Engineering Calculation for Power System Analysis

Purpose of Short-Circuit Studies: A Short-Circuit Study can be used to determine any or all of the following:  Verify protective device close and latch capability  Verify protective d

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Engineering Calculation for Power

System Analysis

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CONTENTS

1 Load Flow Calculation 3-16

Case1-ANSI/IEEE Method

Case2-IEC Method

Case1-Single Machine System

Case2-Multi Machine System

9 Arc Flash Analysis 152-164

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1 LOAD FLOW ANALYSIS

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INTRODUCTION:

Load flow solution analysis is essential for designing a new power system and planning of

the existing one for increased load demand, which determine the steady state operating

condition to calculate,

 Voltage Profile - its magnitude in kV or % of nominal kV

 Current flow throughout the System

 MVA and /or MW plus Mvar power flows throughout each branch of the (i.e

transformer, cables, line or series reactor etc) electrical system

 Voltage drop and Power factor

 Branch Losses i.e MW & Mvar losses on each branch

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Input data required for LFA:

Sl.no Component Required input

2 Power Grid Voltage rating, MVA sc, X/R ratio

3 Syn Generator 1 Swing (slack) – %v and del

2 Voltage Control (PV) – MW, Mvar limits

3 Mvar Control (PQ) -MW, Mvar and Var limits

4 PF Control-MW and PF

4 Transformer Py kV, Sec kV, MVA, %Z Positive sequence Impedance,

%Tap, Tolerance and LTC settings

5 Motor 1 Status- Continuous , Intermittent or Spare

2 Rating-HP and kV

6 Syn Motor 1 Status- Continuous , Intermittent or Spare

2 Rating-HP and kV

7 Static Load 1 Status- Continuous , Intermittent or Spare

2 Rating-kV, MW, Mvar and PF

8 Lump Load 1 Status- Continuous , Intermittent or Spare

2 Ratings- kV, MW, Mvar and PF

3 Load Type-Motor Load or Static load

2 Cable Type- Size, Insulation, kV and #/Cable

3 Impedance/conductor-Positive sequence

8 Transmission Line 1 Length in ft/m/mile/km

2 Parameter-Phase conductor

3 Impedance per phase-positive sequence

10 Protective Devices 2 Circuit Breakers-Rated kV

3 Fuses-Rated kV

4 Switches-Rated kV and amps

5 Contactors- Rated kV and amps

11 Capacitor 1) Status- Continuous , Intermittent or Spare

2) Rating-kV, Max kV, Mvar Bank and No of Banks

2) Impedance- Positive sequence Z and X/R

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STEP by STEP Procedure for Load Flow Analysis

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Step 2 : Input Parameters/data for LFA

Load Type = 100% Motor Load & 0% Static Load

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Step 4 : ETAP WITH HAND CALCULATION

1 To find Voltage Drop(Vd)

Formula: %Voltage drop= Delta V*100 - eq1

To find Delta V:

Delta V= (√ (Vr+ (RP+XQ/Vr) ^2+ (XP-RQ/Vr) ^2))-Vr - eq2

Where:

Vr = Receiving end voltage

R = Resistance of the cable/transmission in p.u

P = Real Power in MW

Q = Reactive Power in Mvar

X = Reactance of the cable in p.u S= Apparent power in p.u

We know that,

Base MVA = 100 MVA Base kV = 11 kV Nominal kV = 11 kV Load PF = 0.8

R Ω/km = 0.0801 Ω

X Ω/km = 0.12736 Ω Length = 0.836 km Assume Vr = 1 p.u

To find Z base:

Zbase = BasekV^2/Base MVA

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To find % voltage drop:

Note Vs ≠Vr so we are finding the new Vr new

Vr new=Vr -Delta V -eq3

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2 To find cable loss:

Formula:

MW =R*S^2*Base MVA*(Base kV/Nominal kV) ^2 - eq 4

Mvar = X*S^2*Base MVA*(Base kV/Nominal kV) ^2 - eq 5

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3 To find Transformer Losses

Formula:

MW =R*S^2*Base MVA*(Base kV/Nominal kV) ^2 - eq 6

Mvar = X*S^2*Base MVA*(Base kV/Nominal kV) ^2 - eq 7

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Step 5 : Run Load Flow Analysis with ETAP software

