06 - Short Circuit_ANSI

Short-CircuitANSI StandardANSI Standard

© 1996-2010 Operation Technology, Inc. – Workshop Notes: Short-Circuit ANSI

Short-Circuit AnalysisTypes of SC Faults

•Three-Phase Ungrounded FaultTh Ph G d d F lt•Three-Phase Grounded Fault

•Phase to Phase Ungrounded Fault•Phase to Phase Grounded Fault•Phase to Ground Fault•Phase to Ground Fault

Fault Current•IL G can range in utility systems from a few percent toIL-G can range in utility systems from a few percent to possibly 115 % ( if Xo < X1 ) of I3-phase (85% of all faults).•In industrial systems the situation I > I is rare•In industrial systems the situation IL-G > I3-phase is rare. Typically IL-G ≅ .87 * I3-phase

•In an industrial system, the three-phase fault condition f f

© 1996-2010 Operation Technology, Inc. – Workshop Notes: Short-Circuit ANSI Slide 2

is frequently the only one considered, since this type of fault generally results in Maximum current.

Purpose of Short-Circuit St diStudies• A Short-Circuit Study can be used to determineA Short Circuit Study can be used to determine

any or all of the following:

V if t ti d i l d l t h bilit– 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


– Selecting ratings or settings for relay coordination

System Components I l d i SC C l l tiInvolved in SC Calculations• Power Company SupplyPower Company Supply

• In-Plant Generators

• Transformers (using negative tolerance)

• Reactors (using negative tolerance)

• Feeder Cables and Bus Duct Systems (at lower temperature limits)


p )

System Components I l d i SC C l l tiInvolved in SC Calculations• Overhead Lines (at lower temperature limit)Overhead Lines (at lower temperature limit)

• Synchronous MotorsSynchronous Motors

• Induction Motors

• Protective Devices

• Y0 from Static Load and Line Cable


Elements That Contribute C t t Sh t Ci itCurrent to a Short-Circuit• Generator• Generator

• Power Grid

• Synchronous Motors

• Induction MachinesInduction Machines

• Lumped Loads(with some % motor load)(with some % motor load)

• Inverters


• I0 from Yg-Delta Connected Transformer

Elements Do Not Contribute C t i P St tiCurrent in PowerStation• Static Loads• Static Loads

• Motor Operated ValvesMotor Operated Valves

• All Shunt Y Connected Branches


Short-Circuit Phenomenon

i(t)v(t)

)tSin(Vmv(t) θω +∗=

i(t)v(t)


i(t)v(t) i(t)( )

(1) ) t Sin(Vmdtdi L Riv(t) +×=+= θω

tVmVm

dt

R-

expression following theyields 1equation Solving

4444 34444 21444 3444 21

Off t)(DCTransientState Steady

t)-sin(

ZVm)-tsin(

ZVmi(t) Le××++×= φθφθω


Offset)(DC

AC Current (Symmetrical) with No AC DecayNo AC Decay

DC CurrentDC Current


AC Fault Current Including the DC Offset (No AC Decay)


Machine Reactance ( λ = L I )

AC Decay CurrentAC Decay Current


Fault Current Including AC & DC Decay


ANSI Calculation Methods1) The ANSI standards handle the AC Decay by varying machine impedance during a fault.

ANSI

2) The ANSI standards handle the the dc offset by applying multiplying factors. The ANSI Terms for this current are:

•Momentary Current•Close and Latch CurrentFirst C cle As mmetrical C rrent


•First Cycle Asymmetrical Current

Sources and Models of Fault C t i ANSI St d d

S

Currents in ANSI StandardsSources•Synchronous Generators•Synchronous Motors & CondensersI d ti M hi•Induction Machines

•Electric Utility Systems (Power Grids)

ModelsAll sources are modeled by an internalvoltage behind its impedancevoltage behind its impedance.

E = Prefault VoltageR = Machine Armature Resistance


X = Machine Reactance (X”d, X’d, Xd)

Synchronous GeneratorsSynchronous Generators are modeled in three stagesin three stages.

Synchronous Motors & CondensersCondensersAct as a generator to supply fault current. This current diminishes as the magnetic field in the machine decays.

Synchronous Reactance

Induction MachinesTreated the same as synchronous motors except they do not contribute to

Transient Reactance

Subtransient Reactance

motors except they do not contribute to the fault after 2 sec.

Electric Utility SystemsTh f lt t t ib ti t d t


The fault current contribution tends to remain constant.

½ Cycle Network

This is the net ork sed to calc late momentar short circ it c rrentThis 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 Network1 ½ to 4 Cycle Network

This network is used to calculate the interrupting short-circuit current and protective device duties 1 5 4 cycles after the faultand 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.


