arc flash incident energy when maintenance activities are taking place. As the results show, arc flash levels are not significantly different for three winding and the equivalent two winding transformers with the difference being less than 1 cal/cm 2. The one exception is the tertiary bus arc fault level of cal/cm 2 for the 11.5 MVA transformer, which is over 5 cal/cm 2 higher than the equivalent two winding transformer with the same MVA and voltage ratings. Arc Flash incident energy is proportional to fault current and tripping time and typically arc flash incident energy is higher on low voltage switchgear than medium voltage switchgear because of higher fault current and lower working distances. Fault current on three winding transformers will be higher than for equivalent two winding transformers and needs to be considered at the design stage. For low voltage (480V) busses with time coordinated tripping the incident energy will not typically be lower than 20 cal/cm 2 where the supply transformer is higher than 300kVA. Arc flash mitigation strategies for switchgear supplied from two winding transformers will be similar to strategies employed for two winding transformers including fast as possible clearing times mentioned above. The other factor in the arc flash incident energy equations is fault current. The assumption in this paper is that transformers will be selected based on economics and best engineering practices for distribution design. It is recognized that transformers can be sized and specified to reduce fault current but only at the expense of economic design and system performance. The transformers referenced in this study are based on the assumption of economic design and design based on best engineering practice for system performance with impedance values specified per ANSI standards. Grounding of the secondary and tertiary systems on three winding transformers is equivalent to two winding transformers including high resistance grounded systems. There is a higher risk of an arc flash incident on a 480V bus where transformers are high resistance grounded and the operational strategy is alarm only. This is because a second ground fault on another phase at the 480V bus could result in an arc flash incident. The recommended mitigation is to trip on ground faults or avoid doing maintenance on switchgear and MCCs with a ground fault on the system. E) Transformer Efficiencies Three winding transformers will have approximately.5% lower efficiency than for an equivalent 2-winding transformers because of higher stray load losses. Copper load losses and no load losses are similar for two winding and three winding transformers F) Voltage Regulation Voltage regulation represents a double edged sword. On the one hand good voltage regulation will mean better motor starting capability. On the other hand, good voltage regulation will mean higher fault levels. The closer V REG approaches zero, the better the voltage regulation. For transformers, the voltage is measured at the transformer secondary or tertiary terminals. Voltage regulation is calculated as follows: [4] VR% = (pr + qx + ((px-qr) 2 /200)) x (OC/RC) (21) VR% = Percentage Voltage Regulation = Power Factor angle of load p = cos ( ) q = sin ( ) r = percent transformer resistance x = percent transformer reactance OC = Operating Current 6

7 RC = Rated Current The percent voltage regulation at the secondary and tertiary terminals is shown in Attachment A. The calculations are based on a 0.9 Power Factor, and efficiencies of 99.2% for two winding transformers and 98.7% for three winding transformers. G) Reliability, Operability, Maintainability and Protection Reliability One three winding transformer will replace 2 x two winding transformers and requires only a single primary circuit breaker and feeder cable where 2 x two winding transformers requires two primary circuit breakers one for each transformer. The reliability of a network utilizing three winding transformers compared with the reliability of an equivalent network using two winding transformers is higher for the network using three winding transformers. This result is expected because there are fewer switching components in the network with three winding transformers and this was confirmed using the Propst and Dong Spreadsheet Electrical Reliability Model [5]. The results are summarized in Fig. 14. Overall maintenance is less with a three winding transformer arrangements because in effect two transformers are maintained at once. There is only one oil sample to take instead of two; there is only one outage instead of two; there is only one primary circuit breaker to maintain instead of two. Protection The protective relaying associated with three-winding transformers is no more involved than with the equivalent twowinding transformers. Primary and secondary overload and overcurrent protection including CT selection is equivalent for two-winding and three-winding transformers. Differential protection is typically not applied for small power transformers less than 10MVA. Where required, three-ended differential protection for three-winding power transformers is commonly