Tip:
Highlight text to annotate it
X
We now move on to discussing another important element in a power system. Our lecture, so
far have been concentrated on modeling a synchronous machine, which itself was a fairly long, and
tedious process. In the last class, we describe some simplified models of a synchronous machine.
Of course, the there is practically no end to the amount of detail one can go in our
discussion of synchronous machine modeling. There are in fact a few topics, which we have
not covered, one of them being the saturation performance of a synchronous machine. How
would you change the modeling, in case saturation exists? We will not discuss this much in detail
in this course. In fact, we will just carry on. I can refer you to the books, which which
I had mentioned right at the beginning of the course. You can refer to them, and there
is some interesting reference is relating to saturation modeling.
Now remember, what is the main issue there? When you try to model the synchronous machine
with saturation considered; remember, it is no longer what is known as a linear machine.
In that sense, you cannot you know get a flux current relationship, which is linear and
as a result of which it becomes difficult to apply the full machinery of d q transformation.
Remember that when we did the modeling of a machine, when we derive the inductance inductance
matrixes, which relate the fluxes in the A winding, and currents in the A, B, C, D winding
and so A, B, C, F, G, H, K windings. You will notice that what we did was of course, you
know try to do some kind of super position of fluxes mmf, etcetera.
You know, we effectively used super position, in order to determine the nature of the inductance
matrixes. You can no longer do so in case saturation exists, and that really quires
the pitch, and as a result of which there there is not a nice or a neat or a mathematically
rigorous way to approach, you know saturation in synchronous machine. Of course, one may
argue that again, you know whenever you have modeling, there is the physical laws are known.
And one should be able to model even saturation by actually computing the electromagnetic
fields at the you know the flux configuration, which exists during saturation of a synchronous
machine. But, that would be really very tough and it
is not justified, when doing the stability kind of studies, when we are studying slow
electromechanical transients but under certain circumstance, it can actually affect the result.
For example, the even the steady state behavior of a machine if one does not consider a saturation,
one can end up with you know a fair degree of error and that is a reason, why people
are worried about it the and although, a very rigorous way of tackling saturation is not
really been discussed in the literature but some ad hoc techniques have been discussed
and I prefer you to the books by the Padiyar, Kundur and good discussion exists also in
Sauer and Pai, which discusses some of the theoretical implications of various saturation
models. So, I the basic model, which we have derived
in the d q reference frame, we tweak it a big, we tweak it, we do not really go ahead
and try to start from scratch ad try to derive a saturation model, which is a absolutely
rigorous but we just simply tweak the d q model to account for saturation and its quite
ad hoc and you can say a pragmatically approach is usually followed. We do not discuss this
any further. I refer you to the books, which I have just mentioned. We move on to today’s
lecture, which is on excitation systems. Now, to look at the role of excitation systems,
let us just look look back what at what we have been doing. We have studied a synchronous
machine connected to a voltage source or an infinite bus. Sooner or later, we will have
to consider synchronous machines connected to other synchronous machines to loads to
a network and try to infer. you know infer How a power system behaves? A integrated power
system behaves. But, even before we interconnect the synchronous
machine to a network and try to study that kind of system. We can look at the two important
inputs, which are there in a synchronous machine, that is one of them is of course, the mechanical
power or the mechanical torque and the second thing is the field voltage. We have been using
the symbols T m and E f din our synchronous machine model. In fact, all the simulation
so far we took E f d to be some kind of constant. In fact, we did simulate step changes in a
the field voltage or E f d but we did not really have any kind of continuous control
over either the mechanical power or the field voltage but these two are essentially the
inputs to our synchronous machine.
Now, if you look at where we are right now, we have done the synchronous generator modeling,
in which we have the rotor mechanical equations and you also have the machine flux equations.
The differential equations in the fluxes of the machine and a rotor mechanical equations
are generally formulated in terms of delta and omega and given the mechanical, the rotor
angle, the mechanical speed and the flux is, one interacts with the network, you can connect
the generator to a network, which you may consist of a voltage source, it may be just
a load or it may be other elements. So, you have typicallya generator connected to a network
and connected to other elements, which could be other synchronous generator, it could be
other loads and so on. Now, the generator itself has two inputs. You have got your mechanical
power or the mechanical torque and you have got the field voltage. These two things are
in fact, things we need to be controlled.
