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Saturday, February 2, 2013

CHAPTER 5 GENERATOR SPEED CONTROL


5.1       GENERAL

All a.c. generators must run as nearly as possible at constant speed in order that the frequency of the generator’s output voltage is held within close limits to the nominal, which on most platforms is 60Hz and on most shore establishments 50Hz.  This applies to both gas-turbine and diesel-driven sets.  The device which achieves this is called a ‘governor

FIGURE 5.1
GOVERNOR CONTROL LOOP

A governor is a speed-sensing device driven by the engine itself or by some mechanical part such as the gearbox, coupled directly to it, or else it derives from the engine a signal which represents the engine speed.  It actuates directly, or through an amplifier, the fuel control to the engine.  When, for example, load on the engine increases, its speed momentarily drops; this is sensed by the governor, which causes the fuel admitted to the engine to be increased.  This in turn increases the engine’s torque and raises its speed against the new load until it is restored near to its former level.  The control system is automatic and forms a closed loop, like any other automatic servo system, and is illustrated in Figure 5.1.

The above description represents the ideal situation; in practice it is achieved only within certain limits of error which are explained below.

It will be seen that the governor must maintain the engine speed at some arbitrary level.  The level can be varied at will by adjusting the governor.  This adjustment is called ‘speed setting’, and the level to which it has been set is the ‘set-point’.  Once so set, the governor maintains the engine speed at the set-point (within limits of error) at all loads within the engine’s rating.

Governors may be mechanical or, more recently, electronic, but the basic control loops for both are the same.



5.2       MECHANICAL GOVERNORS



The mechanical governor is considered first, as it demonstrates the principles more clearly.


FIGURE 5.2
FLY BALL CENTRIFUGAL GOVERNOR

Figure 5.2 shows probably the oldest form of engine governor, namely the ‘flyball’ centrifugal type, used originally on steam and gas engines.  It consists of a pair of heavy balls held by a link mechanism which is driven by the engine.  As the engine rotates, the balls are thrown outwards by centrifugal force against the normal restoring force of gravity.  There is no amplifier in this case.  As the balls move outwards they raise a sleeve which, by a suitable linkage, operates to reduce the opening of the steam or fuel inlet, shown here for simplicity as a butterfly valve.

When the engine is at rest there is no centrifugal force, and the balls hang in the position shown in Figure 5.2(a); the fuel valve is then wide open.  When fuel or steam is admitted the engine starts with a full fuel charge and accelerates.  The balls move outwards, raising the sleeve, and gradually close the valve until the steam or fuel charge just balances the engine load, at which point the speed settles down to a steady value, as shown in Figure 5.2(b).  The level at which it settles depends on the set-point.  This can be adjusted in various ways: in Figure 5.2 it is by adjusting the link between the governor and fuel valve.  Lengthening it opens the valve wider and so raises the set speed; shortening it has the opposite effect.

The steady-running condition is shown in Figure 5.3(a), which is a repeat of Figure 5.2(b).  Once the speed has settled at its set value, any variations of speed without change of load are closely controlled.  An increase causes the balls to move outwards, so closing the valve a little and reducing fuel to check the increase (see Figure 5.3(b)).  When the speed has returned to its set value the valve is once again in its former position.  A similar effect will occur, but in the opposite direction, for any momentary drop in speed.






FIGURE 5.3
EFFECT OF SPEED AND LOAD CHANGES

The reaction to a change of load is different.  If the load on the engine increases, the speed at first drops, causing the balls to move inwards.  This opens the valve further until the increased fuel produces increased torque to balance the higher load.  The deceleration then ceases, and the speed settles at a level somewhat lower than it was before (see Figure 5.3(c)).  The amount of speed loss depends on the characteristic of the fuel valve itself - that is, how much extra fuel it admits for a given loss of speed.

