The excitation of a generator’s field system has already been mentioned in Chapter 2, as it is not possible to describe a.c. generators without referring to their field system and excitation. This chapter discusses the three practical methods of field excitation which may be encountered.
A.C. GENERATOR EXCITATION (1)
Figure 3.1(a) shows the ‘conventional’ method described in the manual ‘Fundamentals of Electricity 2’, where a driven d.c. exciter (in this case belt-driven) feeds its d.c. output through sliprings to the main generator field.
The output voltage is sensed by an automatic voltage regulator (AVR), which regulates the exciter’s field so that the exciter output holds the main field at whatever level is necessary to maintain the generator output voltage constant. AVRs are discussed later in this chapter. It will be seen that the control of voltage is a closed loop, and, like any other closed loop servo mechanism, it is subject to certain errors.
Figure 3.1(b) shows a development where the rotating d.c. exciter is replaced by a static electronic exciter, which usually incorporates the AVR. Voltage sensing and excitation power are derived from the main generator output; excitation current is controlled by the AVR, rectified and fed into the main field through sliprings, just as in the ‘conventional’ case. This is called the ‘static exciter’ method, and it should be noted that it still requires brushes and sliprings. It is not found on platforms but is widely used onshore, although not to any great extent in oil installations.
A further significant development is shown in Figure 3.1(c). Here the shaft-driven rotating exciter has been restored, but it now takes the form of an a.c. generator of the fixed-field type mounted on the main shaft itself. Its a.c. output is taken through connections inside the shaft, through a diode bridge which rotates with the shaft, to the main rotating field of the generator. The field is thus excited by d.c. without the need for brushes and sliprings. It will be seen that this exciter cannot be belt-driven; it must be integral with the main shaft.
As with static excitation, voltage sensing and excitation power are derived from the main generator output. Excitation current is controlled by the AVR, rectified and fed into the fixed field of the a.c. exciter. The a.c. output of the exciter follows the AVR signal, and its output current is rectified by the diodes which rotate with the shaft; the d.c. output from them is in turn passed to the generator’s main field. The field current thus follows the AVR signal almost exactly.
It will be seen that the only link between the fixed and moving parts is the magnetic one between the exciter field and its rotating armature: no sliprings and brushes are needed. The method is for this reason called ‘brushless excitation’, and it will be found, in one form or another, on all platform and onshore main and auxiliary generators.
The principal advantage of brushless excitation over the other two types is that the absence of brushgear and sliprings greatly eases the maintenance problem.
3.5 BEHAVIOUR UNDER SHORT CIRCUIT
In the conventional case (Figure 3.1(a)) excitation power is derived from a separate d.c. generator which is not affected by the voltage on the main generator’s output lines. However, with both static excitation and the brushless excitation described above (Figures 3.1(b) and (c)) excitation power (as well as sensing) is derived from the output of the generator itself - true ‘shunt excitation’.
Under normal conditions this is quite satisfactory, but under short-circuit conditions the generator’s output voltage will drop heavily - it might even vanish. Under this low-voltage output situation the AVR will try to force up the excitation, but, just at the moment it wishes to do so, it has no power available. Under these conditions a collapse of system voltage is possible.
To overcome this a method is employed which makes use of the short-circuit currents themselves to provide the missing excitation.
A.C. GENERATOR EXCITATION (2)
Three heavy current transformers are arranged in the generator output lines as shown in Figure 3.2(a). Their secondary outputs are rectified and passed to the main exciter’s field either in parallel with the normal excitation (as shown) or sometimes to a separate field winding in the exciter. Although they take the form of current transformers, these units, when used in this application, are referred to as ‘short-circuit CTs’.
Under short-circuit conditions when the generator output voltage is very low, the short-circuit CTs pick up the heavy short-circuit currents and, after they have been rectified, use them to boost the main exciter field, and so the main field. This serves to maintain the generator output voltage under short-circuit conditions - a necessary requirement in network operation so that protection may operate reliably.
