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Electrical Transmission Questions

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"Q1) Z= Z * (MVA) /(MVA)pun n bn boParallel MVA = 4 * 30 = 120 MVAZ = per unit impedancepu Z = impedance in nn (MVA) = base MVAb Z = 0.2 * 500/120 = 0.833 p.upun Q2) Transient parallel MVA = 40 + 2 * 15 + 2 * 40 = 150 MVAGrid in feed = 1500 MVAShort ..

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"Q1) Z= Z * (MVA) /(MVA)pun n bn boParallel MVA = 4 * 30 = 120 MVAZ = per unit impedancepu Z = impedance in nn (MVA) = base MVAb Z = 0.2 * 500/120 = 0.833 p.upun Q2) Transient parallel MVA = 40 + 2 * 15 + 2 * 40 = 150 MVAGrid in feed = 1500 MVAShort circuit MVA = base MVA/Xpu Effective reactance = 0.12 + 0.31 + 0.14 = 0.57 p.uShort circuit = 150/0.57= 263 MVA Q3) i) Connects the tapping’s by which the number of turns to be used can be adjustedii) a) Current setting relayb) Plug setting multiplier of relayQ4PSM = fault current in relay coil/pick up currentPick up current of relay = 5 * 150% = 7.5AFault current in the CT primary is 1000APSM = 236/(1000/5)= 31.47b) 100%i) Pick up current of relay = 5*100% = 5AFault current = 6700/(1000/5) = 33.5APSM = relay current/pick up current= 33.5/5 = 6.7 For PSM = 6.7 for operation of relay 2.25 secActual time of relay for PSM 6.7, is 2.25*0.1 = 0.225 sec or 22550%Pick up current relay = 5 * 50% = 2.5AFault current = 6700/(1000/2.5)= 16.75PSM = 16.75/2.5 = 6.7For PSM = 6.7 for operation of relay 2.25 secActual time of relay for PSM 6.7, is 2.25*0.1 = 0.225 sec or 225ii) 0.225 sec = 6.70.4 se = ?PSM = 11.91 "

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