PLT301 /4 Electrical Machines Technology II

1. PLT301 /4Electrical Machines Technology II Synchronous Generator By; SYATIRAH BINTI MOHD NOOR Email: SYATIRAH@unimap.edu.my

2. DESCRIBE, IDENTIFY and EXPLAIN the principles and construction, • A synchronous machine is a double excited machine. Its rotor poles are excited by a dc current and its stator windings (armature winding) are connected to the ac supply. • The air gap flux is the resultant of the fluxes due to both rotor current and stator current.

3. Synchronous generator • Mechanism of ac voltage generation:- • Rotor flux is produced by a dc field current If. • Rotor is driven by a prime mover, producing rotating field in the air gap. • A voltage is induced in the stator winding due to the rotating field. • Induced voltage is sinusoidal due to the sinusoidal distributed flux density in the air gap.

4. Synchronous generator (The Speed of Rotation of a Synchronous Generator) Synchronous generators are by definition synchronous, meaning that the electrical frequency produced is locked in or synchronized with the mechanical rate of rotation of the generator. The rate of rotation of the magnetic fields in the machined is related to the stator electrical frequency is Where fe = electrical frequency (Hz) nm = mechanical speed of the magnetic field, rpm (= speed of rotor) P = number of poles

5. The Internal Generated Voltage of a Synchronous Generator The magnitude of the voltage induced in a stator phase is or Where, NC= no of conductors at an angle of 00 Construction of Three Phase Synchronous Machines • The stator winding of the three phase synchronous machines has a three phase distributed winding similar to that of the three phase induction machine. • Unlike the dc machine, in SG the stator winding, which is connected to ac supply system is called the armature winding. • The rotor winding has a winding called the field winding, which is carries direct current. The field winding on the rotating structure is fed from an external dc source through slip rings and brushes. • Slip rings and brush assembly is the arrangement which used to collect induced e.m.f from the rotating armature and make it available to the stationary circuit.

6. Construction of Three Phase Synchronous Machines Type of Rotor of Synchronous Generator:- • Cylindrical or non salient pole rotor • Unslotted portion of cylinder acts as poles. • one distributed winding and essentially uniform air gap • Mechanically strong, prime mover used are steam turbines, electric motor • Application : High speed machines (1500-3000rpm) • Salient pole rotor • Poles are sticking out from the surface. • concentrated winding on the poles and a uniform air gap • Mechanically weak, prime mover used are water turbines, IC engines • Application: Low speed machines (125-500rpm) Two common approaches to supply the dc supply to the rotor winding (field winding): Supply the power from an external dc source to the rotor by means of slip rings and brushes. Supply the dc power from a special power source mounted directly on the shaft of the generator. On the larger synchronous machine, brushless exciter are used to supply the dc field current to the machines. A brushless exciter is a small ac generator with its field circuit mounted on the stator and its armature circuit mounted on the rotor shaft.

7. Construction of Three Phase Synchronous Machines (A Brushless Exciter Circuit) A small three phase current is rectified and used to supply the field circuit of the exciter, which is located on the stator. The output of the armature of the exciter (on the rotor) is then rectified and used to supply the field current of the main winding.

8. Stator of Synchronous Generator

9. Round Rotor Generator View of a two-pole round rotor generator and exciter Cross-section of a large turbo generator. (Courtesy Westinghouse)

10. Round Rotor Generator Rotor block of a large generator. (Courtesy Westinghouse) Generator rotor with conductors placed in the slots Large generator rotor completely assembled. (Courtesy Westinghouse)

11. Salient pole generator Stator of a large salient pole hydro generator; inset shows the insulated conductors and spacers Large hydro generator rotor with view of the vertical poles

12. IDENTIFY and DETERMINE the equivalent circuit, • The voltage EA is the internal voltage generated produced in one phase of a synchronous generator. However, this voltage EA is not usually the voltage that appears at the terminals of the generator. • There are many factors that cause the difference between EA and VФ. • The distortion of the air gap magnetic filed by the current flowing in the stator called armature reaction. • The self inductance of the armature coils. • The resistance of the armature coils. • The effect of salient pole rotor shapes.