Short-circuit CTs are used generally with medium-sized generators with either static or brushless excitation where no ‘pilot exciter’ is fitted (see below) and where excitation power is drawn from the generator’s output. This applies to most basic services generators on platforms and to some main sets.
With large brushless generators a different method is used. Instead of drawing excitation power from the generator output, the AVR has only a voltage-sensing connection. The arrangement is shown in Figure 3.2(b).
The exciter’s field is powered independently from a separate high-frequency inductor-type generator called a ‘sub-exciter’ or ‘pilot exciter’. It has permanent magnets as rotating field and is driven by the main shaft. It also provides operating power to the AVR itself. Only the voltage-sensing leads to the AVR are taken from the main generator output. The AVR regulates and rectifies the power from the pilot exciter to the main exciter field. This in turn regulates the a.c. exciter output, and thence the d.c. rectified input to the main field through the diodes, to hold the generator output voltage constant.
The pilot exciter is mounted on the main shaft, usually immediately next to the main exciter (not exactly as in Figure 3.2(b) which is schematic only). It is usually arranged in a single enclosure with the main exciter and the diode plates. Figure 3.3 shows this arrangement.
As in the conventional case, the excitation of the generator is now independent of the generator’s output voltage and so is maintained even under short-circuit conditions and without the use of short-circuit CTs. This is the arrangement on almost all platform main generators.
3.8 THE DIODE BRIDGE
In Figures 3.1(c) and 3.2(a) and (b) the diodes are shown for clarity as inside the shaft between the exciter and the main generator. The exciter output is 3-phase, and the diodes are in fact a 3-phase full-wave bridge, requiring six diode elements. Clearly they cannot be buried in the middle of the shaft, and in practice they are mounted on a rotating plate on the extreme end of the shaft at the exciter end, as shown in Figure 3.3 in green. This makes them easily accessible for inspection, testing or replacement.
GENERATOR AND DIODE PLATE
A point on the use of diodes should be noted. If one of the six should fail, either by open-or short-circuiting, harmonic currents flow in the main field circuit. These harmonics are reflected into the field circuit of the main exciter and are detected by a ‘diode failure’ relay tuned to respond to the principal harmonic frequency; the alarm (or trip) signal from this relay is time-delayed by 10 or 15 seconds to prevent false operation.
A diode failure would have no discernible effect, from the consumer’s point of view, on the generator’s output voltage. The reduced d.c. output from the diode bridge with one diode faulty would lower the main field’s d.c. current slightly, and with it the main generator’s output voltage. This would be immediately detected by the AVR, which would increase the excitation until the voltage was restored, and the consumer would not be aware of it. However, the remaining healthy diodes might then be somewhat overloaded, and the situation should be corrected.
With an open-circuited diode the condition would not be serious. The increase of exciter field current would be about 15%, which can be provided by the AVR and carried by the exciter field for some time. Nevertheless, it should be corrected as soon as possible. A short-circuited diode however is more severe, calling for a much greater increase of exciter field current. The AVR and exciter could well be damaged if this condition were allowed to persist.
It is usual for the diode failure relay to give an alarm only, not to trip the breaker and shut down the set. When this alarm appears, generation should be transferred to another machine as soon as opportunity offers; the faulty set should then be stopped and the failed diode replaced. On some sets, however, the diode failure relay actually trips the set.
CAUTION WHEN MEGGER TESTING A GENERATOR FIELD SYSTEM, ALL DIODES MUST FIRST BE DISCONNECTED OR SHORT-CIRCUITED TO PREVENT THE MEGGER VOLTAGE BEING APPLIED ACROSS THEM AND BREAKING THEM DOWN.
A further important point resulting from the use of diodes should be noted. When output voltage falls, it is sensed by the AVR and the exciter field is increased. The increased exciter output voltage is passed by the diodes to appear as an increased d.c. voltage across the main field. This causes an increase of main field current at a rate depending on the R/L ratio of the whole field/exciter loop. Therefore the increase is not instantaneous but, because the exciter resistance is appreciable, R is large enough to allow a reasonably quick response.
