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.