Because the power input to the field coils varies as the square of the field current, this power input is now 5% of 5% of 100 watts or 0.25 watt. Thus, a field power input of 0.25 watts results in the production of a field \( Qa \) as powerful as the field produced directly by a field input of 100 watts. If this flux is used to generate a voltage and this voltage is applied to some external load, the overall power amplification is 400 times that of an ordinary DC generator, that is, a power amplification of 40 000 : 1.
The flux \( Qa \) can be utilized by placing two brushes midway between the short-circuited brushes, as shown in Fig. 25 and connecting these to the external load circuit.
Figure 25
Amplitdyne Generator Showing Load
Circuit and Control-Field Circuit
The \( E \) applied across the load is produced by the conductors cutting the vertical flux \( Qa \) ; the cutting of \( Qc \) does not contribute anything to this \( E \) . A generator excited in this way is called an armature reaction excited machine.
It is necessary to provide a compensating winding, placed in slots in the stator iron and connected in series with the load, to neutralize the armature reaction of the load current because this armature reaction is in direct opposition to \( Qc \) .
The wiring diagram of the Amplitdyne then becomes as shown in Fig. 26.
The high-speed response of the amplitdyne is made possible by the fact that its control field requires only about 1 or 2% of the number of ampere turns used on the field of an ordinary DC generator. Consequently, the inductance of its field winding is much smaller and the rate of change of the field current much more rapid.
Figure 26
Amplitdyne Circuit Showing
the Compensating Winding
Fig. 27 shows two typical field connections using amplidynes.
Figure 27
Amplitdyne Field Control Circuits
Magnetic Amplifier Regulator
To understand this type of regulator, it is necessary to be familiar with the mode of operation of a magnetic amplifier. It consists of windings on a core, the magnetic characteristics of which are very important. The permeability must be high and must be almost constant up to the saturation point. The ideal \( B \) - \( H \) characteristic ( \( B \) is flux density - \( H \) is magnetizing force) is illustrated in Fig. 28.
Figure 28
Ideal B-H characteristic for a Magnetic Amplifier Core
If two coils are wound on a core of this material, with one coil as a series impedance in an AC circuit and the other carrying direct current, impedance of the AC circuit can be varied by altering the current flowing in the DC coil. Consequently, by careful design, small variations in the control current (the name for direct current) can produce large changes of current in the a-c circuit. If the alternating current is rectified after passing through the amplifier windings, it becomes possible to produce large changes in direct current from small changes in another current.
The control winding of the magnetic amplifier is supplied by a voltage transformer connected to the generator terminals. The supply for exciting the machine is taken as a three-phase supply from a shaft alternator, which is connected in series with the AC windings of the magnetic amplifier, rectified, and connected in series with the main exciter field winding. Thus, changes in output voltage directly control the main field current.
The great advantages of this scheme are that there is a minimum of moving parts, no electronic components, the regulator is built entirely of static components, the time response is very rapid, and there is no dead band.
The regulator works from the difference between the generator terminal voltage and a fixed reference voltage derived from the pilot generator. The difference between these two voltages is amplified by magnetic amplifiers in cascade and is fed directly to correct the main exciter field current.
The normal DC pilot exciter is replaced by a 400 Hz self-excited pilot generator, the output being subsequently rectified to feed the main exciter winding. The higher frequency enables the magnetic amplifiers to give a much faster response and reduces the dimensions of the magnetic components. The output of the pilot generator is controlled by an induction regulator working in conjunction with magnetic amplifiers and the rectified output of the combination is fed into the field circuit of the generator. The induction regulator in this design replaces the conventional field rheostat.
Static Excitation System
An automatic voltage regulator (AVR) is a device that causes the excitation of the generator to be automatically increased or decreased so that the terminal voltage can be maintained within acceptable limits. The device must be able to detect a change in voltage and quickly respond to difference between the measured voltage and the desired voltage. Voltage regulation is expressed in percent by the following formula:
$$ \frac{(\text{No-load volts}) - (\text{Full-load volts})}{\text{Full-load volts}} \times 100 $$
Modern voltage regulators maintain voltage within \( \pm 1.5\% \) . Generator voltage regulators are classified by the means of operation.
Direct-acting rheostatic regulators adjust the field strength of a generator directly. This type of AVR was common in older generators. The rheostat varied the resistance and therefore the current, in the exciter circuit. By varying the current, the field strength in the generator is varied. The rheostat was manually operated but automated by using an electromagnet that varied the resistance based on the generator voltage.
The indirect acting rheostatic regulators use an auxiliary device, such as a motor, to operate the field rheostat.
Many older voltage regulators have been replaced by static excitation systems and digital voltage regulators.
As the name suggests, there are no moving parts in a static voltage regulator system. Fig. 29 shows a block diagram for a static excitation system and automatic voltage regulator. Initial voltage buildup in the generator is due to residual magnetism in the field. As voltage builds, a transformer is used to provide power from the generator output terminals to an automatic voltage regulator. Power from the generator terminals is also provided to a thyristor rectifier bridge. The AVR provides a signal to the gates on the thyristors, which modulate the DC voltage to the generator field.
Figure 29
Static Excitation System
Thyristors are similar to diodes but are designed with a gate that allows the amount of rectified DC voltage to be varied. This is shown in Fig. 30.
Figure 30
Thyristors
Objective 5
Describe the cooling systems used for an alternator including circulating air cooling, hydrogen cooling, and stator winding cooling water systems.
