Lecture 31Electro Mechanical System1 Field Excitation The dc field excitation of a large synchronous generator is an important part of its overall design. It must ensure stable terminal ac voltage but must respond quickly to load variations. Three methods of excitation are: 1.Slip rings link the rotor’s field winding to an external dc source. 2.DC generator exciter;  For quickness of the response, two dc generators are used: main exciter and a pilot exciter  Main exciter feeds the exciting I X current to fields of generator.  Main exciter voltage is regulated by pilot excter current I C.  A dc generator is built on the same shaft as the ac generator’s rotor.  A commutator rectifies the current that is sent to the field winding.

Lecture 31Electro Mechanical System2 Field Excitation 3.Brushless exciter  Due to brush wear and tear and slip-ring problems, brushless exciters have been developed.  An ac generator with fixed field winding and a rotor with a three phase circuit.  Diode/SCR rectification supplies dc current to the field windings.

Lecture 31Electro Mechanical System3 Size of synchronous generators  Companies are very conscious about the efficiency of generators.  The efficiency of a 1000MW generating station improves by only 1% means extra revenues of several thousand dollars per day.  The size of the generator is particularly important because its efficiency automatically improves as the power increases.  If a small 1 kilowatt synchronous generator has an efficiency of 50%, a larger, but similar model having a capacity of 10 MW inevitably has an efficiency of about 90%.  Synchronous generators of 1000 MW possess 99% efficiency.  Another advantage of large machines is that the power output per kilogram increases as the power increases.  1 kW generator weighs 20 kg (yielding 1000W/20 kg = 50 W/kg).  10MW generator of similar construction will weigh only kg, thus yielding 500 W/kg.  From a power standpoint, large machines weigh relatively less than small machines; consequently, they are cheaper.  As the size increases, we run into serious cooling problems.

Lecture 31Electro Mechanical System4 No load saturation curve  A 2-pole synchronous generator operating at no-load is shown  driven by a turbine at a constant speed  The generator is Wye connected with terminals A,B,C,N, the variable exciting current I x produces flux in the air gap.  Let us now increase the exciting current gradually, while observing the ac voltage E o between the terminals.  For small value of I x, voltage increase is directly proportional to exciting current.  However when the iron begins to saturate, voltage rise much less for the same increase in I x.  We obtain no load saturation curve of synchronous generator.

Lecture 31Electro Mechanical System5 Synchronous reactance & equivalent cct.  A 3-phase synchronous generator having terminals A, B, C feeding a balanced 3-phase load  The generator is excited by a dc current I x.  The machine and its load are both connected in wye.  Neutrals N, and N 2 are not connected, but they are at the same potential.  Field carries an exciting current which produces a flux .  As the field revolves, the flux induces in the stator three equal voltages E o that are 120° out of phase.  Each phase possesses a resistance R and L.

Lecture 31Electro Mechanical System6 Equivalent Circuit  Induced voltage, E O  Voltage induced as flux cuts across windings  Winding inductance X S = 2  fL where: X S = Synchronous reactance per phase[Ω] f = generator frequency [Hz] L = Inductance of stator winding, per phase [H]  Winding resistance  Usually 1/100 of the size of the reactance  Often neglected in the equivalent circuit

Lecture 31Electro Mechanical System7 Synchronous Reactance  The value of X S can be determined by measurements of the open-circuit and short-circuit tests  Test are conducted under an unsaturated core condition  Open-circuit test is conducted at rated speed with the exciting current I xn adjusted until the generator terminals are at rated voltage, E n  Short-circuit test is conducted at rated speed with the exciting current I xn gradually raised from 0 amps up to the original value used in the open-circuit test  The resulting short-circuit current I sc is measured, allowing the calculation of X S X S = E n /I SC Where: X S = Synchronous reactance per phase[Ω] E n = Rated open circuit voltage line to neutral [V] I SC = Short-circuit current, per phase, using same exciting current I xn that was required to produce E n [A]