Now, if you look at another figure. The power apparatus, which controls the mechanical power,
is essentially that turbine and a boiler. Boiler, of course in case of steam turbine
driven machine. So, you have got a boiler and a turbine in the steam turbine driven
machine, which really gets the mechanical power at the shaft of at the shaft of a generator.
The power apparatus, which generates the field voltage for a synchronous machine is called
as an exciter and it is usually consisting of at least one control power electronic equipment.
So, in fact you will find that the exciter is a control element, by which you can control
the field voltage and it the control is via power electronic convertors.
Now, the valves or the gate control of a synchronous machine really controls the mechanical power
input to the machine. So, you can control both the mechanical power and excitation to
a synchronous machine. Now, there are various ways you can generate this field voltage.
We will consider two of the most common ways. One can generate field voltage for a synchronous
machine. So, we have to actually, when we talk about
excitation system, one one of the thing we have to discuss is a power apparatus, that
is the excitation system and of course, the control of the field voltage itself. We kind
of hinted, when we consider the the simulations in the past two lectures, that if we we need
to change the field voltage we need to change the field voltage as the generator is loaded,
otherwise you may not be able to operate the generator acceptable acceptable. In the the
case which you simulated, we saw that trying to load a synchronous machine without simultaneous
control of the field voltage resulted in a loss of synchronous and the machine was loaded
up to its rated value. So, we do require some kind of control over the felid voltage that
is a very important idea.
Now, the various ways of course, one can obtain the power apparatus or the the configuration
of the power apparatus in order to excite the synchronous machine. Now, the major controllable
element in any of this excitation system is as I mentioned some time a power electronic
convertor. In fact, it is usually a Thyristor based control rectifies. So, most of the generator
you will find I have got controllable element in the excitation system, which is the control
thyristor bridge. So, that is what is essentially there in a
synchronous machine. Now, one of the important thing you should remember at this point is,
that is synchronous machine has got an extremely large synchronous reactants. What would mean
by large? In relative terms, if one wants to understand this.
If you have a synchronous machine, suppose you have got a synchronous machine, which
is unloaded. It is operating at no load. So, the felid voltage, which you would you would
require certain field voltage in order to get rated the rated voltage at the generator
terminals. So, you will have to actually give a field
voltage. It just represented temporarily as a battery. The field excitation you would
have to give, so that you got the rated voltage. Now, the important point here is, if I start
loading synchronous machine if I start loading a synchronous machine, you will find that,
if I load the synchronous machine, you load it and you will find it the voltage keeps
on dropping and this drop can be very very very significant. In the sense, that if you
take a typical synchronous machine, you will find that you will not even be able to load
it to its full value, full rated value. Unless you increase the field voltage, for
example synchronous machine, say with an X d of 2 per unit and let say, it is X d and
X q are equal, then the per unit power per unit power will be equal to if your E f, you
got E f d into the voltage at the load divided by X d into sin delta. So, if I connect the
synchronous machine to a voltage source line to line r m is V, then the per unit power
is this. Now, if I keep my E f d at 1 per unit, V is also 1 per unit, then the amount
of power you can actually push is half, maximum power you can push is half per unit.
So, if you have got a synchronous machine a synchronous machine connected to another
voltage source. Then In that case, the amount of power we can transfer is limited, unless
unless you change E f d. So, this is what is done. When you load a synchronous machine
from no load to full load, you would need to change this E f d and very very significantly.
In fact, you may have to even double the amount of field voltage in order to load, whenever
you load a machine from no load to full load conditions. In fact, if you look at a typical
generator, which is used in the Indian systems, is it 247MVA generator.
This is a typical unit sizing, which is found in a most Indian power systems. You will find
at the no load voltage, field voltage. This is the voltage applied at the field at no
load is 102 volts roughly and current under open circuit conditions is this, the field
current. So, this is under open circuit condition. This gives you 1 per unit at terminals. However,
if you load this machine to its full rating, you will have to apply which is value, which
is more than double, a field voltage; so that, you will get 1 per unit at the terminals again.
So, you see that you need to really change the field very substantially. The reason of
course is, that the X d of a synchronous machine is very very large or in other words, the
armature reaction is very large.