From the above it is clear that, although a governor ideally holds the speed constant at its set value, with an increase of engine load it cannot quite achieve this.  This is one of the ‘errors’ referred to in para. 5.1.  There is in theory no such thing as a truly ‘isochronous’ governor, that is one which keeps the speed absolutely constant at all loads, although modern governors do approach this condition.  Some degree of error, however small, is necessary for a governor (or indeed for any other closed-loop servo) to work at all.

Figure 5.4 shows, in graphical form, a typical relationship between speed and engine load (in kilowatts mechanical).  The fall of speed with increase of load is called ‘droop’ and is typically about 4% from no load to full load for a mechanical governor.  (In Figure 5.4 the slope is exaggerated.)  If the set speed at no load is, say, 1 800 rev/mm, then at full load it will be only 1 730 rev/mm, and the frequency of the driven generator will have fallen from 60Hz to 57.6Hz.  To offset this the nominal speed to achieve 60Hz could be set at half load, so that the frequency varied from 61.2Hz to 58.8Hz from no load to full load (±2%).




FIGURE 5.4
GOVERNOR DROOP

Other sources of error in a mechanical governor include:

(a)   backlash, friction and wear in the flyball and connecting linkages

(b)   time-lag in the flyball mechanism (i.e. inertia time to take up new position)

(c)   time-lag in the amplifier, if fitted

(d)   firing stroke delay (diesel engines only)

(e)   non-linearity of the fuel rack or valve

(f)   twist in the governor drive

(g)   inertia of the rotating parts

All these combine to distort the droop line of Figure 5.4 from its theoretical straight to a somewhat irregular shape.  Random errors produce a tolerance band about the mean, as shown shaded.

The effect of most of these errors is self-evident, but three of them need further explanation.  Error (a) is likely to become worse as wear takes place with the increased life and usage of the engine.  Error (d) occurs only with diesel engines and is due to the next cylinder not necessarily being ready to fire at the moment the governor calls for increased (or decreased) speed.  Error (f) may occur if the drive from the engine shaft to the governor is not solid - for example if the drive is taken from the gearbox.  This may produce lag or even oscillations.

It will be noted that many of the errors are due to time-lags in various parts of the governor system loop.  These all delay the response of the engine to a speed error signal from the governor and, if appreciable, produce the effect of sluggishness.


5.3       MODERN MECHANICAL GOVERNORS


The flyball system of Figure 5.2 is now seldom used.  Instead there are rotating weights on the governor shaft, controlled by springs instead of by gravity.  This system, however, is still a centrifugal one, and the displacement of the weights still actuates the fuel valve or rack.  Instead of the direct linkage of Figure 5.2, most modern mechanical governors use a hydraulic linkage, which is more positive in its action and less liable to backlash and wear.  Oil pressure is obtained from a pump driven by the engine or from an auxiliary motor-driven pump, and it fails safe by causing the fuel valve to close if oil pressure fails.  The hydraulic system acts as the ‘amplifier’ of Figure 5.1 between the speed sensor and the fuel valve.  It operates the valve by a hydraulic actuator, which converts the governor signal into a hydraulic thrust.

5.4       ELECTRONIC GOVERNORS


Because of the unavoidable errors, including the large inherent droop, of mechanical governors an entirely new type was developed and is now in general use throughout all platforms and most shore installations.  This is the ‘electronic governor’, and those which are found on most platforms are of the ‘Woodward’, ‘Speedtronic’ or ‘Rustronic’ type.  It must be emphasised, however, that the governing principles set out in block form in Figure 5.1 apply just as much to an electronic governor as to a mechanical one.

In an electronic governor all linkages, except the final actuator stage, are electrical and therefore not subject to backlash or wear.  Consequently a much greater accuracy can be achieved, and a droop of ½% (as compared with 4% for a mechanical governor) is not unusual.  Moreover, because of lack of wear, an electronic governor is very consistent in its performance.

