GENERATOR CAPABILITY CURVE

1. GENERATOR OPERATION LIMITS
By Pradeep Yadav,
Dy. Director.

2. When a manufacturer designs & construct an
alternator, It will specify the MCR ( max.
continuous rating )of the machine in terms of;
1. Stator current.
The operation of the generator must be such that
none of the design limitations are exceeded as
otherwise damage to the generator may result.
2. Stator voltage.
3. Rotor current.
4. Rotor voltage.
5. MW.
6. MVA.
7. Power factor.

3. There are 5 principal limitations on the
generator operation.
Principal limitations:
A convenient way of presenting these
limitations is in the form of a diagram
called as “ Capability curve”.
1. Stator current limit.
2. Rotor current limit.
3. MW limit.
4. Stability limit.
5. Stator end heating limit.

4. Assumption: Generator terminal voltage is
maintained constant
Erect a voltage vector “OA”.
Capability curve :

5. Principal limitations:
1. Stator current limit : Heat developed in the
winding is limited by the class of Insulation.
At higher current, overheating may damage
insulation.
Class “B” Insulation - 130 Deg. Cel.
Class “F” or “H” - 180 Deg. Cel.

7. 2. MW limit
The Max. continuous MW rating is
dictated by the steam generation
capability of boiler & the H.P rating of the
turbine.
In theory, generators are capable of
generating an output equal to MVA rating.
But generators are designed to operate at a
lagging p.f. ( 0.85 ). So that max.output is
achieved at max.stator current & at rated p.f.
MW = MVA X Cos =MVA at unity p.f.
= V X I X Cos

8. A horizontal line line drawn from point B
will give MW limit boundary

9. 3. Rotor current limit:
Same reasoning as in case of stator current
limit.
Short Circuit Ratio( SCR ) is the ratio of field
current required to induce rated nominal
voltage in the Gen.terminal in open circuit
condition
TO
the field current required to circulate rated
stator current in the Gen. terminals with the
terminals short circuited.

10. Then, GB will represent full load rotor current or
total field current. With GB as radius , draw an
arc. It will be locus of rotor current and its limit.
Angle between GO & GB will be Rotor angle or
stability angle ().
Select a point “G”, such that OG/OB = SCR of
the generator.

11. 4. Stability limit/Minimum rotor current
limit:
For every load condition of the generator, a
minimum rotor current has to be supplied
to maintain magnetic coupling in the air
gap between stator & rotor.A reduction in
field current without any change in steam
admission will lead to break in magnetic
coupling ( increase in rotor angle) with the
subsequent loss of machine stability.

12. Rotor angle ()= 90° - Theoritical stability limit.
Practical stability limit () = 75°.

14. 5. Stator end heating limit:
With high values of stator
current at leading power factor , heating
of the end sections of the stator core tend
to occur.
The end heating limit would normally only
be reached if the stability limits were
exceeded.

15. 1.Variation of terminal voltage:
Voltage(KV) 14.18 14.96 15.75 16.54 16.7 16.85 17.01 17.17 17.32
-------------------------------------------------------------------------------------------------
Stator Current(KA)9.5 9.5 9.05 8.6 8.37 8.14 7.92 7.56 7.24
2.Frquency variation: +5%, -5%
3. Overloading:
Stator current(KA) 9.95 10.41 10.86 11.31 11.76 12.22 12.67 13.57
Rotor current(KA) 2.75 -- -- --- 3.12 -- 3.9 5.2
Time ( Sec. ) 3600 900 360 300 240 180 120 60
4.Operation at different Hydrogen pressure:
Hydrogen Pr.(Kg./Cm2) M.W. Duration
3.0 200 Continous
2.5 170 Not more than 5 Hrs.
2.0 140 -------do------
“Operation in air medium is not permitted”

16. 6.One gas cooler out of service – 175 MW continously.
More than one gas cooler out – operation not permitted.
Coolent: Normal Limit Property
Cold Hydrogen gas temp. 44 Deg.cel 55Deg.cel 3.5Kg/cm2. 99% pure
Stator water temp. 45 Deg.cel 48 Deg.cel 27m3/hr.