If you just connect a synchronous machine, you connect synchronous machine to a resistive
load. This a just a schematic representation of that and you go on increasing the load
by decreasing the resistance, you will find that the amount of power you can actually
deliver has a maximum and that maximum is very low value. In fact, the equivalent of
a synchronous machine, electrical equivalent of synchronous machine in steady state if
it does not have any saliency is, simply voltage source of magnitude E f d, X d and the resistance
R. And you know that if X d is large, the maximum
power transfer is going to be limited. You are not going to; rather I should say the
power transfer in this situation is limited, unless I change E f d. So, I hope I made a
good case, that you would really need to have a system, in which typically if a large generators
you need to have when excitation system, which is very well controlled and as got very large
range as well. So, from no load to full load you really need to change the felid substantially.
So, one example of exciter, the power apparatus if you look at, is a the static excitation
system and what you really have here is the voltage, which appears at the terminal of
the main generator, that is our generator which we which we are studying in fact, is
rectify after stepping it down by a control thyristor base rectifier and then, the DC
value is fed to the field of the generator. Remember of course, this is the control rectifier.
So, I can control the DC value. The DC value of it is fed back into the generator and this
is one way you can excite a generator. In fact, it may somewhat you know worry you worry
you initially because the the voltage, which is required to be rectified in order to feed
a DC voltage to the field winding is in fact being obtain form the terminals of the main
generator itself and of course, if the main generator is an under open circuit condition,
one may argue that there is no voltage in the terminals of a synchronous machine, if
no felid voltage is provided. So, as a result you will get no AC voltage
at the terminals of control rectifier and the DC voltage is also not going to will not
really have any value, will be 0.So, this whole system may not work but actually if
you look at it, it is a some kind of positive feedback system. If there is some residual
magnetism available in the generator, it can generate a small AC voltage. If that AC voltage
is enough to forward by is the device is used in the control rectifier usually thyristor,
then that would cause small DC voltage. The DC voltage would cause some field current,
which will enhance the existing. If it enhances the existing residual voltage, residual flux,
which is there in machine. You will find it the voltage increase and some kind of a you
know positive feedback mechanism will ensure that the machine self excites.
So, this actually can happen in a practical situation of course, because you know you
require adequate residual fluxes to generate. Initially, an AC voltage you will forward
by thyristors. We do not actually connect the synchronous machine from scratch in this
fashion. What we usually do is, start it up with battery. So, your field voltage, felid
is initially fed wire or battery and then, after that this particular configuration the
system switches over to this configuration, in which the voltage generated at the terminals
of a generator itself, is used to power the field, in this fashion.
Now, this in fact can be actually shown in a laboratory using simple diode rectifier.
So, if you actually replace the control rectifier by a diode rectifier in just, you know feed
the output of a generator back on to its felid winding through a diode rectifier. You find
that the voltage builds upon its own. You know it is a it is a kind of spontaneous increase.
In fact, it is an interesting exercise for you to show that in fact, this if you write
down the differential equations or the dynamical equations corresponding to this scenario,
you should be able to show that in fact this is a unstable system and therefore, itself
excites. So, if you give any non-zero initial condition, it builds up the voltage, builds
up on itself. One small caution again. In a real system,
the residual voltage is may not be residual fluxes in the synchronous machine, may not
be adequate to generate, just that initial kick to start this self excited system and
as a result of which, you may actually have you may actually have to use the battery is
in a power station to initially excite the machine and then, switch over to this configuration.
So, whatever you do now, we just show you a small video clip of how one can simply excite
a synchronous machine by simply connecting its output back on to the field, wire a diode
rectifier. The diode rectifier in fact they nota control rectifier.
So, we will not be able to achieve much control over the voltage, which we are getting. I
will just show you that the voltage suddenly kicks and the machine self excites. So, what
I will do is, connect the diode rectifier or the input the of the diode rectifier to
the output the terminals of a synchronous machine in the laboratory. Then, the DC terminals
after rectifier, I will feed it back to the field field of synchronous machine, then I
will rote start rotating the machine slowly and at a particular point, you will find that
this whole system excites on its own. So, let see that video clip. So, this is a
our set up set up in the laboratory. What I have done is, I have connected initially
the static control rectifier is connected to the field winding but I also have a diode
rectifier, which right now is open. So, that is what was shown to you. The input of the
diode rectifier is of course, the terminals of the A, B, C terminals of the generator
itself.
