Wednesday, January 23, 2013

CHAPTER 2 A.C. GENERATORS




2.1       GENERAL


The principle of a.c. generation is fully covered in the manual ‘Fundamentals of Electricity 2’, where it is developed from Faraday’s Law of Electromagnetic Induction to the idea of a modern generator with a rotating field and a stationary armature.  This chapter assumes familiarity with that concept and deals with the actual hardware. Chapter 3 discusses the various methods of excitation.

Figure 2.1 shows, in cutaway form, a typical a.c. generator in the 15-megawatt (20 000 hp) size range.  The generator proper is enclosed in a box or ‘hood’; this is both to exclude noise and to contain the closed ventilation system.  It also assists purging before starting if gas has been present. The rotating parts are coloured yellow and the stator blue.


FIGURE 2.1
TYPICAL A.C. GENERATOR

The armature (normally the stator) windings carry the load current, which varies with the loading.  These windings have resistance and generate heat at a rate proportional to the square of the current (W = I2R).  The field’s exciting winding (normally on the rotor) also carries current. It too has resistance and generates I2R heat.  These two sources of heat, together with iron loss heating, combine to raise the temperature of the machine.  All the heat must be taken away by the cooling system if the temperature rise is to be held below the designed limit.

Since the stator heating varies with the square of the load current, doubling the load current gives rise to a four-fold increase in the stator heat generated.  It is important therefore that the machine never becomes excessively overloaded.  If it does, the cooling system may be unable to handle the heat, and dangerously high temperatures may result.

The generator is cooled by a shaft-driven fan which circulates air in a closed air circuit through all the windings.  The air, in circulating, passes through an air/water heat exchanger.  Here the heated water is discharged and the cooled air recirculates, as shown by the arrows in the figure.  Temperature detectors at various points give warning of overheating; if it is seriously high and continues unchecked, the whole set is usually shut down.

If the cooling system should break down for any reason, panels in the hood can be removed and the machine cooled by natural ventilation through the fan.  Under these circumstances however the loading on the generator may have to be curtailed to a value well below its normal rating.

The stator (armature) carries a 3-phase winding consisting of insulated conductors in slots round the inside face.  These conductors must be insulated up to the full working voltage of the system.  Serious or sustained excess temperature of the winding will cause this insulation to deteriorate or even to break down completely, resulting in an internal flashover and possibly complete write-off of the generator.

The rotor windings, which provide the field, operate at a much lower voltage - of the order of 70V d.c. - so insulation is less of a problem.  Nevertheless, if the automatic voltage regulator calls for too much voltage and therefore too much field current, it is still possible to overheat and damage the rotor.

The limitations imposed by overheating the stator and rotor are further discussed in the manual ‘Electrical System Control’, Chapter 1, under the heading ‘Capability Diagram

2.2       ROTOR CONSTRUCTION


A.C. generators with rotating fields have rotors which fall into two types: salient pole and cylindrical. They are both shown in Figure 2.2.

The salient-pole type is illustrated in Figure 2.2(a).  It is by far the most common with offshore generators and also with the smaller sizes onshore.  It consists of a solid iron rotor body (square in the case of a 4-pole rotor) onto which pole pieces are bolted.  Each pole piece carries one of the field windings as shown in the figure.  The poles terminate in pole shoes which spread out the magnetic field in the air gap, but it should be noted that with the salient-pole arrangement the air gap, and so the air gap flux, is far from uniform.  Some rotors have damper windings embedded in the pole shoes, but these are not shown in Figure 2.2(a).

The salient-pole rotor is commonly used with 4-pole generators.  Where there are six or more poles, this is the only type which is practical.



FIGURE 2.2
A.C. GENERATOR ROTORS

The cylindrical rotor (sometimes also called ‘turbo type’) is, as the name implies, completely cylindrical and has no projections.  It is illustrated in Figure 2.2(b).  The field windings are embedded and wedged into slots in the rotor surface in a similar way to the stator slots.  (The overhang of the end windings has been exaggerated in the figure to make the construction clearer.)  The rotor slots cover only part of the surface and are disposed either side of the poles, the whole field winding forming a spiral around each pole centre.