ALTERNATOR COOLING SYSTEMS
Cooling systems for alternators fall into the following fours types depending on the volt-amp output of the unit:
- • direct air
- • enclosed air
- • hydrogen
- • water
Direct Air Cooling
Small alternators use fans mounted on the rotor shaft. Ambient air is drawn in through one end of the frame and discharged out the opposite end. This type of cooling is not well suited for dusty environments. Fig. 31 shows a photograph of a small alternator. Notice the ventilation slots on each end of the frame.
Figure 31
Small Alternator
In larger alternators, there are considerably higher \( I^{2}R \) losses and magnetic losses. These types of losses produce heat that must be properly dissipated. The physical size and structure of alternators are determined by the manner in which the machine is cooled.
Fig. 32 shows a side view of a generator with direct-cooling fans on both ends of the rotor. The top part of the drawing shows an end view of the two fans. The arrows on the drawing show the paths the cooling air takes.
Figure 32
Generator with Direct Cooling Fans
Enclosed Air Cooling
Air-cooled alternators are limited in size due to the inability of air to cool the rotor and stator to acceptable ranges. For larger generators, in the range of 25 to 325 MVA (mega-volt-amps), the generator is totally enclosed with separate fans supplying cooling air. Fig. 33 shows a side and end views of a typical air-cooled arrangement.
The air is filtered before it is supplied to the generator. Air from two fans is blown through air coolers into the stator and field windings and the exciter cabinet. Air return ducts are provided to guide the exhaust air back to the inlet of the fans.
The air coolers consist of finned tubes through which cooling water is circulated. The air is cooled as it is blown through channels between the finned tubes.
Figure 33
Arrangement of Alternator, Exciter and Sliprings Air-cooling System
Hydrogen Cooling
Hydrogen has properties that allow it to be a better coolant for larger alternators. Hydrogen is used on units that have a capacity in the range of 200 - 1000 MVA. The greater cooling ability of hydrogen vs. air allows for smaller physical dimensions of alternators. But, there is a greater cost associated with hydrogen cooling. Fig. 34 is a photograph of 1120 MVA hydrogen cooled generator manufactured by Toshiba Corporation.
The increase in rated output obtained with hydrogen gas can be 20% - 30% based on a hydrogen pressure of 3.5 kPa. Further increases can be realized with greater hydrogen pressure. Alternators are commonly operated at pressures of 100 kPa.
Hydrogen gas has several advantages over air:
- • High heat transfer coefficient, absorbs heat rapidly as it is flows through the generator windings
- • Rejects this same heat more rapidly in the cooling stages of the ventilation circuit
- • Less dense than air and requires less energy to drive it through the different generator components
- • Offers less braking effect (windage) to the rotating parts of the alternator.
Figure 34
1120 MVA Hydrogen Cooled Generator
(Toshiba Corporation)
Overheating in generators can cause fires. This can be a serious problem with air ventilation systems since air supports combustion. The fire hazard with hydrogen is reduced. The airtight system prevents buildup of dirt that may cause fires.
The absence of air in the ventilation system also prevents the formation of acids that otherwise may attack the insulation on the rotor and stator windings.
However, there is a risk of explosion if hydrogen is mixed in the proper proportions with air. This is prevented by air tight casings and proper seals around the rotor shaft.
Hydrogen must be kept free from moisture because it increases the viscosity of the hydrogen and decreases its ability to carry away excess heat. Moisture also deteriorates the seals on the rotating shaft.
Normally, the hydrogen gas circulates in a closed loop through a water-cooled heat exchanger and a molecular sieve or desiccant dryer before returning to the generator enclosure.
Fig. 35 shows a schematic diagram of the equipment layout and piping diagram for a hydrogen cooling system.
| Symbol | Key to Symbols |
|---|---|
|
Image: Symbol for Shut-Off Valve: a circle with a diagonal line and a crossbar. |
Shut-Off Valve |
|
Image: Symbol for Non-Return Valve: a circle with a diagonal line and a triangle. |
Non-Return Valve |
|
Image: Symbol for Electrical Connections: a dashed line. |
Electrical Connections |
Figure 35A
Hydrogen Cooled Generator System
Figure 35B
End View of Hydrogen Cooled Generator System
The gas flow paths for hydrogen cooling vary with manufacturers' designs. In some cases, the gas enters at each end of the rotor and leaves at the centre. Other designs have the gas enter one end and exit at the opposite end. Some manufacturers allow the gas to enter special rotor ventilation slots and then escape radially through slotted conductors.
Water Cooling
Some alternator designs use water to cool the stator windings. In this design the water is circulated through hollow conductors. Usually pure water such as deionized water is used. The cool water enters the winding through a distribution header at one end of the generator and the warm water is discharged at the opposite end.
Cooling water cooling systems are more complex and more costly than conventional systems. However, at higher ratings, this cost is offset by a significantly smaller physical size.
Fig. 36 shows the water-cooling ducts around the stator frame and stator windings in a generator manufactured by Alsaldio Energia of Italy.
Figure 36
Generator Water-Cooling Ducts
(Alsaldo Energia-Italy)
The stator windings, shown in Fig. 36, are made up of hollow non-magnetic stainless steel conductors that carry the cooling water. These hollow conductors are manufactured from solid conductor material to make the bars that carry the stator current.
Objective 6
Describe shaft sealing arrangements for an alternator.
SHAFT SEALING ARRANGEMENTS
Hydrogen cooled machines must have seals on the rotor shaft to prevent gas from leaking outwards. Fig. 37 shows a radial clearance type that prevents gas from escaping along the radial circumference of the shaft. Note how it is fitted with the shaft journal.
This cross-sectional diagram illustrates a radial clearance type shaft seal. A central shaft journal is shown at the bottom. A sealing ring is mounted on the journal, held in place by a helical garter spring. The sealing ring is in contact with lapped surfaces on a stationary seal housing. An oil feed line is shown entering the seal housing to provide lubrication.
