Differences in ANSI-IEEE and IEC Short Circuit Calculations and Their Implications

Institute of Integrated Electrical Engineers of the Philippines, Inc.41 Monte de Piedad St., Cubao, Quezon City

A Technical Report on

Differences in ANSI/IEEE and IECShort Circuit Calculations and Their Implications

___________________________________________________________________

Prepared by:Institute of Integrated Electrical

Engineers, Inc. (IIEE) Standards Committee 2011

iDifferences in ANSI/IEEE and IECShort Circuit Calculationsand Their Implications

Prepared by:IIEE Standards CommitteeMarch 19, 2011

ii

DisclaimerIt is not the intention of this paper to endorse over another the compared short-circuitcalculations and standards.All discussions in this report are based on the featured system one line diagram only. Thesame parameters were considered for the American National Standards Institute/Institute ofElectrical and Electronics Engineers (ANSI/IEEE) and the International Electro-technicalCommission (IEC) calculations for result comparison. The values of these parameters,however, may vary from every project in terms of available utility short circuit levels, powersystem configuration, wiring method and all applicable factors to consider.IIEE and this Committee will not be responsible for any disputes that may arise out ofreferencing from this paper.

iii

PrefaceThis technical report focuses on two of the most widely used short circuit calculationmethods and standards/guidelines namely: American National Standards Institute/Institute ofElectrical and Electronics Engineers (ANSI/C37/IEEE std 551) and the International Electro-technical Commission (IEC 60909). To fully understand the analytical techniques of shortcircuit current analysis in industrial and commercial power system using both methods, arepresentative network model was exemplified and a comprehensive comparison between thetwo calculation methods was presented. For expediency, a short circuit calculating softwarewas employed and the results were presented and evaluated at the end of the analysis.This technical report provides information and inculcates awareness to electrical practitionersin the country on the difference in the procedure of short circuit calculations and itsimplication between the standards cited. It is not intended to show the detailed short circuitcurrent calculation for both methods. The reader is still recommended to consult technicalbooks for reference on a complete and accurate calculation procedure.This paper starts off with a brief introduction on the current scenario in the Philippines andthe importance of short circuit calculation in Chapter I and expounds on its basic principle inChapter II. The equivalent short circuit schematic diagram is also available for analysis insimple calculation.Chapter III discusses the asymmetry current application focusing on the importance ofdetermining the total available short circuit current in the design of electrical equipment suchas circuit breakers, switches, transformers and fuses that are subjected to fault current.Chapter IV shows the different components in determining the short circuit calculation basedon the two standard/guidelines, the ANSI/IEEE and the IEC. This is followed by Chapter Vpresenting the comparative matrix on both standards Calculation Method and MultiplyingFactors with reference to the X/R ratio.Chapter VI clearly tabulates a comparison between the standards parameters particularly thedevice type, device capability and the calculated short circuit duty.In Chapter VII, an illustration of a sample network was configured consisting of two powertransformers connected to a 13.2 kV bus wherein two different results from the ANSI/IEEEand IEC calculations were generated with the aid of short circuit calculating software.Chapter VIII presents the protective devices selection and evaluation focusing on the X/Rratio for breaker evaluation and on the short circuit test parameters while Chapter IXdiscusses the findings and results of the ETAP Total Bus Fault Short Circuit Study. Thetables on the short circuit calculation clearly show the difference in values for the sameparameters between ANSI/IEEE and IEC.This technical report was developed through the initiative of the IIEE Standards Committee.Any concern or contention as to its applicability, accuracy and completeness shall beaddressed to the Institute of Integrated Electrical Engineers of the Philippines, Inc. for furthervalidation and interpretation.

iv

ParticipantsThe following are the working group members of the Institute of Integrated Electrical Engineers of thePhilippines, Inc. (IIEE) under the Standards Committee:Chairman:Gem J. TanFuji-Haya Audit Inspection and Maintenance CorporationMembers:Arjun G. AnsayTechnological University of the Philippines ManilaArturo M. ZabalaAC-DC-KV and AssociatesEdwin V. PangilinanTotal Power Box Solution, Inc.Frumencio T. TanSafety ConsultantGenesis A. RamosDepartment of EnergyGideon S. TanYu Eng Kao Electrical Supply and Hardware, Inc.Jaime S. JimenezMeralco

Jesus C. SantosJC Santos and AssociatesMarites R. PangilinanLJ Industrial Fabrication, Inc.Roderick T. KhuAirnergy and Renewables, Inc.Samson D. PadenDepartment of Trade and Industry-Bureau of Product StandardsVincent E. JimenezDelta Power Engineering and ConsultingWilson T. YuStandards Committee Member

Advisers:Arthur A. LopezPrivate ConsultantIIEE former president - year 2000Willington K.K.C. TanColumbia Wire and Cable CorporationIIEE former president - year 1990

Approved by the members of the IIEE Board of Governors on March 19, 2011:Armando R. Diaz, PresidentJules S. Alcantara, VP- Internal AffairsGregorio R. Cayetano, VP- External AffairsAlex C. Cabugao, VP- Technical AffairsMa. Sheila C. Cabaraban, SecretaryLarry C. Cruz, TreasurerFlorigo, C. Varona, AuditorFrancis R. Calanio, Region I

Virgilio S. Luzares, Region IIRoselyn C. Rocio, Region IVRonaldo D. Ebrada, Region VMarlon T. Marcuelo, Region VILelanie T. Mirambel, Region VIIRey G. Paduganan, Region VIIIVictorianito E. Teofilo, Region IXGregorio Y. Guevarra, Immediate former President

vTable of ContentsChapter Title Page

I. Introduction 1

II. Basic Short-Circuit Discussion 1Figure 1 : Current Model for Asymmetry 1Figure 2 : Maximum Peak Asymmetrical Short Circuit Current 2

III. Asymmetry Current Application 2

IV. Short-Circuit Current Calculation Standard/Guideline 3

V. Calculation Comparison 3Table 1 : Comparison Matrix of ANSI/IEEE and IEC 4

VI. Comparison of Device Duty Rating and Short-Circuit Duty 5Table 2 : ANSI/IEEE Parameter 5Table 3 : IEC Parameter 5Table 4 : ANSI/IEEE vs. IEC Parameter 5

