CHAPTERS

Introduction:

Synchronous machines are principally used as alternating current (AC) generators. They supply the electric power used by all sectors of modern societies: industrial, commercial, agricultural, and domestic. Synchronous machines are sometimes used as constant- speed motors, or as compensators for reactive power control in large power systems. The synchronous machine is an important electromechanical energy converter. Rotating electrical machine ,generators and motors,are device that transform mechanical power into electrical power,and vice-versa. Electrical power from a central station can be transmitted and subdivided very efficiently and conveniently. This article explains the constructional features and the four operating principles of the synchronous machine will be briefly presented below. These principle are not difficult to understand,and illuminate most of the reasons for the stage in the historical development of electrical power, and especially of electrical railways.

Synchronous generators usually operate together (or in parallel), forming a large power system supplying electrical energy to the loads or consumers. For these applications synchronous machines are built in large units, their rating ranging from tens to hundreds of megawatts. For high-speed machines, the prime movers are usually steam turbines employing fossil or nuclear energy resources. Low-speed machines are often driven by hydro-turbines that employ water power for generation. Smaller synchronous machines are sometimes used for private generation and as standby units, with diesel engines or gas turbines as prime movers.Synchronous machines can also be used as motors, but they are usually built in very large sizes. The synchronous motor operates at a precise synchronous speed, and hence is a constant-speed motor. Unlike the induction motor, whose operation always involves a lagging power factor, the synchronous motor possesses a variable-power-factor characteristic, and hence is suitable for power-factor correction applications. A synchronous motor operating without mechanical load is called a compensator. It behaves as a variable capacitor when the field is overexcited, and as a variable inductor when the field is under excited. It is often used in critical positions in a power system for reactive power control. This page discuss motors in general ,but the specific application to electric locomotive is emphasized. Electricity is the medium that carries power from the prime over, whether at a central power station or on the locomotive,to it‘s point of application at the rail,and allows it to be controlled conveniently.

Power is rate of doing work. One horsepower means lifting 550 pound by one foot in one second. Mechanical power is force times speed. One watt is a current of one amper(A)flowing in a potential difference or voltage of one volt(V).Electrical power is current (in

amperses,A) time voltage (in Volts,V).746W is equivalent to 1 hp. A medium-sized electric locomotive might have a rating of 2000KW,or 2680 hp. At 85%efficincy ,and a voltage of 15 KV,157A is drawn from the overhead contact wire. Torque is the rotational equivalent of force,often useful in speaking of motors. It is force times perpendicular distance,and power is torque times rotational speed in radians per second. The first principle (shown in fig.1.1)is that an electrical current causes a magnetic filed which surrounded it like a whirlpool,and this filed,is not material but rather a region of influence on other current and magnets,is guided and greatly strengthened (by more than a thousand times) by passing through iron. When the current reverses in direction so does the magnetic filed. Currents deep in the earth cause its magnetic filed,and the energy to drive them comes from either the rotation of the earth or the flow of heat within the earth. The filed acts on the compass needle,which is a magnet. This principle can be called "electromagnetic action".

Fig. 1.1. Priciple 1: a current creates a magnetic filed surrounding it that is strengthened by passing through an iron core.

The second 9shown in fig. 1.2) is that an electrical current in a magnetic filed (produced by some other currents) experiences a force perpendicular to both the direction of the current and the direction of the magnetic filed ,and reverse if either of these reverses in direction. The force is proportional to the current and to the strength of the magnetic filed.This principle can be called :motor action".

Fig. 1.2. Principle 2: a force is exerted on a current in a magnetic filed perpendicular to the place of the magnetic filed and the current.

The third (shown on fig. 1.3) is that an electrical conductor,such as a copper wire moving in a magnetic filed has an electrical current induced in it. This is expressed by the creation of an electromotive force or voltage, which causes current to flow just as the voltage of a battery does. The effect is maximum when the wire, the motion,and the magnetic filed are all mutually perpendicular. This principle can be called "generator action".

Fig. 1.3. Principle 3: a voltage is induced in a conductor moved in a magnetic filed. Note that the voltage is opposite to the current causing a force in the direction of motion by principle 2.

And the fourth principle (shown in Fig. 1.4) is that a changing magnetic filed causing voltage in any circuit through which it passes. The change can be caused by changing the current producing the magnetic filed, or by moving the source of the magnetic filed. This principle can be called "transformer action".

Fig. 1.4. Principle 4 " a changing magnetic filed induced a voltage however the change is produced. Only a change induces,not a constant magnetic filed.