Figure 37
Radial Clearance Type Shaft Seal
Fig. 38 shows an axial clearance type that fits against the shaft. This type of seal prevents gases from escaping radially from the shaft.
This cross-sectional diagram shows an axial clearance type shaft seal. A shaft collar is mounted on the shaft. A sealing ring is positioned against the collar, held by helical springs. The sealing ring has a white-metal face that creates a seal against the seal housing. An oil feed is provided to the sealing area.
Figure 38
Axial Clearance Type Shaft Seal
Fig. 39 shows a thrust ring type. These seals are designed to move slightly to compensate for the radial and axial forces (thrust) that the shaft experiences as it rotates from stopped to a full-load speed.
Figure 39
Thrust Ring Type Shaft Seal
The seals prevent hydrogen from escaping outwards by forcing oil inwards against the seal. Seal oil is circulated from the main machine lubricating oil system, through the seals, to the hydrogen detraining tanks and then back to the main oil tank. The hydrogen detraining tanks allow any gas that may become entrained with the circulated oil to be removed before it is returned to the main storage tank. Fig. 40 shows the equipment and piping layout for a seal-oil system for a hydrogen cooled alternator.
The diagram illustrates the seal oil system for an H 2 cooled alternator. At the top, the Generator is shown with oil lines entering from the bottom. The oil flows through a series of components: a Seal Oil Cooler (connected to a To Water System ), a Seal Oil Filter , and then splits into two pumps: a D.C. Seal Oil Pump and an A.C. Seal Oil Pump . Both pumps are connected to a Starter . The oil from the pumps returns to the Turbine Oil Tank . A line labeled Main Power Oil also connects to the Turbine Oil Tank. On the right side, there are two Hydrogen Detaining Tanks connected to the system. Various valves are indicated with symbols: NC (Normally Closed), Shut off Valve (X symbol), Non Return Valve (checkmark symbol), and Adjustable Orifice (two parallel lines symbol). Electrical connections are shown with dashed lines.
| NC | Indicates Normally Closed |
| Indicates Shut off Valve | |
| Indicates Non Return Valve | |
| Indicates Adjustable Orifice | |
| - - - | Indicates Electrical Connection |
Figure 40
Seal Oil System for H
2
Cooled Alternator
Objective 7
Explain the theory and significance of alternator synchronization and parallel operation including the impact on power factor.
POWER FACTOR
The power factor (pf) of the system supplied by the generators is determined by the characteristics of the load connected. In the case where one alternator only supplies the load, its power factor is also that of the load.
Where two or more alternators supply a system (by far the more common case), the pf of each can be controlled individually by altering of its excitation.
If the generator is under excited, its pf becomes leading and the current output again increases without change in kW output.
Control of alternator pf is usually carried out by hand operation of a trimming resistance in the voltage regulator circuit.
PARALLEL OPERATION
If the excitation of a generator operating in parallel with others is increased beyond the amount required to give normal terminal voltage for the load condition, its pf will change in the lagging direction. Its current output increases without appreciable change in kW load output.
Synchronizing
Alternating-current power systems consist of several generating machines connected in parallel to common bus bars which supply the system load. Moreover it is common to have interconnection between local systems through transmission lines. Thus, any generator which is to be switched into the system must fulfill the following synchronizing conditions.
Condition 1
The alternator terminal voltage must equal that of the system. This condition is fulfilled by adjusting the alternator-field rheostat after running the machine up to approximately full speed, until the terminal voltage matches the system voltage. Implicit in this condition is also a demand that the shape of the incoming machine voltage wave will correspond closely with that of the system.
Condition 2
The alternator frequency must approach that of the system within close limits. The alternator frequency is controlled by adjusting the speed of the prime mover. In most cases this means control of steam supply to the turbine.
Condition 3
Phase rotations of alternator and system must be the same. That is, if the system bus bars are designated red, white and blue and the maximum of the voltage waves of these three phases occur in the sequence red, white, blue, then the incoming machine (which is to be connected red to red, blue to blue, etc.) must also have voltage maximums occurring in the phase sequence red, white, blue.
This condition can be checked by lamps or a phase rotation meter. This condition can only be altered by disconnection so that once the phase rotation of a generator is proved correct with relation to the system, it will not be necessary to repeat the test. Normal switching on and off load is carried out without changing the generator phase rotation.
The series of operations required to bring about the above conditions and to close the switch are known as synchronizing . The process of synchronizing may be illustrated by the following diagrams of the incoming machine and the system voltage waves, as shown on Fig. 41.
Fig. 41(a) shows the existing system voltage wave (one phase only shown).
Referring to Fig. 41(b), the machine voltage wave is shown dotted and is out of phase and frequency.
The generator's output voltage is slowly increased to equal the system's maximum voltage. This is accomplished by adjusting the field rheostat.
Fig. 41(c) shows that the machine and system voltages are now equal. The voltages are out of phase but the frequency is being increased by increasing the speed of the prime mover.
In Fig. 41(d), the machine and system:
- • voltages are equal
- • frequencies are equal
- • are both in phase
The synchroscope shows 12 o'clock and the switch can now be closed.
(a) Existing system voltage wave. (One phase only shown.)
(b) Machine voltage wave shown dotted. Out of phase and frequency. Being built up to equal the system max. volts by adjustment of field rheostat.
(c) Machine voltage now equal to system. Voltage waves out of phase but frequency being increased by increasing speed of prime mover.
(d) Machine voltage now equal to system, in phase and with equal frequency. Synchroscope shows 12 o'clock. Switch can now be closed.
