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some Exam problems

Discussion in 'Electronic Basics' started by Lisa Thomson, Jun 13, 2004.

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  1. Lisa Thomson

    Lisa Thomson Guest

    next week we have a exam in Power Systems. Our Professor gave us some
    problems to practise but without solutions. If anybody wants to help
    me here are the problems:

    A shops receives power through a 2.5km long, 4160V transmission line.
    Following is the list of machines with their ratings:
    Power(kW) Power Factor
    nr1 60 0.76(lagging)
    nr2 50 0.75(lagging)
    nr3 37.5 0.78(lagging)
    nr4 60 0.8(lagging)

    a) What is the overall power factor?
    b) Design a delta connected capacitor bank across the 4160V terminals
    to raise the power factor to 0.9(lagging).
    Calculate the power rating, capacitance per phase and the rated
    per-phase voltage of the capacitor bank.
    c) Calculate the savings on energy and its cost over a period of 1
    year if the per phase resistance of the 4160V transmission line is 1.5
    Ohm/km and the shop is in operation everyday, 24h hours a day. The
    cost of electrical energy is 7.8cents/kWh.

    | |
    TL2 TL3
    | |
    B3 B4
    | |
    T3 T4
    | |
    G3 G4

    Generator G1=G2 30MVA 6.3kV Xd"=12%
    Transformer T1=T2 35MVA 6.3kV/110kV Xt=7%
    Generator G3 150MVA 12kV Xd"=15%
    Generator G4 50MVA 13.8kV Xd"=10%
    Transformer T3 150MVA 12kV/110kV Xt=8%
    Transformer T4 50MVA 13.8kV/110kV Xt=7%

    Transmission line 1 TL1 X=10 Ohm
    Transmission line 2 TL2 X=18 Ohm
    Transmission line 3 TL3 X=26 Ohm
    The above system has been operation at no load when a 3 phase power
    short circuit occurs on Bus B4. Calculate the short circuit current in
    amperes on Bus B4. Choose Vb=110kV and Sb=100MVA as the base values in
    the TL 2.


    G1 12MVA, 6.3kV Y (neutral grounded) X"d=X2=14%; X0=5%
    G2 50MVA, 12kV Y (neutral grounded) X"d=X2=16%; X0=6%
    G3 45MVA, 13.8kV Y (neutral grounded) X"d=X2=15%; X0=4%
    T1 15MVA, 6.3kV Y (neutral grounded)/138kV Y(neutral grounded)
    T2 50MVA, 12kV Delta connected/138kV Y(neutral grounded) Xt=X2=X0=8%;
    T3 50MVA, 13.8kV Delta connected/138kV Y(neutral grounded)
    TL1 X1=X2=38 Ohm, X0=105 Ohm
    TL2 X1=X2=26 Ohm, X0=58 Ohm

    Consider 50MVA and 138kV as the bases in the transmission line

    In a thounderstorm day the circuit breaker CB of the above system
    opened and remained open after a 3 phase short circuit in the system.
    Another lightning hits a phase conductor and caused a single phase to
    ground fault at Bus B2.
    Determine the fault current in amperes, and phase voltages in volts at
    Bus B2. neglect the pre fault current in the system.
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