VII. Sample Calculation using ANSI/IEEE and IEC 6Figure 3 : Single Line Diagram of the Sample Network 6Figure 4 : Single Line Diagram to consider IEC SC Result 7Figure 5 : Single Line Diagram to consider ANSI SC Result 16Figure 6 : Impedance Diagram for ANSI/IEEE SC Method 23

VIII. Protective Devices Selection and Evaluation 26Table 5 : Circuit Breakers Short Circuit Breaking Capacity 26Table 6 : Circuit Breakers Interrupting Capacity 27

IX. Findings and Results 27Table 7 : IEC Short Circuit Calculation 27Table 8 : ANSI Short Circuit Calculation 27

X. Conclusion and Recommendation 28

XI. References 29

AppendixABB MCB S200 Technical Features 30

1I. IntroductionIn the emerging world market place, Electrical Engineers should be familiar with the basic differences between theAmerican National Standards Institute (ANSI) and the International Electro-technical Commission (IEC) withregards to short circuit current calculation procedures. Both the ANSI and the IEC Standards developed theseprocedures to provide rating for electrical equipment. These two standards are currently being applied by theelectrical practitioners in the Philippines and it is important to determine the differences between these standardsso that a more logical evaluation and breaker rating selection can be appropriated. IEC procedure requiressignificantly more detailed modeling of the power system short circuit contribution than ANSI.A short circuit calculation is an important task undertaken by a professional in power systems planning andoperation. Circuit breaker and switchgear selection, protection settings and coordination require a comprehensive,detailed and accurate short-circuit calculation. The report focuses on the guidelines found in the following short-circuit standards: the North American ANSI/IEEE standard and its European counterpart, IEC.

II. Basic Short Circuit DiscussionTo come up with a precise short circuit calculation requires a very complex computation. What is important is thatwhatever the short circuit calculation method used, it should be compared with the assigned (tested) fault currentrating of the protective devices.The final equivalent short circuit schematic diagram is shown below.

Figure 1: Current Model for AsymmetryThe circuit constitutes a series of resistance, inductance, and a switch connected to an ideal sinusoidal voltagesource. The fault is simulated by closing the switch and the magnitude of the rms symmetrical short circuitcurrent, I, is determined by the equation below.

ZEI

where:I = short circuit current (rms symmetrical)E = driving voltage (rms)Z = Thevenins equivalent system impedance from the fault point back to and including

the source or sources of short-circuit currents for the distribution system.The duration and magnitude of the asymmetrical current depends on the following parameters:

a) The X/R ratio of the faulted circuitb) The phase angle of the voltage waveform at the time the short circuit occur

R L

2 Esin(t + )

i(t)

~

2The asymmetrical fault current decay time is longer when X/R ratio is greater at the fault point. For specific X/Rratio, the angle of the applied voltage at the time of short-circuits initiation determines the degree of fault currentasymmetry that will exist for that X/R ratio. The maximum asymmetrical short-circuit current occurs at the faultinception when the voltage sine wave is at zero point and not necessarily at the highest dc component.

Figure 2: Maximum Peak Asymmetrical Short Circuit Current

III. Asymmetry Current ApplicationFrom the equipment design and application point of view, the phase with the largest fault peak current should beof major interest. This current subjects the equipment to the most severe magnetic force. The maximum magneticforce produced on a circuit element, such as a breaker, occurs at the instant the fault current through the circuitelement is at a maximum. The largest fault peak typically occurs in the first cycle when the initiation of the short-circuit current is near or coincident with the applied voltage passing through zero. This condition is called thecondition of maximum asymmetry.Electrical equipment such as circuit breakers, switches, transformers and fuses that are subjected to carry faultcurrent, the total available short circuit current must be determined. For correct equipment application, knowledgeof the minimum test X/R ratio or maximum power factor of the applied fault current used in the acceptance test byANSI, NEMA, UL and IEC is also required. Knowledge of peak fault current magnitudes are significant for somedevices, such as low voltage breakers, while asymmetrical rms current magnitudes are equally significant for highvoltage circuit breakers. This leads to the need to develop an X/R ratio dependent short circuit calculation forproper comparison to the equipment being applied. To determine the maximum peak or rms current magnitudethat can occur in a circuit, every fault current calculation must consider the symmetrical ac component and thetransient dc component of the calculated fault current. When the calculated fault X/R ratio is greater than theequipment X/R ratio, the higher X/R ratio must also be considered when evaluating or selecting the equipment.

3IV. Short Circuit Calculation Standard / GuidelineANSI C37/IEEE Std. 551The ANSI/IEEE method calls for determining the momentary network fault impedance which makes it possible tocalculate the close and latch rating of the breaker. It also calls for identifying the interrupting network faultimpedance which makes it possible to calculate the interrupting duty of the breaker. The interrupting network faultimpedance value differs from the momentary network in that the impedance increases from the sub-transient totransient level. The IEEE standard permits the exclusion of 3 phase induction motors below 50 hp and all singlephase motors. Hence no reactance adjustment is required for these sizes of motors. For detailed calculationrequirements please refer to the applicable standards.

IEC60909

The IEC method calls for the adjusted network impedance in calculating the symmetrical three phase fault (Ik) ata voltage higher than the nominal rating by a factor (c). The result is further manipulated to calculate peak currentip which is then compared to the breakers making capacity (Icm). Also, further manipulation of the calculatedthree-phase fault current Ik will result to the interrupting rating requirement that is compared to the selectedbreakers interrupting capacity (Ib). For detailed calculation requirements please refer to the applicable standards.

V. Calculation ComparisonTable 1 presents a brief comparison of the ANSI/IEEE and IEC with regards to short circuit current calculationmethod and multiplying factors.