Figure 41
Steps Taken to Synchronize an Incoming A-C Generator to the Supply System
Fig. 42 shows one method of phasing out polyphase alternators. If the phase rotation is correct on the incoming alternator, the lights will all be dark or bright simultaneously. If the phase rotation is wrong, the lights will never all be bright or dark at the same time.
The phase rotation may also be checked with a small polyphase induction motor, connected alternately to the system and the incoming machine, as shown in Fig. 43. If the direction of rotation of the motor is the same for both incoming alternator and system, then the phase rotation is the same.
Figure 42
Phasing Out Polyphase Alternators
Figure 43
Checking of Phase Rotation
Condition 4
The alternator voltage must be in correct phase relationship with the system, that is, both reach maximum at the same instant. Synchronizing the phase relationship of incoming machine and the system requires the use of a synchronizing device such as an indicator, which may be a bank of lamps or a synchroscope. Modern large machines always use the latter because indication by lamps is not accurate enough.
If lamps are used for synchronizing, they are connected as in Fig. 44.
Assuming that phase rotation has been checked and is correct and that the machine is at full-volts and close to synchronizing speed, the remaining small difference in frequency between incoming machine and system causes varying displacement between the two sets of voltages.
If lamp indication is being used on all three phases, all lamps will become bright and dark together. As the voltages of alternator and system become more nearly in step, the
flickering becomes slower. The main switch may be closed when all lamps are dark. It is always good practice to close the switch at a time when the incoming machine speed is increasing because this machine tends to pick up a little load and is more stable in operation.
Figure 44
Synchronizing With Lamps
The synchroscope gives a much clearer picture of the phase relationship between the two voltages (incoming machine and system). Rotation of the indicating pointer in the direction marked fast shows the incoming machine frequency to be faster than the system and vice versa. If the frequency of the incoming machine is equal to the system frequency, the pointer will not revolve.
Its position relative to 12 o'clock indicates the angle of phase displacement between incoming machine and system in electrical degrees. The incoming machine should be paralleled at the instant the synchroscope pointer passes the zero (12 o'clock) position while revolving slowly in the fast direction.
Once paralleled the pointer no longer revolves. So to prevent overheating of its coils, the synchroscope is switched off when not in use.
The synchroscope operates by the interaction of magnetic fields from two circuits, one connected to the alternator and one to the system upon a soft iron vane or disc with the pointer attached.
Many modern alternators are fitted with automatic synchronizing equipment. The basic operating principle is that agreement between coils supplied from alternator and system produces sufficient field strength to operate a switch-closing relay.
Power Factor Control
When two alternators are running in parallel, a transfer of load between them results in a voltage differential. The alternator with the reduced kilowatt loading has its internal voltage increased because of the lessened voltage drops within its stator windings. Hence, the machine is said to be over-excited for the new value of load. The alternator with the increased kW loading has its internal voltage decreased because of the increased voltage drops in the stator. Hence this machine is said to be under-excited for the new value of load.
If the pf of the system is unity, this voltage differential causes reactive cross-current between the machines. If the system pf is lagging however, the effect is to cause a disproportionate division of reactive power between the alternators. The over-excited machine will supply more Vars (volt-amps reactive) than the under-excited machine. A pf meter would show lagging on the over-excited machine and leading on the under-excited machine.
The balancing of Vars between the machines is accomplished by adjusting the field excitation of each machine.
The field rheostat of the under-excited machine is turned in the raise voltage direction, and field rheostat of the over-excited machine is turned in the lower voltage direction until the kVA meter indication or the pf meter indication is the same on each machine.
Objective 8
Explain efficiency and power losses in AC generators.
AC GENERATOR EFFICIENCY AND LOSSES
The voltage at the terminals of an alternator is affected considerably by the pf of the load it is supplying. For this reason the rated voltage, stated on the nameplate, is always given for rated kVA at a specific pf and field current.
The average-system load includes induction, resistance and some capacitance and is between unity and 0.8 lagging. Alternators for general service usually have the terminal voltage for rated kVA at 0.8 pf lagging stamped upon the nameplate.
The short-circuit ratio is often included in the nameplate data. This gives an indication of the design of the machine with regard to the quantity of copper and iron in the electrical and magnetic parts. The machine with high short-circuit ratio will contain more materials.
Short-circuit ratio is defined as the ratio of the field current required to produce rated voltage at rated speed and no-load to the field current required to circulate rated stator current when operating at rated speed under sustained short-circuit conditions. The standard figure quoted for a turbo-alternator is between 0.8 and 1.0.
The affect of high short-circuit ratio in the design of an alternator gives it improved stability at times of load changes.
Alternator efficiency is dependent upon the losses in the machine. Not all of the power input to a generator does useful work. Some of the energy input is lost as:
- • friction
- • windage
- • \( I^2R \) losses in the stator and field windings
- • hysteresis and eddy current losses in the iron of the stator and rotor field circuits
The efficiency of an alternator operating at 0.8 lagging pf can be expected to reach 98%.
Chapter Questions
B3.3
- 1. With the aid of simple sketches, explain the differences between AC and DC generators.
- 2. Explain the relationship between alternator speed, frequency, and number of pole pairs.
- 3. With the aid of a simple sketch, explain the sequence of operation for a brushless excitation system for a modern alternator.
- 4. With the aid of a simple sketch, explain the operating principle of a direct-acting type of regulator with rolling contacts.
- 5. Explain the advantages hydrogen cooling has over air cooling for alternators.
- 6. With the aid of a simple sketch, describe the seal oil system that is used for a hydrogen cooled alternator.
- 7. With the aid of a simple sketch, explain the steps that are taken to synchronize an incoming AC generator to the supply system.
