View source for Reactances of synchronous machines

The '''reactances of synchronous machines''' comprise a set of characteristic constants used in the theory of [[synchronous machine]]s.{{sfn|Park|Robertson|1928|p=514}} Technically, these constants are specified in units of the [[electrical reactance]] ([[ohm]]s), although they are typically expressed in the [[per-unit system]] and thus [[dimensionless]]. Since for practically all (except for the tiniest) machines the resistance of the coils is negligibly small in comparison to the reactance, the latter can be used instead of ([[Complex number|complex]]) [[electrical impedance]], simplifying the calculations.{{sfn | Klempner | Kerszenbaum | 2004 | p=144}}

== Two reactions theory ==
[[File:Salientpole axii.png|thumb|A diagram of a salient pole machine with direct (d) and quadrature (q) axes]]
The [[air gap (electrical machine)|air gap]] of the machines with a [[salient pole rotor]] is quite different along the pole axis (so called ''direct axis'') and in the orthogonal direction (so called ''quadrature axis''). [[Andre Blondel]] in 1899 proposed in his paper "Empirical Theory of Synchronous Generators" the '''two reactions theory''' that divided the [[Armature (electrical)|armature]] [[magnetomotive force]] (MMF) into two components: the direct axis component and the quadrature axis component. The '''direct axis''' component is aligned with the magnetic axis of the rotor, while the '''quadrature''' (or '''transverse''') '''axis''' component is perpendicular to the direct axis.{{sfn | Gieras | Shen | 2022 | p=211}} The relative strengths of these two components depend on the design of the machine and the operating conditions. Since the equations naturally split into direct and quadrature components, many reactances come in pairs, one for the direct axis (with the index d), one for the quadrature axis (with the index q). In the machines with a [[cylindrical rotor]] the air gap is uniform, the reactances along the d and q axes are equal,{{sfn | Deshpande | 2011 | p=315}} and d/q indices are frequently dropped.

== States of the generator ==
The [[flux linkage]]s of the generator vary with its state. Three states are considered:{{sfn | Machowski | Bialek | Bumby | 1997 | pp=102-103}}
# the ''steady-state'' is the normal operating condition with the armature [[magnetic flux]] going through the rotor;
# the ''sub-transient state'' is the one the generator enters immediately after the fault (short circuit). In this state the armature flux is pushed completely out of the rotor. The state is very brief, as the current in the [[damper winding]] quickly decays allowing the armature flux to enter the rotor poles only. The generator goes into transient state;
# in the ''transient state'' the flux is still out of the [[field winding]] of the rotor. The transient state decays to steady-state in few [[cycle (AC current)|cycles]].{{sfn | Ramar | Kuruseelan | 2013 | p=20}}

The sub-transient and transient states are cheracterized by significantly smaller reactances.

== List of reactances ==
Das{{sfn|Das|2017|pp=180-182}} identifies the following reactances:
* leakage reactance <math>X_l</math>;
* synchronous reactance <math>X_d</math> (also <math>X_S</math>{{sfn | Klempner | Kerszenbaum | 2004 | p=144}}); 
* transient reactance <math>X'_d</math>;
* subtransient reactance <math>X''_d</math>;
* quadrature axis reactances <math>X_q</math>, <math>X'_q</math>, <math>X''_q</math>, counterparts to <math>X_d</math>, <math>X'_d</math>, <math>X''_d</math>;
* [[negative sequence]] reactance <math>X_2</math>;
* [[zero sequence]] reactance <math>X_0</math>;
* Potier reactance <math>X_P</math>.

== Synchronous reactances ==
The synchronous reactances are exhibited by the [[armature (electrical)|armature]] in the steady-state operation of the machine.{{sfn|Das|2017|p=181}} The three-phase system is viewed as a superposition of two: the direct one, where the maximum of the phase current is reached when the pole is oriented towards the winding and the quadrature one, that is 90° offset.{{sfn|Park|Robertson|1928|p=515}}

The per-phase reactance can be determined in a mental experiment where the rotor poles are perfectly aligned with a specific angle of the phase field in the armature (0° for <math>X_d</math>, 90° for the <math>X_q</math>). In this case, the reactance {{mvar|X}} will be related with the [[flux linkage]] <math>\Psi</math> and the phase current {{mvar|I}} as <math>X = \omega \frac \Psi I</math>, where <math>\omega</math> is the [[circular frequency]]. {{sfn|Prentice|1937|p=7}} The conditions for this mental experiment are hard to recreate in practice, but:
* when the armature is short-circuited, the flowing current is practically all reactive (as the coil resistance is negligible), thus under the short-circuit condition the poles of the rotor are aligned with the armature [[magnetomotive force]];
* when the armature is left open-circuit, the voltage on the terminals is also aligned with the same phase and is equal to <math>\omega \Psi</math>. If saturation is neglected, the flux linkage is the same.
Therefore, the direct synchronous reactance can be determined as a ratio of the voltage in open condition <math>V_{OPEN}</math> to short-circuit current <math>I_{SC}</math>: <math>X_d = \frac {V_{OPEN}} {I_{SC}}</math>. These current and voltage values can be obtained from the [[open-circuit saturation curve]] and the [[synchronous impedance curve]].{{sfn|Prentice|1937|p=8}}