4Table 1: Comparison of ANSI/IEEE and IECANSI/IEEE IEC

Standard North America Europe PredominantCalculation Method 1. Voltage Source is equivalent

to the pre-fault voltage at thelocation

2. Machines are represented bytheir internal impedances

3. Line capacitance and staticloads are neglected

4. Bolted Fault is assumed hencearc resistance is neglected

5. System impedances areassumed balanced 3-phase

6. Uses symmetrical componentsfor unbalanced faultcalculations

7. Momentary calculates throughsub-transient impedancenetwork at half cycle

8. Interrupting calculates throughtransient impedance networkat 1.5 4 cycles

9. Steady-State calculatesthrough steady-stateimpedance network at andbeyond 30 cycles

1. Pre-fault voltage is automaticallyadjusted by a factor ( c )

2. Machines are represented by theirinternal impedances

3. Line capacitance of transmissionlines and static loads are consideredfor unbalanced ground faultsfollowing a Shunt Admittance Model

4. System impedances are assumedbalanced 3-phase

5. Uses symmetrical components forunbalanced fault calculations

6. (Ik) Initial RMS Symmetrical SCCcalculates through adjustedimpedance network of synchronousmachine Zk

7. (ip) Peak Short circuit current =k1*sqrt 2* Ik where k is determinedby Method A, B or C

8. (Ib) Symmetrical Short-CircuitBreaking Current = Ik for neargenerator faults and = u*Ik for synchmachines = u*q*Ik for asynch.machines

9. Asymmetrical SC Breaking Current =Ik + Idc component current

10.Steady State SC current (Ik)accounts for power grid, generator andsynch machine contributions(ip) Peak Short circuit current = k1*sqrt.2 * Ik

Multiplying Factors1. MF(m) Momentarymultiplying factor I mom.rms asym = I mom. rms sym *MF(m)

2. MF(p) Peak multiplying factor I mom. peak = I mom. rms.Sym * MF(p)

1. C pre-fault voltage factor (takenfrom IEC)

2. k factors determined by IECmethod A, B or C

5VI. Comparison of Device Duty Rating and Short-Circuit DutyThe tables below show the different parameters used in evaluating a protective device in terms of calculated shortcircuit duty of the ANSI/IEEE and IEC Standards.

Table 2: ANSI/IEEE Parameter

DEVICE TYPE DEVICE CAPABILITYCALCULATED SHORT-

CIRCUIT DUTY(Momentary Duty)

HV BUS BRACING Asymm. KA rms Asymm. KA rmsSymm. KA rms Symm. KA rmsLV BUS BRACING Symm. KA rms Symm. KA rmsAsymm. KA rms Asymm. KA rms

HVCBC and L Capability KA rms Asymm. KA rmsC and L Capability KA Crest Asymm. KA Crest

Interrupting KA Adjusted KALVCB Rated Interrupting KA Adjusted KA

Table 3: IEC Parameter

DEVICE TYPE DEVICE CAPABILITYCALCULATED

SHORT-CIRCUITDUTY (Momentary

Duty)MVCB Making ipAC Breaking Ib ,symmLVCB Making ipBreaking Ib ,symmFuse Breaking Ib ,symm

Table 4: The ANSI/IEEE vs. IECDEVICE TYPE DEVICE CAPABILITY CALCULATED SHORT

CIRCUIT DUTYANSI IEC ANSI IEC ANSI IECHVCB MVCB C and L cap. KA rms Making (ip) Asymm. KArms ip

C and L cap. KA rest n/a Asymm. KArestInterrupting KA AC breaking (Isc) Adjusted KA Ib symm

Ib asymm Ib asymmIdcIohm Ish

LVCB LVCB Rated interrupting KA Breaking (Ib symm)ICUIb symmor Ik

Making peak (ICM) ipIb asymm Ib asymmIsh Ish

6VII. Sample Calculation using ANSI/IEEE and IECDescription of Sample networkThe sample network consists of two power transformers connected to a 13.2 KV bus. One of the transformers feedsa bus at a nominal voltage of 240 V, while the other transformer feeds a bus at a nominal voltage of 2.3 KV. Thedata of the transformer and other equipment and their principal characteristics are presented in Fig. 3. For thepurpose of presenting a discussion on fault calculation, points Bl and B2 are selected to have experienced a 3phase bolted fault.

Figure 3: Single Line Diagram of the Sample Network

7A. IEC SHORT CIRCUIT RESULT

Figure 4: Single Line Diagram to consider IEC SC Result

8Location:

Engineer: Study Case: SC

6.0.0C Page: 1

SN: FUJIHAYAPH

Filename: sample

Project: ETAP

Contract:Date: 10-19-2010

Revision: BaseConfig.: Normal

Electrical Transient Analyzer Program

IEC 60909 Standard3-Phase Fault Currents

Short-Circuit Analysis

Maximum Short-Circuit Current

Total

Number of Buses:Swing V-Control Load Total

Number ofBranches:

XFMR2 Reactor Line/Cable Impedance Tie PDXFMR3 Total

Number ofMachines:

Generator Motor MachinesSynchronous Synchronous Induction LoadLumped

1 0 7

2 0 00 0 5

0 0 5 11

8

7

7

PowerGrid

System Frequency: 60 HzUnit System: EnglishProject Filename: sampleOutput Filename: D:\Etap6.0 Projects\SC sample attachments_2010_10_18\Untitled.SI1

9Adjustments

Tolerance ApplyAdjustmentIndividual/Global Percent

Transformer Impedance: Yes IndividualReactor Impedance: Yes IndividualOverload Heater Resistance: NoTransmission Line Length: NoCable Length: No

Temperature Correction ApplyAdjustmentIndividual/Global Degree C

Transformer Resistance: Yes Global 20Cable Resistance: Yes Global 20

Bus Input Data

Bus Initial VoltageID Type Nom. kV Base kV Sub-sys %Mag. Ang.B1 Load 0.240 0.240 1 100.00 0.00B2 Load 2.300 2.300 1 100.00 0.00Bus4 Load 0.240 0.240 1 100.00 0.00Bus5 Load 2.300 2.300 1 100.00 0.00Bus6 Load 2.300 2.300 1 100.00 0.00Bus7 Load 2.300 2.300 1 100.00 0.00Bus8 Load 2.300 2.300 1 100.00 0.00UB SWNG 13.200 13.200 1 100.00 0.00

8 Buses TotalAll voltages reported by ETAP are in % of bus Nominal kV.Base kV values of buses are calculated and used internally by ETAP

2-Winding Transformer Input Data

Transformer Rating Z Variation % Tap Setting Adjusted Phase ShiftID MVA Prim.kV Sec. kV %Z X/R +5% -5% %Tol. Prim. Sec. % Z Type AngleT1 1.500 13.200 0.240 5.75 7.10 0 0 0 0 0 5.7500 Std Pos. Seq. 0.0T2 5.000 13.200 2.300 7.15 12.14 0 0 0 0 0 7.1500 Std Pos. Seq. 0.0

Branch Connections

CKT/Branch Connected Bus ID % Impedance, Pos. Seq., 100 MVAbID Type From Bus To Bus R X Z YT1 2W XFMR UB B1 51.58 366.13 369.74T2 2W XFMR UB B2 11.76 142.82 143.31CB6 Tie Breaker B1 Bus4CB7 Tie Breaker B2 Bus5CB8 Tie Breaker B2 Bus6CB9 Tie Breaker B2 Bus7CB10 Tie Breaker B2 Bus8

10

Induction Machine Input Data

Induction Machine Connected Bus Rating % Impedance(Motor Base) mFact.