A rotating electric machine consist of a filed and an armature that rotate with respect to each other. The armature is the part of the

machine in which the energy conversion take place. The filed provides the magnetic filed to aid this process. In DC machines,the filed is stationary (the stator) and the armature rotates within it (the rotor),because the rotation is necessary to switch the armature connection by means of commutator ,but it is only the relative motion that counts. In an alternator, the armature is stationary and the filed rotates. The filed consist of an iron core to carry the magnetic filed ,and a winding to excite the magnetic filed by current passing through it (first principle). The magnetic filed is a passive but essential component in the operation of the machine. Like the filed, the armature also consist of iron to complete the magnetic circuit, and is separated by a short air gap from the iron of the filed. It is important that the air gap be as small as possible and remain uniform as the armature rotates.

The armature also has windings. In a generators, these conductors are moved in the magnetic filed producing a voltage (generator action). If a circuit is completed and current flows in these windings, a force is produced resisting the rotation of the armature (motor action) so that the driving machinery experience a mechanical resistance and does work, which is being transformed into electrical energy. In a motor, these conductors are supplied with an electrical current,so that a force acts on them in the magnetic filed (second principle), and this force can do external work. When the armature rotates while exerting the force, work is done, but a voltage is also produced opposite the applied voltage, resisting the flow of current in the armature (third principle),implying a change of electrical work into mechanical work.

This voltage generated when the armature if a motor is called counter-electromotive force. It might seem that it resists current flow through the motor, and od course it does, but it is really the essential factor in turning electrical into mechanical energy. Only the current that is driven into a counter-emf appears as mechanical work at the motor shaft: all else is wasted, the energy going into heat instead of mechanical work. Early inventors of electric motors did not realize this, and tried simply to get as much current into the motor as possible, which only burned the motor up without producing any mechanical effect.

Current is supplied to the armature through sliding contacts formed by graphite blocks (called brushers because originally brushes of phosphor bronze wire used instead) pressing against copper rings. It is usually necessary to change the connections of the armature windings as they rotate with respect to the magnetic filed, and this can conveniently be done by making the copper rings in segments. The result is the rotary switch called the commutator. These days, semiconductors switches can be used for this purpose in small motors, eliminating the commutator, but the principle is the same.

The commutator and brushes are the only parts of a machine that normally requires maintenance, except for the bearings and other mechanical elements. Is it is not necessary to switch the current, as in AC machines, the moving contacts are called slip rings.

Contemplate now the complete chain of energy flow from the prime mover, a team or internal combustion engine, to the point where the mechanical power is finally applied. The transformation at each end must take place with a smooth mutual reaction, based on the second and third principle. This was not properly understood until the 18880‘, so that practical electric transmission of power was delayed until that time. Power that is not delivered to the load is lost as heat in electrical resistance., which is equivalent to mechanical friction. Heat is produced in the generator, transmission lines, and motor, and limits the amount of power that can be handled. Electrical motors were invented early , in 1830 ‘s as soon as the magnetic effects of electrical current and the magnetic properties of iron became known. The motors of Christie and Pixii are typical of these, which used the repulsion and attraction between electromagnetic poles switched by a commutator. Small motors of this kind are still made for classroom demonstration. Attempts in the 1840‘ s to make these motors more powerful and larger failed completely, because the magnetic forces do not scale proportionally to the distance, and the significant of counter-emf was not known. The motors of davenport and Long in the United States are example of these unsuccessful attempts to scale up classroom demonstration to practical size.

More success was encountered in making generators, usually by moving permanent magnetic ( thereby creating a moving magnetic filed) with respect to coils or wire wound around iron cores, to generate alternating current for supplying arc lights and direct currents for electrolysis tanks (transformer action). These generators all ran quite hot because of their lack of efficiency, but supplied the greater required for these applications more cheaply than chemical batteries . This industry evolved into the electrical power industries of later years.

Siemens and Gramme solved the problem of efficiency in the late 1870 ‘s by introducing magnetic circuits that did not change as the armature rotated, so that the electrical reactions were smooth and constant. Siemens‘ first machine ( a generator ) of 1866 is shown on Fig . 1.5, and a Gramme dynamo, which could also serve as motor if the brushes were repositioned, is shown in Fig. 1.6. These mechanical had smooth armature with conductors on their surfaces. It was still though that conductors actually head to be immersed in the magnetic filed to produce forces. Soon it was discovered that if the conductor were put into slots in the armature surface, the same result was obtained. This was far superior mechanically and also

made a smaller air gap possible.