The air gap is uniform, and consequently the air gap flux due to the field winding is almost purely sinusoidal around the gap, being maximum opposite each pole centre.  The smooth surface also results in low windage resistance.

Cylindrical rotors are very sound mechanically and are favoured for large, high-speed generators (3 000 or 3 600 rev/mm), where centrifugal forces on a salient-pole rotor would present severe problems.  Consequently cylindrical rotors are common with 2-pole generators and are sometimes used with 4-pole types.  They are never used with six poles or more, where the rotor construction would become far too difficult.

2.3       HARMONICS


Because the rotor’s magnetic field does not have a pure sine-wave shape, the emf which it generates in the armature is not a pure sine-wave either; this is particularly so with a salient-pole rotor.

Although steps are taken in the stator slot arrangements to offset this effect as much as possible and to restore the emf to near sine-wave form, this is only partly achieved, and some impurity remains.  It shows up as harmonic voltages in the emf waveform, and it is the odd-numbered harmonics which prevail.  In a 3-phase system the third-harmonic voltages (at 150 or 180Hz) are all in phase with each other and cause equal currents through the loads which all return through the neutral conductor if there is one.  These third-harmonic currents are sometimes confused with earth-leakage currents since they may, if sufficiently strong, actuate the earth-fault protection in the neutral line.  They can be distinguished, however, because their frequency is three times nominal.

In a 3-wire system, where there is no neutral conductor as such, third-harmonic currents cannot flow through the load and generator windings (unless there is an earth fault) because there is otherwise no neutral return path.  They can, however, circulate between paralleled generators through their common star-point earths, even without an earth fault, causing additional heating of the stators.  The effect of these harmonics increases with the generator loading.

At one time steps used to be taken to restrict the earthing of paralleled generators to one machine only, in order to prevent such circulation.  Modern generators, however, produce less harmonics than did the older ones, and they are now usually designed to absorb such circulating currents, so permitting multiple earthing - that is, the individual earthing of each generator.

Technical Specification for Synchronous A.C. Generators requires that the voltage waveform shall be in accordance with the international IEC Publication 34-1(7).

This requires that the ratio of the net r.m.s. value of all the harmonic voltages present shall not exceed 5% of the fundamental voltage for small machines up to 1 000kVA, falling to 1.5% for machines greater than 5 000kVA.  When calculating the net r.m.s. value, each separate harmonic voltage is ‘weighted’ by a factor (λn, for the nth harmonic) depending on its degree of interference with communications.  λn varies from about 0.001 for a 100Hz to 1.4 for a 1 000Hz harmonic.  Thus, if E2, E3, E4. . . . are the 2nd, 3rd, 4th .... harmonic r.m.s. voltages and λ 2, λ 3, λ 4 …. the weighting factors, then the net r.m .s. harmonic voltage is


and this must not exceed the stated percentages of the fundamental r.m.s. voltage.

A further cause of harmonic currents in a generator can be due to the load itself and has nothing to do with the voltage waveform.  Loads which include rectifiers are a particularly severe source of harmonic currents.  Where an offshore drilling plant is fed direct from the platform’s main generating system (as distinct from having separate diesel-driven drilling generators), the SCR units which convert to d.c. for the drilling motors are a considerable source of a wide range of harmonic currents.  Because of the action of rectifiers, this range consists entirely of the odd-numbered harmonics.  And because the 3rd harmonic currents (and multiples of the 3rd, namely 6th, 9th, 12th ….) are all in phase with one another, those currents cannot flow in a 3-wire system with no neutral return.