ID Type Qty ID HP/kW kVA kV Amp PF R X" R/X" MW/PPM2 Motor 1 Bus5 500.00 440.28 2.300 110.52 90.82 2.96 15.41 0.19 0.19M3 Motor 1 Bus6 500.00 440.28 2.300 110.52 90.82 2.96 15.41 0.19 0.19M4 Motor 1 Bus7 500.00 440.28 2.300 110.52 90.82 2.96 15.41 0.19 0.19M5 Motor 1 Bus8 500.00 440.28 2.300 110.52 90.82 2.96 15.41 0.19 0.19M1 Motor 1 Bus4 125.00 110.12 0.240 264.91 91.51 4.62 16.01 0.29 0.05

Total Connected Induction Machines ( = 5 ): 1871.3 kVA

Lumped Load Input DataLumped Load Motor Loads

Lumped Load Connected Bus Rating % Load Loading % ImpedanceMachine Base m Fact.ID ID kVA kV Amp % PF MTR STAT kW kvar R X" R/X" MW/PPL1 B1 1000.0 0.240 2405.63 85.00 60 40 510.0 316.1 6.46 15.37 0.42 0.51

Total Connected Lumped Loads ( = 1 ): 1000.0 kVA

Power Grid Input Data

Power Grid Connected Bus Rating % Impedance 100MVA BaseID ID MVAsc kV R X" R/XU1 UB 720.000 13.200 0.00014 13.88889 0.00

Total Connected Power Grids ( = 1 ): 720.000 MVA

11

SHORT - CIRCUIT REPORT

3-Phase fault at bus: B1Nomimal kV = 0.240Voltage c Factor = 1.10 (Maximum If)Peak Value = 181.348 kA Method ASteady State = 68.754 kA rms

Contribution Voltage and Initial Symmetrical Current (rms)From Bus To Bus % V kA kA X/R kA

ID ID FromBus Real Imaginary Ratio MagnitudeB1 Total 0.00 13.406 -78.631 5.9 79.766UB B1 96.12 9.232 -68.170 7.4 68.792L1 B1 100.00 3.690 -8.782 2.4 9.526M1 Bus4 100.00 0.485 -1.680 3.5 1.748Bus4 B1 0.00 0.485 -1.680 3.5 1.748

Breaking and DC Fault Current (kA)

Based on Total Bus Fault CurrentTD (S) Ib sym Ib asym Idc0.01 78.916 101.347 63.5880.02 78.315 86.941 37.7560.03 77.529 80.547 21.8430.04 76.761 77.794 12.6370.05 76.017 76.406 7.7040.06 75.656 75.79 4.5040.07 75.301 75.347 2.6330.08 74.952 74.968 1.5390.09 74.610 74.616 0.9390.10 74.275 74.277 0.5510.15 73.582 73.582 0.0390.20 72.918 72.918 0.0030.25 72.285 72.285 0.0000.30 72.260 72.260 0.000

12

3-Phase fault at bus: B2Nomimal kV = 2.300Voltage c Factor = 1.10 (Maximum If)Peak Value = 51.136 kA Method ASteady State = 17.417 kA rms


ID ID From Bus Real Imaginary Ratio MagnitudeB2 Total 0.00 1.882 -20.420 10.9 20.506

UB B2 90.44 1.297 -17.377 13.4 17.425M5 Bus8 100.00 0.146 -0.761 5.2 0.775M4 Bus7 100.00 0.146 -0.761 5.2 0.775M3 Bus6 100.00 0.146 -0.761 5.2 0.775M2 Bus5 100.00 0.146 -0.761 5.2 0.775

Bus5 B2 0.00 0.146 -0.761 5.2 0.775Bus6 B2 0.00 0.146 -0.761 5.2 0.775Bus7 B2 0.00 0.146 -0.761 5.2 0.775Bus8 B2 0.00 0.146 -0.761 5.2 0.775


Based on Total Bus Fault CurrentTD (S) Ib sym Ib asym Idc

0.01 20.014 28.877 20.8160.02 19.722 25.04 15.4290.03 19.441 22.464 11.2540.04 19.174 20.857 8.2080.05 18.921 19.955 6.3420.06 18.799 19.373 4.6790.07 18.681 18.997 3.4530.08 18.566 18.74 2.5480.09 18.456 18.564 2.0010.10 18.349 18.409 1.4860.15 18.145 18.148 0.3360.20 17.954 17.954 0.0760.25 17.775 17.775 0.0170.30 17.769 17.769 0.004

13

3-Phase fault at bus: Bus4

Nomimal kV = 0.240Voltage c Factor = 1.10 (Maximum If)Peak Value = 181.348 kA Method ASteady State = 68.754 kA rms


ID ID From Bus Real Imaginary Ratio MagnitudeBus4 Total 0.00 13.406 -78.631 5.9 79.766

M1 Bus4 100.00 0.485 -1.680 3.5 1.748UB B1 96.12 9.232 -68.170 7.4 68.792L1 B1 100.00 3.690 -8.782 2.4 9.526B1 Bus4 0.00 12.921 -76.952 6.0 78.029


Based on Total Bus Fault CurrentTD (S) Ib sym Ib asym Idc

0.01 78.916 101.347 63.5880.02 78.315 86.941 37.7560.03 77.529 80.547 21.8430.04 76.761 77.794 12.6370.05 76.017 76.406 7.7040.06 75.656 75.790 4.5040.07 75.301 75.347 2.6330.08 74.952 74.968 1.5390.09 74.61 74.616 0.9390.10 74.275 74.277 0.5510.15 73.582 73.582 0.0390.20 72.918 72.918 0.0030.25 72.285 72.285 0.0000.30 72.26 72.260 0.000