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Step 6: Summary Report

Hand Calculation ETAP Result

MW Mvar MW Mvar Cable Losses

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2 SHORT CIRCUIT ANALYSIS

(IEEE / ANSI AND IEC METHOD)

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CASE 1:-ANSI / IEEE METHOD INTRODUCTION:

The ETAP Short-Circuit Analysis program analyzes the effect of 3-phase, line-to-ground,

line-to-line, and line-to-line-to-ground faults on electrical distribution systems The program

calculates the total short circuit currents as well as the contributions of individual motors,

generators, and utility ties in the system Fault duties are in compliance with the latest

editions of the ANSI/IEEE Standards (C37 series)

Purpose of Short-Circuit Studies:

A Short-Circuit Study can be used to determine any or all of the following:

 Verify protective device close and latch capability

 Verify protective device Interrupting capability

 Protect equipment from large mechanical forces (maximum fault kA)

 I2t protection for equipment (thermal stress)

 Selecting ratings or settings for relay coordination

Elements that Contribute Current to a Short-Circuit:

 Inverters

 I0 from Yg-Delta Connected Transformer

Elements that do Not Contribute Current in PowerStation:

 Static Loads

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 All Shunt Y Connected Branches

Short-Circuit Phenomenon:

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Steady

t )

sin(

-Z

Vm )

t sin(

-Z

Vm i(t)

(1) )

t Sin(

Vm dt

di L Ri v(t)

L

R -

theyields1equation Solving

i(t)

v(t)

Symmetrical Faults:

The ETAP Short-Circuit Analysis program analyzes the effect of 3-phase

Symmetrical faults on electrical distribution systems The program calculates the total short

circuit currents as well as the contributions of individual motors, generators, and utility ties in

the system Fault duties are in compliance with the latest editions of the C37 series which

calculates

 Momentary symmetrical fault current in kA

 Interrupting symmetrical fault current in kA

Unsymmetrical Faults:

The ETAP Short-Circuit Analysis program also analyzes the effect of 3-phase

unsymmetrical Faults like

 Line to Ground fault

 Double line fault

 Double line to ground fault

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½ Cycle Network

This is the network used to calculate momentary short-circuit current and protective device

duties at the ½ cycle after the fault

1 ½ to 4 Cycle Network

This network is used to calculate the interrupting short-circuits current and protective device

duties 1.5-4 cycles after the fault

30-Cycle Network

This is the network used to calculate the steady-state short-circuit current and settings for

over current relays after 30 cycles of the fault

Device Duty and Usage of Fault Currents from Different Networks:

½ Cycle Currents (Sub transient Network)

1 ½ to 4 Cycle Currents (Transient Network)

HV Circuit Breaker Closing and Latching

Capability

Interrupting Capability

LV Circuit Breaker Interrupting Capability -

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ANSI Calculation Standard Compliance:

ETAP short circuit calculation per ANSI/IEEE Standards fully complies with the latest

ANSI/IEEE and UL Standards, as listed below:

IEEE C37.13 1990 Standard for Low-Voltage AC Power Circuit Breakers

Used in Enclosures IEEE C37.013 1997 Standard for AC High-Voltage Generator Circuit

Breakers Rated on a Symmetrical Current Basis IEEE C37.20.1 1993 2002 Standard for Metal Enclosed Low-Voltage Power

Circuit Breaker Switchgear IEEE Std 399 1990 & 1997 Power System Analysis – the Brown Book

IEEE Std 141 1986, 1993, 2002 Electric Power Distribution for Industrial Plants – the

Red Book IEEE Std 242 1986 & 2001 IEEE Recommended Practice for Protection and

Coordination of Industrial and Commercial Power Systems – the Buff Book

UL 489_9 1996, 2000, 2002 Standard for Safety for Molded-Case Circuit Breakers,

Molded-Case Switches, and Circuit-Breaker Enclosures

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Input data required for SCA:

