FAULT LEVEL CALCULATION, ENGINEERING PROJECTS, ELECTRICAL EQUIPMENT RATING DESIGNDESIGN FACTORS CONSIDERED

A)The tolerance on 6.6kV/0.433kV Transformer impedance is taken as5% at principal tap. Actual minimum impedance at the lowest tap of other transformers is considered.B)Impedance of cables is neglected.

C)First cycle asymmetrical peak short circuit (S/C) current (motor contribution as well as grid incomer)= 2.55symmetrical RMS first cycle S/C current.

D)A factor of 1.1 is taken into account for the increase in source fault feeding voltage. A factor of 1.06 is also taken into account for persistent system overvoltage during fault conditions for 6.6kV motor contribution.

E)Subtransient Impedance of motors, Zdis calculated on the following basis:

VOLTAGE LEVELMOTOR IDSUBTRANSIENT IMPEDANCE (Zd)

6.6KV

ALL MOTORS

415VLumped motor Load (having rating >= 37KW

F)The tolerance on generator reactance (Xd) is taken as-15% as per IEC-34.

G)Power factor is taken as 0.85 for 6.6kV drives and 0.8 for 415V drives.H)Efficiency is taken as 0.9 for 6.6kV motors and 0.85 for 415V motors..

I)The values of Zmom(momentary) & Zintt(interrupting)for motors arederived as under

Zmom = 1.2 xZd (For motor 750 kWand below)Zintt= 3.0 xZd (For motor 750 kWand below)Zmom = 1 xZd (For motorabove750 kW)Zintt= 1 xZd (For motorabove750 kW)415V FAULT LEVEL:Basic Considerations:1. The fault level calculations for three-phase fault and single line to ground faults are carried out for all the threetypes of 6.6KV/0.433KV LT Transformers in service at XYZ PLANT2MVA,1.6MVA,1MVA2. A negative tolerance of 5% is taken on transformer impedance at nominal tap for lowest tap imp.value for fault current calculations.3. X/R values as per transformer test report.4. Fault is considered on the 415V Bus fed from HT board connected to Level1 6.6KV Board HT212 as this is the strongest source to the 41V faults.5. Motor contribution is calculatedfor largest lumped motor load for each transformer ratingfor motor ratings equal to or greater than 37KW fed from the switchboard and downstream MCCs connected to the switchboard.6. Only case 1 is considered as in all the cases the fault current will not change considerably.7. Impedance of cable and bus duct connections is neglected.8. Source over-voltage factor of 1.06 is considered.Base MVA=10Base KV=6.6KV/0.433KVBase Current=10/(3x0.433)=13.33KAINPUT DATATransformer Data:1.2MVA, 6.33% imp,-5% for lowest tap ;X/R=7.522.1.6MVA,6.19% imp,-5% for lowest tap;X/R=6.333.1MVA, 4.95% imp ,-5% for lowest tap;X/R=6 (assumed)

Source:1.Symmetrical momentary fault current=35.86KAShort circuit MVA=3x6.6x35.86=410MVA2.Symmetrical interrupting fault current=29.35KAShort circuit MVA=3x6.6x29.35=335MVA3.X/R Ratio=15 (assumed)

Lumped motor data:1. Largest Lumped motor Load fed from2MVA transformerto boardPC-331-40B assuming that incomer to the adjacent bus PC-331-41M is off and all motor load is fed from the same incomer through the coupler.

SL#BOARD NAMEFEEDER NAMERATED KW

1PC-331-40B(SS8)9-PAM-004B110

2PC-331-40B(SS8)9-PAM-015B90

3PC-331-40B(SS8)COOLING TOWER FAN-A90

4PC-331-40B(SS8)COOLING TOWER FAN-C90

5PC-331-41MB(SS8)9-KAAM-00390

6PC-331-40B(SS8)43-PAM-CF-006A75

7PC-331-40B(SS8)43-PAM-CF-001A75

8PC-331-41MB(SS8)43-PAM-CF-006B75

9PC-331-41MB(SS8)43-PAM-CF-001B75

10PC-331-41MB(SS8)9-PAM-005A37

TOTAL(KW)807

Rating=807KWMotor MVA=807/(0.8x0.85x1000)Whereas, pf=0.8 and efficiency=0.85=1.187Zd=10/(6x1.187)=1.404puZdmom=1.2x1.404=1.685puZdintt=3x1.404=4.212puX/R= 6(assumed)Rmom=1.685/[(62+12)]=0.277puXdmom=0.277x6=1.66puRintt=4.212/[(62+12)]=0.693puXdintt=0.693x6=4.156pu

1. Largest Lumped motor Load fed from1.6MVA transformerto boardPC-306-11Bassuming that incomer to the adjacent bus PC-306-11B is off and all motor load is fed from the same incomer through the couplerSL#BOARD NAMEFEEDER NAMERATED KW

