Reference:: Etap Training By Oti Etap Help User

Reference : ETAP Training by OTI ETAP Help User

Short-Circuit Analysis 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

Fault Current •IL-G can range in utility systems from a few percent to possibly 115 % ( if Xo < X1 ) of I3-phase (85% of all fault s). •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 conditio n is frequently the only one considered, since this typ e of fault generally results in Maximum current.

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

System Components Involved in SC Calculations • Power Company Supply • In-Plant Generators

• Transformers (using negative tolerance) • Reactors (using negative tolerance)

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

System Components Involved in SC Calculations • Overhead Lines (at lower temperature limit) • Synchronous Motors • Induction Motors • Protective Devices

Elements That Contribute Current to a Short-Circuit • Generator • Power Grid • Synchronous Motors • Induction Machines • Lumped Loads (with some % motor load) • Inverters

Elements Do Not Contribute Current in PowerStation • Static Loads • Motor Operated Valves

• All Shunt Y Connected Branches

Short-Circuit Phenomenon

i(t)

v(t)

v(t) Vm Sin( t

)

v(t)

i(t)

di Vm Sin( t ) (1) dt Solving equation 1 yields the following expression v(t)

Ri L

i(t)

Vm sin( t Z SteadyState

- )

Vm sin( - ) Z Transient (DC O ffset)

t

AC Current (Symmetrical) with No AC Decay

DC Current

AC Fault Current Including the D C Offset (No AC Decay)

Machine Reactance ( λ = L I )

AC Decay Current

Fault Current Including AC & DC Decay

ANSI Calculation Methods 1) 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 Current •First Cycle Asymmetrical Current

Sources and Models of Fault Currents in ANSI Standards Sources •Synchronous Generators •Synchronous Motors & Condensers •Induction Machines •Electric Utility Systems (Power Grids)

Models All sources are modeled by an internal voltage behind its impedance. E = Prefault Voltage R = Machine Armature Resistance X = Machine Reactance (X”d, X’d, Xd)

Synchronous Generators Synchronous Generators are modeled in three stages.

Synchronous Motors & Condenser s Act as a generator to supply fault curre nt. This current diminishes as the mag netic field in the machine decays.

Induction Machines

Transient Reactance

Treated the same as synchronous mot ors except they do not contribute to the fault after 2 sec.

Subtransient Reactance

Electric Utility Systems

Synchronous Reactance

The fault current contribution tends to r emain constant.

½

Cycle Network

This is the network used to calculate momentary short-circuit current an d 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-circuit current an d 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 curre nt and settings for over current relays after 30 cycles.

Reactance Representation for Utility and Synchronous Machine ½ Cycle

1 ½ to 4 Cycle

30 Cycle

X”d

X”d

X”d

X”d

X”d

X’d

Hydro-Gen with Am ortisseur winding

X”d

X”d

X’d

Hydro-Gen without Amortisseur windin g

0.75*X”d

0.75*X”d

X’d

X”d

X”d

X”d

1.5*X”d

Utility

Turbo Generator

Condenser

Synchronous Motor

Reactance Representation for Induction Machine ½ Cycle

1 ½ to 4 Cyc le

>1000 hp , <= 1800 r pm

X”d

1.5*X”d

>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

Note: X”d = 1 / LRCpu

Device Duty and Usage of Fault Currents from Different Networks ½ Cycle Currents (Subtransient Network)

1 ½ to 4 Cycle Currents (Transient Network)

HV Circuit Breaker

Closing and Latching Capability

Interrupting Capability

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 Factor

MFm is calculated based on:

• Fault X/R (Separate R & X Networks) • Location of fault (Remote / Local generation) Comparisons of Momentary capability (1/2 Cycle) SC Current Duty

Device Rating

HV CB

Asymmetrical RMS Asymmetrical Crest

C&L RMS C&L RMS

HV Bus

Asymmetrical RMS Asymmetrical Crest

Asymmetrical RMS

Symmetrical RMS Asymmetrical RMS

Symmetrical RMS Asymmetrical RMS

LV Bus

Crest

Interrupting Multiplying Factor

MFi is calculated based on:

• Fault X/R (Separate R & X Networks) • Location of Fault (Remote / Local generatio n) • Type and Rating of CB

Comparisons of Interrupting Capability (1 ½ to 4 Cycle) SC Current Duty

Device Rating

Adj. Symmetrical RMS*

Adj. Symmetrical RMS*

Adj. Symmetrical RMS***

Symmetrical RMS

HV CB

LV CB & Fuse

HV CB Closing and Latching Capability 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

MV CB Interrupting Capability Calculate 1½ to 4 Cycle Current (Imom, rms, sym) using ½ Cycle Network. • Determine Local and Remote contributions (A “local” contribution is f ed predominantly from generators through no more than one transfo rmation or with external reactances in series that is less than 1.5 tim es generator subtransient reactance. Otherwise the contribution is d efined as “remote”). • Calculate no AC Decay ratio (NACD) and multiplying factor MFi NACD = IRemote / ITotal ITotal = ILocal + IRemote (NACD = 0 if all local & NACD = 1 if all remote) • Calculate Iint, rms, adj = MFi * Iint, rms, Symm

LV CB Interrupting Capability • LV CB take instantaneous action. • Calculate ½ Cycle current Irms, Symm (I’f) from the ½ cycle network. • Calculate X/R ratio (IEEE method or ETAP method) and MFi (based on CB type). • Calculate adjusted interrupting current Iadj, rms, symm = MFi * Irms, Symm

Fuse Interrupting Capability Calculate ½ Cycle current Iint, rms, symm from ½ Cycle Network.

• Same procedure to calculate Iint, rms, asymm as for CB.

L-G Faults

L-G Faults Symmetrical Components

Sequence Networks

L-G Fault Sequence Network Connections If

3 Ia 0

If

3 VPrefault Z1 Z 2 Z0

if Zg

0

L-L Fault Sequence Network Connections Ia 2

I a1

If

3 VPrefault Z1 Z2

L-L-G Fault Sequence Network Connections Ia 2 If

I a1 I a 0

0 Ia

VPrefault Z0 Z 2 Z1 Z0 Z 2

if Zg

0

Transformer Zero Sequence Connections

Solid Grounded Devices and L-G Faults Generally a 3 - phase fault is the most severe case. L - G faults can be greater if : Z1 Z 2 & Z 0 Z1 If this conditions are true then : I f3 I f 1 This may be the case if Generators or Y/ Connected transformer are solidly grounded.

Thank You