Utilities

Electrical Generation

Generators

The Alternator

The synchronous machine consists essentially of (a) a field system excited by direct current and (b) an armature. Almost invariably the armature is the stationary member and the field system the rotating member. The induced e.m.f. in the armature winding is a motionally induced e.m.f. and its mode of production identical with that of the D.C. machine. The only difference is that it is the magnetic field which moves whereas the armature conductor is stationary.


As with a D.C. machine the e.m.f. induced in an individual armature coil ia an alternating e.m.f. and consequently by bringing the winding out to fixed terminals, the e.m.f. between these will be alternating also. The complete fixed armature, that is magnetic core and windings, is called the stator, and the rotating field system the rotor. The general constructional features of a salient pole alternator are shown below


As the field system rotates and carries its flux with it, each portion of the stator core will experience reversals of magnetisation, and therefore, as in a direct current machine, the core has to be laminated. For ventilation purposes, a series of radial ventilating ducts are provided. Since the field system rotates, its exciting winding has to be fed by means of two slip rings, but as the excitation voltage is low and the power taken by the field winding small, these present no difficulties


A salient pole has one field coil per pole, very like a D.C. machine. For the very high speeds of turbine driven alternators it is necessary to adopt a cylindrical construction for the rotor and in such a case the field winding has to be housed in a number of slots. A simplified form shows the cross section of a four pole turbo alternator , the dispostion of the rotor field of a turbo alternator rotor may be as high as 40,000 ft per min or 200 metres per sec. The stresses due to centrifugal force are exceedingly high. The rotors are thus made from steel forging, or in some cases from thick steel discs bolted together.



High speed rotor


The axial length is normally considerably greater than the diameter. Has the advantage of great strength and stiffness. The exciting current is carried by bar type conductors in the groups of slots shown below. All currents in one group are in the same direction , those on the next group on the opposite direction. Flux produced is distributed over surface approximately according to sine law.



Details of stationary armature alternators


Armature stampings pressed out of sheets of special magnetic iron or steel alloy. In the smaller sizes the stampings are pressed out in compete rings


Section through top stator of salient pole machine. The armature core is built up of laminations which are held tightly together by end clamping rings. Spacing strips inserted at intervals leave ducts for cooling air to pass through. The air is driven through by the fan action of the rotor and escapes via the apertures in the cast iron supporting frame


Types of armature slot. The filled slot has round wires but it is common to have rectangular conductors to economise slot space.


Sectional simplified diagram of a turbo alternator

The rotor is turned from a steel forging ans slotted to carry the exciting windings the slots being arranged as shown above. Because of the high running speed, alternators for large outputs have a considerable axial length compared with rotor diameter.


Layouts of A.C. generators


Conventional excitation scheme (Rotary)

Separately excited D.C. exciter (Out dated)


Brushless excitation scheme using shaft mounted diodes (Rotary)

Indirect self excitation (Error)


Comparison of the value required to control with a fixed value. When the variable differs from a fixed reference value an 'error' exists and the function of the controlling medium is to restore equilibrium e.g. if the voltage output falls on the brushless rotary excited alternator the a.v.r. controls the exciter field to restore equilibrium.

Modern compound scheme (static)

Direct self excited (Functional)


Control of the voltage to a set value is achieved by the inherent characteristics of the machine.


A compound wound d.c. generator with a level compound characteristic has additional current in the series field under load conditions. In the self excited compound alternator there is a constant amount of excitation required for no load condition. Additional excitation due to more current form the current transformers is obtained in response to extra external demands



Recovery graphs for functional and 'error' layouts


Shaft driven generating system


Methods of drive

  1. Belt or chain driven
  2. Direct coupling engaging the propeller shaft
  3. Power taken from the main gearbox.
  4. Power taken from the free end of the engine

With D.C. auxiliaries power can be taken by either a chain or belt drive from the propulsion system with an A.V.R. maintaining constant voltage.