Therefore rectifier equipments tend to draw, in addition to the fundamental, the following harmonic currents:

5th
typically
12% of the fundamental
7th
typically
10% of the fundamental
11th
typically
  6% of the fundamental
13th
typically
  5% of the fundamental
17th
typically
  4% of the fundamental
19th etc
typically
  3% of the fundamental

These harmonic currents are additional to the fundamental current and cause extra heating in the stator.  Their net heating effect is obtained by adding their squares together and taking the square root of the sum (i.e. the net r.m.s. value).

This means that the stator has to carry more current than it would have if its load had been a normal one without harmonics - that is, it must have a higher kVA rating for the same kW active output.  Therefore, if the rectified load forms a sizeable part of a generator’s capacity, the machine must be under-run in terms of active output if its kVA rating is not to be exceeded, or a special design of generator with a low rated power factor (e.g. 0.6) must be used.

Harmonic currents due to the load, in flowing through the reactance of the generator, cause volt-drops at harmonic frequencies and therefore distortion of the terminal voltage waveform.  Due to their higher frequencies passing through the same reactance, these harmonics produce distortion far greater than their magnitudes would suggest.  If excessive, this distortion may cause trouble to other consumers, and for this reason Supply Authorities apply rigid limits on the amount of rectified load that may be put onto their systems.

2.4       INSULATION


Generator windings are insulated against the highest voltages to which they may be subjected, and the insulation must withstand a certain specified maximum temperature without deteriorating.  There are many insulating materials with different - and often conflicting - properties.  They are grouped into a number of classes, depending on the maximum temperature to which they will be exposed and on the insulating material used. 
The classification is as follows (in accordance with BS 2757 :1956).

Class
Typical Insulating Material
Ultimate
Temperature
Y
Cotton, silk, paper, etc., unimpregnated
90oC
A
Impregnated cotton, silk, etc.; paper; enamel
105oC
E
Paper laminates; epoxies
120oC
B
Glass fibre, asbestos (unimpregnated); mica
130oC
F
Glass fibre, asbestos, epoxy impregnated
155oC
H
Glass fibre, asbestos, silicone impregnated
180oC
C
Mica, ceramics, glass, with inorganic bonding
>180oC


It should be noted that the classification letters do not follow an alphabetical sequence.  This is because there were originally only three classes - ‘A’ ‘B’ and ‘C’.  Later intermediate classes were added, and it was decided not to disturb the original well-understood three.  Most platform and shore-installed generators are Class ‘B’ or ‘F’.

Certain of the higher-temperature insulation materials may be hygroscopic and therefore not always suitable in any particular environment, particularly where dampness is severe.

It should be particularly noted that the classification depends on the ultimate temperature to which the insulating material may be subjected, for it is this which determines whether or not it will suffer damage when heated.  It does not depend on temperature rise alone: if for instance, the ambient temperature is 40oC, a Class ‘B’ material may be used if the designed temperature rise will not exceed 90oC, so making the ultimate maximum temperature 130oC.  Designed temperature rises therefore must take into account the greatest expected ambient temperature in which the machine will operate.

2.5       COOLING


All generators used on platforms and in shore installations are air cooled.  The air is circulated past the stator and rotor windings by a fan on the generator shaft.  The warmed air itself may be discharged to atmosphere and not used again (‘Circulating Air’ or CA) or it may be water cooled in a separate cooler with a forced water circulation (‘Circulating Air, Forced Water’ or ‘CAFW’);or in a radiator-type cooler (‘Circulating Air, Natural Water or ‘CANW’)  There are usually alarms if the air or water temperatures exceed certain limits.  All the largest gas-turbine generators are CAFW cooled.