What I will do is the, I will disconnect the existing control rectifier and instead of
that, I will connect the output the DC terminals of the diode rectifier to the synchronous
synchronous machine field. So, that is what you have seeing here. So, the diode bridge
is connected to the generator terminals on the AC side and the DC terminals are connected
to the synchronous machine field winding. So, this is what I have done. Of course, if
I do not rotate the machine, there will be no voltage induce and nothing will happen
actually by doing this but what I will do now is slowly start rotating the machine.
Remember that I am not separately exciting it. The output of the output voltage at the
terminals of the DC machine itself of the synchronous machine itself is being used to
excite the machine. What I am doing now is starting the prime mover, which is the DC
machine. What you have seen here is, I am applying the field voltage to the DC machine.
Now, I am applying the armature voltage to the DC to the DC machine is the prime mover
to the AC machine. As soon as I start the started this way you will find at the machines
starts rotating. You will shortly see the machine rotating. What really we wish to show
you is that after the certain speed, the voltage kicks and the machine the synchronous machine
self excites. So, what you see is that there is some voltage
at the terminals of the machine. Also, you see there is a field synchronous machine felid
current and a synchronous machine field voltage. So, the machine of course is rotating at a
low speed. In fact, at a very low speed itself you find that there is enough voltage to trigger
self excitation. So, just by connecting the output of a machine to a diode bridge rectifier
and feeding it back to the synchronous machine felid, we are able to in fact demonstrate
that self excitation can occur.
So, let me just repeat what I said, just draw schematic of what I showed you. So, what I
had is a synchronous machine. Its output was simply rectified using diode rectifier and
would have said to the field winding and you saw that after the certain speed is acquired
by synchronous machine of course, synchronous machine is driven by DC machine, you find
that there is a adequate voltage to trigger a kind of positive feedback or you know this
self excitation phenomena. Right now, remember that in order for this to work well, the voltage
here the residual flux in the machine should be enough to forward bias the diode diodes
in the bridge at least at some speed. So, there at some speed the voltage magnitude
here should be adequate to trigger self excitation. Otherwise of course, one will have to use
the station battery is for what is known as felid plashing initially and after that one
can switch over to this configuration. So, what you have here is of course, a static
excitation system. A static excitation system requires brushes and slip ring to in fact,
two slip rings and brushes to convey the felid voltage to the field winding, which is usually
rotating in a typical synchronous machine.
So, if you look at some interesting pictures, which I have got here, which is courtesy,
the western regional power committee, Mumbai. You will find it, this is a snap short of
a synchronous machine. What you see here, right at the end are inflect the slip ring,
brush arrangement. You see, this is actually luckily it is exposed
for us here. So, you can see that the brushes and slip rings. So, this is a snap short of
that, you get close up also. So, you see those brushes rubbing against the slip ring. So,
this is the end region of a end, this is on one side of this synchronous machine.
In fact, if you look at this another snap short, what you see here is on one side is
the place, where the slip rings are. On one side, you have got the slip rings through
which the field voltage is conveyed. In fact the exciter itself is in another room.
And the voltage is a conveyed to the slip rings wire, the brushes. On one at one end
you see these green structures here. The blue structure here is of course, the synchronous
generator itself. The green structure here are in fact the turbine, which really control
the mechanical power input to this synchronous machine. Another kind of excitation system
is what is known is the brushless excitation system. This slightly looks a bit more complicated
than our static excitation system, in which the exciter itself is static. It does not
move and the voltage, which the exciter gives this conveyed wires, slip rings and brushless
arrangement.
Now, a brushless excitation system on the other hand has got a slightly different structure.
What you have essentially, I will just try to you know describe it here. A rotating permanent
magnet, a rotating permanent magnet is there in a permanent magnet generator. So, rotating
permanent magnet causes voltage to be induced in the stator, the stationary part of a permanent
magnet generator. The voltage output of the permanent magnet generator itself is fed.
This is a this is fed to a control rectifier, this is again a thyristor based rectification
system. This is a control rectifier, in the sense that the DC voltage is a function of
the AC voltage as well as the control signals, which is essentially the firing angle delays,
which is obtained from a control system. We shall discuss this control system, it is also
called a voltage regulation system. So, this is the control rectifier by which
we can actually control the output, which eventually goes through the main generator
but of course, there are unlike a static excitation system, there are several steps before this
is actually done. What you have here is of course, the control rectifier, which controls
the output of, this is now DC. The output of the control rectifier is DC. The permanent
magnet stator, the control rectifier are both stationary whereas, the magnet of the permanent
magnet generator of course, is rotating. The DC output of the control rectifier is fed
to the to the field winding of an of an AC generator. Now, this is not the main generator,
which we are talking of. This is a generator of the excitation system.
