Synchronous Generator Introduction
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Synchronous Generator Introduction
Synchronous Generator or Alternator
A two-pole round rotor generator and exciter.
http://elektro.fs.cvut.cz/en/SSem/2141025/Synchronous_Machine.pdf
Cross Section of a Large Turbo Generator
(Courtesy Westinghouse) http://elektro.fs.cvut.cz/en/SSem/2141025/Synchronous_Machine.pdf
Round Rotor of a Large Generator
(Courtesy Westinghouse) http://elektro.fs.cvut.cz/en/SSem/2141025/Synchronous_Machine.pdf
Round Rotor with Conductor Placed
http://elektro.fs.cvut.cz/en/SSem/2141025/Synchronous_Machine.pdf
Four Pole Salient Pole Rotor
http://elektro.fs.cvut.cz/en/SSem/2141025/Synchronous_Machine.pdf
A Large Salient Pole Hydro Generator (1)
rotor
http://elektro.fs.cvut.cz/en/SSem/2141025/Synchronous_Machine.pdf
A Large Salient Pole Hydro Generator (2)
http://elektro.fs.cvut.cz/en/SSem/2141025/Synchronous_Machine.pdf
stator
Two Methods to Provide DC Field Current
1. Supply the DC power from an external DC source to the rotor by means of slip rings and brushes. - used in small synchronous generators, cost effective- brushes need to be checked for wear regularly- brush voltage drop can be significant power loss
2. Supply the DC power from a special DC power source (exciter)mounted on the shaft of the synchronous generator.
Rotor with Slip Rings
2 Stack Synchronous Generator System
3-Stack Synchronous Generator System (1)
(completely independent of any external power source)
3-Stack Synchronous Generator System (2)
Aircraft Synchronous Generator
J. F. Gieras, Advancements in Electric Machines, Springer, 2008.
Synchronous Generator PhasorDiagram and Power Flow
Generator Phasor Diagram – Lagging Power Factor
A s A s AR jX E V I I
Over excited ( )AE V
Generator Phasor Diagram – Unity Power Factor
A s A s AR jX E V I I
Slightly over excited ( )AE V
Generator Phasor Diagram – Leading Power Factor
A s A s AR jX E V I I
Maybe under excited ( )AE V
Generator Power Flow
s A s A AR jX V I I E
3 cosconv ind m A AP T E I
A A E I
conv out CuP P P
Output Power
, since A s A s A s A s sR jX jX R X E V I I V I
S
Aconvout X
EVPP
sin3
3 cosout AP V I sincos A
As
EIX
Induced Torque
3 sinAconv ind m
S
V EP T
X
3 sinAind
m S
V ET
X
sinind net RT kB B
Synchronous Generator Model Parameter Measurement
Measurement of Model Parameters
1. The relationship between field current and EA2. The synchronous reactance3. The armature resistance
Open Circuit Characteristics (1)
Saturation for large field current
, 0 2
ˆ ˆ8 2 a f
A rms e feff
N NDlE f Ig P
(or ET )
Open Circuit Characteristics (2)
Measurement Procedure:
1. Drive rotor at rated synchronous speed.2. Increase field current If upward from 03. Take data of Voc and plot Voc (or EA) vs If
Short Circuit Characteristics (1)
0netB
0V
SR BB
No saturation0V
AA
s sR jX
EI
2 2A
A
s s
EIR X
(or IL )
Short Circuit Characteristics (2)
Measurement Procedure:
1. When = 0 and If = 0, short the machine through three ammeters
2. When If = 0, drive the machine to rated synchronous speed
3. Increase If , take data (rms values) from three ammeters and plot IA,sc vs If
Take three rms values in average and get
,13A sc a b cI I I I
Measurement of Synchronous Reactance
, 2 2A A
A scss s
E EIXR X
,
As
A sc
EXI
Therefore, an approximate method for determining the synchronous reactance Xsat a given field current is:
1. Get the internal voltage EA from the OCC at that field current.2. Get the short-circuit current flow IA,SC at that field current from the SCC.3. Find Xs by applying (1).
(1)
Unsaturated Synchronous Reactance
AEscAI ,
uSX ,
,/s A A scX E I
The unsaturated synchronous reactance Xs,u can be found simply by applying
at any field current in the linear portion (on the airgap line) of the OCC curve.
Saturation for large field current
No saturation
Follow equation,
As
A s c
EXI
Short-Circuit Ratio Short Circuit Ratio: The ratio of the field current required for the rated voltage at open circuit to the field current required for the rated armature current at short circuit.
rated voltage at OC
rated current at OC
||
fV
fI
ISCR
I
SCR is inversely proportional to Xs
AE
,ratedV
,
, , , ,
1
fV
fV ratedx xb
fI A rated rated A rated s I
I VI ISCR ZI I V I X
Measurement of Armature Resistance
The armature resistance Rs can be approximately measured by applying a DC voltage to the windings while the machine is in stationary and measuring the resulting current flow. Using DC voltage means that the reactance of the windings will be zero during the measurement process.
This technique is not perfectly accurate, since the AC resistance will be slightly larger than the DC resistance (as a results of the skin effect at higher frequencies).
EA versus If under Load
Round rotor generator rated at 93.75 MVAand 0.8 power factor lagging under two load conditions.
Note: EA is not quite proportional to Ifunder load since the extrapolation results in an intercept not at origin.
For many practical applications, people Develop a useful approximate proportional relationship between EA and If .
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