One essential difference of detail is that speed is sensed by an inductor-type tacho-generator consisting of an iron toothed wheel rotating past fixed coils.  The varying flux as 

FIGURE 5.5
ESSENTIAL ELEMENTS OF AN ELECTROHYDRAULIC GOVERNOR



the teeth pass the coils induces in them an emf at a frequency directly proportional to the speed.  The other main difference is that the former mechanical or hydraulic linkage is replaced by simple electrical connections (apart from an electrohydraulic actuator referred to below); these have no backlash and are not subject to friction or wear.

The varying-frequency signal is processed and amplified by electronic circuits, and also mixed with certain other signals, to give an electrical output signal representative of the fuel input required.  It is converted to a hydraulic signal through a pilot solenoid valve in an electrohydraulic actuator.  This is, in effect, a further amplifying stage, and the actuator drives the liquid fuel or fuel-gas valve.  The hydraulic oil pressure is derived from an engine-driven pump when the set is running, and from an auxiliary pump when it is at rest or running slowly.

The basic system is shown in Figure 5.5.  The closed loop should be compared with the block diagram of Figure 5.1.

5.5       TYPICAL SINGLE SHAFT GAS TURBINE GOVERNOR


The foregoing is a general description of an electronic governor.  As applied to various gas turbines and diesel engines the arrangements differ in detail, but the principles remain the same.



FIGURE 5.6
TYPICAL SINGLE SHAFT TURBINE SPEED CONTROL


The installation for a typical single-shaft gas turbine is shown in simplified form in Figure 5.6.  It uses the magnetic speed-sensing pick-up and hydraulic actuator already described.  A special feature, however, is the refinement of load sensing.

It has already been explained how various time-lags in the control loop delay the response of the system to a control signal.  One of these is the inertia of the rotating parts. In a single-shaft set this is considerable, consisting as it does of the compressor and turbine unit (running at high speed), the gearbox and the generator itself.  When there is a sudden increase of load, the rotating mass decelerates relatively slowly because of its high inertia, and there is an appreciable delay before the speed has dropped sufficiently for the sensor to initiate governing action; there are further time-lags in the control loop before the hydraulic actuator admits more fuel.  All this amounts to a slow response.  By making the governor sense the load change when it occurs, and so anticipate the speed drop, the correcting action can be started earlier and the overall speed of response quickened.

In the system illustrated the electronic processor, in addition to receiving the speed signal from the sensor, receives also a load signal from a wattmetric element on the output side of the generator, as shown in Figure 5.6.  These two signals are mixed in the correct proportions - an advantage of electronics - and are balanced against the ‘set speed’ signal put on manually.  The difference, or ‘error’, causes the actuator to respond to the combined speed/load error signal.

The gas turbine illustrated here is assumed to be dual-fuelled, and the hydraulic actuator controls both the liquid fuel and fuel-gas valves.

5.6       SINGLE SHAFT OVERSPEED PROTECTION


The normal governor control system should prevent excessive speed, but it is covered by a ‘back-up overspeed’ system in case it should fail.  This is similar to the normal speed governor system, except that it operates a shutdown valve instead of the actuator.  There is a completely separate magnetic pick-up and electronic amplifier, with a speed setting of about 110%.  At this speed the amplifier produces a signal which shuts down the whole engine by allowing the contactors in the circuits of the solenoid-operated shutdown fuel valves to open (a ‘fail-safe’ arrangement).  This is shown in Figure 5.6.

5.7       TYPICAL TWO SHAFT GAS TURBINE GOVERNOR


The installation for a typical two-shaft gas turbine is shown in simplified form in Figures 5.7 and 5.8.  Most of such sets installed on platforms operate on single fuel (gas) only, but some have been modified to run on gas or liquid fuel.  The governor uses a magnetic speed-sensing pick-up and an electrohydraulic actuator as already described, but as the turbine has two shafts it has certain other refinements added.