1 Bus Nominal kv

3 Syn Generator 1 Swing – %v and del

2 Voltage Control – MW, Mvar limits

3 Mvar Control-MW, Mvar and Var limits

5 Imp/Model-Impedance(Xd’’,Xd’, Xd, Xo, X2 and X/R)

4 Transformer 1 Rating-Py kV, Sec kV, MVA

2 Impedance-%Z (+Ve & -Ve sequence Impedance)

3 Tolerance

4 Tap-Fixed Tap and LTC settings

5 Grounding-Py and Sec

2 Rating-HP and kV and select MFR details

3 Model-%LRC, %PF

4 Model-Parameters-X0, X/R

3 Model-Impedance(Xd’’,Xo,Xd’,X2, X/R)

9 Cable 1 Length in ft/m/mile/km

9 MOV 1 Rating-HP, kV and rated Torque

10 Protective Devices 1 Circuit Breakers-Rated kV, Amps, Interrupting kA

2 Fuses-Rated kV, Size, Amps, Interrupting kA

3 Switches-Rated kV , BIL ratings and amps

4 Contactors- Rated kV, Interrupting kA and amps

11 Capacitor 1 Status- Continuous , Intermittent or Spare

2 Rating-kV, Max kV, Mvar Bank and No of Banks

12 Impedance 1 Rating-Amps and kV

2 Impedance- Positive sequence Z and X/R

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Step 1: Build the single line diagram using ANSI / IEEE method

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Step 2 : Input Parameters/data for short circuit Analysis

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Step 3 : Apply Fault

Apply fault at the bus 4 by opening the short circuit study case editor and open the

motor side circuit breaker, because the Calculation 1 was done without motor contribution

First step to find Grid, Cable and Transformer reactance Sum of this will get the

Total reactance, using this we can find out the symmetrical fault current at the faulted bus

Calculation 1: Without motor contribution

Formulae:

I symm =MVAsc/√3*kV -eq 1 MVAsc=symm fault level *1.6 -eq 2 Symm fault level=1/ X TOTAL - eq 3

To find XTOTAL

X TOTAL = Xgrid+ Xcable+ X TFR - eq 4

i.e total reactance of all components in the system

Xgrid =Base MVA/MVAsc

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Total reactance

X TOTAL = Xgrid+ Xcable+ X TFR

=0.0013+0.00485+0.009587

X TOTAL =0.011485 -eq 5

To find Symmetrical fault level substitute 5 in eq 3

Symm fault level =1/ X TOTAL

=1/ XTOTAL =1/0.011485 =87.68MVA -eq 6

Then find MVAsc at the faulted bus

For this substitute eq 6 in eq 2

MVAsc=symm fault level *1.6

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Step 5 : Run Short Circuit Analysis without motor contribution

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Step 6: Build the single line diagram using ANSI / IEEE method

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Step 7: Compare hand calculation with ETAP software

Calculation 2: With motor contribution

Formulae:

Isymm=MVAsc/√3*kV -eq 8MVAsc=Base MVA/ XM -eq 9

XM = Xd1'' / rated input MVA -eq 10 Xd''=1/LRC p.u -eq12Where

MVAsc =Short Circuit MVA at the faulted bus

kV =Nominal kV at the faulted bus

Xm = Motor reactance

Xd'' = Dynamic reactance of the motor

LRC =Locked Rotor Current

Then find the reactance of the motor 1

X M1 = Xd 1 '' / rated input MVA

= 0.25/5.676

X M1 =0.0440

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Then find the MVA at the faulted bus

MVAsc = Base MVA/ X M1

= 1/0.0440

MVAsc=22.70

Substitute the above values in eq 8

Then to find the short circuit current by motor 1 contribution

Then find the reactance of the motor 2

X M2 = Xd2'' / rated input MVA

= 0.1818/1.282

X M2 =0.1418

Then find the MVA at the faulted bus

MVAsc = Base MVA/ X M2

= 1/0.1418

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MVAsc=7.051

Then to find the short circuit current by motor 2 contributions

KA m2 =MVAsc/ (√ (3)*kV)

=7.051 / (√ (3)*6)