In order to improve the response time when there is a drop of output voltage, the AVR is made to give the main exciter field a considerable boost, causing a big jump in its a.c. output voltage and so a large rise in the d.c. voltage applied through the diodes to the main field. It helps to overcome the field’s natural sluggishness and to build it up more quickly. This is known as ‘field forcing’. When the field has reached its new value and the output a.c. voltage is restored, the AVR removes the excess forcing current from the exciter’s field.
However, if there is an output voltage rise (for example due to the throwing-off of a large load) it is sensed by the AVR and the exciter field is reduced. The reduced exciter output voltage is now lower than that of the main field, and it is blocked by the diodes. The main field current, flowing in the highly inductive field system, ‘flywheels’ round the closed circuit formed by the field and the diodes. It decays slowly because it is damped only by the comparatively small resistance of the main field itself (small R/L ratio, and therefore longer time constant L/R).
Thus in a brushless system, whereas the response to a drop in output voltage is reasonably quick, reaction to a rise is appreciably slower. This is particularly significant after a short-circuit has been cleared. During the period of the fault the voltage will have dropped and the AVR will have forced up the excitation, probably to its limit. When the fault is cleared, this overexcitation shows as a large overvoltage on the whole system, which is comparatively slow to recover. This could involve a risk of burn-out of lamps or delicate apparatus.
AVRs are of many different makes, and various types are found on platforms and onshore installations.
All, however, have certain features in common when used with brushless generators. They are nowadays entirely electronic; they take their operating power from either the main output or the shaft-driven high-frequency sub-exciter (typically at 400Hz), but they sense the voltage to be controlled from the output side of the generator before the circuit-breaker terminals. In high-voltage generators this sensing circuit is taken through a measuring voltage transformer of at least Class 0.5 accuracy (see Chapter 4 of the manual ‘Electrical Distribution Equipment, Part B’).
The detailed electronic circuits are not discussed here, but power from the main output or the high-frequency sub-exciter is rectified through thyristors, which are controlled by the voltage-sensing circuits to provide the correct d.c. current to the field of the main a.c. exciter.
Like any closed-loop servo, an automatic voltage regulating system holds the voltage constant, within stated errors, at whatever level it has been set. This level is referred to as the ‘set-point’.
In an electronic AVR the set-point is adjusted by a variable resistance, or rheostat, in the appropriate part of the circuit. On some generators this rheostat is outside the AVR proper and is mounted on the adjacent generator control panel for manual control; it is usually marked ‘Raise Volts/Lower Volts’. On other makes of generator it is arranged for remote control from some distant panel. In such a case the rheostat is motor-driven, the motor being controlled forward or backward by a 2-way-and-off spring-loaded switch marked as above.
When used with a single generator the AVR set-point control does indeed regulate the machine’s voltage output, but when used on a generator running in parallel with others, the prime function of the AVR control is not so much to regulate voltage but to adjust the sharing of reactive load between the generators, despite the marking of the control knob or switch. It does, however, have some effect on voltage level, but this is only secondary. The action is fully explained in the manual ‘Electrical System Control’.
When a load is applied to the terminals of a generator previously running at no load and without AVR control, the terminal voltage will drop by an amount which depends on the nature of the load. This drop of voltage is called the ‘regulation’ of the generator at that load. It is usually quoted at full rated load - that is, at the full-load rated current and rated power factor and is expressed as a percentage of the no-load or system voltage. Thus, if V0 is the no-load voltage and V the generator terminal voltage at full rated load and power factor and with the excitation unaltered, then
is the percentage full-load regulation.
In practice of course the reduced voltage V would be immediately detected by the AVR, which would increase the excitation until the terminal voltage was restored to the system value V0 .
The determination of the generator’s internal voltage drops and their effect on regulation is covered fully in Chapter 4.