Because the power input to the field coils varies as the square of the field current, this power input is now 5% of 5% of 100 watts or 0.25 watt. Thus, a field power input of 0.25 watts results in the production of a field \( Qa \) as powerful as the field produced directly by a field input of 100 watts. If this flux is used to generate a voltage and this voltage is applied to some external load, the overall power amplification is 400 times that of an ordinary DC generator, that is, a power amplification of 40 000 : 1.
The flux \( Qa \) can be utilized by placing two brushes midway between the short-circuited brushes, as shown in Fig. 25 and connecting these to the external load circuit.
Figure 25
Amplitdyne Generator Showing Load
Circuit and Control-Field Circuit
The \( E \) applied across the load is produced by the conductors cutting the vertical flux \( Qa \) ; the cutting of \( Qc \) does not contribute anything to this \( E \) . A generator excited in this way is called an armature reaction excited machine.
It is necessary to provide a compensating winding, placed in slots in the stator iron and connected in series with the load, to neutralize the armature reaction of the load current because this armature reaction is in direct opposition to \( Qc \) .
The wiring diagram of the Amplitdyne then becomes as shown in Fig. 26.
The high-speed response of the amplitdyne is made possible by the fact that its control field requires only about 1 or 2% of the number of ampere turns used on the field of an ordinary DC generator. Consequently, the inductance of its field winding is much smaller and the rate of change of the field current much more rapid.
Figure 26
Amplitdyne Circuit Showing
the Compensating Winding
Fig. 27 shows two typical field connections using amplidynes.
Figure 27
Amplitdyne Field Control Circuits
Magnetic Amplifier Regulator
To understand this type of regulator, it is necessary to be familiar with the mode of operation of a magnetic amplifier. It consists of windings on a core, the magnetic characteristics of which are very important. The permeability must be high and must be almost constant up to the saturation point. The ideal \( B \) - \( H \) characteristic ( \( B \) is flux density - \( H \) is magnetizing force) is illustrated in Fig. 28.
Figure 28
Ideal B-H characteristic for a Magnetic Amplifier Core
If two coils are wound on a core of this material, with one coil as a series impedance in an AC circuit and the other carrying direct current, impedance of the AC circuit can be varied by altering the current flowing in the DC coil. Consequently, by careful design, small variations in the control current (the name for direct current) can produce large changes of current in the a-c circuit. If the alternating current is rectified after passing through the amplifier windings, it becomes possible to produce large changes in direct current from small changes in another current.
The control winding of the magnetic amplifier is supplied by a voltage transformer connected to the generator terminals. The supply for exciting the machine is taken as a three-phase supply from a shaft alternator, which is connected in series with the AC windings of the magnetic amplifier, rectified, and connected in series with the main exciter field winding. Thus, changes in output voltage directly control the main field current.
The great advantages of this scheme are that there is a minimum of moving parts, no electronic components, the regulator is built entirely of static components, the time response is very rapid, and there is no dead band.
The regulator works from the difference between the generator terminal voltage and a fixed reference voltage derived from the pilot generator. The difference between these two voltages is amplified by magnetic amplifiers in cascade and is fed directly to correct the main exciter field current.
The normal DC pilot exciter is replaced by a 400 Hz self-excited pilot generator, the output being subsequently rectified to feed the main exciter winding. The higher frequency enables the magnetic amplifiers to give a much faster response and reduces the dimensions of the magnetic components. The output of the pilot generator is controlled by an induction regulator working in conjunction with magnetic amplifiers and the rectified output of the combination is fed into the field circuit of the generator. The induction regulator in this design replaces the conventional field rheostat.
Static Excitation System
An automatic voltage regulator (AVR) is a device that causes the excitation of the generator to be automatically increased or decreased so that the terminal voltage can be maintained within acceptable limits. The device must be able to detect a change in voltage and quickly respond to difference between the measured voltage and the desired voltage. Voltage regulation is expressed in percent by the following formula:
$$ \frac{(\text{No-load volts}) - (\text{Full-load volts})}{\text{Full-load volts}} \times 100 $$
Modern voltage regulators maintain voltage within \( \pm 1.5\% \) . Generator voltage regulators are classified by the means of operation.
Direct-acting rheostatic regulators adjust the field strength of a generator directly. This type of AVR was common in older generators. The rheostat varied the resistance and therefore the current, in the exciter circuit. By varying the current, the field strength in the generator is varied. The rheostat was manually operated but automated by using an electromagnet that varied the resistance based on the generator voltage.
The indirect acting rheostatic regulators use an auxiliary device, such as a motor, to operate the field rheostat.
Many older voltage regulators have been replaced by static excitation systems and digital voltage regulators.
As the name suggests, there are no moving parts in a static voltage regulator system. Fig. 29 shows a block diagram for a static excitation system and automatic voltage regulator. Initial voltage buildup in the generator is due to residual magnetism in the field. As voltage builds, a transformer is used to provide power from the generator output terminals to an automatic voltage regulator. Power from the generator terminals is also provided to a thyristor rectifier bridge. The AVR provides a signal to the gates on the thyristors, which modulate the DC voltage to the generator field.
Figure 29
Static Excitation System
Thyristors are similar to diodes but are designed with a gate that allows the amount of rectified DC voltage to be varied. This is shown in Fig. 30.
Figure 30
Thyristors
Objective 5
Describe the cooling systems used for an alternator including circulating air cooling, hydrogen cooling, and stator winding cooling water systems.
