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Synchronous Machine Transent parameters from sudden 3phase Short Circuit

  1. Oct 24, 2015 #1
    Hi, So we're doing this Lab on Transient reactance of a synch generator and I'm reading Sarma's classic: Electric Machines Steady-state theory and Dynamic performance p. 474-479

    But I can't see how to get the actual exponential time constants for the sub-transient and transient reactances?
  2. jcsd
  3. Oct 25, 2015 #2
    Does the constant 0.368 have any significance? It's almost like he's saying to progress 36.8% from the initial transient peak, then that time is the time constant for the transient?
    Also how do you plot the envolope around a waveform?

    Lastly, how to you work out the asymmetrical DC component? Fourier or something?
  4. Oct 25, 2015 #3


    Staff: Mentor

  5. Oct 31, 2015 #4
    There are some standards in order to calculate the short-circuit currents.
    The most importants ,I think, are IEEE 551/2006 –this is the last edition I know- and IEC 60909.
    In IEEE 551/2006 Calculating Short-Circuit Currents in Industrial and Commercial Power Stations the synchronous machine reactance is treated in CH.5 and the short circuit current
    at the terminals of this machine [formula 5.1]:
    Isc=(E/X”dv-E/X’dv)e^(-t/T”do)+ (E/X’dv-E/Xdv)e^(-t/T’do)+E/xd
    Where :
    X’’dv=Direct-axis saturated subtransient reactance
    X’dv= Direct-axis saturated transient reactance
    Xd =synchronous reactance
    T’’do=subtransient time constant
    T’do= transient time constant
    In IEC 60909-2 ch. 2.3 there are Data on typical synchronous machines [Table 2]
  6. Oct 31, 2015 #5
    Hi, thanks.
    Right, but how do I work out the Direct-axis saturated subtransient reactance, Direct-axis saturated transient reactance, synchronous reactance or the sub or transient, timeconstants?

  7. Nov 1, 2015 #6
    Usually there are manufacturer data. Never-the-less in some standards are approximate formulae as for instance:
    From IEC 61363-1 ch.6.2.6 Time constants
    “If the a.c. and d.c. time constants are unavailable, they can be calculated from the machine
    open-circuit time constants and impedance as follows:
    Subtransient time constant (usually of the order of 1 ms to 30 ms):
    1. T"d = (X"d / X'd) T"do (32)
    2. Transient time constant (usually of the order of 20 ms to 1200 ms):
      T'd = (X'd / Xd) T'do (33)
      c) d.c. time constant (usually of the order of 15 ms to 300 ms):
      Tdc = X"d /(2.pi.fr.Ra)
  8. Nov 1, 2015 #7
    Hmm, because I took the current waveform for the field and phases on the CRO using a solid state switch on the zero crossing. So I have the transient oscillations of the current, but no manufacturer data.
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