13. The Development of a Model for Armature Reaction Figure (a) shows a two pole rotor spinning inside a three phase stator. A rotating magnetic field produces the internal generated voltage EA. There is no load connected to the stator. The rotor magnetic field BR produces an internal generated voltage EA whose peak value coincides with the direction of BR. With no load on the generator, there is no armature current flow, and EA will be equal to the phase voltage VФ. Figure (b): The resulting voltage produces a lagging current flow when connected to a lagging load

14. The Development of a Model for Armature Reaction Figure (c): The stator current produces its own magnetic filed BS, which produces its own voltage Estat in the stator windings of the machine The current flowing in the stator in the stator windings produces a magnetic filed of its own. This stator magnetic filed is called BS and its direction is given by the right hand rule. The stator magnetic filed Bs produces a voltage of its own in the stator, and this voltage is called Estat. Figure (d): The field BS adds to BR, distorting it into Bnet. The voltage Estat adds to EA, producing VФ at the output of the phase. With two voltages present in the stator windings, the total voltage in a phase is just the sum of the internal generated EA and the armature reaction voltage Estat:

15. The Development of a Model for Armature Reaction • The net magnetic field Bnet is just the sum of the rotor and the stator magnetic fields: • Since the angles of EA and BR are the same and the angles of Estat and Bs, are the same, the resulting magnetic field Bnet will coincide with the net voltage VФ. • We know, the voltage Estatis directly proportional to the current IA. If X is a constant of proportionality, then the armature reaction voltage can be expressed as: • The voltage on a phase is

16. The Development of a Model for Armature Reaction In addition to the effects of armature reaction, the stator coils have a self inductance and a resistance. If the stator self inductance is called LA (and its corresponding reactance is called XA) while the stator resistance is called RA, then the total difference between EA and VФ is given by Combine the armature reaction effects and the self inductance in the machine So

17. The Development of a Model for Armature Reaction If the machine is Wye (Y ) connection If the machine is Delta (Δ) connection The Per Phase Equivalent Circuit of a Synchronous Generator

18. Per Unit System Definition: Base value (in normal): – Choose rated power for base value of power – Choose rated voltage for base value of voltage Other variables: Select then

19. Per Unit System Equivalent circuit in per unit system Usually VT,pu= 1.0, which is the rated voltage of the generator

20. DESCRIBE and EXPLAIN the power & torque characteristics, Input mechanical power Power converted from mechanical to electrical is Where γ is the angle between EA and IA

21. Power and Torque in Synchronous Generator • If the armature resistance RA is ignored (since Xs >> RA) Since the resistances are assumed to be zero Where torque angle, δ is the angle between VФ and EA The power of the generator is maximum when δ = 900 The maximum power indicated by this equation called static stability limit of the generator. The induced torque is

22. CALCULATE the parameter of the synchronous generator , The effect of load changes Measuring synchronous generator model parameter The behavior of a real synchronous generator is determine by • The relationship between field current and flux (and therefore between field current and EA) • The synchronous reactance, Xs • The armature resistance, RA • The quantities above are determined by Open Circuit Test and Short Circuit Test.

23. Open Circuit Test First step: • To perform this test, the generator is turned at the rated speed. • The terminals are disconnected from all loads. • The field current is set to zero. Second step: • The field current is gradually increased in steps, and the terminal voltage is measured at each step along the way with the terminals open. (IA = 0, so EA is equal to VФ) Plot EA or VA versus IF from this information • The curve almost perfectly linear, until some saturation is observed at high field currents. • The unsaturated iron in the frame of the synchronous machine has a reluctance several thousand times lower than the air gap reluctance, so at the first almost all the magnetomotive force is across the air gap, and the resulting flux increase is linear. • When the iron finally saturates, the reluctance of the iron increases dramatically, and the flux increases much more slowly with an increase in magnetomotive forces. The linear portion of an OCC is called the air gap line of characteristic. This plot called open circuit characteristics

24. Short Circuit Test Adjust the field current to zero again and short circuit terminals of the generator through a set of ammeters. Then the armature current IA or the line current IL is measured as the field increased. When the terminals are short circuited, the armature currents IA is Its magnitude is Refer to Figure (b), BS almost cancels BR, the net magnetic field Bnet is very small, so the machine is unsaturated and the SCC is linear.

25. Short Circuit Test The internal machine impedance is If XS >> RA, this equation reduces to Therefore, an approximate method for determining the synchronous reactances at a given field current is • Get the internal generated voltage EA from the OCC at the field changing. • Get the short circuit current flow IA,SC at that field current from SCC. • Find XS by equation above. The saturated synchronous reactance may also found by taking the rated terminal voltage (line to line) measured on the OCC and dividing by the current read from SCC corresponding to the field current that produces at rated terminal voltage.

