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

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

A Technical Report on

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

______________________________ ____________________________________________ ___________________________ _______________________ __________

Prepar Prepared ed by: Institut Institutee of Integr Integrate atedd Electr Electrica icall



Disclaimer It is not the intention of this paper to endorse over another the compared short-circuit calculations and standards. All discussions discussions in this report are are based on the featured featured system one line line diagram only. only. The same parameters parameters were considered for the American National Standards Standards Institute/Institut Institute/Institutee of Electri Electrical cal and Electro Electronic nicss Enginee Engineers rs (ANSI/ (ANSI/IEE IEEE) E) and the Internat Internationa ionall Electro-t Electro-tech echnic nical al Commission Commission (IEC) (IEC) calculations calculations for result comparison. comparison. The values of these parameters, parameters, however, may vary from every project in terms of available utility short circuit levels, power system configuration, wiring method and all applicable factors to consider. IIEE and this Committee will not be responsible for any disputes that may arise out of referencing from this paper.

Preface This technical report focuses on two of the most widely used short circuit calculation methods and standards/guidelines namely: American National Standards Institute/Institute of Electrical and Electronics Engineers (ANSI/C37/IEEE std 551) and the International Electrotechnical Commission (IEC 60909). To fully understand the analytical techniques of short circuit current analysis in industrial and commercial power system using both methods, a representative network model was exemplified and a comprehensive comparison between the two calculation methods was presented. For expediency, a short circuit calculating software was employed and the results were presented and evaluated at the end of the analysis. This technical report provides information and inculcates awareness to electrical practitioners in the country on the difference in the procedure of short circuit calculations and its implication between the standards cited. It is not intended to show the detailed short circuit current calculation for both methods. The reader is still recommended to consult technical books for reference on a complete and accurate calculation procedure. This paper starts off with a brief introduction on the current scenario in the Philippines and the importance of short circuit calculation in Chapter I and expounds on its basic principle in Chapter II. The equivalent short circuit schematic diagram is also available for analysis in simple calculation. Chapter III discusses the asymmetry current application focusing on the importance of determining the total available short circuit current in the design of electrical equipment such as circuit breakers, switches, transformers and fuses that are subjected to fault current. Chapter IV shows the different components in determining the short circuit calculation based on the two standard/guidelines, the ANSI/IEEE and the IEC. This is followed by Chapter V

Participants The following are the working group members of the Institute of Integrated Electrical Engineers of the Philippines, Inc. (IIEE) under the Standards Committee: Chairman: Gem J. Tan

Fuji-Haya Audit Inspection and Maintenance Corporation Members: Arjun G. Ansay

Jesus C. Santos

Arturo M. Zabala

Marites R. Pangilinan

Edwin V. Pangilinan

Roderick T. Khu

Frumencio T. Tan

Samson D. Paden

Technological University of the Philippines – Manila AC-DC-KV and Associates Total Power Box Solution, Inc. Safety Consultant

JC Santos and Associates LJ Industrial Fabrication, Inc. Airnergy and Renewables, Inc. Department of Trade and IndustryBureau of Product Standards

Genesis A. Ramos

Department of Energy

Vincent E. Jimenez

Delta Power Engineering and Consulting Gideon S. Tan

Yu Eng Kao Electrical Supply and Hardware, Inc.

Wilson T. Yu

Standards Committee Member Jaime S. Jimenez

Meralco

Advisers: Arthur A. Lopez

Table of Contents Chapter

Title

Page

I.

Introduction

1

II.

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

1 1 2

III.

Asymmetry Current Application

2

IV.

Short-Circuit Current Calculation Standard/Guideline

3

V.

Calculation Comparison Table 1 : Comparison Matrix of ANSI/IEEE and IEC

3 4

VI.

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

5 5 5 5

VII.

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

6 6 7 16 23

I.

Introduction

In the emerging world market place, Electrical Engineers should be familiar with the basic differences between the American National Standards Institute (ANSI) and the International Electro-technical Commission (IEC) with regards to short circuit current calculation procedures. Both the ANSI and the IEC Standards developed these procedures to provide rating for electrical equipment. These two standards are currently being applied by the electrical practitioners in the Philippines and it is important to determine the differences between these standards so that a more logical evaluation and breaker rating selection can be appropriated. IEC procedure requires significantly 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 and operation. 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 shortcircuit standards: the North American ANSI/IEEE standard and its European counterpart, IEC.

