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# Generator Reactance(2)

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 prc (Electrical) 10 Mar 12 6:13
 I find the following terms when referring to a turbogenerator(400MVA)data sheet.1) Direct axis synchronous reactance2) Direct axis transient reactance3) Direct axis  subtransient reactance4) Quadrature axis synchronous reactance5) Quadrature axis transient reactance6) Quadrature axis subtransient reactance.Hope the percentage reactances mentiond should be with respect to the rated full load rating, at  rated  lagging power factor and maximum hydrogen temperature.Can the experts  briefly say what it is,significance for operation,which ones come in to effect during short circuit, normal operation and load sharing (if loading to a common bus.
 7anoter4 (Electrical) 10 Mar 12 10:06
 I'm not pretending to be an "expert" in Electrical Power System Analysis.So, simplifying the basic equations I think we may say the following:Ik=U/Xd Permanent short-circuit current[ss c]I'k=U/X'd Transitory  sscI"k=U/X"d Supertransitory  sscWhere:U=Generator terminal voltagedelta=load angle [between Eo and U]Eo=no-load emfE'do=transitory emfPa=Eo*U/Xd*sin(delta)+U^2/2*(Xd-Xq)/(Xd*Xq) Stationary regime active power.P"a=E'do*U/X'd*sin(delta)+U^2/2*(X'd-Xq)/(X'd*Xq) Transitory regime active powerA  phasor diagram may be more explicating
 7anoter4 (Electrical) 10 Mar 12 10:55
 I forgot the phase-to-ground short-circuit.For phase-to-ground short-circuit we need Z2. X2=(X"d+X"q)/2
 Skogsgurra (Electrical) 10 Mar 12 10:58
 Would that mean that "Direct" means resistive and "Quadrature" neans reactive (90 degrees)? Gunnar Englundwww.gke.org--------------------------------------Half full - Half empty? I don't mind. It's what in it that counts.
 Hoxton (Electrical) 10 Mar 12 12:52
 Would that mean that "Direct" means resistive and "Quadrature" means reactive (90 degrees)?  - yesI usually explain all this to my mechanical engineer colleagues by saying that reactances do not exist.....  If you put a short circuit across the terminals of a generator, the current will rapidly increase and then gradually decrease.To use this in calculations you have to imagine a generator with a voltage source and a variable reactance.When operating normally, the reactance is large (synchronous)When the fault is applied, it is very small (subtransient)And the then rises (transient)As regards the direct and quadrature components, if you look at a salient pole generator (like fitted to a 500kVA standby diesel).  The magnetic path between the rotor pole face and the stator is easier (only interupeted by the air gaps) than if you took the path between the poles where the air gap is large. Basically that explains the difference.The reactances are usually referred to the rated VA of the generator.
 7anoter4 (Electrical) 10 Mar 12 12:54
 The Salient Poles Synchronous Generator Rotor [inductor] magnetic field is deformed by stator [induced] reaction field. Mathematically we can decompose this field in 2 sinusoidal magnetic fields produced by stator one acting in the pole and another  between poles. The reactance produced by these fields will be xad [direct armature reactance in pole axis ]  and xaq [Quadrature axis armature reactance]The stator magnetic field losses produces a loss reactance Xs. The direct axis synchronous reactance then will be Xd=Xs+Xadand  Quadrature axis synchronous reactance will be Xq=Xs+XaqAs we know the pole is provided with excitation winding and [usually] oscillation damping [I don't know if my translation is correct name]winding and in a short-circuit case these fields will produce an other change in synchronous reactance and we'll get X'd and X"d [respectively X'q and X"q]
 dpc (Electrical) 10 Mar 12 14:55
 http://en.wikipedia.org/wiki/Dqo_transformationSome info on Park's (or Dq0) transformation.
 prc (Electrical) 11 Mar 12 0:44
 Thank you gentlemen.Then what is the significance of following:1) Stator Leakage Reactance2) Zero sequence reactance3) Negative Sequence Reactance
 7anoter4 (Electrical) 11 Mar 12 13:52
 First of all I'm sorry I didn't notice the first sentence of the o.p.:"I find the following terms when referring to a turbogenerator(400MVA)data sheet."So, as a turbogenerator has a cylindrical [not a salient pole] rotor and Xd=Xq. Second, I translated wrong instead of  "field leakage reactance " into "field losses reactance" so what I noted Xs it is actually Xla [armature-stator-leakage reactance].I have to apologize for my bad English!Stator leakage field is "a lost field" indeed, since it does not flow with the main flux but it remains in the air space as slot and open winding ends. This includes also the magnetic field harmonics[ as differential leakage].The negative and zero sequence reactance are used in the connection with short-circuit case as in steady state balanced regime the generator only generatepositive sequence voltage and negative and zero sequence voltages are zero.Nevertheless, negative sequence currents can arise whenever there isany unbalance present in the system. Their effect is toset up a field rotating in the opposite direction to themain field generated by the rotor winding, so subjectingthe rotor to double frequency flux pulsations. This givesrise to parasitic currents and heating.If a machine is operating with an earthed neutral, asystem earth fault will give rise to zero sequencecurrents in the machine. This reactance represents themachine's contribution to the total impedance offered tothese currents. In practice it is generally low and oftenoutweighed by other impedances present in the circuit. If Y point is grounded through impedance  Zg  , then  3*Zg will have to be added to zero sequence impedance of generator.

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