The above letter coding was formerly in general use and is well understood.  Recently however a new international coding system for cooling methods has been  introduced for all rotating machines (BS 4999, Part 21) and is likely to be met with on modern drawings.  It consists of the letters ‘IC’ followed by two digits. The meanings of these digits are given below for typical platform or shore-installed generators:


    First Digit
    Second Digit

0
Free circulation
0
Free convection

1
Inlet duct ventilated
1
Self-circulation

2
Outlet duct ventilated
2
Integral component mounted on separate shaft

3
Inlet and outlet duct ventilated

4
Frame surface cooled
3
Dependent component mounted on the machine

5
Integral heat exchanger  (using surrounding medium)

5
Integral independent component

6
Machine-mounted heat exchanger (using surrounding medium)
6
Independent component mounted on the machine

7
Integral heat exchanger (not using surrounding medium)
7
Independent and separate device or coolant system pressure

8
Machine-mounted heat exchanger (not using surrounding medium)
8
Relative displacement


9
Separately mounted heat exchanger




Where it is desired to specify the nature of a coolant the following letter code is used in conjunction with the cooling code:





Gases

air
A
hydrogen
H
nitrogen
N
carbon dioxide
C
helium
L



Liquids

water
W
oil
U

When nothing but air is used, the letter ‘A’ may be omitted.

Thus a generator cooled by air with an internal fan and with an air/water heat exchanger using pressurised water from the platform system would be classified IC87, or IC8A/7W, instead of the former CAFW.

The larger generators also have thermocouple-type temperature detectors embedded at various points in the windings.  If any one of them exceeds a certain temperature, an alarm is given on the control panel.  The panel also has facilities for the operator to scan all the detectors in turn and to read off the actual temperatures.

2.6       BEHAVIOUR UNDER FAULT


Figure 2.3 shows the general construction of a rotating field a.c. generator.  For simplicity a 2-pole machine has been chosen.  The rotating poles are shown with their exciting field windings and damper windings, and the flux paths are drawn in blue.  Outside the field system is the stator carrying the stationary armature winding in slots.  The condition depicted is the normal one, with the generator delivering steady load current and with its normal excitation flux.

FIGURE 2.3
A.C. GENERATOR - MAGNETIC FLUX PATH


The complete magnetic path is from one field pole (N), through its damper winding, through one air gap, through the armature (stator) coils, through the yoke, back through a second air gap, back through the damper windings and the opposite pole, and finally through the rotor body to the original field pole.  The route is indicated in blue in Figure 2.3.

Consider now that a sudden external short circuit is applied.  The pattern of current and the flux disturbance which then follows is quite complicated.  The sequence of the process is shown in Figure 2.4.

The first step is that the armature current suddenly rises, limited only by the reactance due to the stator iron and air-gap magnetic path.  The new air-gap flux due to the sudden increase of armature current (shown in red) tries to penetrate the field poles but is prevented from doing so by the eddy currents set up in the solid-pole shoes, aided by damper windings embedded in the face of the shoe if fitted (see Figure 2.4(a)).  The eddy currents induced oppose the flux change (Lenz’ Law), and the short-circuit flux from the armature (stator) is deflected along the air gap.  Its return path now has a very long air gap, so the reactance due to the magnetic path is very much lower than before.  This is called the ‘subtransient stage’; the reduced reactance at the beginning of the stage is called the ‘subtransient reactance’ (symbol Xd), and the increased current which it allows to pass is called the ‘subtransient current’, as shown in Figure 2.4(d).  It persists for a few cycles.  It is during this period that the system protection normally operates to trip the generator breaker, so the breaker must be capable of breaking the highest subtransient current.

After the eddy currents and damper currents have subsided owing to resistance in the pole shoe and dampers, the armature short-circuit flux will have penetrated the outside of the main pole body (usually laminated) - see Figure 2.4(b).  Here again it meets opposition, because the changing flux in the pole body induces an emf in the pole winding which


FIGURE 2.4
A.C. GENERATOR ON SHORT CIRCUIT

causes a current to flow in the closed pole winding/exciter loop to oppose the change (Lenz’ Law again).  The direction of that opposing current is the same as that of the main exciting current, so the effect of a short circuit is initially to cause a sudden increase in the exciter and main field current.  Once again this opposing current is slowly damped out by the resistance of the exciting loop, and the short-circuit flux from the armature gradually penetrates the main poles, so increasing the reactance due to the magnetic path and steadily reducing the short-circuit current.  This is called the ‘transient stage’, and it lasts somewhat longer than the subtransient.  The increased reactance at the beginning of the stage (i.e. at the end of the subtransient stage) is called the ‘transient reactance’ (symbol Xd), and the reduced short-circuit current which it allows to pass is called the ‘transient current’.  It is lower and lasts longer than the subtransient, as shown in Figure 2.4(d).