KA m2 =0.678 kA, short circuit current due to motor2 contribution

Note:-Sum of all the short circuit current without motor contribution and with motor

contribution gives the total fault current in the faulted bus

From the above results w.k.t

Symmetrical fault current = 13.49 kA without motor contribution - eq 13

Short circuit current by motors = 2.858 kA ( KA m1 + KA m2) - eq 14

Sum of above two equations 13 & 14

We get the total Symmetrical fault current

KAsc Total =13.49+2.858

=16.45 kA

Symmetrical fault current (I symm ) =16.45 kA (with motor contribution)

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Step 8: Run short circuit Analysis with motor contribution

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Step 9: Comparison Table

With Contribution

Without contribution

With contribution

Without contribution

ANSI/IEEE

Method

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CASE 2:- IEC METHOD

INTRODUCTION:

The ETAP Short-Circuit Analysis program analyzes the effect of 3-phase, line-to-ground,

line-to-line, and line-to-line-to-ground faults on electrical distribution systems The program

calculates the total short circuit currents as well as the contributions of individual motors,

generators, and utility ties in the system Fault duties are in compliance with the latest

editions of the IEC Standards (IEC 60909 and others)

Purpose of Short-Circuit Studies:

A Short-Circuit Study can be used to determine any or all of the following:

 Verify protective device close and latch capability

 Verify protective device Interrupting capability

 Protect equipment from large mechanical forces (maximum fault kA)

 I2t protection for equipment (thermal stress)

 Selecting ratings or settings for relay coordination

Elements that Contribute Current to a Short-Circuit:

 Inverters

 I0 from Yg-Delta Connected Transformer

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Elements that do Not Contribute Current in PowerStation:

 Static Loads

 Motor Operated Valves

 All Shunt Y Connected Branches

IEC Short-Circuit Calculation (IEC 909):

 Initial Symmetrical Short-Circuit Current (I"k)

 Peak Short-Circuit Current (ip)

 Symmetrical Short-Circuit Breaking Current (Ib)

 Steady-State Short-Circuit Current (Ik) ETAP checks the protective device rated making and breaking capacities against the fault

currents and flags inadequate devices

Types of SC Faults

 Three-Phase Ungrounded Fault

 Three-Phase Grounded Fault

 Phase to Phase Ungrounded Fault

 Phase to Phase Grounded Fault

 Phase to Ground Fault

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Types of Short-Circuits

Near-To-Generator Short-Circuit

 This is a short-circuit condition to which at least one synchronous machine

contributes a prospective initial short-circuit current which is more than twice the

generator’s rated current, or a short-circuit condition to which synchronous and

asynchronous motors contribute more than 5% of the initial symmetrical

short-circuit current ( I"k) without motors

Far-From-Generator Short-Circuit

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 This is a short-circuit condition during which the magnitude of the symmetrical ac

component of available short-circuit current remains essentially constant

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IEC Calculation Standard Compliance:

Standard Pub Year Title

IEC 62271-100 2003 voltage switchgear and control gear – Part 100:

High-voltage alternating-current circuit breakers IEC 62271-200 2003 High-voltage switchgear and control gear – Part 200: AC

metal-enclosed switchgear and control gear for rated voltages above 1 kV and up to and including 52 kV

IEC 62271-203 2003 High-voltage switchgear and control gear – Part 203:

Gas-insulated metal-enclosed switchgear for rated voltages above

52 kV IEC 60282-2 1997 High-voltage fuses – Part2: Expulsion fuses

IEC 61363-1 1998 Electrical installations of ships and mobile and fixed offshore

units – Part 1: Procedures for calculating short-circuit currents

in three-phase a.c

IEC 60909-0 2001 Short-Circuit Currents in Three-phase a.c systems - Part 0:

Calculation of Currents (including 2002 corrigendum 1) IEC 60909-1 2002 Short-circuit currents in three-phase a.c systems - Part 1:

Factors for the calculation of short-circuit currents according

to IEC-60909-0 IEC 60909-2 1992 Electrical equipment - Data for short-circuit current

calculations in accordance with IEC 909 (1988) IEC 60909-4 2000 Short-circuit currents in three-phase a.c systems Part 4:

Examples for the calculation of short-circuit currents IEC 60947-1 2004 Low voltage switchgear and control gear, Part 1: General

rules IEC 60947-2 2003 Low voltage switchgear and control gear, Part 2: Circuit

breaker

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