available from most differential relay manufacturer s. V. SUMMARY Are Three Winding transformers of the types discussed in this paper equivalent in performance to the Two Winding transformers they replace? There are some differences in performance that need to be understood when specifying a three winding transformer and the differences vary depending on the size, voltage rating and application. A) Type 1 Three Winding Transformers Fig. 14 Percent Availability of Substations with Different Numbers of Transformers A three winding transformer carries the equivalent load of 2 x two winding transformers and a trip or outage for a three winding transformer will result in a larger loss of load than is the case of the loss of a two winding transformer. The trade off here is frequency of outages verses severity of outages. In typical process plants the frequency of outages is often worse because the process is sequential and an outage to any part impacts the entire process. Operability Another consideration is operability as more switching operations are required to take a three winding transformer out of service than for a two winding transformer as there is one additional isolating point for the three winding transformers. Maintainability The preferred application for this type of transformer is to replace two equally sized 2-winding transformers that have similar load profiles. For this application a 3-Winding transformer will have equivalent performance to the 2-winding transformers provided they are specified to the same requirements, including size, voltages, BIL, and impedances. For example consider Case 1B. These transformers have an impedance of 13% on the primary base rating of 5000 kva from the primary to secondary terminals and also 13% from the primary to tertiary terminals. These impedances are stated as 6.5% when referred to the secondary and tertiary base ratings of 2500 kva. This transformer replaces 2 x two winding transformers each with a rating of 2500kVA and an impedance of 6.5%. In all cases impedance values for three winding Type 1 Transformers can be specified equivalent to the two winding transformers they replace. The fault current at the secondary and tertiary buses for Case 1B is approximately 46kA without motor contribution and is identical to a two winding Case 1A transformers with comparable impedance. The fault current on the tertiary bus with full motor contribution (2.5 MVA of running motor load at the time of the fault) from the secondary bus is approximately 50 ka which is not very significant and shows that the 7

8 impedance from secondary to tertiary is very high for this transformer design. The fault current on the secondary and tertiary busses with the maximum motor contribution from each bus is approximately 64 ka. For the equivalent two winding transformer the fault current is approximately 60 ka with maximum motor contribution. The difference in the fault current between the three winding and two winding transformers is because of the added motor contribution from the non-faulted bus for the three winding transformer. Both the three winding and two winding transformer designs will typically have an off load primary tap changer. If the transformer loading on each bus for all transformers is approximately the same all transformers will have the same setting on the tap changer for the typical industrial secondary selective system. For two winding transformers it is possible to have a different tap setting on each transformer while this is not the case for a three winding transformers. There is more flexibility in the tap changer settings with individual transformers. If the tap changer settings are the same, which is often the case, the three winding transformer will have marginally higher regulation at each bus as the two winding transformers. Type 1 three winding transformers are very comparable in performance to two winding transformers. The main performance differences will be: 1) Marginally lower efficiency due to higher transformer stray losses 2) Marginally higher fault currents. 3) Marginally higher voltage regulation. B) Type 2 Three Winding Transformers In general with Type 2 three winding transformers the low voltage tertiary winding will tend to have higher fault currents and the secondary medium voltage winding will tend to have low fault currents than the equivalent two winding transformers. Increasing the fault current by design on the secondary medium voltage winding will result in an increase of fault current on the tertiary low voltage winding. The reverse is also true; lowering the fault current by design on the tertiary low voltage winding will result in a lower fault level on the secondary medium voltage winding. The transformer designer is constrained by having to choose a volts-per-turn value that works for both the secondary and tertiary voltage. It is imperative that impedance requirements be discussed with the transformer vendors. In the interim impedance values for three winding Type 2 transformers can be specified within the range and ratios of values given in Attachment A. Motor contribution to fault current is higher on