II.

Basic Short Circuit Discussion

To come up with a precise short circuit calculation requires a very complex computation. What is important is that whatever the short circuit calculation method used, it should be compared with the assigned (tested) fault current rating of the protective devices. The final equivalent short circuit schematic diagram is shown below. R

L

i(t)

~

2 Esin(ωt + Ø)

The asymmetrical fault current decay time is longer when X/R ratio is greater at the fault point. For specific X/R ratio, the angle of the applied voltage at the time of short-circuits initiation determines the degree of fault current asymmetry that will exist for that X/R ratio. The maximum asymmetrical short-circuit current occurs at the fault inception 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

IV.

Short Circuit Calculation Standard / Guideline

ANSI C37/IEEE Std. 551

The ANSI/IEEE method calls for determining the momentary network fault impedance which makes it possible to calculate the close and latch rating of the breaker. It also calls for identifying the interrupting network fault impedance which makes it possible to calculate the interrupting duty of the breaker. The interrupting network fault impedance value differs from the momentary network in that the impedance increases from the sub-transient to transient level. The IEEE standard permits the exclusion of 3 phase induction motors below 50 hp and all single phase motors. Hence no reactance adjustment is required for these sizes of motors. For detailed calculation requirements please refer to the applicable standards. IEC60909

The IEC method calls for the adjusted network impedance in calculating the symmetrical three phase fault (I” k ) at a voltage higher than the nominal rating by a factor (c). The result is further manipulated to calculate peak current i p which is then compared to the breaker’s making capacity (I cm). Also, further manipulation of the calculated three-phase fault current I”k will result to the interrupting rating requirement that is compared to the selected breaker’s interrupting capacity (I b). For detailed calculation requirements please refer to the applicable standards.

V.

Calculation Comparison

Table 1 presents a brief comparison of the ANSI/IEEE and IEC with regards to short circuit current calculation method and multiplying factors.

Table 1: Comparison of ANSI/IEEE and IEC ANSI/IEEE Standard Calculation Method

IEC

North America 1. Voltage Source is equivalent to the pre-fault voltage at the location

Europe Predominant 1. Pre-fault voltage is automatically adjusted by a factor ( c ) 2. Machines are represented by their internal impedances 2. Machines are represented by 3. Line capacitance of transmission their internal impedances lines and static loads are considered for unbalanced ground faults 3. Line capacitance and static following a Shunt Admittance Model loads are neglected 4. System impedances are assumed balanced 3-phase 4. Bolted Fault is assumed hence 5. Uses symmetrical components for arc resistance is neglected unbalanced fault calculations 6. (I”k ) Initial RMS Symmetrical SCC 5. System impedances are calculates through adjusted assumed balanced 3-phase impedance network of synchronous machine Zk 6. Uses symmetrical components 7. (i p) Peak Short circuit current = for unbalanced fault calculations k1*sqrt 2* I”k where k is determined by Method A, B or C 7. Momentary calculates through 8. (I b) Symmetrical Short-Circuit sub-transient impedance Breaking Current = I” k for near network at half cycle generator faults and = u*I” k for synch

VI.

Comparison of Device Duty Rating and Short-Circuit Duty

The tables below show the different parameters used in evaluating a protective device in terms of calculated short circuit duty of the ANSI/IEEE and IEC Standards. Table 2:

ANSI/IEEE Parameter

DEVICE TYPE

HV BUS BRACING LV BUS BRACING HVCB LVCB Table 3:

LVCB Fuse

Asymm. KA rms Symm. KA rms Symm. KA rms Asymm. KA rms C and L Capability KA rms C and L Capability KA Crest Interrupting KA Rated Interrupting KA

Asymm. KA rms Symm. KA rms Symm. KA rms Asymm. KA rms Asymm. KA rms Asymm. KA Crest Adjusted KA Adjusted KA

IEC Parameter

DEVICE TYPE

MVCB

DEVICE CAPABILITY

CALCULATED SHORTCIRCUIT DUTY (Momentary Duty)

DEVICE CAPABILITY

CALCULATED SHORT-CIRCUIT DUTY (Momentary Duty)

Making AC Breaking Making Breaking

i p I b ,symm i p I b ,symm

Breaking

I b ,symm

VII. Sample Calculation using ANSI/IEEE and IEC Description of Sample network

The sample network consists of two power transformers connected to a 13.2 KV bus. One of the transformers feeds a bus at a nominal voltage of 240 V, while the other transformer feeds a bus at a nominal voltage of 2.3 KV. The data of the transformer and other equipment and their principal characteristics are presented in Fig. 3. For the purpose of presenting a discussion on fault calculation, points Bl and B2 are selected to have experienced a 3 phase bolted fault.