1PC-306-11B(SS4)23-PM-001A110.00

2PC-306-11B(SS4)23-PM-001C110.00

3PC-306-12B(SS4)23-PA-CF-001B110.00

4PC-306-12B(SS4)23-PA-CF-001D110.00

5PC-306-11B(SS4)47-PM-111A45.00

6PC-306-11B(SS4)90-PM-101A45.00

7PC-306-11B(SS4)90-PM-101B45.00

8PC-306-12B(SS4)90-PM-101C45.00

9PC-306-12B(SS4)47-PM-111B45.00

TOTAL(KW)665

Zdmom=2.045puZdintt=5.113puX/R= 6(assumed)Rmom=0.336puXdmom=2.017puRintt=0.841puXdintt=5.043pu

1.Largest Lumped motor Load fed from1MVA transformerto boardPMCC-341-1B assuming that incomer to the adjacent bus PMCC-341is off and all motor load is fed from the same incomer through the coupler

SL#BOARD NAMEFEEDER NAMERATED KVA/KW

1PMCC-34147-PM-102A125

2PMCC-341-1B47-PM-103A125

TOTAL(KW)250

Zdmom=5.44puZdintt=13.6puX/R= 6(assumed)Rmom=0.894puXdmom=5.366puRintt=2.24puXdintt=13.41pu

Source Imp:Zs=10/410=0.0244puRs=0.0244/[(152+12)]=0.0016puXs=15x0.0016=0.0243pu

Transformer imp.1.2MVA Transformer (-5% tolerance)

ZT1=0.06331x0.95x10/2=0.3007puRT1=0.3007/[(7.522+12)]=0.0396puXT1=7.52x0.0396=0.2981puRT0=90% of RT1=0.0356puXT0=90% of XT1=0.268pu2.1.6MVA Transformer (-5% tolerance)

ZT1=0.0619x0.95x10/1.6=0.3868puRT1=0.3868/[(6.332+12)]=0.06puXT1=6.33x0.06=0.3821puRT0=90% of RT1=0.054puXT0=90% of XT1=0.344pu3.1MVA Transformer (-5% tolerance)

ZT1=0.0495x0.95x10/1=0.47puRT1=0.47/[(62+12)]=0.077puXT1=6x0.077=0.464puRT0=90% of RT1=0.069puXT0=90% of XT1=0.418pu

Equivalent circuit calculations

3- PHASE FAULT:The equivalent reactance and resistance diagram is shown in sh. 26 of diagram " Fault Level Calculations"

1.2MVA Transformer:Momentary:Equivalent Resistance, Req=0.0359puEquivalent Reactance, Xeq=0.2501puEquivalent imedance for 3 phase fault:Zeq=(0.25012+0.03592)=0.2724puThree phase short Circuit current in K Amps with over voltage factor of 1.06::If=1.06x(1/0.2605)x13.334=51.9KarmsHowever, this value is not expected to go beyond desined value of 50Kamp(1 sec) due to following reasons:as the calculated value will reduce after taking into account cable impedance.All level transformers are not expected to work at the least impedance tap, resulting into higher impedance during fault.

Peak Value of short circuit current is as per following formula:Ipeak=2xImomx(1+e-t/)= R/L;t=10msX/R=0.2501/0.0359=6.966L/R=6.966/(2f)=0.0239

Ipeak=2x51.9x(1+e-0.01/0.0239)=121.7KAmpPeak2.1.6MVA Transformer:Momentary:Equivalent Resistance, Req=0.0323puEquivalent Reactance, Xeq=0.3383puEquivalent imedance for 3 phase fault:Zeq=0.342puThree phase short Circuit current in K Amps with over voltage factor of 1.06:If=41.3KArms

This is found OK as the design Value is 50KA.Peak Value of short circuit currentL/R=0.0206

Ipeak=2x41.3x(1+e-0.01/0.0206)=94K Amps Peak

1.1MVA Transformer:Momentary:Equivalent Resistance, Req=0.0725puEquivalent Reactance, Xeq=0.4475puEquivalent imedance for 3 phase fault:Zeq=0.4533puThree phase short Circuit current in K Amps with over voltage factor of 1.06:If=31.2KArms

This is found OK, as the design Value is 50KA.Peak Value of short circuit currentL/R=0.0196

Ipeak=2x31.2x(1+e-0.01/0.0196)=70K Amps Peak

SINGLE PHASE TO EARTH FAULT:The impedance diagram for single phase to Earth fault is shown in sh. 27 of diagram " Fault Level Calculations". After network reduction, the current calculations are as follows:1. 2MVA transformerEquivalent impedance to single line to Ground Fault:Zeq= 0.9208pu

hence,FaultMVA=1.06x(3x(1/0.9208))x10=34.5Fault Current=34.5/(3x0.415)=46KA

1. 1.6MVA transformerEquivalent impedance to single line to Ground Fault:Zeq= 1.171puhence,FaultMVA=1.06x(3x(1/1.171))x10=27.2Fault Current=27.2/(3x0.415)=36KA2. 1MVA transformerEquivalent impedance to single line to Ground Fault:Zeq= 1.484puhence,FaultMVA=1.06x(3x(1/1.484))x10=21.4Fault Current=21.4/(3x0.415)=28.6KA