Reactance Representation forUtility and Synchronous Machine

½ Cycle 1 ½ to 4 Cycle 30 Cycle

Utility X”d X”d X”d

Turbo Generator X”d X”d X’dTurbo Generator

Hydro-Gen with Amortisseur

winding

X”d X”d X’d

winding

Hydro-Gen without Amortisseur

winding

0.75*X”d 0.75*X”d X’d

Condenser X”d X”d α

Synchronous X”d 1 5*X”d


Synchronous Motor

X”d 1.5*X”d α

½ Cycle 1 ½ to 4

Reactance Representation for Induction Machine

½ Cycle 1 ½ to 4 Cycle

>1000 hp , <= 1800 X”d 1.5*X”drpm

>250, at 3600 rpm X”d 1.5*X”d

All others, >= 50 hp 1.2*X”d 3.0*X”d

< 50 hp 1.67*X”d α

Note: X”d = 1 / LRCpu


Device Duty and Usage of Fault Currentsfrom Different Networks

½ Cycle Currents(Subtransient

Network)

1 ½ to 4 Cycle Currents

(Transient Network)

HV Circuit Breaker Closing and LatchingCapability

InterruptingCapability

LV Circuit Breaker Interrupting Capability ---

---Fuse Interrupting

Capability

SWGR / MCC Bus Bracing ---

Relay Instantaneous Settings

---


30 Cycle currents are used for determining overcurrent settings.

Momentary Multiplying F tFactor

MF is calculated based on:MFm is calculated based on:• Fault X/R (Separate R & X Networks)• Location of fault (Remote / Local generation)

SC Current Duty Device Rating

HV CB Asymmetrical RMS C&L RMS

Comparisons of Momentary capability (1/2 Cycle)

yCrest C&L RMS

HV Bus Asymmetrical RMSCrest

Asymmetrical RMSCrest


LV Bus Symmetrical RMSAsymmetrical RMS

Symmetrical RMSAsymmetrical RMS

Interrupting Multiplying F tFactor

MFi is calculated based on:F lt X/R (S t R & X N t k )• Fault X/R (Separate R & X Networks)

• Location of Fault (Remote / Local generation)• Type and Rating of CB

SC Current Duty Device Rating

Comparisons of Interrupting Capability (1 ½ to 4 Cycle)

HV CBAdj. Symmetrical RMS* Adj. Symmetrical RMS*


LV CB & FuseAdj. Symmetrical RMS*** Symmetrical RMS

HV CB Closing and L t hi D tLatching Duty

Calculate ½ Cycle Current (Imom, rms, sym) using ½ Cycle Network.

• Calculate X/R ratio and Multiplying factor MFm

• Imom, rms, Asym = MFm * Imom, rms, sym


HV CB Interrupting Duty

Calculate 1½ to 4 Cycle Current (Imom, rms, sym) using ½ Cycle Network.

• Determine Local and Remote contributions (A “local” contribution is fed predominantly from generators through no more than one transformation or with external reactances in series that is less thantransformation or with external reactances in series that is less than 1.5 times generator subtransient reactance. Otherwise the contribution is defined as “remote”).

• Calculate no AC Decay ratio (NACD) and multiplying factor MFi

NACD = IRemote / ITotale o e o aITotal = ILocal + IRemote

(NACD = 0 if all local & NACD = 1 if all remote)


• Calculate Iint, rms, adj = MFi * Iint, rms, Symm

HV CB Interrupting C bilit• CB Interrupting kA varies between Max kA and Rated kA

Capabilityg

as applied kV changes – MVAsc capability.

• ETAP’s comparison between CB Duty of AdjETAP s comparison between CB Duty of Adj. Symmetrical kA and CB capability of Adjusted Int. kA verifies both symmetrical and asymmetrical rating.

• The Option of C37.010-1999 standard allows user to specify CPT.

• Generator CB has higher DC rating and is always compared against maximum through SC kA


compared against maximum through SC kA.

LV CB Interrupting Duty

• LV CB take instantaneous action.

• Calculate ½ Cycle current I (I’ ) from the ½• Calculate ½ Cycle current Irms, Symm (I f) from the ½ cycle network.

C / ( C )• Calculate X/R ratio and MFi (based on CB type).

• Calculate adjusted interrupting current Iadj rms symm = j p g adj, rms, symm MFi * Irms, Symm


Fuse Interrupting Duty

Calculate ½ Cycle current I from ½ Cycle NetworkCalculate ½ Cycle current Iint, rms, symm from ½ Cycle Network.

S d t l l t I f CB• Same procedure to calculate Iint, rms, asymm as for CB.