For A.C. systems methods used include the use of a D.C. generator with an D.C./A.C. converter, or direct A.C. generation. With the latter either a constant speed drive is required or a frequency converter. With either method the revolutions at which the shaft alternator can be used is limited. In this way direct drive systems will generally be fitted in conjunction with a C.P. system which maintains constant engine speed under full away conditions.

Advantages

Disadvantages


Air Gap

If the air gap around a rotor is not uniform the motor may not start in certain positions. Because the rotor is not centred, probably due to worn bearings, there is an out of balance magnetic pull.

Radial play in between the shaft and the housing should be detected by hand and bearing wear detected by feeler gauge between the rotor and the stator, or armature and field poles may be measured at three or four fairly equidistant points around the machine. If possible one measurement should be made at the bottom of the machine and another in line with the drive. Compare with previous records to check wear. At minimum air gap. Clearance of the bearings should be renewed to avoid the possibility of the rotor rubbing on the stator.

On small machines two feelers on opposite sides of the rotor should be used to avoid error caused by rotor movement from normal position when only one feeler gauge is used.

In synchronous motors and D.C. motors sparking may occur if the radial air gaps between the armature and the field poles are unequal. If necessary renew bearings or add or remove soft iron shims from under the pole shoes. Unequal field strength has a similar effect of sparking at the brushes. This might be due to short circuit or earth fault on the field coils, or a short circuit on the shunt and field coils.

An increase of air gap gives an increase in 'reluctance'.
In a salient pole A.C. generator this fact may be used to produce a sinusoidal flux density curve by gradually increasing the length if the air gap towards the pole tips.


In the induction motor the air gap should be as small as possible if the motor is to act with a high power factor. An increase in air gap increases the reactance of the motor and lowers its power factor. Small motors are accurately machined and centring of the rotor is very important so ball or roller bearings are fitted.


Air gap  Motor size  

0.25mm  1kW  

0.75mm  10kW  

2.0mm  100kW  



Parallel operation of generators


D.C. generators

For compound wound D.C. generators it is usually sufficient to ensure that the voltages of the incoming generator is the same as the bus bar voltage. The equalising connection joining the junctions between the armatures and their series fields is incorporated in the circuit breaker in such a way that the equalising connection is automatically closed before and opens after, the main contacts. By adjustment of the shunt field regulator the load sharing may be controlled

A.C. alternators

To parallel alternators the following conditions are required;

Same voltage-checked with the voltmeter

Same frequency-checked with the frequency meter and synchroscope

Same phase angle-checked with synchroscope

Same phase rotation-checked with rotation meter. Only important when connecting shore supply, or after maintenance on switchgear or alternator.


Load Sharing Of Alternators In Parallel

Alternators in parallel must always run at the same speed. After a machine has been paralleled and is required to take up its share of the load, this will not be achieved by adjusting the field excitation current. Although the increase in e.m.f. will cause a current to flow in the busbars, and this will show on the machines ammeters, this is a reactive current that lags the e.m.f. by 90o and produces a reactive (kVAr) but not kW. Its only effect is to alter the operating power factor of the alternator.

More power may be obtained at the bus bars from the incoming alternator only by supplying more power to its prime mover. This increase of steam or fuel supply is achieved by altering the governor setting either electrically or manually.

After adjusting the governor the incoming machine takes up its desired amount of the kW loading and this is recorded on the machines watt meter. However, if the kW loading is shared equally between two machines it may be found that the Load Current of the incoming machine is more or less than the other machine. This is fue to the incoming machine having a different power factor. This may be corrected by adjusting the excitation of the incoming alternator.

Thus after paralleling an alternator;

Adjust prime mover governor until kW loading is correct

Adjust field excitation current until current sharing is correct.

If the alternators have similar load characteristics, once adjusted, the load will continue to be shared. If the load characteristics of alternators vary, the kW loading and load current sharing may require readjusting under different load conditions.