A. IEC SHORT CIRCUIT RESULT

Figure 4: Single Line Diagram to consider IEC SC Result

7

ETAP

Project: Location: Contract: Engineer: Filename: sample

Page: 1 Date: 10-19-2010 SN: FUJIHAYAPH Revision: Base Config.: Normal

6.0.0C

Study Case: SC

Electrical Transient Analyzer Program Short-Circuit Analysis IEC 60909 Standard 3-Phase Fault Currents

Maximum Short-Circuit Current

Number of Buses:

Number of

Swing 1

V-Control 0

Load 7

XFMR2 2

XFMR3 0

Reactor 0

Synchronous

Total 8

Line/Cable Impedance 0 0

Power Synchronous Induction

Lumped

Tie PD 5

Total 7

Adjustments

Tolerance Transformer Impedance: Reactor Impedance: Overload Heater Resistance: Transmission Line Length: Cable Length: Temperature Correction Transformer Resistance: Cable Resistance:

Apply Adjustment Yes Yes No No No

Individual /Global Individual Individual

Apply Adjustment Yes Yes

Individual /Global Global Global

Percent

Degree C 20 20

Bus Input Data Bus

ID B1 B2 Bus4 Bus5 Bus6 Bus7 Bus8 UB

Type Load Load Load Load Load Load Load SWNG

Initial Voltage

Nom. kV 0.240 2.300 0.240 2.300 2.300 2.300 2.300 13.200

Base kV 0.240 2.300 0.240 2.300 2.300 2.300 2.300 13.200

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

Sub-sys 1 1 1 1 1 1 1 1

%Mag. 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

Ang. 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Power Grid Input Data % Impedance 100 MVA Base

Power Grid

Connected Bus

Rating

ID

ID

MVAsc

kV

R

X"

R/X

U1

UB

720.000

13.200

0.00014

13.88889

0.00

Total Connected Power Grids ( = 1 ): 720.000 MVA

Induction Machine Input Data Induction Machine

Connected Bus

ID

Type

Qty

M2

Motor

1

M3

Motor

M4

% Impedance (Motor Base)

Rating

ID

mFact.

HP/kW

kVA

kV

Amp

PF

R

X"

R/X"

MW/PP

Bus5

500.00

440.28

2.300

110.52

90.82

2.96

15.41

0.19

0.19

1

Bus6

500.00

440.28

2.300

110.52

90.82

2.96

15.41

0.19

0.19

Motor

1

Bus7

500.00

440.28

2.300

110.52

90.82

2.96

15.41

0.19

0.19

M5

Motor

1

Bus8

500.00

440.28

2.300

110.52

90.82

2.96

15.41

0.19

0.19

M1

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 Data Lumped Load

Motor Loads

Lumped Load

Connected Bus

Rating

ID

ID

kVA

kV

Amp

L1

B1

1000.0

0.240

2405.63

Total Connected Lumped Loads ( = 1 ): 1000.0 kVA

% Load

% Impedance Machine Base

Loading

Fact.

% PF

MTR

STAT

kW

kvar

R

X"

R/X"

MW/PP

5.00

60

40

510.0

316.1

.46

15.37

0.42

0.51

SHORT - CIRCUIT REPORT

3-Phase fault at bus: Nomimal kV Voltage c Factor Peak Value Steady State

B1

= = = =

0.240 1.10 181.348 68.754

(Maximum If) kA Method A kA rms

Contribution

Voltage and Initial Symmetrical Current (rms)

From Bus

To Bus Total

%V From Bus 0.00

ID

ID

B1 UB L1 M1 Bus4

B1 B1 Bus4 B1

96.12 100.00 100.00 0.00

kA

kA

X/R

kA

Real

Imaginary

Ratio

Magnitude

13.406

-78.631

5.9

79.766

9.232 3.690 0.485 0.485

-68.170 -8.782 -1.680 -1.680

7.4 2.4 3.5 3.5

68.792 9.526 1.748 1.748

Breaking and DC Fault Current (kA) Based on Total Bus Fault Current

TD (S)