The governor control system for a two-shaft turbine was shown basically in Chapter 1, Figure 1.7.  Speed is sensed from the output shaft driven by the power turbine, but fuel control is applied to the gas generator only, which runs at a speed different from that of the power turbine, the difference depending on the generator loading.  As the gas-generator turbine runs free, it has relatively low inertia and responds quickly to fuel input changes, so load sensing as applied to the single-shaft turbine is not needed.

The main control requirement is to maintain constant speed of the power turbine.  Other limitations are, however, necessary to ensure that the various limits of the engine’s rating are not exceeded.  It is arranged that the limitation requiring the least power from the engine, and therefore least fuel, is the one in control.



FIGURE 5.7
TYPICAL TWO SHAFT TURBINE SPEED CONTROL (INPUTS)

Figures 5.7 and 5.8 show, in block form, the speed control system and the limitations which are applied to it.  They also show how the predominant speed control signal is taken through a chain of electronic and hydraulic units to position the fuel-gas control valve.

At the top of Figure 5.7 is the Power Turbine Speed element.  It has the automatic magnetic speed-sensing unit which gives a signal proportional to the power turbine speed.  It is compared with the demanded, or ‘set-point’, speed, and a difference signal is passed on to the output chain.

Next is the Manual Speed Control element.  When this is selected the manually set speed signal is passed direct to the output chain.

The third element is the Gas-generator Speed limitation.  Its speed is sensed by a magnetic pick-up similar to that on the power turbine shaft, but it is purely an overspeed device.  The sensed speed signal is compared with a preset maximum allowable speed setting for the gas generator.  If the actual speed exceeds the allowable, the signal to the output chain is reduced to a level which reduces the gas-generator speed to below the allowable limit.


The fourth element is the Temperature Control limitation.  Thermocouples disposed around the gas-generator exhaust duct detect exhaust temperature.  Their signals are averaged and compared with a preset maximum allowable temperature.  If the actual temperature exceeds the allowable, the signal to the output chain is reduced to a level which keeps the temperature below the allowable limit.

The fifth element is the Acceleration limitation.  If during starting the acceleration is too great, it not only causes excessive temperature but may take other quantities outside their designed limits.  This unit, whose level is preset, takes over only during the acceleration period and limits the fuel rate until the speed has reached 94% of maximum.  At that point it is taken out of circuit, leaving the other limiting circuits to exercise their individual controls.

The sixth element is the Base Load limitation.  If two or more generators are running in parallel and it is desired that one of them (the ‘base load’ machine) should carry a constant load and that the others should carry all loads above a certain base load maximum, this element is used.  It is brought into action only when the ‘Base Load’ switch is made.  The maximum base load level is preset and, as soon as the load reaches this point, the signal to the output chain is locked so that the machine can deliver no further load even though in parallel with the others.  Generators on platforms are not usually operated in this manner; it is preferred that they share the load equally or in proportion to their sizes (see manual ‘Electrical System Control’).  This feature, though available, is not therefore used.



FIGURE 5.8
TYPICAL TWO SHAFT TURBINE SPEED CONTROL (ELECTROHYDRAULIC LOOP)


The seventh element is the Peak Load limitation.  It is similar to the Base Load Limit but is preset to the maximum allowable peak load for the turbine.  If any attempt is made to exceed this load, the signal is locked, permitting no further increase of fuel.

At the bottom is the element for the Idle Clamp (Gas Generator) limitation.  This is in operation while the speed of the gas generator is below idling level - e.g. when starting from rest.  At such speeds it prevents all other controls operating.  When the gas generator has reached its correct idling speed (15%) the set is ready for acceleration.  The Idle Clamp is then automatically disconnected, allowing all the other controls to work normally.