ALTERNATOR COOLING SYSTEMS
Cooling systems for alternators fall into the following fours types depending on the volt-amp output of the unit:
- • direct air
- • enclosed air
- • hydrogen
- • water
Direct Air Cooling
Small alternators use fans mounted on the rotor shaft. Ambient air is drawn in through one end of the frame and discharged out the opposite end. This type of cooling is not well suited for dusty environments. Fig. 31 shows a photograph of a small alternator. Notice the ventilation slots on each end of the frame.
Figure 31
Small Alternator
In larger alternators, there are considerably higher \( I^{2}R \) losses and magnetic losses. These types of losses produce heat that must be properly dissipated. The physical size and structure of alternators are determined by the manner in which the machine is cooled.
Fig. 32 shows a side view of a generator with direct-cooling fans on both ends of the rotor. The top part of the drawing shows an end view of the two fans. The arrows on the drawing show the paths the cooling air takes.
Figure 32
Generator with Direct Cooling Fans
Enclosed Air Cooling
Air-cooled alternators are limited in size due to the inability of air to cool the rotor and stator to acceptable ranges. For larger generators, in the range of 25 to 325 MVA (mega-volt-amps), the generator is totally enclosed with separate fans supplying cooling air. Fig. 33 shows a side and end views of a typical air-cooled arrangement.
The air is filtered before it is supplied to the generator. Air from two fans is blown through air coolers into the stator and field windings and the exciter cabinet. Air return ducts are provided to guide the exhaust air back to the inlet of the fans.
The air coolers consist of finned tubes through which cooling water is circulated. The air is cooled as it is blown through channels between the finned tubes.
Figure 33
Arrangement of Alternator, Exciter and Sliprings Air-cooling System
Hydrogen Cooling
Hydrogen has properties that allow it to be a better coolant for larger alternators. Hydrogen is used on units that have a capacity in the range of 200 - 1000 MVA. The greater cooling ability of hydrogen vs. air allows for smaller physical dimensions of alternators. But, there is a greater cost associated with hydrogen cooling. Fig. 34 is a photograph of 1120 MVA hydrogen cooled generator manufactured by Toshiba Corporation.
The increase in rated output obtained with hydrogen gas can be 20% - 30% based on a hydrogen pressure of 3.5 kPa. Further increases can be realized with greater hydrogen pressure. Alternators are commonly operated at pressures of 100 kPa.
Hydrogen gas has several advantages over air:
- • High heat transfer coefficient, absorbs heat rapidly as it is flows through the generator windings
- • Rejects this same heat more rapidly in the cooling stages of the ventilation circuit
- • Less dense than air and requires less energy to drive it through the different generator components
- • Offers less braking effect (windage) to the rotating parts of the alternator.
Figure 34
1120 MVA Hydrogen Cooled Generator
(Toshiba Corporation)
Overheating in generators can cause fires. This can be a serious problem with air ventilation systems since air supports combustion. The fire hazard with hydrogen is reduced. The airtight system prevents buildup of dirt that may cause fires.
The absence of air in the ventilation system also prevents the formation of acids that otherwise may attack the insulation on the rotor and stator windings.
However, there is a risk of explosion if hydrogen is mixed in the proper proportions with air. This is prevented by air tight casings and proper seals around the rotor shaft.
Hydrogen must be kept free from moisture because it increases the viscosity of the hydrogen and decreases its ability to carry away excess heat. Moisture also deteriorates the seals on the rotating shaft.
Normally, the hydrogen gas circulates in a closed loop through a water-cooled heat exchanger and a molecular sieve or desiccant dryer before returning to the generator enclosure.
Fig. 35 shows a schematic diagram of the equipment layout and piping diagram for a hydrogen cooling system.
| Symbol | Key to Symbols |
|---|---|
|
Image: Symbol for Shut-Off Valve: a circle with a diagonal line and a crossbar. |
Shut-Off Valve |
|
Image: Symbol for Non-Return Valve: a circle with a diagonal line and a triangle. |
Non-Return Valve |
|
Image: Symbol for Electrical Connections: a dashed line. |
Electrical Connections |
Figure 35A
Hydrogen Cooled Generator System
Figure 35B
End View of Hydrogen Cooled Generator System
The gas flow paths for hydrogen cooling vary with manufacturers' designs. In some cases, the gas enters at each end of the rotor and leaves at the centre. Other designs have the gas enter one end and exit at the opposite end. Some manufacturers allow the gas to enter special rotor ventilation slots and then escape radially through slotted conductors.
Water Cooling
Some alternator designs use water to cool the stator windings. In this design the water is circulated through hollow conductors. Usually pure water such as deionized water is used. The cool water enters the winding through a distribution header at one end of the generator and the warm water is discharged at the opposite end.
Cooling water cooling systems are more complex and more costly than conventional systems. However, at higher ratings, this cost is offset by a significantly smaller physical size.
Fig. 36 shows the water-cooling ducts around the stator frame and stator windings in a generator manufactured by Alsaldio Energia of Italy.
Figure 36
Generator Water-Cooling Ducts
(Alsaldo Energia-Italy)
The stator windings, shown in Fig. 36, are made up of hollow non-magnetic stainless steel conductors that carry the cooling water. These hollow conductors are manufactured from solid conductor material to make the bars that carry the stator current.
Objective 6
Describe shaft sealing arrangements for an alternator.
SHAFT SEALING ARRANGEMENTS
Hydrogen cooled machines must have seals on the rotor shaft to prevent gas from leaking outwards. Fig. 37 shows a radial clearance type that prevents gas from escaping along the radial circumference of the shaft. Note how it is fitted with the shaft journal.
This cross-sectional diagram illustrates a radial clearance type shaft seal. A central shaft journal is shown at the bottom. A sealing ring is mounted on the journal, held in place by a helical garter spring. The sealing ring is in contact with lapped surfaces on a stationary seal housing. An oil feed line is shown entering the seal housing to provide lubrication.