Ib sym

Ib asym

Idc

0.01 0.02 0.03 0.04 0.05 0.06 0.07 0 08

78.916 78.315 77.529 76.761 76.017 75.656 75.301 74 952

101.347 86.941 80.547 77.794 76.406 75.79 75.347 74 968

63.588 37.756 21.843 12.637 7.704 4.504 2.633 1 539

3-Phase fault at bus: Nomimal kV Voltage c Factor Peak Value Steady State

B2

= = = =

2.300 1.10 51.136 17.417

(Maximum If) kA Method A kA rms

Contribution


From Bus ID B2

To Bus ID Total

%V From Bus 0.00

kA Real 1.882

kA Imaginary -20.420

X/R Ratio 10.9

kA Magnitude 20.506

UB

B2

90.44

1.297

-17.377

13.4

17.425

M5

Bus8

100.00

0.146

-0.761

5.2

0.775

M4

Bus7

100.00

0.146

-0.761

5.2

0.775

M3 M2

Bus6 Bus5

100.00 100.00

0.146 0.146

-0.761 -0.761

5.2 5.2

0.775 0.775

Bus5

B2

0.00

0.146

-0.761

5.2

0.775

Bus6

B2

0.00

0.146

-0.761

5.2

0.775

Bus7

B2

0.00

0.146

-0.761

5.2

0.775

Bus8

B2

0.00

0.146

-0.761

5.2

0.775

Breaking and DC Fault Current (kA)

Based on Total Bus Fault Current TD (S)

Ib sym

Ib asym

Idc

0.01

20.014

28.877

20.816

0.02

19.722

25.04

15.429

0.03

19.441

22.464

11.254

3-Phase fault at bus:

Bus4

Nomimal kV

= 0.240

Voltage c Factor

= 1.10

(Maximum If)

Peak Value

= 181.348

kA Method A

Steady State

= 68.754

kA rms

Contribution


From Bus ID

To Bus ID

%V From Bus

kA Real

kA Imaginary

X/R Ratio

kA Magnitude

Bus4

Total

0.00

13.406

-78.631

5.9

79.766

M1

Bus4

100.00

0.485

-1.680

3.5

1.748

UB

B1

96.12

9.232

-68.170

7.4

68.792

L1

B1

100.00

3.690

-8.782

2.4

9.526

B1

Bus4

0.00

12.921

-76.952

6.0

78.029



Ib sym

Ib asym

Idc

0.01

78.916

101.347

63.588

0.02

78.315

86.941

37.756

0.03

77.529

80.547

21.843

0.04

76.761

77.794

12.637

0.05

76.017

76.406

7.704


UB

Nomimal kV

= 13.200

Voltage c Factor

= 1.10

(Maximum If)

Peak Value

= 90.256

kA Method A

Steady State

= 31.492

kA rms

Contribution


From Bus ID

To Bus ID

%V From Bus

kA Real

kA Imaginary

X/R Ratio

kA Magnitude

UB

Total

0.00

0.142

-32.117

226.5

32.117

B1

UB

13.64

0.060

-0.167

2.8

0.178

B2

UB

13.86

0.081

-0.458

5.7

0.465

U1

UB

100.00

0.000

-31.492

99999.0

31.492



Ib sym

0.01

32.048

55.226

44.977

0.02

32.004

55.166

44.934

0.03

31.959

54.943

44.692

0.04

31.915

54.723

44.452

0.05

31.874

54.998

44.82

Ib asym

Idc

Short Circuit Summary Report

3-Phase FaultCurrent Bus ID B1

Device Capacity (kA)

Device kV

ID

Type

Making Peak

Ib sym

Ib asym

Short-Circuit Current (kA) Idc

I"k

ip

79.766

181.348

Ib sym

Ib asym

Idc

Ik

0.240

1

Bus

0.240

CB2

CB

220.000

100.000

102.111

79.766

181.348

77.920

83.044

28.718

0.240

CB6

CB

176.000

80.000

80.426

79.766

181.348*

77.219

77.219

17.549

B2

2.300

2

Bus

20.506

51.136

17.417

Bus4

0.240

us4

Bus

79.766

181.348

68.754

0.240

CB6

CB

79.766

181.348*

13.200

B

Bus

32.117

90.256

UB

176.000

80.000

80.426

68.754

77.219

79.188

17.549 31.492

ip is calculated using method A Ib does not include decay of non-terminal faulted induction motors Ik is the maximum steady state fault current Idc 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-Phase Short-Circuit