the tertiary 480V bus because of motor contribution from the secondary 4160V side. In general the fault current on the tertiary due to maximum motor contribution will be 40 percent higher than the fault level without motor contribution. The fault current with motor contribution on the equivalent two winding transformer is 30 percent higher than the fault level without motor contribution. Another consideration is that the voltage on the tertiary follows the voltage on secondary. This is evident when you look at the equivalent circuit where Z S is always very low compared with Z P and Z T. Therefore, for example, the voltage drop due to starting a motor on the secondary will be seen in the same proportion on the tertiary. In this regard, the performance is comparable to a 2-winding transformer sub fed from another larger two winding transformer. This does not occur with Type 1 transformers where the voltage drop on one side due to motor starting is not seen on the other side. Type 2 two winding transformers have more restrictions than Type 1 three winding and will not be suitable in all cases. If the rating of the secondary medium voltage winding is required to be rated in excess of 9 MVA then a two winding transformer should be specified. The upper limit for three winding transformers of this type is nominally 11.5 MVA primary rating as stray losses start to become unreasonable above 11.5 MVA which could cause local hot spots thereby reducing transformer life. It is therefore recommended to either split the loads to allow smaller three-winding power transformers to be specified, or to specify larger standard two-winding power transformers should the anticipated load exceed 11.5 MVA. VI. ACKNOWLEDGEMENTS The authors wish to thank Ms. Annie Lam for providing the Reliability Calculations referenced in this paper. VII. REFERENCES [1] SKM Application Guide Three Winding Transformers [2] ABB Electrical Transmission and Distribution Reference Book 5 th Edition Sep. 1997, Ch. 5 pp [3] R.F. Ammerman, P.K. Sen, J.P. Nelson. Arc Flash azard Incident Energy A istoric Perspective and Comparative Study of the Standards: IEEE 1584 and NFPA 70E, IEEE Petroleum and Chemical Industry Conference, Calgary, AB, Sep [4] ABB Electrical Transmission and Distribution Reference Book 5 th Edition Sep. 1997, Ch. 5 pp [5] J.E. Propst, D.O. Koval, Z. Dong, An Update on the Electrical Spreadsheet Reliability Model, IEEE Petroleum and Chemical Industry Conference, Calgary, AB, Sep VIII. VITA Douglas G. Brooks, P.Eng. (S 76-M 97-SM 07) received his B.Sc. in Electrical Engineering from the Royal Military College in Kingston, Ontario in e is presently the Chief Electrical Engineer for Bantrel in Calgary, Alberta. e is the Vice Chair of the Young Engineers Development Subcommittee of the IEEE PCIC and he is a member of the Association of Professional Engineers, Geologists, and Geophysicists of Alberta (APEGGA). 8

9 Doug has co-authored two previous IEEE PCIC papers in 2000 and Donald E. Morency, P,Eng. is a graduate of the University of Alberta (80) with a B.Sc. in Electrical Engineering. e is presently a Lead Electrical Engineer with Suncor Energy Services. e has 20 plus years of experience in both consulting and operations in the fields of industrial, mining and utility power systems. Pascal Tang, P,Eng. is a graduate of the University of Ottawa (77) with a B. Applied Sc. in Electrical Engineering. e is presently a Senior Design Engineer at Northern Transformers Inc. and has 26 plus years of experience in the design of single and three phase liquid filled units. e is a member of CSA C88 M90 (Power Transformers), C2 (Distribution Transformers), C227.3 (Single Phase Pad Mounted Transformers) and C227.4 (3 Phase Pad Mounted Transformers) working committees. 9

10 ATTACMENT A TRANSFORMER DATA Cases 1A 1B 2A 2B 3A 3B Ratings KVA Primary 2,500 5,000 4,000 6,500 9,000 11,500 KVA Secondary 2,500 2,500 4,000 4,000 9,000 9,000 KVA Tertiary 2,500 2,500 2,500 KV Primary KV Secondary KV Tertiary Loads Secondary Load (kva) 2,500 2,500 4,000 4,000 9,000 9,000 Tertiary Load (kva) 2,500 2,500 2,500 Impedances %Z PS (primary base KVA) 6.50% 13.00% 6.85% 11.13% 8.72% 11.14% %Z PT (primary base KVA) 13.00% 19.11% 21.85% %Z ST (primary base KVA) 20.56% 7.80% 9.86% %Z PS (secondary base KVA) 6.50% 6.50% 6.85% 6.85% 8.72% 8.72% %Z PT ( secondary base KVA) 6.50% 11.76% 17.10% %Z ST (tertiary base KVA) 10.28% 3.00% 2.14% %Z MS (primary base KVA) 22.00% 22.00% 19.00% 19.00% 19.00% 19.00% %Z MT(primary base KVA) 22.00% 22.00% 22.00% Secondary Bus Bolted Three Phase Fault Levels Fault without motor contribution (ka) Fault w non-fault bus motor contribution (ka) Fault with full motor contribution (ka) Tertiary Bus Bolted Three Phase Fault Levels Fault without motor contribution (ka) Fault with non-fault bus motor contribution (ka) Fault with full motor contribution (ka) Arc Flash Incident Energy w Motor Contribution Secondary Bus (cal/cm2) Tertiary Bus (cal/cm2) Voltage Regulation Secondary Bus (%) 5.97% 6.04% 6.28% 6.35% 7.96% 8.04% Tertiary Bus (%) 6.04% 4.04% 2.37% Total installed Costs TIC x $1,000 $1,197 $996 $1,293 $1,002 $1,505 $1,121 10