Now, the field winding of this particular AC generator, remember is stationary. So,
on the rotor of the synchronous of this particular synchronous generator, the rotor has got the
armature windings. The stator has got the field winding. So, the three phase armature
windings are in fact on the rotor of this machine. This is called an excitation system
generator. Now, the output of this synchronous generator is three phase AC. So, you have
got rotating armature windings. So you have getting rotating winding, which are in which
three phase AC is induced AC voltages are induced. Now, these three phase AC voltages
are fed to a rotating, this also rotating; uncontrolled rectifier, which is nothing but
a diode bridge, a three phase diode bridge, which is also rotating. So, this R on top
here indicates rotating structure. So, you have got rotating structure, a diode
bridge is rotating. The output of this diode is fed to the field winding, which is also
rotating. So, the final field voltage is conveyed to the field winding of the main generator.
The generator which we really interested in. Directly, you do not have to have a slip ring
brush arrangement because the rotating, the diode bridge is also rotating along with a
field winding. So, it is a direct connection. You do not really have to have a slip ring
brush arrangement. So, I will just repeat this again. You have got a permanent magnet
generator, in which you get three phase voltage is induced on the stator. The three phase
voltage output of the permanent magnet generator itself is rectified using a controlled rectifier.
The output of the controlled rectifier is fed to the stationary field winding of a excitation
alternator or excitation synchronous generator.
The three phase voltage is of this generator are excited on the rotor of the machine. They
are on the rotor of the machine. The output of that is fact to Diode Bridge, which is
rectifier and fed to the main field and does not require slip rings. So, some large generators
in fact have this kind of arrangement. Now, of course I have been talking of control rectifier
and so on. What exactly is a control rectifier? It is in fact, an arrangement of thyristors
typically, it is the kind of rectifiers, which are used in most excitation system or using
thyristor. So, controlled rectifiers are made out of
thyristors. If you look at a thyristor three phase Thyristor Bridge, it is made out of
6 thyristors. The input of course, is a the three phase AC wind AC input and the output
is DC and that is what I have been representing as this box here. So, this is equivalent to
a box with three phase inputs and the DC input in this fashion or DC output in this fashion.
One important point which you should note at this point. I do not know whether it is
visible on this screen. So, I will just redraw here.
So, if you take a thyristor bridge, which is schematically denoted as I showed you sometime
back. Remember that a thyristor bridge as if you denote the voltages and currents in
this fashion, the V d c can be positive or negative but i d c is always positive.
So, that is one important thing, which you should remember that this particular rectifier
does not allow current to flow in the negative direction. Now, we have a small video clip,
which shows you controlled rectifier operation. In fact, you can by manual you can manually
in the in the video clip, we are showing we are showing the voltages, which are developed
in a synchronous machine. Due to application of this excitation voltage, the excitation
voltage itself is the output of the control rectifier and the control rectifier is controlled
by controlling the delay angle of the thyristor bridge. So, you can actually by doing that
you can control the DC component of the voltage, which appears across the Thyristor Bridge.
Now, remember one small point, which you should you know this thyristor bridge if it is uses
6; what you call thyristor bridge consisting of 6 thyristors as I shown some time back,
you can look at it again. In that case, the DC voltage is V d c will have this 6 thermionic.
It is a DC voltage with 6 thermionic and DC a component itself. So, you have got a DC
6 thermionic, 12 thermionic and so on. No lower order harmonics are present other than
of course, the DC component itself. So, you have got the DC components 6th, 12th and so
on. So, this is known as often called a 6 pulse thyristor bridge.
So, what we will do is now, see a simple situation here. A simple video clip, which we will demonstrate
to you, how thyristor thyristor bridge voltage, the output DC voltage can be changed by controlling
the delay angles.
So, that will be done manually in the video clip, which will be shown to you. We do not
start the machine for this purpose, we just switch on the excitation system and applied
DC voltage to the field winding. The aim of course, is to show you 6 pulse operation.
This is the static excitation box, in which we have got 6 pulse bridge. You will have
to pay attention to the ammeter and voltmeter on your right. What we will do now, is to
reduce the firing angle from greater than 90 to less than 90 after the point than average
DC voltage, which is greater than 0 appears and continuous current is established.