B. ANSI SHORT CIRCUIT RESULT

FIGURE 5:

Single Line Diagram to consider ANSI SC Result

16

Project: ANSI Calc Total Bus Fault Peak Current Location: Contract: Engineer: Filename: sample

ETAP

Page: 1 Date: 10-19-2010 SN: FUJIHAYAPH Revision: Ansi Breaker Config.: Normal

6.0.0C

Study Case: SC

Electrical Transient Analyzer Short-Circuit Analysis ANSI Standard 3-Phase Fault Currents

Number of Buses:

Number of

Swing 1

V-Control 0

Load 7

XFMR2 2

XFMR3 0

Reactor 0

Total 8

Line/Cable Impedance Tie PD 0 0 5

Synchronous Power Synchronous Induction Lumped

Total 7

Adjustments Tolerance Transformer Impedance: Reactor Impedance: Overload Heater Resistance: Transmission Line Length: Cable Length: Temperature Correction Transformer Resistance: Cable Resistance:

Apply Adjustment Yes Yes No No No

Individual /Global Individual Individual

Apply Adjustment Yes Yes

Individual /Global Global Global

Percent

Degree C 20 20

Bus Input Data Bus

ID B1 B2 Bus4 Bus5 Bus6 Bus7 Bus8 UB

Type Load Load Load Load Load Load Load SWNG

Initial Voltage

Nom. kV 0.240 2.300 0.240 2.300 2.300 2.300 2.300 13.200

Base kV 0.240 2.300 0.240 2.300 2.300 2.300 2.300 13.200

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

Sub-sys 1 1 1 1 1 1 1 1

%Mag. 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

Ang. 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Branch Connections CKT/Branch

Connected Bus ID

% Impedance, Pos. Seq., 100 MVAb

ID

Type

From Bus

To Bus

R

X

Z

Y

T1

2W XFMR

UB

B1

53.48

379.58

383.33

T2

2W XFMR

UB

B2

11.74

142.52

143.00

CB6

Tie Breaker

B1

Bus4

CB7

Tie Breaker

B2

Bus5

CB8

Tie Breaker

B2

Bus6

CB9

Tie Breaker

B2

Bus7

CB10

Tie Breaker

B2

Bus8

Power Grid Input Data % Impedance 100 MVA Base

Power Grid

Connected Bus

Rating

ID

ID

MVASC

kV

X/R

R

X

U1

UB

720.000

13.200

99999

0.00014

13.88889

Total Connected Power Grids ( = 1 ): 720.000 MVA

Induction Machine Input Data Induction Machine

Connected Bus

Rating

X/R Ratio

% Impedance (Motor Base)

SHORT - CIRCUIT REPORT


B1

Prefault voltage = 0.240 =

=

100.00% of nominal bus kV ( 0.240 kV ) 100.00% of base ( 0.240 kV )

Contribution

1/2 Cycle

From Bus

To Bus

ID

ID

B1

%V

kA

kA

Imag.

kA Symm.

Real

Imaginary

/Real

Magnitude

Total

From Bus 0.00

10.893

-67.489

6.2

68.362

UB

B1

96.57

8.165

-60.047

7.4

60.600

L1

B1

100.00

2.577

-6.134

2.4

6.653

M1

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 breaker If faulted bus is involved in loops formed by protection devices, the short-circuit contribution through these PDs will not be reported



B4

Prefault voltage = 0.240

= =


Contribution

1/2 Cycle

From Bus ID Bus4

To Bus ID Total

%V From Bus 0.00

kA Real 10.893

kA Imaginary -67.489

Imag. /Real 6.2

kA Symm. Magnitude 68.362

M1 UB L1

Bus4 B1 B1

100.00 96.57 100.00

0.150 8.165 2.577

-1.307 -60.047 -6.134

8.7 7.4 2.4

1.316 60.6 6.653

*B1

Bus4

0.00

10.743

-66.181

6.2

67.048



UB

Prefault voltage = 13.200

=


Momentary Duty Summary Report

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

Bus

Device

Momentary Duty

Device Capability

ID

kV

ID

Type

Symm. kA rms

X/R Ratio

M.F.