LL--G FaultsG Faults


L-G Faults

Symmetrical Components


Sequence Networks


L-G Fault Sequence N t k C tiNetwork Connections

I3I ×=

V3I

I3I

efaultPr

af 0

×=

×=

0ZZZ

I021

f

=++

=

Zif 0gZif


L-L Fault Sequence Network C tiConnections

aa

V3

II12

×

−=

21

efaultPrf ZZ

V3I+

×=


L-L-G Fault Sequence N t k C tiNetwork Connections

I0III aaaa 012==++

ZZZ

VI20

efaultPrf

⎟⎞

⎜⎛

=

0ZZ

Z20

201 ⎟⎟

⎠⎜⎜⎝ +

+

Zif 0=gZif


Transformer Zero Sequence Connections


Solid Grounded Devices d L G F lt

theisfault phase-3 aGenerally and L-G Faults

: ifgreater becan faultsG -L case. severemost

I: then trueare conditions thisIf

& 1021 <=

I

ZZZZ

solidlareertransformConnectedY/or Generators if case thebemay This

I 1f3

Δ

< φφ fI

grounded.solidlyareer transformConnectedY/Δ


Unbalanced Faults Display & R t

Complete reports that include individual branch contributions for:

& Reports

•L-G Faults

•L-L-G Faults

•L-L FaultsL L Faults

One-line diagram displayed results that include:include:

•L-G/L-L-G/L-L fault current contributions

•Sequence voltage and currents


•Sequence voltage and currents

•Phase Voltages



SC Study Case Info Page


SC Study Case Standard PPage


SC Study Case Adjustments P

Tolerance Adjustments

PageLength Adj t t

•Transformer Impedance

•Reactor R i t

Adjustments

•Cable Length

•Transmission Li L thResistance

•Overload Heater Resistance T t

Line Length

Resistance Temperature Corrections

•Transmission Line Resistance

Adjust Fault Impedance Line Resistance

•Cable Resistance•L-G fault Impedance


Tolerance Adjustments

ToleranceZZ T fT f )1(*' ±=

T lL thL thToleranceLengthLength

ToleranceZZ

CableCable

rTransformerTransforme

)1(*')1(*'

)1(

±±=

±=

ToleranceLengthLength onLineTransmissionLineTransmissi )1(*' ±=

Positive tolerance value is used for IEC Minimum If calculation.f

Negative tolerance value is used for all other calculations.

Adjustments can be applied Individually or Globally


Temperature Correction

TcRR BASEC+

=)5.234(*''

TbTcRR

TbRR

BASEAlumi

BASECopper

++

=

+=

)1228()1.228(*'

)5.234(

retempereatubaseatResistanceR

Tb

BASE =

+ )1.228(

C in etemperatur base ConductorTbetemperatur operating at ResistanceR'

pBASE

==

C in limit etemperatur ConductorTc =

Temperature Correction can be applied


Temperature Correction can be appliedIndividually or Globally

System for SC StudyPower Grid U1X/R = 55

Gen1Voltage Control

Transformers

gDesign Setting:%Pf = 85MW = 4 Max Q = 9Min Q = 3T1 X/R

PS =12PT =12ST =12T2 X/R = 12

Min Q = -3

T2 X/R 12

Lump1Y open grounded


System for SC Study

Tmin = 40, Tmax = 90


System for SC Study


Short-Circuit Alerts

• Bus Alert

• Protective Device Alert

• Marginal Device Limit


Bus SC Rating

Type of Device Monitored Parameter Condition Reported

MV Bus (> 1000 Volts)Momentary Asymmetrical. rms kA Bracing Asymmetrical

Momentary Asymmetrical crest kA Bracing Crest

Bus SC Rating

Momentary Asymmetrical. crest kA Bracing Crest

LV Bus (<1000Volts)Momentary Symmetrical. rms kA Bracing Symmetrical

Momentary Asymmetrical. rms kA Bracing Asymmetrical

P t ti D i R tiDevice Type ANSI Monitored Parameters IEC Monitored Parameters

LVCB Interrupting Adjusted Symmetrical. rms kA Breaking

Protective Device Rating

HV CB

Momentary C&L MakingMomentary C&L Crest kA N/A

Interrupting Adjusted Symmetrical. rms kA Breaking

Fuse Interrupting Adjusted Symmetrical. rms kA Breaking

SPDT Momentary Asymmetrical. rms kA MakingSPST Switches Momentary Asymmetrical rms kA Making


SPST Switches Momentary Asymmetrical. rms kA Making

3-Phase Duty SC ResultsRun a 3-phase Duty SC calculation for a fault on Bus4. The display shows the Initial Symmetrical Short-Circuit Current.


Unbalance Fault Calculation


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