The first long distance transmission of electrical power took place in 1886 over the 8 km between Kriegstetten and Solothurn in Switzerland. Two Gramme machine in series were used as generator, and two similar machine in series as the other end were used as motors. The line voltage was 2000V, and wire 6mm in diameter (1/4"). The shaft-to-shaft efficiency was 75%, and installation remained in service until 1908.

Edison’s famous Z-type dynamos (as direct-circuit generator are often called) appeared in 1879 to supply his carbon-filament incandescent lamps see Fig. 1.7. These had long fields on the mistaken assumption that this gave a more powerful magnet (like a longer lever), showing how little magnetic circuits were understood at the time. This arrangement allowed a great deal of magnetic leakage between the long arms, and made the flux distribution in the armature nonuniform. Hopkinson, an engineer with Edison’s British company,rationalized the field geometry,making a very good generator of modern type a few years later. The filed was symmetrical with respect to the armature, and short . A closely related type, the Manchester dynamo, is shown in Fig. 1.8. Compare its compact and short magnetic circuit with that of the Edison Z. Note the brush holder and the brass commutator on the armature. This is a two-pole machine, because the field has one N pole and one S pole.

One thing that may worry you if you examine an electrical machine closely also worried early designers. They put the wires on the surface of the armature where they would actually be in the magnetic field and experience motor or generator action, in the way we have explained it here by our principles. However, wires are not always placed in slots cut in the armature iron, allowing the air gap to be made smaller and the magnetic circuit much more efficiency. The overall result is the same as if the wire were actually in the magnetic filed, but the mechanism is slightly different. Now the armature current in the motor magnetizes the armature iron, and interaction of this magnetic with the field poles provides the force. In a generator, the field magnetized the armature iron, and this field moves past the conductors as the armature rotates, with an effect like a transformer. Siemens, I believed, was the one who first saw this and great improvement it could make in electrical machines.

The commercial birth of the alternator (synchronous generator) can be dated back to August 24, 1891 shown in Fig. 1.9. {1,2} .

Introduction In which a brief history and rating of different synchronous

generator system are presented.

Synchronous generatorIn this chapter, construction, operation, characteristics and efficiency of the synchronous generator are presented.

Excitation system and voltage controlHere, types of excitation system and AVR are discussed.

Prime movers and frequency controlIn this chapter, steam turbines, gas and diesel engines with their speed governors are presented.

Synchronous of alternatorsHere, the concept of synchronization, conditions and methods of synchronization parallel operations of alternators and synchronization with infinite bus-bar are discussed.

Simulation resultsIn this chapter, simulation oscillograms for different cases, obtained using SIMPLORER software, are presented. A method is developed in the simulation scheme to control the terminal voltage and frequency of the synchronous generator whether isolated or synchronized.

Summary and conclusionIn which summary and conclusion of the project work is presented. Moreover suggestion for further work is also given.

Reference

CHAPTER3

EXCITATION SYST"EM AND VOLTAGE CONTROL

EXCITATION SYSTEM AND VOLTAGE CONTROL

For the generator to function as a generator, magnetizing current (or “excitation”)must be supplied to the generator rotor winding. The excitation system providesthis function. The system is designed to control the applied voltage, and thus thefield current to the rotor, which in turn gives control of the generator output orterminal voltage. Subsequently this is what provides reactive power and powerfactor control between the generator and the system.Voltage requirements range from very low dc levels up to 700 volts dc forthe larger generators. The dc field currents may approach 8000 amps dc on thelarger turbogenerators.Excitation response time must be fast so that the automatic voltage regulatorcan control the generator during system disturbances or transients in which rapidchanges of excitation are necessary. Field forcing is the term generally used forthis mode of operation and requires the exciter to be capable of field forcingvoltages from 2 to 3 times the rated dc field voltage. Therefore the rotor windingmust be insulated for these voltage levels. The excitation system of a synchronous generator contains the exciter and an automatic voltage regulator (AVR) shown in Fig. 3.1.{2}

3. 1. Exciter

The exciter is, in fact ,the DC power supply that delivers controlled power to SG excitation (filed) winding. The exciter power is supplied either from a permanent-filed pilot exciter, a separate excitation transformer, an auxiliary winding of self-excite. The excitation may be classified into the following:1) Rotating Exciter.2) Static Exciter.