14


Based on Total Bus Fault CurrentTD(S) Ib sym Ib asym Idc0.01 32.048 55.226 44.9770.02 32.004 55.166 44.9340.03 31.959 54.943 44.6920.04 31.915 54.723 44.4520.05 31.874 54.998 44.820.06 31.853 54.889 44.7010.07 31.833 54.781 44.5830.08 31.814 54.674 44.4640.09 31.795 55.076 44.9710.10 31.777 55.024 44.9210.15 31.74 54.801 44.6740.20 31.706 54.581 44.4270.25 31.673 54.362 44.1820.30 31.672 54.164 43.939

3-Phase fault at bus: UB

Nomimal kV = 13.200Voltage c Factor = 1.10 (Maximum If)Peak Value = 90.256 kA Method ASteady State = 31.492 kA rms


ID ID From Bus Real Imaginary Ratio MagnitudeUB Total 0.00 0.142 -32.117 226.5 32.117

B1 UB 13.64 0.060 -0.167 2.8 0.178B2 UB 13.86 0.081 -0.458 5.7 0.465U1 UB 100.00 0.000 -31.492 99999.0 31.492

15

ip is calculated using method AIb does not include decay of non-terminal faulted induction motorsIk is the maximum steady state fault currentIdc is based on X/R from Method C and Ib as specified above

LV CB duty determined based on ultimate rating.Total through current is used for device duty.*Indicates a device with calculated duty exceeding the device capability.# Indicates a device with calculated duty exceeding the device marginal limit ( 95 % times device capability)

Short Circuit Summary Report

3-PhaseShort-Circuit

Device Capacity CurrentBus ID Device ID 1thr (kA) Tkr (sec.) Ith (kA)B1 CB2 100.000 1.00 75.613B1 CB6 65.000 1.00 75.613*Bus4 CB6 65.000 1.00 75.613*

1thr = Rated short-circuit withstand currentTkr = Rated short-timeIth = thermal equivalent short-time current*Indicates a device with calculated duty exceeding the device capability.# Indicates a device with calculated duty exceeding the device marginal limit ( 95 % times device capability )

Short Circuit Summary Report3-Phase FaultCurrent

Bus Device Device Capacity (kA) Short-Circuit Current (kA)MakingPeakID kV ID Type Ib sym Ib asym Idc I"k ip Ib sym Ib asym Idc Ik

B1 0.240 B1 Bus 79.766 181.348 68.7540.240 CB2 CB 220.000 100.000 102.111 79.766 181.348 77.920 83.044 28.7180.240 CB6 CB 176.000 80.000 80.426 79.766 181.348* 77.219 77.219 17.549

B2 2.300 B2 Bus 20.506 51.136 17.417Bus4 0.240 Bus4 Bus 79.766 181.348 68.754

0.240 CB6 CB 176.000 80.000 80.426 79.766 181.348* 77.219 79.188 17.549UB 13.200 UB Bus 32.117 90.256 31.492

16

B. ANSI SHORT CIRCUIT RESULT

FIGURE 5: Single Line Diagram to consider ANSI SC Result

17

Location:

Engineer: Study Case: SC

6.0.0C Page: 1

SN: FUJIHAYAPH

Filename: sample

Project: ANSI Calc Total Bus Fault Peak Current ETAPContract:

Date: 10-19-2010

Revision: Ansi BreakerConfig.: Normal

Electrical Transient AnalyzerProgram

ANSI Standard3-Phase Fault Currents

Short-Circuit Analysis

Total

Number of Buses:Swing V-Control Load Total

Number ofBranches:

XFMR2 Reactor Line/Cable Impedance Tie PDXFMR3 Total

Number ofMachines:

Generator Motor MachinesGridSynchronous Synchronous Induction LoadLumped

1 0 7

2 0 00 0 5

0 0 5 11

8

7

7Power

System Frequency: 60 HzUnit System: EnglishProject Filename: sampleOutput Filename: D:\Etap6.0 Projects\SC sample attachments_2010_10_18\Untitled.SA1

18

AdjustmentsTolerance ApplyAdjustment

Individual/Global Percent

Transformer Impedance: Yes IndividualReactor Impedance: Yes IndividualOverload Heater Resistance: NoTransmission Line Length: NoCable Length: No

Temperature Correction ApplyAdjustmentIndividual/Global Degree C

Transformer Resistance: Yes Global 20Cable Resistance: Yes Global 20

Bus Input DataBus Initial Voltage

ID Type Nom. kV Base kV Sub-sys %Mag. Ang.B1 Load 0.240 0.240 1 100.00 0.00B2 Load 2.300 2.300 1 100.00 0.00Bus4 Load 0.240 0.240 1 100.00 0.00Bus5 Load 2.300 2.300 1 100.00 0.00Bus6 Load 2.300 2.300 1 100.00 0.00Bus7 Load 2.300 2.300 1 100.00 0.00Bus8 Load 2.300 2.300 1 100.00 0.00UB SWNG 13.200 13.200 1 100.00 0.00

8 Buses TotalAll voltages reported by ETAP are in % of bus Nominal kV.

Base kV values of buses are calculated and used internally by ETAP

2-Winding Transformer Input DataTransformer Rating Z Variation % Tap Setting Adjusted Phase Shift

ID MVA Prim.kV Sec. kV %Z X/R +5% -5% %Tol. Prim. Sec. % Z Type AngleT1 1.500 13.200 0.240 5.75 7.10 0 0 0 0 0 5.7500 Std Pos. Seq. 0.0T2 5.000 13.200 2.300 7.15 12.14 0 0 0 0 0 7.1500 Std Pos. Seq. 0.0

19

Branch ConnectionsCKT/Branch Connected Bus ID % Impedance, Pos. Seq., 100 MVAb

ID Type From Bus To Bus R X Z YT1 2W XFMR UB B1 53.48 379.58 383.33T2 2W XFMR UB B2 11.74 142.52 143.00CB6 Tie Breaker B1 Bus4CB7 Tie Breaker B2 Bus5CB8 Tie Breaker B2 Bus6CB9 Tie Breaker B2 Bus7CB10 Tie Breaker B2 Bus8