And what you see here is the output of 6 Pulse Bridge near about 70 or 80 degrees. Now, we
are increasing the DC voltage by decreasing the delay angle and you see of course, the
repel comes down and it average value seems to going up. You can see the ammeter, voltmeter
as well as the CRO on our left. Now, what we will do is, well of course increase the
voltage and make firing angle near about 30 or 40 degrees. We will do now by increasing
the firing angle again, we can decrease the DC voltage and now you see again the voltage
is coming down the DC voltage and the repel of course increases. You would have notice
of course, if the repel is a of course, it is not very clear on the CRO it is a 6th thermionic
repel. Now, before I go ahead and you know you know
discuss something more about the excitation system itself I mentioned to you that there
is a need to have some kind of continuous control over the excitation system because
the excitation voltage, which is given to the synchronous machine because the synchronous
machine as got very poor regulation because of having large value of X d. So, this something
which I discussed just some time back. So, what I will do now, we show you a small second
third video clip of the drop to a very precipitous drop in voltage, once you start loading synchronous
machine by resistive loads. So, what I will do is, start the synchronous
machine, set the field voltage at a certain field voltage. So, that you get roughly the
rated voltage at the generator terminals. Then, what I will do is, load the synchronous
machine by a resistive load, as simply a passive resistive load. As I try to decrease the résistance
from if it is open of course, the resistance the infinity but as I start loading it, that
is I reduce the resistance from a certain value I go on decreasing the resistance and
load the synchronous machine, you will find what happens is, that this terminal voltage
of synchronous machine drops and as a result of that, you will find that in fact the synchronous
machine is not able to take on much power because the voltage drop so much, that it
goes beyond the maximum power point of you know of this particular situation of this
particular source. So, as I go on decreasing the resistance,
I have try to load the machine but unfortunately the terminal voltage goes on dropping. So,
eventually the machine does not get loaded at all. So, this is what you will see in the
next video clip.
So, the machine has been started, wire the prime mover. Now, what I do is, that adjust
the tool winding. Right now, there is no voltage, which there across the field. I change the
delay angle of this thyristor bridge and gradually try to develop some voltage across the felid
and therefore, the terminal voltage will appear. yeah So, I have a kind of try to change the
delay angle. So that, I get the rated voltage at the terminals
of the synchronous machine. Now, what I will do is, I load the machine by connecting resistive
load to it. So, this is been done gradually. So, you can keep an eye on the power meter.
yeah Now, the we can see that load is slightly increasing as I introduce the resistance but
what is very striking is the voltage initially, which was 230 volts, decreases as a go on
loading the machine. In fact, if I go on loading the machine, the voltage in fact drops almost
to half of what the rated value was. So, if do not touch the field winding voltage, what
I am showing you now is, I am readjusting the field voltage so I as to and readjusting
the field voltage so I as to get back to the rated value.
So, unless I do that, the voltage will drop to very low value and we will not be able
to load the machine adequately. So, what I will just repeat what I did. I loaded the
machine and you saw that if I kept the felid voltage constant, you find that the terminals
voltage of the machine drops and in fact you not able to take on the complete power. The
actual power, which deliver to the load is not we cannot increase it beyond the point
because is the large source impedance in the form of X d but if I increase E f d, that
is the field voltage, I can get back the voltage back to the rated value.
So, that is what is really need to do. If you look at what needs to be done in a typical
excitation system, is to have some kind of continuous control. We did not have it in
our the demonstration, which I showed to you so far. In fact, you can see that the there
is no continuous control; I had to manually adjust the field voltage of the excitation
system. This is not desirable because the load could change suddenly and then, you may
have sudden dip in the voltage. So, you always need to have the excitation system in continuous
control mode. So, what you need to do? For example, the most simplest thing would you
need to do is monitor the terminal voltage of a synchronous machine. In case, it drops
to keep adjusting the field voltage. So that, the you know the voltage is maintained.
So, you adjust the field voltage in such way. Now, this really brings us to a new dimension
so to speak in our course. We will be we have been so far talking about modeling of power
apparatus. We can of course, talk in terms of now, how you really introduce control system,
which themselves may be dynamical systems mind you. So, how do you have control continuous
feedback systems introduced into our models, so that we can accurately describe their effects.
So, for example, right now one of the various one of the things you would probably do is,
measure the terminal voltage of the generator.