Finally, when the armature short-circuit flux has fully penetrated all the pole bodies (see Figure 2.4(c)), all the iron of both stator and rotor is in the magnetic circuit, and the reactance due to its path is at its greatest; the short-circuit current then settles down to its steady value.  This is the ‘synchronous stage’ and may continue indefinitely if allowed to do so.  The reactance is called the ‘synchronous reactance’ (symbol Xd), and the steady-state current the ‘synchronous (short-circuit) current’, as seen on the right-hand side of Figure 2.4(d).

When this steady state has been reached, the armature short-circuit flux (red), in penetrating the main poles, has partly demagnetised them, since it is in opposition to the main exciting flux (blue).  This phenomenon of demagnetisation of the field by the load current is called ‘armature reaction’.  By weakening the air-gap flux it reduces the generated emf and so reduces the current still further.  The steady short-circuit synchronous current may well then be even less than the machine’s normal full-load current.  This, of course, supposes that the excitation is constant and that no automatic voltage regulation is applied to compensate for the loss of voltage.

In practice Automatic Voltage Regulators (AVRs) are always fitted, but in general they do not act quickly enough to affect the short-lived subtransient stage.  As this period gives rise to the fiercest short-circuit currents which the switchgear has to break, the effect of the AVR is not taken into account when calculating fault currents for switchgear. (See also the manual ‘Electrical Protection’.)

To sum up: when a short-circuit is suddenly applied to a generator, the ensuing current goes through three definable stages - subtransient, transient and synchronous - so long as it is allowed to continue.  At the beginning of each stage the current is determined by one of three reactances, as follows:

             Subtransient reactance                  Xd     (Current typically 6 times full load)
             Transient reactance                       Xd     (Current typically 3 times full load)
             Synchronous reactance                Xd      (Current typically two-thirds full load)
The above currents assume fixed excitation and no AVR action.

The following points on the three types of reactance should be noted:

Subtransient Reactance determines the initial current peaks following a disturbance and, in the case of a sudden fault, is of importance for selecting the capacity ratings of the associated circuit-breakers.

Transient Reactance covers the behaviour of a generator in the period 0.1 to 3.0 seconds after a disturbance.  This generally corresponds to the speed of changes in a system and is usually employed in studies of transient stability.

Synchronous Reactance is a measure of the steady-state stability of the set.  The smaller its value, the more stable the machine.

One effect of the heavy short-circuit current on the generator itself is that, if it persisted for more than a few seconds, winding temperatures would rise to a point where insulation could be permanently damaged or may even break down.  Automatic protection is therefore provided (see the manual ‘Electrical Protection’) to clear the fault as quickly as possible after its onset.  Nevertheless, most large generators are designed to carry their short-circuit current for three seconds.

A further, equally important, effect of short-circuit currents is the intense mechanical stresses which they produce by electromagnetic reaction between the current-carrying conductors.  These occur from the very first cycle of a fault, and no protection is quick enough to prevent them.  The most severe forces occur in the overhang at the ends of the windings.  All generators must therefore be constructed to withstand these forces, and the overhangs are specially braced.  If movement does occur, this is the most likely place to find it.

2.7       DIRECT AXIS AND QUADRATURE AXIS REACTANCES


All that has been said so far about the generator reactances has assumed that the air gap is uniform.  This in turn assumes a generator with a cylindrical rotor.