Load sharing of alternators

No1 on load


No1 on load, No2 synchronised and taking 100kW


No1 and No2 sharing load after adjusting governor settings, excitation adjusted to prevent excessive volt drop in No2


No1 and No2 sharing load with balanced power factors by adjusting excitation


The effects of altering Torque and Excitation on single phase alternator plant-and by extrapolation a 3-phase circuit


Before paralleling, by varying Rb, adjust the excitation current in the rotor field of 'B' until Va=Vb. When in phase and at the same frequency synchronising may take place.

If there was no external load on the bus bars the torque on the prime movers of A and B is only that required by its own alternator and Ra and Rb are adjusted so that Ea and Eb are equal.


Relative to the bus bars Ea and Eb are acting in the same direction with each other making the top bar positive with respect to the bottom bar.

Varying the driving torque



If the driving torque of 'B' is reduced (less fuel supplied) the rotor falls back by an angle say p.f.(b) giving a resultant e.m.f. of Ez in the closed circuit.

The e.m.f. Ez circulates a current I which lags behind Ez by angle p.f.(a).
This circulating current Iis more or less in phase with Ea and in opposition to Eb.
This means that A is generating power to motor B and this will compensate for any loss of power in the prime mover of B.
Once the power increase in A equals the power loss of B balance is restored and A and B continue to run in synchronism.

Therefore the power is shared by adjusting the torque ( fuel input.)

Slight loss of power in B-is taken up by an increase in power from A. The terminal voltage will not vary and the speed and frequency will stay the same or drop only very slightly.


Large loss of power in B-with a large circulating current from A to B the alternator A will try to drive B as a synchronous motor. The amount of full load power required to drive an alternator as a motor is only 2 to 3% for a turbine and 10 to 12% for diesel engine.
As the circulating current flows from A to B the reverse power trip on B will operate after about 3 to 5 seconds.
All the load now falls on A which will probably cause the overload trip to operate and 'black out' .

Varying excitation



Consider A and B are exerting the torque required by its alternator and the generated e.m.f. Ea and Eb are equal. There is no circulating current.
By reducing Rb the excitation current in the field of B can be increased and Eb will increase. Ez is the resultant difference (Eb - Ea) which will give a circulating current I through the synchronous impedances of the two alternators. As the machines are similar the impedance drop in each will be 1/2Ez so the terminal voltage

V1 = Eb - Н Ez = Ea + Н Ez

Therefore increasing the excitation current will increase the terminal voltage

As p.f.(a) is almost 90o the Power circulating from B to A is very small

Ez I Cos [ p.f.(a)] approx equals Zero (Cos 90o = Zero)

Effect of reducing Excitation

By increasing Rb the reduction of the field excitation current of B will reduce the terminal voltage

Ea>Eb terminal Voltage V = Ea - Н Ez = Eb + Н Ez

The circulating current I from A to B will have a large 'Wattless' component. Machine A now has more of the lagging reactive current and its power factor is reduced. Too large a reduction in excitation current in B with subsequent increase in load current in A could cause the current overload trip of A to operate. This could be followed by the low voltage or the overload trip of B operating causing a black out.

Voltage regulation


The graph demonstrates that excitation must be increased (generally) with increasing load to maintain terminal voltage


The worse the power factor the worse the terminal voltage change during load change.

Voltage regulation = DV when load removed/ Full load terminal voltage

At 1.0 p.f. = AC/ OA

At 0.8 p.f = AD/ OA

Therefore lower p.f. = greater voltage regulation


Limiting voltage dip and response time under impact loading

The effect of a large load suddenly switched on to a small power installation such as a ships plant will be an instantaneous dip in the generator voltage.

This effect, due to the transient reactance on starting, cannot be obviated either in a self regulated machine, or in a conventional generator with A.V.R.

The sluggish response of the excitation systems limits the speed of voltage recovery.

In a self excited generator the dip is less and the recovery time greatly improved. (say 0.3s against 0.7s)

In order to maintain constant voltage, under varying conditions, excitation must be varied.

Variation of voltage at constant excitation


Variation of excitation at constant voltage