บทที 11 การคํานวณกระแสลัดวงจร

1 11 11 11 2 - ( Normal Current )- ( Short Circuit Current ) 23 - - - 4 ( )- ( Mechanical Stress ) ( Thermal Stress ) - 35 - - 6- IEC 60909 " Short-circuit Current Calculationin Three-phase System 47- IEC 60909

2 2 1. ( Far-from-generatorShort Circuit )2. ( Near-to-generator Short Circuit )8 2 1. 2. 59 10 4 1. ( Balanced Three-Phase Short Circuit )2. ( Line to Line Short Circuit Without Earth Connection)3. ( Line to Line Short Circuit With Earth Connection )4.

3 ( Line to Earth Short Circuit )611 12- ( Equivalent Voltage Source )- 713- ( ) ( )14 815- F c ( Voltage Factor ) Un ( L- L )- c Voltage Factor cNote.

4 - cUaShould Not Exceed the Highest Voltage Umfor Equipment of Power System917 Voltage Factor c - LV 230 / 400 V , 3 ph , 4 w- LV 240 / 416 V , 3 ph , 4 w- MV 22 kV , 3 ph , 3 w18 Voltage Factor c- LV 230/400 V,3 ph ,4 w ; Cmax= LV 240/416 V,3 ph ,4 w ; Cmax= MV 22 kV , 3 ph ,3 w ; Cmax= 3 Symmetrical Components Z( 1 )Positive Sequence ImpedanceZ( 2 )Negative Sequence ImpedanceZ( 0 )Zero Sequence ImpedanceZ( 1 ) Z( 1 ), Z( 2 ), Z( 0 ) 20 a) Network Feederb) Two-wining Transformersc) Overhead Lines and Network Feeder- ( Initial Symmetrical Short-circuit Power ) S"kQ - ( Infinite Bus )

5 = 022 1223 tr= HV / LV24 35 kV ( OH Lines )- ZQ ZQ= j XQ1325 - RQ RQ = XQ XQ = ZQ S"kQ - 22 kV 24 kV S"kQ = 500 MVA 26 RQ= XQ XQ ZQ1427 28 S"kQ= 500 MVA 22 kV , RQ= XQ 22 kV / 230-400 V RQt XQt 1529 = m 30 XQt= ZQ= x m RQt= XQt= m 230/400 V.

6 ZQt= m XQt = m RQt= m Two-winding TransformersZT= RT+ j XT 32 :UrT= IrT= SrT= PkrT= ukr= uRr= 1733- Zero Sequence ImpedanceZ( 0 ) T= R( 0 ) T+ j X( 0 ) T - Delta-Wye R( 0 ) T= RTX( 0 ) T= XT34 Taps Impedance IEC 60909 Impedance correction factors ( KT )

7 KT ZTK= KTZT1835 ZTK= KTZT 1000 kVA 22 kV / 400V KT = 2000 kVA 22 kV / 400V KT = KT = 2000 kVA , 22 kV/230 - 400V,% Uk= 6 , Load Loss = 24 kW RT, XT ZT 1937 = x 10-3 = m 38= m 2039= m ZT = + j m 40 230/400 V (kVA)IFL (A)%UkPL(kW)ZT (m )RT (m )XT (m ) ( Overhead Lines and Cables Impedance ) ZL = RL+ j XL 42 R'L 20oC qn 2243RL= / qn.

8 44 X'L d = r = Bundle r R n = Bundle2345 0= 4 10-4H / km ( Permeability ) 46 PVC Overhead Line Aluminium 95 mm2 X L 150 mm48 95 mm2A = r2= mm2549= 189 mm50= / km2651 IEC 01 50 mm2 X L52 50 mm2A = r2= mmd = mm2753= / km PVC

9 PVC ( mm2 ) ( m / m ) ( m /m ) 4 56 2957 Impedance 58 k1 ( Line to Earth Short Circuit ) ( Balanced Three-phase Short Circuit ) ( Positive Sequence Impedance ) 60.

10 = Rk= Xk= Zk= 3161 Ib= Ik= I k ip 62 R / X X / R 3263 64 Ib Ib= Ik= I ( Line to Line Short Circuit Without Earth Connection ) I"k2 cUn/ 3 66== Z( 1 )= Z( 2 ) =3467 ( Minimum Fault Current.)