Handbook of Electrical Engineering For Practitioners in the Oil, Gas ...

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FAULT CALCULATIONS AND STABILITY STUDIES 279transformers is simply the arithmetic sum of their individual ratings S ti .S te =n∑i=1S tiThe equivalent impedance Z te of the transformers may be found from,Z te =S ten∑S tii=1Z ti11.5.2.6 Worked exampleThree transformers feed a load from a main switchboard. Their ratings and impedances are,n∑i=1Transformer No. 1 S ti = 10 MVAZ ti = 0.008 + j0.09 puTransformer No. 2 S t2 = 15 MVAZ t2 = 0.009 + j0.1 puTransformer No. 3 S t3 = 25 MVAZ t3 = 0.01 + j0.12 puThe total capacity S te = 10.0 + 15.0 + 25.0= 50.0 MVAS tiZ ti=10.00.008 + j0.09 + 15.00.009 + j0 + 1 + 25.00.01 + j0.12= 40.432 − j465.9350.0Z te == 0.0092 + j0.1065 pu40.432 − j465.9311.6 CALCULATE THE SUB-TRANSIENT SYMMETRICAL RMS FAULTCURRENT CONTRIBUTIONSThe method adopted below is based upon the principles set out in IEC60363 and IEC60909, bothof which describe how to calculate sub-transient and transient fault currents, and are well suited tooil industry power systems. The method will use the per-unit system of parameters and variables.Choose the base MVA to be S base .
280 HANDBOOK OF ELECTRICAL ENGINEERINGIt is customary to assume that all the generators are operating and that they are heavily loaded.In which case the emf E g ′′ behind the sub-transient reactance X′′dis about 5 to 10% above the ratedterminal voltage, hence assume E g ′′ is 1.1 pu. This emf drives the fault current around the circuit. InIEC60909 the elevation in driving emf, or voltage, is given in Table I as ‘factor c’ and discussed inClause 6 therein.The contribution of fault current I g ′′ from the generators is,g = E g′′( X′′d+ X ) putgS baseS gen S tg1.1= ( X′′d+ 0.08 )S baseS gen K tg S genI ′′I ′′g =1.1S gen(Xd ′′ + 0.08 )(11.1)S baseK tgThe contribution from the high voltage motors is found as follows.It may be assumed that the average ratio of starting current to rated current (I s /I n )ofthemotor is,I s= 6.0 pu for high voltage motorsI nConsequently the sub-transient impedance Zhm ′′ of the motors is,Z hm ′′ = 1.06.0 = 0.167 pu (at S hm)For typical high voltage motors the starting power factor is between 0.15 and 0.2 lagging,hence assume 0.2. The sub-transient impedance becomes,Z hm ′′ = 0.033 + j0.164 puThe equivalent impedance Z tm of the motor unit transformers is 0.06 pu at a total capacityof S tm .Z tm = 0.0 + j0.06 puThe emf Ehm ′′ behind the motor sub-transient impedance is the air-gap emf and will in practicebe slightly less than 1.0 pu, hence it is reasonable and conservative to assume it to be 1.0 pu.The contribution of fault current Ihm ′′ from the main switchboard motors isI hm ′′ = Ehm′′( Z′′hm+Z ) putmS baseS hm K tm S hm=1.0S hm(0.033 + j0.164 + j0.06 )(11.2)S baseK tm
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Handbook ofElectrical EngineeringFo
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ContentsForewordPrefaceAcknowledgem
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4 Automatic Voltage Regulation 834.
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9 Cables, Wires and Cable Installat
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References 308Further Reading 30912
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xv14.2.3 Pole amplitude modulated m
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20 Summary of the Generalised Theor
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ForewordThe oil, gas and petrochemi
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xxiiThe appendices contain comprehe
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About the AuthorThe author began hi
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3Synchronous Generators and Motors3
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3.2.2 Transient State Armature Reac
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SYNCHRONOUS GENERATORS AND MOTORS 6
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Table 3.1.generatorsGeneratorrating
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5Induction Motors5.1 PRINCIPLE OF O
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INDUCTION MOTORS 101Figure 5.1Commo
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INDUCTION MOTORS 103Figure 5.310 MW
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INDUCTION MOTORS 105Figure 5.4 Torq
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INDUCTION MOTORS 107slip. This is s
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INDUCTION MOTORS 109Figure 5.6 Sens
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INDUCTION MOTORS 111The shunt compo
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INDUCTION MOTORS 113The shunt compo
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Table 5.2.motorsRated power(kW)INDU
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Table 5.7.Rated power(kW)INDUCTION
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INDUCTION MOTORS 119R 22N , X 22N ,
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INDUCTION MOTORS 121500 to 516 give
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Table 5.12. Limits to motor ratings
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INDUCTION MOTORS 125• The use of
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INDUCTION MOTORS 127Figure 5.7Circu
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INDUCTION MOTORS 1295.10.5 Part Win
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9Cables, Wires and CableInstallatio
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CABLES, WIRES AND CABLE INSTALLATIO
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Table 9.8.Conductors Insulation Scr
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Table 9.10.ActualCSA (mm 2 )CABLES,
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Table 9.12.CABLES, WIRES AND CABLE
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EF = IX sin ØDF = IX cos ØCABLES,
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9.4.3.3.1 Worked exampleCABLES, WIR
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Table 9.25.CABLES, WIRES AND CABLE
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13Earthing and Screening13.1 PURPOS
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EARTHING AND SCREENING 353is usuall
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EARTHING AND SCREENING 35513.2.2.1
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EARTHING AND SCREENING 357Figure 13
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EARTHING AND SCREENING 363construct
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EARTHING AND SCREENING 365that have
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EARTHING AND SCREENING 367moulded c
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EARTHING AND SCREENING 369Annex A o
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EARTHING AND SCREENING 37113.4 CONS
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EARTHING AND SCREENING 373with a ve
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EARTHING AND SCREENING 377Find I c
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EARTHING AND SCREENING 379If the ca
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EARTHING AND SCREENING 383REFERENCE
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20Summary of the Generalised Theory
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GENERALISED THEORY OF ELECTRICAL MA
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f) The rotor equations involving th
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Appendix EList of Document Types to
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LIST OF DOCUMENT TYPES TO BE PRODUC
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LIST OF DOCUMENT TYPES TO BE PRODUC
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Appendix FWorked Example for Calcul
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WORKED EXAMPLE FOR CALCULATING THE
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Step 12.WORKED EXAMPLE FOR CALCULAT
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608 INDEXCables (Continued)Derating
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610 INDEXDirty engine, gas turbine
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612 INDEXFault making capacity 151F
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614 INDEXInductionRotational type o
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616 INDEXMotors (Continued)DC shunt
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618 INDEXProtection schemes and sys
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620 INDEXShaft torque and shaft pow
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622 INDEXTacho-generator for feedba
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624 INDEXVoltage (Continued)Standar
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