Figure 37
Radial Clearance Type Shaft Seal
Fig. 38 shows an axial clearance type that fits against the shaft. This type of seal prevents gases from escaping radially from the shaft.
This cross-sectional diagram shows an axial clearance type shaft seal. A shaft collar is mounted on the shaft. A sealing ring is positioned against the collar, held by helical springs. The sealing ring has a white-metal face that creates a seal against the seal housing. An oil feed is provided to the sealing area.
Figure 38
Axial Clearance Type Shaft Seal
Fig. 39 shows a thrust ring type. These seals are designed to move slightly to compensate for the radial and axial forces (thrust) that the shaft experiences as it rotates from stopped to a full-load speed.
Figure 39
Thrust Ring Type Shaft Seal
The seals prevent hydrogen from escaping outwards by forcing oil inwards against the seal. Seal oil is circulated from the main machine lubricating oil system, through the seals, to the hydrogen detraining tanks and then back to the main oil tank. The hydrogen detraining tanks allow any gas that may become entrained with the circulated oil to be removed before it is returned to the main storage tank. Fig. 40 shows the equipment and piping layout for a seal-oil system for a hydrogen cooled alternator.
The diagram illustrates the seal oil system for an H 2 cooled alternator. At the top, the Generator is shown with oil lines entering from the bottom. The oil flows through a series of components: a Seal Oil Cooler (connected to a To Water System ), a Seal Oil Filter , and then splits into two pumps: a D.C. Seal Oil Pump and an A.C. Seal Oil Pump . Both pumps are connected to a Starter . The oil from the pumps returns to the Turbine Oil Tank . A line labeled Main Power Oil also connects to the Turbine Oil Tank. On the right side, there are two Hydrogen Detaining Tanks connected to the system. Various valves are indicated with symbols: NC (Normally Closed), Shut off Valve (X symbol), Non Return Valve (checkmark symbol), and Adjustable Orifice (two parallel lines symbol). Electrical connections are shown with dashed lines.
| NC | Indicates Normally Closed |
| Indicates Shut off Valve | |
| Indicates Non Return Valve | |
| Indicates Adjustable Orifice | |
| - - - | Indicates Electrical Connection |
Figure 40
Seal Oil System for H
2
Cooled Alternator
Objective 7
Explain the theory and significance of alternator synchronization and parallel operation including the impact on power factor.
POWER FACTOR
The power factor (pf) of the system supplied by the generators is determined by the characteristics of the load connected. In the case where one alternator only supplies the load, its power factor is also that of the load.
Where two or more alternators supply a system (by far the more common case), the pf of each can be controlled individually by altering of its excitation.
If the generator is under excited, its pf becomes leading and the current output again increases without change in kW output.
Control of alternator pf is usually carried out by hand operation of a trimming resistance in the voltage regulator circuit.
PARALLEL OPERATION
If the excitation of a generator operating in parallel with others is increased beyond the amount required to give normal terminal voltage for the load condition, its pf will change in the lagging direction. Its current output increases without appreciable change in kW load output.
Synchronizing
Alternating-current power systems consist of several generating machines connected in parallel to common bus bars which supply the system load. Moreover it is common to have interconnection between local systems through transmission lines. Thus, any generator which is to be switched into the system must fulfill the following synchronizing conditions.
Condition 1
The alternator terminal voltage must equal that of the system. This condition is fulfilled by adjusting the alternator-field rheostat after running the machine up to approximately full speed, until the terminal voltage matches the system voltage. Implicit in this condition is also a demand that the shape of the incoming machine voltage wave will correspond closely with that of the system.
Condition 2
The alternator frequency must approach that of the system within close limits. The alternator frequency is controlled by adjusting the speed of the prime mover. In most cases this means control of steam supply to the turbine.
Condition 3
Phase rotations of alternator and system must be the same. That is, if the system bus bars are designated red, white and blue and the maximum of the voltage waves of these three phases occur in the sequence red, white, blue, then the incoming machine (which is to be connected red to red, blue to blue, etc.) must also have voltage maximums occurring in the phase sequence red, white, blue.
This condition can be checked by lamps or a phase rotation meter. This condition can only be altered by disconnection so that once the phase rotation of a generator is proved correct with relation to the system, it will not be necessary to repeat the test. Normal switching on and off load is carried out without changing the generator phase rotation.
The series of operations required to bring about the above conditions and to close the switch are known as synchronizing . The process of synchronizing may be illustrated by the following diagrams of the incoming machine and the system voltage waves, as shown on Fig. 41.
Fig. 41(a) shows the existing system voltage wave (one phase only shown).
Referring to Fig. 41(b), the machine voltage wave is shown dotted and is out of phase and frequency.
The generator's output voltage is slowly increased to equal the system's maximum voltage. This is accomplished by adjusting the field rheostat.
Fig. 41(c) shows that the machine and system voltages are now equal. The voltages are out of phase but the frequency is being increased by increasing the speed of the prime mover.
In Fig. 41(d), the machine and system:
- • voltages are equal
- • frequencies are equal
- • are both in phase
The synchroscope shows 12 o'clock and the switch can now be closed.
(a) Existing system voltage wave. (One phase only shown.)
(b) Machine voltage wave shown dotted. Out of phase and frequency. Being built up to equal the system max. volts by adjustment of field rheostat.
(c) Machine voltage now equal to system. Voltage waves out of phase but frequency being increased by increasing speed of prime mover.
(d) Machine voltage now equal to system, in phase and with equal frequency. Synchroscope shows 12 o'clock. Switch can now be closed.