Asymm kA rms

Asymm. kA Crest

B1

0.240

B1

Bus

68.362

6.9

1.342

91.741

157.859

B2

2.300

B2

Bus

17.840

13.1

1.496

26.68

45.067

Bus4

0.240

Bus4

Bus

68.362

6.9

1.342

91.741

157.859

UB

13.200

UB

Bus

31.901

999.9

1.728

55.139

90.088

Symm kA rms

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 d evice capability

Interrupting Duty Summary Report

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

Bus

Device

Interrupting Duty

Device Capability

Asymm. kA rms

Asymm kA Crest

C. ANSI/IEEE Short Circuit Method (for Manual Calculation)

Impedance Diagram Development @ 100 MVA base

XU R T2

R T1 XT1 R M1 XM1

where:

B2

B1

XT2 R M3

R M2

R L XL

XM3

XM2

R M4 XM4

R M5 XM5

MOTOR (MVA) = HP(0.746) (Eff)(pf)(100) 500(0.746) M2 = (1000)(0.9325)(0.9082) M2 = 0.440 MVA

Figure 6: Impedance Diagram

 j

1.

XU

2.

R T1



3.

X T1



4.

R T2



5

X

100

 j 0.13889 720 100  0.575 cos tan 7.1  0.53463 1.5 100  0.575 sin  tan 7.1  j 3.79587 1.5 100  0.0715 cos tan 12.14  0.11739 5 100  0 0715 sin tan 12 14 j1 42157

Solving for ½ cycle 3 phase fault at 

Z T B1

 Z

T2

B1

  Z M2 Z M3 Z M4 Z M5   Z U   Z T1  Z M1 Z L

 0.56271  j3.44496  3.49062 80.72



I T3 B

1

100 x 10

6

 68.9 kA

3  240  3.49062 

Solving for ½ cycle, 3phase fault at

B2

 Z M2 Z M3 Z M4 Z M5

Z T B2

 Z

M1

Z L   Z T1   Z U   Z T2 

 0.10787  j1.40240  1.40655 85.6

I T3 B

 2

100 x 10 6 3  2300 1.40655 

 17.85 kA

IEC Short Circuit Calculation

1. XUK 2. R T1K K

 j0.13889  0.534630.965  0.51592  0.965 asper60909IECformula C

 

Solving for 3 phase fault at Z TK B1





B1

, I"K

Z T2K   Z M2K Z M3 K Z M4K Z M5K  Z UK   Z T1K  Z M1K Z L

 0.54641  j3.22821  3.27412 80.39

I"K B

 1

100 x 10 6 1.1 3  240  3.27412 

Solving for 3 phase fault at Z TK B 2

I"K B

, I"K

 Z M2K Z M3K Z M4K Z M5K



 0.10307

 1.32948 85.6

 2

B2

 80 kA

 j1.32547

100 x 10 6 1.1 3  2300 1.32948 

Z M1K Z LK   Z T1K 

Z UK   Z T2K 

 17.85 kA

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

VIII.

Protective Devices Selection and evaluation

X/R Ratio for Breaker Evaluation

The 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 the circuit breaker withstands and interrupting time duties. ANSI/IEEE C37.010-1999 recommends a separate R and jX network reduction to determine the fault point X/R ratio while IEC 61909 allows several methods to provide a conservative X/R ratio. The peak current calculation that yields a very close approximation to the exact peak current and is conservative for most values of circuit X/R ratios greater than 0.81. The non-conservative errors for circuit X/R ratio around 10 are negligible. Please refer to equation below:     X / R Half cycle  I ac peak 1  e  or  

ANSI / IEEE

    X / R  2 I ac rms 1  e   

3    X / R Half cycle  I ac peak 1.02  0.98 e  or  

IEC 60909 3    X / R  2 I ac rms 1.02  0.98 e   

Based on NEMA AB1/UL489, the circuit breakers interrupting capacity, lagging power factor should be in accordance with Table 6. Table 6: Circuit Breakers Interrupting Capacity

MCCB and ICB Power Circuit Breaker (Unfuse) Power Circuit Breaker (Fuse)

IX.