Power Grid Input DataPower Grid Connected Bus Rating % Impedance100 MVA Base

ID ID MVASC kV X/R R XU1 UB 720.000 13.200 99999 0.00014 13.88889

Total Connected Power Grids ( = 1 ): 720.000 MVA

Induction Machine Input Data

InductionMachine Connected Bus Rating X/R Ratio

% Impedance(Motor Base)

ID Qty ID HP/kW kVA kV RPM X"/R X'/R R X" X'MotorsM2 1 Bus5 500.00 440.28 2.300 1800 10.89 10.89 2.21 24.05 36.08M3 1 Bus6 500.00 440.28 2.300 1800 10.89 10.89 2.21 24.05 36.08M4 1 Bus7 500.00 440.28 2.300 1800 10.89 10.89 2.21 24.05 36.08M5 1 Bus8 500.00 440.28 2.300 1800 10.89 10.89 2.21 24.05 36.08M1 1 Bus4 125.00 110.12 0.240 1800 8.71 8.71 2.30 20.00 50.00

Total Connected Induction Machines ( = 5 ): 1871.3 kVA

Lumped Load Input Data

Lumped Load Motor Loads Static LoadsLumped Load Connected Bus Rating % Load Loading X/R Ratio % Imp. (Machine Base) Loading

ID ID kVA kV MTR STAT kW kvar X"/R X'/R R X" X' kW kvar

L1 B1 1000.0 0.240 60 40 510.00 316.1 2.38 2.38 8.403 20.00 50.00 340.00 210.71

Total Connected Lumped Loads ( = 1 ): 1000.0 kVA

20

SHORT - CIRCUIT REPORT

3-Phase fault at bus: B1Prefault voltage = 0.240 = 100.00% of nominal bus kV ( 0.240kV )

= 100.00% of base ( 0.240kV )Contribution 1/2 Cycle

From Bus To Bus % V kA kA Imag. kASymm.ID ID FromBus Real Imaginary /Real MagnitudeB1 Total 0.00 10.893 -67.489 6.2 68.362

UB B1 96.57 8.165 -60.047 7.4 60.600L1 B1 100.00 2.577 -6.134 2.4 6.653M1 Bus4 100.00 0.150 -1.307 8.7 1.316

*Bus4 B1 0.00 0.150 -1.307 8.7 1316

NACD Ratio = 1.00# Indicates a fault current contribution from a three-winding transformer* Indicates a fault current through a tie circuit breakerIf faulted bus is involved in loops formed by protection devices, the short-circuit contribution through these PDs will not be reported

3-Phase fault at bus: B2

Prefault voltage = 2.300 = 100.00% of nominal bus kV ( 2.300 kV )= 100.00% of base ( 2.300 kV )

Contribution 1/2 Cycle 1.5 to 4 CycleFrom Bus To Bus % V kA kA Imag. kASymm. % V kA kA Imag.

kASymm.

ID ID FromBus Real Imaginary /Real Magnitude From Bus Real Imaginary /Real MagnitudeB2 Total 0.00 1.368 -17.787 13.0 17.840 0.00 1.311 -17.177 13.1 17.227

UB B2 91.20 1.201 -15.965 13.3 16.010 91.19 1.199 -15.962 13.3 16.007M5 Bus8 100.00 0.042 -0.456 10.9 0.458 100.00 0.028 -0.304 10.9 0.305M4 Bus7 100.00 0.042 -0.456 10.9 0.458 100.00 0.028 -0.304 10.9 0.305M3 Bus6 100.00 0.042 -0.456 10.9 0.458 100.00 0.028 -0.304 10.9 0.305M2 Bus5 100.00 0.042 -0.456 10.9 0.458 100.00 0.028 -0.304 10.9 0.305

* Bus5 B2 0.00 0.042 -0.456 10.9 0.458 0.00 0.028 -0.304 10.9 0.305* Bus6 B2 0.00 0.042 -0.456 10.9 0.458 0.00 0.028 -0.304 10.9 0.305* Bus7 B2 0.00 0.042 -0.456 10.9 0.458 0.00 0.028 -0.304 10.9 0.305* Bus8 B2 0.00 0.042 -0.456 10.9 0.458 0.00 0.028 -0.304 10.9 0.305

21


3-Phase fault at bus: B4


Contribution 1/2 CycleFrom Bus To Bus % V kA kA Imag. kA Symm.

ID ID From Bus Real Imaginary /Real MagnitudeBus4 Total 0.00 10.893 -67.489 6.2 68.362M1 Bus4 100.00 0.150 -1.307 8.7 1.316UB B1 96.57 8.165 -60.047 7.4 60.6L1 B1 100.00 2.577 -6.134 2.4 6.653

*B1 Bus4 0.00 10.743 -66.181 6.2 67.048


3-Phase fault at bus: UB


Contribution 1/2 Cycle 1.5 to 4 CycleFromBus

ToBus % V kA kA Imag. kA Symm. % V kA kA Imag.

kASymm.

ID ID FromBus Real Imaginary /Real Magnitude From Bus Real Imaginary /Real MagnitudeUB Total 0.00 0.068 -31.901 470.9 31.901 0.00 0.037 -31.742 863.6 31.742

B1 UB 11.24 0.041 -0.121 2.9 0.128 4.81 0.018 -0.052 2.8 0.055B2 UB 9.44 0.026 -0.288 11.0 0.289 6.50 0.018 -0.198 11.0 0.199U1 UB 100.00 0.000 -31.492 99999.0 31.492 100.00 0.000 -31.492 99999.0 31.492


22

Momentary Duty Summary Report

3-Phase Fault Currents: (PrefaultVoltage = 100% of the Bus NominalVoltage

Bus Device Momentary Duty Device Capability

ID kV ID Type Symm.kA rmsX/RRatio M.F.

AsymmkA rms

Asymm.kA Crest

SymmkArms

Asymm.kA rms

AsymmkACrest

B1 0.240 B1 Bus 68.362 6.9 1.342 91.741 157.859B2 2.300 B2 Bus 17.840 13.1 1.496 26.68 45.067Bus4 0.240 Bus4 Bus 68.362 6.9 1.342 91.741 157.859UB 13.200 UB Bus 31.901 999.9 1.728 55.139 90.088

Method : IEEE - X/R is calculated from separate R and X networks.

