FAULT LEVEL CALCULATION

Fault Level

• Maximum Apparent power developed under dead short circuit condition

Sub Transient Reactance (Xd”):

• Sub Transient Reactance (Xd”): Reactance during the first Half cycle (It is least and SC Current is high because the flux crossing the air gap is high during first half cycle.

• Xd” → 20%• Valid for T< 100 mili Second• Used in Fault level calculation• Used in Breaker & insulator Rating calculation

Transient Reactance (Xd’)

• After first cycle stage, the reactance is called sub transient reactance

• Xd’ → 25%

• Valid for 0.1<T<1.0 sec– In this stage breaker contacts are operated– Used in relay coordination calculation– Used in motor starting

Synchronous Reactance (Xd)

• The reactance after transient dies out & the current reaches a steady sinusoidal state, called direct axis synchronous reactance

• Xd → 200%• Valid for T>>1 Sec

– Xd= If (at rated FLC on sustained symmetrical SC) / (If at Open Circuit Voltage on the air gap line)

– Xd > Xd’ > Xd’’

• In Generator Fault current does not fall to zero but reaches steady state value = 1.0/Xd

• In motors fault current falls to nearly zero after 200 to 300 mili sec

• Motor contribution For Breaker sizing calculations– Significant for make duty (Currents that breaker has

to carry after ½ cycle – Less significant for break duty: Current that breaker

has to break at around 5 cycles– Motor Contribution for relay coordination studies :

Insignificant and can be ignored

Effect of Xd

• High Xd -- Voltage Regulation is poor (Greater changes in voltage under fluctuations)

• High Xd -- Lower Stability limit

Max O/P Power = EV/Xd

• High Xd -- Lower synchronizing power

=EV Cosδ/Xd

Effect of Xd

• High Xd: - This is lead to large voltage on Open circuit produced by self excitation owing to large capacitive currents drawn by the transmission line

• Small Xd-- High value of Short Circuit current• Small Xd:-Has Long air gap and large

reluctance which means that the mmf required by field is large and M/c having lower Xd is costlier to build.

Contribution of Xd and Xq in fault level

Short circuit current of the alternator lags behind the voltage by 90’, the reactance involved are only direct axis reactance

Terminology in Fault Level Calculation

• Base MVA • Base Impedance• PU value = Actual Value of the quantity (in any ckt)

/ The Base value in the same circuit=% Quantity/100

• P.U. Impedance = Actual Imp/ Base Imp• PU2 = PU1 x Base MVA2 / Base MVA1• Base Current = Base MVA/ √Base Voltage

Motor Contribution Calculation

• Name Plate :- KW, PF, V, FLC &Efficiency• Starting Current = 6 x FLC• MVA = KW/ (PF x Efficiency)• Fault MVA = 6 x Rated MVA• % Motor Imp = Base MVA / (Motor

MVA Contribution)

Formulas in Fault Level Calc

• Addition of Series Impedance Zeqt

= Z1+Z2+Z3+……..

• Addition of Parallel Impedance 1/Zeqt

= 1/Z1 + 1/Z2 + 1/Z3 + …….

Procedure of fault Level Calculation

1. Draw SLD showing rating & impedance

2. Select Base MVA

3. Convert all impedances in pu on common base

4. Draw impedance diagram with Imp. In pu

5. Find equivalent Imp

6. Evaluate Fault MVA = Base MVA/Zpu

7. Fault Current on the Bus = Fault MVA/ √3 x Bus Volt

Thanks