kA I ≤ 10 10 < I ≤ 20 20 < I All All

lagging pf 0.45 – 0.5 0.25 – 0.30 0.15 – 0.20 0.15 0.20

X/R 1.98 – 1.73 3.87 – 3.17 6.59 – 4.90 6.59 4.90

Findings and Results

From the result of ETAP Total Bus Fault Short Circuit Study, the following results were found: Table 7: IEC Short Circuit Calculation

Bus ID Device B1

CB2

Device Capacity (kA) Making I b sym I b asym Peak 176

80

80.426

Short Circuit Calculation Result (kA) I" b

i p

I b sym

I b asym

79.8

181.348*

77.219

79.188

Note: Method A, Total Bus Fault

From the data in Table 7, the calculated short circuit current level of 79.8kA (I” k ) is within the circuit breaker capacity of 80 kA (I b sym), however, other parameters such as peak short circuit current ( i p) is 181.348 kA exceeded the circuit breaker rating equivalent to 176 kA (making peak) only. Therefore, the selected IEC rated

MF 

1 e 1 e



 R       X  C  R       X  T

1 e 1e

 1       6.9  C  1       4.9  T

 1.07

Where:

 R     X  T

 R     X  C

MF

X.

- Break test R/X ratio - Calculated R/X ratio at the point fault - Multiplying factor

Conclusion and Recommendation

From the above short circuit calculation examples, IEC method shows a higher value of short circuit current as compared to ANSI/IEEE calculation method. This is due to the differences in the consideration as mentioned above. 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 power factor or X/R ratio based on their product standard (e.g. UL/NEMA/ANSI or IEC). The example above

XI. References 1 IEEE Std 551-2006, IEEE Recommended Practice for Calculating Short-Circuit currents in Industrial and Commercial Power System. 2 IEEE Papers, Simplifying IEEE/ANSI and IEC Fault Point X/R Ratio for Breaker Evaluation by Ketut Dartawan and Conrad St. Pierre 3 IEC 60497-1:2009, Low-voltage switchgear and control gear - Part 1: General rules 4 IEC 60497-2:2009, Low-voltage switchgear and control gear - Part 2: Circuit-breakers 5 ANSI C37.5-1989, Calculation of Fault Currents for Application of Power Circuit Breakers Rated on a Total Current Basis 6 UL 489-1986, Molded Case Circuit Breaker and Circuit Breakers Enclosure 7 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

Appendix Courtesy of ABB Phil., Inc.

proM Compact

Technical features

S 200

of MCBs S 200 series

Series

S 200

S 200 M

Characteristics

B,C,D

B,C,D

Rated current

[A]

Breaking capacity

[kA]

Reference standard IEC 23-3/EN 60898 IEC/EN 60947 - 2 Alternating current

Nr. Poles lcs lcu

1, 1P + N 2,3,4 2,3,4

lcs

1, 1P + N

S 200 P B,C,D

B,C,D

S 280 B,C,D

B,C

K,Z

K,Z

K,Z

K,Z

0.5 ≤ ln ≤ 63

0.5 ≤ ln ≤ 63

0.2 ≤ ln ≤ 25

32 ≤ ln ≤ 40

50 ≤ ln ≤ 63

80≤ LN ≤ 100

6

10

25

15

15

6

133 230 230 400 500 690

20 10 20 10

25➇ 15➇ 25➇ 15➇

40 25 40 25

25 15 25 15

25 15 25 15

15 6 10 6

133

15

18.7 ➇

20

18.7

18.7

15

Ue [V] 230 / 400

30

K,Z

(continued …)

2,3,4 2,3,4

7.5 15 ➀ 7.5

11.2 ➇ 18.7 ➇ 11.2 ➇

12.5 20 12.5

11.2 18.7 11.2

7.5 18.7 7.5

6 10 6

20 10

10

15

10

10

10

20 10

10

15

10

10

10

24 60 125 250

20 10

10

15

10

10

10

48 125 250 500 600 800 375

20 10

10

15

10

10

10

230 230 400 500 690

IEC/EN 60497 - 2 Direct current T= lR≤ 5ms for all series except S280 UC and S800S-UC where T = lR <15ms

lcu

1, 1P + N

2

3,4

lcs

1, 1P + N

2

3,4

24 60 125 250 48 125 250 500 600 800 375 500 750 1000 1200

31

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

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