Figure 41
Steps Taken to Synchronize an Incoming A-C Generator to the Supply System
Fig. 42 shows one method of phasing out polyphase alternators. If the phase rotation is correct on the incoming alternator, the lights will all be dark or bright simultaneously. If the phase rotation is wrong, the lights will never all be bright or dark at the same time.
The phase rotation may also be checked with a small polyphase induction motor, connected alternately to the system and the incoming machine, as shown in Fig. 43. If the direction of rotation of the motor is the same for both incoming alternator and system, then the phase rotation is the same.
Figure 42
Phasing Out Polyphase Alternators
Figure 43
Checking of Phase Rotation
Condition 4
The alternator voltage must be in correct phase relationship with the system, that is, both reach maximum at the same instant. Synchronizing the phase relationship of incoming machine and the system requires the use of a synchronizing device such as an indicator, which may be a bank of lamps or a synchroscope. Modern large machines always use the latter because indication by lamps is not accurate enough.
If lamps are used for synchronizing, they are connected as in Fig. 44.
Assuming that phase rotation has been checked and is correct and that the machine is at full-volts and close to synchronizing speed, the remaining small difference in frequency between incoming machine and system causes varying displacement between the two sets of voltages.
If lamp indication is being used on all three phases, all lamps will become bright and dark together. As the voltages of alternator and system become more nearly in step, the
flickering becomes slower. The main switch may be closed when all lamps are dark. It is always good practice to close the switch at a time when the incoming machine speed is increasing because this machine tends to pick up a little load and is more stable in operation.
Figure 44
Synchronizing With Lamps
The synchroscope gives a much clearer picture of the phase relationship between the two voltages (incoming machine and system). Rotation of the indicating pointer in the direction marked fast shows the incoming machine frequency to be faster than the system and vice versa. If the frequency of the incoming machine is equal to the system frequency, the pointer will not revolve.
Its position relative to 12 o'clock indicates the angle of phase displacement between incoming machine and system in electrical degrees. The incoming machine should be paralleled at the instant the synchroscope pointer passes the zero (12 o'clock) position while revolving slowly in the fast direction.
Once paralleled the pointer no longer revolves. So to prevent overheating of its coils, the synchroscope is switched off when not in use.
The synchroscope operates by the interaction of magnetic fields from two circuits, one connected to the alternator and one to the system upon a soft iron vane or disc with the pointer attached.
Many modern alternators are fitted with automatic synchronizing equipment. The basic operating principle is that agreement between coils supplied from alternator and system produces sufficient field strength to operate a switch-closing relay.
Power Factor Control
When two alternators are running in parallel, a transfer of load between them results in a voltage differential. The alternator with the reduced kilowatt loading has its internal voltage increased because of the lessened voltage drops within its stator windings. Hence, the machine is said to be over-excited for the new value of load. The alternator with the increased kW loading has its internal voltage decreased because of the increased voltage drops in the stator. Hence this machine is said to be under-excited for the new value of load.
If the pf of the system is unity, this voltage differential causes reactive cross-current between the machines. If the system pf is lagging however, the effect is to cause a disproportionate division of reactive power between the alternators. The over-excited machine will supply more Vars (volt-amps reactive) than the under-excited machine. A pf meter would show lagging on the over-excited machine and leading on the under-excited machine.
The balancing of Vars between the machines is accomplished by adjusting the field excitation of each machine.
The field rheostat of the under-excited machine is turned in the raise voltage direction, and field rheostat of the over-excited machine is turned in the lower voltage direction until the kVA meter indication or the pf meter indication is the same on each machine.
Objective 8
Explain efficiency and power losses in AC generators.
AC GENERATOR EFFICIENCY AND LOSSES
The voltage at the terminals of an alternator is affected considerably by the pf of the load it is supplying. For this reason the rated voltage, stated on the nameplate, is always given for rated kVA at a specific pf and field current.
The average-system load includes induction, resistance and some capacitance and is between unity and 0.8 lagging. Alternators for general service usually have the terminal voltage for rated kVA at 0.8 pf lagging stamped upon the nameplate.
The short-circuit ratio is often included in the nameplate data. This gives an indication of the design of the machine with regard to the quantity of copper and iron in the electrical and magnetic parts. The machine with high short-circuit ratio will contain more materials.
Short-circuit ratio is defined as the ratio of the field current required to produce rated voltage at rated speed and no-load to the field current required to circulate rated stator current when operating at rated speed under sustained short-circuit conditions. The standard figure quoted for a turbo-alternator is between 0.8 and 1.0.
The affect of high short-circuit ratio in the design of an alternator gives it improved stability at times of load changes.
Alternator efficiency is dependent upon the losses in the machine. Not all of the power input to a generator does useful work. Some of the energy input is lost as:
- • friction
- • windage
- • \( I^2R \) losses in the stator and field windings
- • hysteresis and eddy current losses in the iron of the stator and rotor field circuits
The efficiency of an alternator operating at 0.8 lagging pf can be expected to reach 98%.
Chapter Questions
B3.3
- 1. With the aid of simple sketches, explain the differences between AC and DC generators.
- 2. Explain the relationship between alternator speed, frequency, and number of pole pairs.
- 3. With the aid of a simple sketch, explain the sequence of operation for a brushless excitation system for a modern alternator.
- 4. With the aid of a simple sketch, explain the operating principle of a direct-acting type of regulator with rolling contacts.
- 5. Explain the advantages hydrogen cooling has over air cooling for alternators.
- 6. With the aid of a simple sketch, describe the seal oil system that is used for a hydrogen cooled alternator.
- 7. With the aid of a simple sketch, explain the steps that are taken to synchronize an incoming AC generator to the supply system.