The synchronous reactance is a sum of the leakage reactance <math>X_l</math> and the reactance of the armature itself (<math>X_a</math>): <math>X_d = X_l + X_a</math>.{{sfn | Machowski | Bialek | Bumby | 1997 | p=104}}

== References ==
{{Reflist}}

== Sources ==
* {{cite journal | last1=Park | first1=R. H. | last2=Robertson | first2=B. L. | title=The Reactances of Synchronous Machines | journal=Transactions of the American Institute of Electrical Engineers | publisher=Institute of Electrical and Electronics Engineers (IEEE) | volume=47 | issue=2 | year=1928 | issn=0096-3860 | doi=10.1109/t-aiee.1928.5055010 | pages=514–535}}
* {{cite journal | last=Prentice | first=B. R. | title=Fundamental Concepts of Synchronous Machine Reactances | journal=Transactions of the American Institute of Electrical Engineers | publisher=Institute of Electrical and Electronics Engineers (IEEE) | volume=56 | issue=12 | year=1937 | issn=0096-3860 | doi=10.1109/t-aiee.1937.5057505 | pages=1–21}}
* {{cite web |last1=Heydt |first1=G. |last2=Kalsi |first2=S. |last3=Kyriakides |first3=E. |title=A Short Course on Synchronous Machines and Synchronous Condensers |url=https://documents.pserc.wisc.edu/documents/general_information/presentations/presentations_by_pserc_university_members/heydt_synchronous_mach_sep03.pdf |publisher=[[Arizona State University]], [[American Superconductor]] |date=2003}}
* {{cite journal | last1=El-Serafi | first1=A.M. | last2=Abdallah | first2=A.S. | title=Saturated synchronous reactances of synchronous machines | journal=IEEE Transactions on Energy Conversion | publisher=Institute of Electrical and Electronics Engineers (IEEE) | volume=7 | issue=3 | year=1992 | issn=0885-8969 | doi=10.1109/60.148580 | pages=570–579}}
* {{cite book | last=Das | first=J. C. | title=Short-Circuits in AC and DC Systems: ANSI, IEEE, and IEC Standards | publisher=CRC Press | year=2017 | isbn=978-1-4987-4542-0 | url=https://books.google.com/books?id=PjwPEAAAQBAJ&pg=PA180 | access-date=2023-07-02}}
* {{cite book | last1=Gieras | first1=J.F. | last2=Shen | first2=J.X. | title=Modern Permanent Magnet Electric Machines: Theory and Control | publisher=CRC Press | year=2022 | isbn=978-1-000-77700-0 | url=https://books.google.com/books?id=KvGaEAAAQBAJ&pg=PA211 | access-date=2023-07-03}}
* {{cite book | last=Deshpande | first=M.V. | title=Electrical Machines | publisher=Prentice Hall India Pvt., Limited | year=2011 | isbn=978-81-203-4026-8 | url=https://books.google.com/books?id=JKsH_3AzYMMC&pg=PA315 | access-date=2023-07-03}}
* {{cite book |last1=Klempner |first1=Geoff |last2=Kerszenbaum |first2=Isidor |title=Operation and Maintenance of Large Turbo-Generators |date=2004 |publisher=John Wiley & Sons |isbn=978-0-471-61447-0 |url=https://books.google.com/books?id=DPdkLp_crLAC&pg=PA144 |language=en}}
* {{cite book | last1=Machowski | first1=J. | last2=Bialek | first2=J. | last3=Bumby | first3=J.R. | title=Power System Dynamics and Stability | publisher=Wiley | year=1997 | isbn=978-0-471-95643-3 | url=https://books.google.com/books?id=L7o4TitajRIC&pg=PA104 | access-date=2023-07-04}}
* {{cite book | last1=Ramar | first1=S. | last2=Kuruseelan | first2=S. | title=Power System Analysis | publisher=PHI Learning | year=2013 | isbn=978-81-203-4733-5 | url=https://books.google.com/books?id=ahntULiMFgwC&pg=PA19 | access-date=2023-07-04}}
{{Characteristics of synchronous machines}}
[[Category:Electrical engineering]]
[[Category:Electrical generators]]