26. Effect of Load Changes on a Synchronous Generator What will happen if we change the load on the generator??? • An increase in the load is an increase in the real/reactive power drawn from the generator. • As the load increase, it will increase the load current as well. • Since the field resistor is not changed, the field current will be constant, so that the flux (φ) is also constant. • If the speed of the generator is constant, hence the magnitude of the internal generated voltage EA is constant. • If unity loads (no reactive power load) are added, Vφ will decrease slightly. The Phasor Diagram of A Synchronous Generator c) Unity Power Factor

27. Load Changing Effect on Synchronous Generator • If lagging loads (inductive power load, +Q) are added, Vφ will decrease. • The Phasor Diagram of A Synchronous Generator (a) Lagging Power Factor

28. Load Changing Effect on Synchronous Generator • If leading loads (capacitive power load. -Q) are added, Vφ will increase. • The Phasor Diagram of A Synchronous Generator (b) Leading Power Factor

29. Synchronous impedance test • This method is used to determined values of R and XLS for alternator. • The line of single-phase alternator are short circuited through an ammeter when switch s closed. • The field excitation current increases until the current in the armature is nearly 150% of the rated full-load current. This value of current is recorded. Then switch s is opened. • The reading of a voltmeter connected across the generator terminals is noted. • The field excitation current and the alternator speed are keep at constant values.

30. To determine the synchronous impedance of the armature winding. • The voltage for an open circuit condition is divided by the current for short-circuit condition. and

31. Synchronous impedance of a Wye-Connected Generator test • The three-pole switch shorting switch. • The field excitation current increases until the current in the armature is nearly 150% of the rated full-load current. This value of current is recorded. Then three-pole switch is opened. • The reading of a voltmeter connected across the generator terminals is noted. • The field excitation current and the alternator speed are keep at constant values.

32. To determine the synchronous impedance of the armature winding. • The voltage for an open circuit condition is divided by the current for short-circuit condition. and

33. Voltage Regulation of Synchronous Generator The voltage regulation of the generator can be determined by the equation below which it can be used to compare the voltage behaviour among the generators. Generally, the synchronous generator operating at lagging PF has large positive VR, a synchronous generator operating at leading PF has negative VR while a synchronous generator operating at unity PF has always small positive VR.

34. EXPLAIN the Parallel operation condition of Synchronous generator, • The following condition must be observed: • The output voltages of alternator must be equal. • The frequencies of the alternator must be same. • The output voltages of the alternators must be in phase. • The following steps describe the actual process of synchronizing two three-phase alternator: • Assume that alternator 1 is supplying energy to the bus bas of the station at rated voltage and frequency. • An incoming machine , alternator 2, is to be synchronized with alternator 1 for the first time. The speed of alternator 2 is increased until it turns at the value required to give the desired frequency. The voltage of generator 2 is adjusted by means of its field rheostat until it is equal to that of generator 1. • The three voltages of the incoming generator must be in phase with the respective voltages of generator 1. To accomplish this, the phase sequence of the two alternators and their frequencies must be the same. The use of synchronizing lamps is a simple way to check these relationships.

35. Synchronizing Two Alternators • The three lamps go on and off in unison depending on the frequency difference between the two alternator. • A slight adjustment in the speed of the prime mover for the alternator coming on line will make the frequency of this machine the same as the alternator presently on the line. • When all three lights go out, the instantaneous electrical polarity of the second machine will equal that of the alternator on the line. The second machine can be brought on line, and the generators will be paralleled • Three lamps methods • Three lamps dark method • Two bright, one dark method • Use of the Synchroscope Three lamps methods Three lamps dark method • This method is used to determine the phase sequence of an alternator. • One the phase sequence is known, permanent connection can be made between stator windings, the switching equipment, and the station bus bars. It is not necessary to determine the phase sequence each time the alternator is parallel once the equipment is marked correctly. • This method is also used to indicate when alternators are synchronism.

36. This method is never used to determine the phase sequence. • It is used to indicate the synchronism of two alternator. • When the incoming alternator is in synchronism (ready to be paralled), the two lamps in line wires 1 and 2 will have a maximum brightness, and the lamp in line wire 3 will be dark. Two bright, one dark method • Once the phase sequence is known to be correct and permanent connections are made, a synchroscope can be used. • The single-phase synchroscope indicates synchronism accurately. • A synchroscope gives an accurate indication of the differences in the frequency phase relationship between two voltages. • A pointer rotates over a dial face. When the pointer stops, the frequencies of the two alternators are the same. • When the pointer stops in a vertical upward position, the frequencies are equal and the voltages are in phase. This means that the alternators are in synchronism and the alternator switch can be closed to parallel both machines. Use of the Synchroscope

37. Alternator nameplate data • Consist; -The full-load terminal voltage, - The rated full-load current per terminal, -The number of phases, - The frequency, -The speed in r/min, -The operating power factor, -The dc field current and voltage, -The maximum temperature rise