So, you measure the terminal voltages of a generator. So, what you do is of course, have
a p t and feed it to a regulator. The regulator in fact, gives the appropriate signals to
the control rectifier of your excitation system. It could be a brushless excitation system
or static excitation system. And that feeds voltage to the generator field.
So, this excitation system could be a brushless excitation or static excitation system but
in both cases you do have a control rectifier. The signals to the control rectifier to enhance
the voltage or reduce it or in fact obtain from the voltage regulator.
Now, what is the voltage regulator? The voltage regulator itself is some kind of control system.
It is a dynamical system but it is not power apparatus. It is basically consisting of some
hardware, which tries to implement certain mathematical functions. Remember, that the
control signal to a thyristor bridge is not what is known as high power signal. It is
just a it is has enough strength to convey to a thyristor or to the gate of the thyristors
to delay there firings. So, the power levels, which which are here
which are used by control system are much much lower than the actual power rating of
the apparatus it is trying to control. So, in that sense although we will be modeling
these regulators etcetera there are also going to be differential equations and dynamical
systems in most cases but they are not power apparatus. They are in fact low power apparatus.
It is essentially signals. So, for example, you could give some set point to rather set
point is the value would like the terminal voltage of your synchronous generator to be,
if the value at which it should run. You measure the actual voltage, which is generator; step
it down and get it measureable or signal levels; a low power signal you can say. You have comparator.
This is some hardware. It is some kind of built in hardware, comparator or that is using
analog electronics, you make comparator or using even digital systems, you can implement
is using some software. So, we will of course, discuss these things
a bit later, may be in the next class. So, by comparing this, you will know the error
and then, by some control law. The control law could be just a simple gain or an amplifier.
You could determine the control signal, which is to be given to the control rectifier, which
is the power apparatus. So, the this is the power apparatus.
So, the output of this is a control system is a signal, which is given to this power
apparatus. Now, this power apparatus interprets this signal appropriately and appropriately
changes the output of the voltage of this rectifier. Now, there is some mapping of course,
between the value, which is obtained here and how much change it causes here. So, I
one way of doing that is, if I the output of this change is by delta alpha, how much
is the voltage change here. So, that is something you should know before
and before you design this control system. So, what this is one way of controlling the
voltage. If there is a larger the error, the larger this correction, which you make here.
So, of course, of course if the voltage is low, if V is less than V ref, this error will
be present and you should of course, control design your control system. So, that it drives
the power apparatus to rectify the situation, correct the situation. So, it changes the
signal given to the control rectifier. So that, the voltage increases. So, this is
what it will do under circumstance where voltages are low. So, it is a continuously acting of
course, is very important the continuously acting control system. So, this is one thing,
which you should remember, you need to do this in addition to just having the power
apparatus. You have continuously control control system. Now, remember that once one of the
things, which I hinted to talk you about is, you need to know how much you need to change
the output of a control system. And map it to what the power apparatus or
what the power the excitation system the way it behaves. So, for example, in a brushless
excitation system, which I had shown you some time back. You should have in your hand or
mapping of how this control signal change in this control signal changes the DC output
of this. How that change in DC output of this changes the AC voltage output of this synchronous
generator and correspondingly, how this change reflex here in the final change in the field
voltage and of course, once you change the field voltage, we know how it affects this
main generator by just the synchronous machine equation, which we have been discussing all
this while, in the previous lectures. So, you know how a synchronous machine behaves
but you need to model all these components here. That is the rectifier itself, then the
AC generator. It is also a generator. So, you may wish to model this in detail. I mean
the amount of the detail something, which is based on our engineering judgment. You
also need to model a behavior of a diode bridge rectifier. So, you need to model not only
you know these are not just algebraic relationship you put in the input and the output. You may
actually have to model some of these components at least as dynamical system rather differential
equations. So, what you have it this power apparatus,
which is the synchronous machine itself. Now, you have got another power apparatus, which
is the excitation system, which you need to model in some detail. Now, the amount of detail
depends on the kind of studies you are doing. So, as I mentioned some time back, in a brushless
excitation system, you have a generator, the excitation system itself has a small generator
there. It cannot have large of course, as the main generator. That itself may have to
be model in some detail, but their of course, on the other hand there are some studies,
which really do not require you to model this things in great amount of detail. You will
get more or less the same results even if you use simplified model. So, these ends some
other issues will be discussed in our next lecture.