3.1.1Rotating Exciter are classified to : a) Brush Exciter:1)Where the exciter is dc generator. The brush-type exciter can be mounted on the same shaft as the AC generator armature or can be housed separately from , but adjacent to ,the generator. When it is housed separately, the exciter is rotated by the AC generator through a drive belt.The distinguish feature of the rush-type generator is that stationary brushes are used to transfer the DC exciting current to the rotating generator filed. Current transfer is made via rotating slip rings (collector rings ) that are in contact with the brushes. Each collector ring is a hardened-stell forging is mounted on the exciter shaft. Two collector rings are used on each exciter, each ring is fully insulated from the shaft and other . The two slip rings on the shaft are connected to one end of the DC rotor winding and number of brushes ride on each slip ring. The positive end of the DC voltage is connected to one slip ring,and the negative end is connected to the second,this ensure that the same DC voltage is applied to the field winding regardless of the angular position or speed of the rotor. Slip rings and brushes require high maintenance regularly, also because the voltage drop across the brushes can be the cause of large power losses when filed current are high.

Fig. 3.2 Typical excitation system for large synchronous machine i. Direct excitation system with separately excited E mounted

on the shaft of the synchronous machine SM.ii. Direct excitation system with self-excited exciter driven by

SG-shaft via reduction gear.iii. Indirect excitation system in which motor M driving the

exciter is fed from an auxiliary generator G mounted on SM-shaft.

A typical alternator with dc exciter is shown in Fig. 3.3. The exciter is a dc, shunt-wound, self-excited generator. When the exciter armature is rotated in the exciter-filed flux, voltage is induced in the exciter armature windings. The output from the exciter commutator is connected through brushes and slip rings to the alternator filed. Since this is direct current already converted by the exciter commutator, the current always flows I one direction through the alternator filed. Thus a fixed-polarity magnetic filed is maintained at all times in the alternator filed windings. When the alternator filed is rotated, its magnetic flux is passed through and across the alternator armature windings.

2)Auxiliary Exciter ( pilot exciter ): The DC exciter (Fig.3.4.), still in existence for many SGs below 100 MVA per unit, consists of twoDC commutator electric generators: the main exciter (ME) and the auxiliary exciter (AE). Both are placedon the SG main shaft. The ME supplies the SG field winding (Vf ), while the AE supplies the ME fieldwinding.Only the field winding of the auxiliary exciter is supplied with the voltage Vcon

controlled by the AVR.The power electronics source required to supply the AE field winding is of very low power rating, as thetwo DC commutator generators provide a total power amplification ratio around 600/1.The advantage of a low power electronics external supply required for the scope is paid for by thefollowing:• A rather slow time response due to the large field-winding time constants of the two excitationcircuits plus the moderate time constants of the two armature windings• Problems with brush wearing in the ME and AE• Transmission of all excitation power (the peak value may be 4 to 5% of rated SG power) of theSG has to be through the slip-ring brush mechanism• Flexibility of the exciter shafts and mechanical couplings adds at least one additional shaft torsionalfrequency to the turbine-generator shaftThough still present in industry, DC exciters were gradually replaced with AC exciters and static exciters.

3.1.2. Static Exciter Modern electric power plants are provided with emergency power groups for auxiliary services that maybe used to start the former from blackout. So, an auxiliary power system is generally available.This trend gave way to static exciters, mostly in the form of controlled rectifiers directly supplying thefield winding of the SG through slip-rings and brushes (Figure 6.23a and Figure 6.23b). The excitationtransformer is required to adapt the voltage from the auxiliary power source or from the SG terminals(Figure 6.23a).It is also feasible to supply the controlled rectifier from a combined voltage transformer (VT) andcurrent transformer (CT) connected in parallel and in series with the SG stator windings (Figure 6.23b).This solution provides a kind of basic AC voltage stabilization at the rectifier input terminals. This way,short-circuits or short voltage sags at SG terminals do not much influence the excitation voltage ceilingproduced by the controlled rectifier.Power electronics (static) exciters are characterized by fast voltage response.

Fig.3.4. Typical DC Exciter (pilot exciter).

3.1.2 Brushless Exciter (AC Exciter) Large generators used brushless exciters or (AC exciter) for supplying DC filed current to the rotor. They consist of a small AC generator having its filed circuit mounted on the stator and its armature circuit mounted on the rotor shaft. The exciter generator output (three-phase alternating current ) is converted to direct current by a three-phase rectifiers circuit also main generator can be controlled by the small DC filed current of the exciter generator, which is isolated on the stator Fig. 3.6 (a) and (b).