In practice all offshore, and many onshore, generators are of the salient-pole type which do not have a uniform air gap.  This adds a complication which leads to the idea of two different types of reactance: one where the fluxes due to the stator current are opposite the field pole centres - called the ‘direct axis’ - and the other where the fluxes due to the stator current are opposite the centre of the gap between two adjacent poles - called the ‘quadrature axis’. The direct-axis reactances (subtransient, transient and synchronous) are termed Xd, Xd and Xd, whereas the corresponding quadrature reactances are Xq, Xq and Xq.  The two types are combined mathematically in various machine calculations.

In cylindrical rotor machines the quadrature-axis reactances are practically the same as the direct-axis and need not be taken into account separately.  This is not so with salient-pole machines; in their case both types of reactance should be used when making a rigorous calculation.  However, in practice the error introduced into the calculation of short-circuit currents (see the manual ‘Electrical Protection’) by using only direct-axis values is not significant, and indeed it errs on the safe side.  For this reason only direct-axis quantities have been used in the previous description of behaviour under fault.

2.8       EXCITATION AND VOLTAGE CONTROL


The different forms of excitation and automatic voltage control are dealt with in Chapter 3.

2.9       NEUTRAL EARTH ING RESISTOR


The star-points of all high-voltage generators on platforms are earthed through a current-limiting ‘neutral earthing resistor’ (NER).  Its purpose is to limit the fault current flowing through the generator if an earth fault develops anywhere on the system (see the manual ‘Electrical Protection’).

The NER is separately mounted near the generator and usually consists of a frame containing a heavy grid-type resistance element capable of carrying a large current for a short time.  This short-time rating is possible because any heavy fault current will be quickly cleared by the earth-fault protection.

Neutral earthing resistors are therefore given a maximum current rating for a maximum time - for example, ‘200A for 30 s’.  They may also have a continuous current rating - for example 25A cont.’ - to cover small earth-leakage and harmonic currents which are not large enough to operate the protection.  Their ohmic value goes down to about 10 ohms for the largest offshore generators.

The NER unit sometimes contains also a current transformer to measure the presence of any earth-fault current in order to initiate the protection.

Low-voltage generators are usually solidly earthed without a neutral earthing resistor.

2.10     INSULATED BEARINGS


Bearings of a large machine are often insulated to prevent stray currents from circulating through them.  Such currents can arise from emfs being generated in the rotor shaft due to stray magnetic fields. Under fault conditions these stray fields can be very large. Figure 2.5(a) shows how such currents may flow through the bearings.




FIGURE 2.5
INSULATION OF BEARINGS

These currents, if allowed to flow, would arc across the bearing surface and cause small craters which would eventually destroy the bearings.  Figure 2.5 shows pedestal sleeve bearings, but the same principles apply to ball and roller bearings.

The current path of Figure 2.5(a) can be broken by insulating one or both bearings: the insulation may be at the bearing housing or, more commonly, beneath the pedestal where it seats on the bedplate stool as shown in Figure 2.5(b).  The insulation of only one bearing is more usual, but insulating both allows the insulation to be checked.

For reasons of safety the shaft must be at earth potential.  Consequently on most machines one bearing (the uninsulated end if only one is insulated) is fitted with an earth strap, one end of which terminates in a brush running on the dry shaft.  If the generator is of the ‘overhung’ type with only one outboard bearing, such as with certain diesel-generator sets, this bearing is insulated and the earthing of the rotor shaft is made through the engine and coupling.

The insulation of the pedestal is carried out by a shim of insulating material between the base of the pedestal and its stool.  The holding-down bolts are bushed with insulating material.  Sometimes two insulating shims are used with a thin metal sheet between them.  This enables the insulation resistance of each part to be measured separately, since the shaft and bed plate are normally both at earth potential.

It is important that, where a bearing pedestal is insulated, no waste material or tools should be allowed to lean against it, as they would short-circuit the insulation.
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