Protective device duty is calculated based on total fault current* Indicates a device with momentary duty exceeding the device capability

Interrupting Duty Summary Report

3-Phase Fault Currents: (Prefault Voltage = 100% of the Bus Nominal Voltage

Bus Device Interrupting Duty Device CapabilityCPT X/R Adj. Sym. Test Rated Adjusted

ID kV ID Type (Cy) Ratio M.F. kA rms kV PF Int. Int.B1 0.240 CB6

MoldedCase 68.362 6.9 1.070 73.118 0.240 20.00 85.000 85.000

B2 2.300 17.227 13.1Bus4 0.240 CB6

MoldedCase 68.362 6.9 1.070 73.118 0.240 20.00 85.000 85.000

UB 13.200 31.742 99184.7

Method: IEEE - X/R is calculated from separate R and X networks.HV CB interrupting capability is adjusted based on bus nominal voltageShort-Circuit multiplying factor for LV Molded Case and Insulated Case Circuit Breakers is calculated based on peak current.Generator protective device duty is calculated based on maximum through fault current. Other protective device duty is calculatedbased on total fault current.* Indicates a device with interrupting duty exceeding the device capability.

23

C. ANSI/IEEE Short Circuit Method (for Manual Calculation)Impedance Diagram Development @ 100 MVA base

Figure 6: Impedance Diagram

91214.12888.10tansin24051.044.0

100.

43227.54888.10tansin24051.044.0100.

73089.3038.2tansin2.06.00.1100.

91214.1238.2tancos2.06.00.1100.

63078.180707.8tansin2.011.0100.

74547.20707.8tancos2.011.0100.

42157.114.12tansin0715.05100.

11739.014.12tancos0715.05100.

79587.31.7tansin575.05.1100.

53463.01.7tancos575.05.1100.

13889.0720100.

M5M2

M5M2

L

L

M1

M1

T2

T2

T1

T1

U

R11

X10

X9

R8

X7

R6

X5

R4

X3

R2

X1

j

j

j

j

j

jj

XU

RT1XT1 XT2

RM1

XM1

RLXL

RM2

XM2

RM3

XM3

RM4

XM4

RM5

XM5

RT2B2B1

HP(0.746)(Eff)(pf)(100)MOTOR (MVA) =

500(0.746)(1000)(0.9325)(0.9082)

where:

M2 = 0.440 MVA

M2 =

24

Solving for cycle 3 phase fault at

kA968490623240310x100I

728049062344496j3562710ZZZZZZZZZZ

6T3

LM1T1UM5M4M3M2T2T

1B

1B

..

....

Solving for cycle, 3phase fault at

kA8517406551230310x100I

68540655140240j1107870ZZZZZZZZZZ

6T3

T2UT1LM1M5M4M3M2T

2B

2B

..0

....

IEC Short Circuit Calculation

199.364.099929.4.837.3464.043227.54.66.2377.073089.30.

942.977.091214.12.505.1448.063078.180.

596.168.074547.20.42517.10.142517.1.11739.00.111739.0.66301.3965.079587.3.

11.1;6.01965.0

965.051592.0965.053463.0.

13889.0.

M5KM2K

M5KM2K

LK

LK

M1K

M1K

T2

T2K

T1K

max

TKT

max

T1K

UK

R11X10X9R8X7R6X5R4X3

060909ofTableCKZZX

CKformulaIEC60909perasK

R2X1

jjj

jj

jj

jj

j

B1

B2

25

Solving for 3 phase fault at , I"K

kA802741232403

1110x100I

398027412322821j3546410ZZZZZZKZZZZ

6K

LM1KT1KUKM5KM4KM3M2KT2KTK

1B

1B

.."

....

Solving for 3 phase fault at , I"K

kA85173294812303

1110x100I

68532948132547j1103070ZZZZZZZZZZ

6K

T2KUKT1KLKM1KM5KM4KM3KM2KTK

2B

2B

..0."

....

* It is therefore the basic inclusion of factors Cm and k that increases the calculated short-circuit of IECmethod when being compared to the result of the ANSI method.

B1

B2

26

VIII. Protective Devices Selection and evaluationX/R Ratio for Breaker EvaluationThe fault point X/R ratio is a critical factor in the calculation of short circuit current when evaluating breakers.The X/R ratio determines the amount of dc component in the short circuit current and in the application to thecircuit breaker withstands and interrupting time duties. ANSI/IEEE C37.010-1999 recommends a separate R andjX network reduction to determine the fault point X/R ratio while IEC 61909 allows several methods to provide aconservative X/R ratio.The peak current calculation that yields a very close approximation to the exact peak current and is conservativefor most values of circuit X/R ratios greater than 0.81. The non-conservative errors for circuit X/R ratio around 10are negligible. Please refer to equation below:

RX

RX

RX

RX

e

e

e

e

/

/

/

/

..

..

3rmsac

3peakac

rmsac

peakac

980021I2

or980021IcycleHalf

1I2

or1IcycleHalf

Circuit Breaker Short Circuit Test ParametersBased on IEC 60947-2, the circuit breakers short circuit breaking capacity, power factor and ratio, , betweenshort circuit making capacity and short circuit breaking capacity should be in accordance with Table 5.

Table 5: Circuit Breakers Short Circuit Breaking CapacityShort circuit breaking

capacity,Ib, kA rms

LaggingPowerfactor X/R

Minimum value required forShort-circuit making capacityShort-circuit breaking capacity

4.5 I 66 < I 1010 < I 2020 < I 5050 < I

0.70.50.30.250.2

1.021.733.183.874.9

1.51.72.02.12.2

Ratio between Short-circuit making capacity and Short-circuit breaking capacity and related power factor or X/Rratio (for ac circuit breaker).

=

ANSI / IEEE

IEC 60909

27

Based on NEMA AB1/UL489, the circuit breakers interrupting capacity, lagging power factor should be inaccordance with Table 6.