All these individual control signals are fed into a ‘Least Gate’, shown tinted in Figure 5.7.  This is a unit consisting of diodes which select whichever of the eight controls listed above is producing the greatest signal.  This is normally the speed signal.  That signal alone is automatically selected, and all the others are blocked off by the diodes.  (All the control signals are in fact of negative polarity, and the stronger the signal, the less negative it becomes - hence the name ‘Least Gate’.)  For example, if the set is being controlled automatically, as normal, by the power turbine speed (through diode D1), only that diode is conducting, and all the others are blocked.  If now the temperature limit is exceeded, diode D4 conducts and takes over, and all the others, including the speed diode D1, are blocked.

The selected output signal from the Least Gate is passed through an Increase Rate Limit network, which limits the rate at which a power increase can be demanded without interfering with the normal governor action.  From here it passes to the main Output Amplifier where it is compared with a feedback or ‘reset’ signal from the pressure of the gas-generator exhaust, which is proportional to the demanded speed.  The difference between these two, or ‘error’, is fed to a Power Amplifier and thence to the Electrohydraulic Actuator which converts the electrical into a hydraulic pressure signal.  This moves a hydraulic Servo Limiter (a hydraulic relay) which in turn moves the fuel valve (or fuel valves in the case of dual-fuelled turbines) to admit more, or less, fuel to the turbine.  It continues to move the valve until the gas-generator pressure feedback just balances the governor’s demand signal and the error disappears.  The fuel valve then stops in its new position.

5.8       TWO SHAFT AUTO AND MANUAL SPEED CONTROL


If the automatic system described above becomes faulty or if for any other reason direct control is desired, the governor can be cut out (but not the other limitations) and the speed controlled by hand.  This is done by an ‘Auto/Manual’ changeover switch and ‘Raise’ and ‘Lower’ pushbuttons on the local control panel.

There is a risk, when changing from auto to manual, that the manual setting may not at that moment match that of the automatic, and there would follow a gross change of speed as the fuel valve setting changed abruptly.  There is a built-in automatic circuit to prevent this, but normally when changing over from auto to manual the manual setting should first be trimmed to match the auto.  Matching is normally indicated by an Auto/Manual Balance Meter on the Local Control Panel.

5.9       TWO SHAFT OVERSPEED PROTECTION


Overspeed protection of this particular gas generator is already covered in para. 5.7 and Figure 5.7, diode D2.  For the power turbine there are two separate overspeed devices, one electronic and one mechanical, both completely independent of the main speed control.  Both are shown in Figure 5.8.


The electronic overspeed system consists of a separate magnetic pick-up similar to that for normal speed sensing.  The signal from this is compared in an amplifier with a preset voltage representing the critical overspeed level; if it exceeds it a signal is passed to an Electronic Overspeed Trip unit which de-energises the solenoid-operated valve by allowing two contactors in series to open and cut off the fuel (a ‘fail-safe’ arrangement).  These contactors and the solenoid are all energised while the set is running.

Separate from this is a mechanical overspeed unit which is basically a centrifugal type.  At an overspeed setting slightly in excess of that for the electronic one, a spring-restrained bolt flies out under centrifugal force.  It actuates a trip lever mechanically and thence a relay and the main tripping contactors.  The mechanical overspeed trip acts as a back-up for the electronic.

Either one can, if required for example for maintenance, be isolated by a ‘3-position’ switch on the local control panel for testing, while leaving the other operative.  The switch is marked: ‘Mech. Override/Normal/Elec. Override’.  The machine is thus never left without overspeed protection.

5.10     LOAD SHARING


Electronic governors have very small inherent droops, typically ½%; they are often referred to as ‘isochronous’ though of course they cannot be exactly so.  This is excellent for single machine running, as speed is held closely controlled, but for load sharing with parallel generators some droop is essential.  When so required, droop can be injected artificially into the electronic circuits by operating a ‘Droop/Isoch’ switch on some makes of governor.  This biases them from the load signal which is provided for load sensing (see dotted line of Figure 5.6) and provides an artificial droop.  On other types of governor the amount of droop can be varied by direct adjustment of the governor circuits.  Load sharing is dealt with more fully in the manual ‘Electrical System Control’.

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