Table 6: Circuit Breakers Interrupting CapacitykA lagging pf X/R

MCCB and ICB I 1010 < I 2020 < I

0.45 0.50.25 0.300.15 0.20

1.98 1.733.87 3.176.59 4.90

Power Circuit Breaker (Unfuse) All 0.15 6.59Power Circuit Breaker (Fuse) All 0.20 4.90

IX. Findings and ResultsFrom the result of ETAP Total Bus Fault Short Circuit Study, the following results were found:

Table 7: IEC Short Circuit Calculation

BusID Device

Device Capacity (kA) Short Circuit Calculation Result (kA)MakingPeak Ib sym Ib asym I"b ip Ib sym Ib asym

B1 CB2 176 80 80.426 79.8 181.348* 77.219 79.188Note: Method A, Total Bus Fault

From the data in Table 7, the calculated short circuit current level of 79.8kA (Ik) is within the circuit breakercapacity of 80 kA (Ib sym), however, other parameters such as peak short circuit current (ip) is 181.348 kAexceeded the circuit breaker rating equivalent to 176 kA (making peak) only. Therefore, the selected IEC ratedcircuit breaker is not suitable for this particular application.The reason for this difference is that the calculated X/R ratio at 3-phase fault at point B1 is 5.9, which is greaterthan the device capacity X/R ratio of only 4.9 (see ETAP IEC method result above) applied during the testing ofcircuit breaker interrupting capacity or the Icu rating of the circuit breaker.

Table 8: ANSI Short Circuit Calculation

BusID Device

Device Capacity (kA) Short Circuit Calculation Result (kA)Rated Int. Adj. Int Symrms X/R ratio M.F

Adj. Symrms

B1 CB2 85.0 85.0 68.362 6.9 1.07 73.118

From the data in Table 8, the calculated symmetrical rms current of 68.362 kA needs to be adjusted by themultiplying factor (MF) of 1.070 (see computation below) resulting to 73.118 kA. This is because the calculatedX/R ratio 6.9 is greater than the X/R ratio used in testing the circuit breaker interrupting capacity which is only 4.9(Table 6). Comparing the adjusted symmetrical rms value of 73.118 kA against the selected NEMA rated deviceinterrupting capacity of 85kA, the selected circuit breaker is suitable for the particular application.

28

071

11

11MF

T

C

T

C

941961

XRXR

.

.

.

ee

ee

Where:TX

R

- Break test R/X ratio

CXR

- Calculated R/X ratio at the point fault

MF - Multiplying factor

X. Conclusion and RecommendationFrom the above short circuit calculation examples, IEC method shows a higher value of short circuit current ascompared to ANSI/IEEE calculation method. This is due to the differences in the consideration as mentionedabove. Both methods are being used and internationally acceptable.In any electrical system, it is important to know the short circuit level of each of the protective equipment.However, we should not forget to verify the X/R ratio of the faulted bus against the circuit breaker test powerfactor or X/R ratio based on their product standard (e.g. UL/NEMA/ANSI or IEC). The example aboveillustrates clearly the importance of X/R ratio in evaluating or selecting the circuit breaker. Understanding therelationship between the product standards and electrical codes is of utmost importance.It is up to the engineers/designers to decide which method of short circuit calculation they are more comfortablewith provided they have to take note of the different considerations in the selection of the protective equipment.

29

XI. References

1 IEEE Std 551-2006, IEEE Recommended Practice for Calculating Short-Circuit currents inIndustrial and Commercial Power System.

2 IEEE Papers, Simplifying IEEE/ANSI and IEC Fault Point X/R Ratio for Breaker Evaluationby Ketut Dartawan and Conrad St. Pierre

3 IEC 60497-1:2009, Low-voltage switchgear and control gear - Part 1: General rules4 IEC 60497-2:2009, Low-voltage switchgear and control gear - Part 2: Circuit-breakers5 ANSI C37.5-1989, Calculation of Fault Currents for Application of Power Circuit BreakersRated on a Total Current Basis

6 UL 489-1986, Molded Case Circuit Breaker and Circuit Breakers Enclosure7 IEC 60909-0, Corrigendum 1 - Short-circuit currents in three-phase A.C. systems - Part 0:Calculation of currents

8 Electrical Transient Analyzer Program (ETAP) Software version 6.0

30

AppendixCourtesy of ABB Phil., Inc.

proM Compact Technical features S 200of MCBs S 200 series

Series S 200 S 200 M S 200 P S 280Characteristics B,C,D B,C,D B,C,D B,C,D B,C,D B,C

K,Z K,Z K,Z K,Z K,ZRated current [A] 0.5 ln 63 0.5 ln 63 0.2 ln 25 32 ln 40 50 ln 63 80 LN 100Breaking capacity [kA]

Reference standardNr.Poles Ue [V]

IEC 23-3/EN 60898 lcs 230 / 400 6 10 25 15 15 6

IEC/EN 60947 - 2 lcu1, 1P +

N 133 20 25 40 25 25 15Alternating current 230 10 15 25 15 15 6

2,3,4 230 20 25 40 25 25 10400 10 15 25 15 15 6

2,3,4 500690

lcs1, 1P +

N 133 15 18.7 20 18.7 18.7 15

31

(continued )

230 7.5 11.2 12.5 11.2 7.5 62,3,4 230 15 18.7 20 18.7 18.7 10

400 7.5 11.2 12.5 11.2 7.5 62,3,4 500

690

IEC/EN 60497 - 2 lcu1, 1P +

N 24 20Direct current 60 10 10 15 10 10 10T= lR 5ms for all 125series 250except S280 UC and 2 48 20S800S-UC 125 10 10 15 10 10 10where T = lR

32

(continued )50075010001200

UL 1077 / C22.2 lnt.1, 1P +

N 120 10 10 10 10No 235 cap. 277 6 10 10 10Alternating current 2,3,4 240 10 10 10 10

480Y /277 6 10 10 10

UL 1077 / C22.2 lnt.1, 1P +

N 60 10No 235 cap. 2,3,4 125 10Direct currentUL 489/ C22.2 lnt. 1 240No 5 cap. 277Alternating current 2,3,4 240

480y /277

IEC / EN 60947 - 3 lcw 2 8003,4 1200

Date post:	24-Nov-2015
Category:	Documents
Upload:	albert-tuazon
View:	694 times
Download:	56 times

Differences in ANSI-IEEE and IEC Short Circuit Calculations and Their Implications

Documents