Note: Descriptions are shown in the official language in which they were submitted.
~ 1073980
This invention relates to electrical logic circuits
and ~o automatic electrical control systems incorporating the
same.
Control of the steam inlet valves of a large steam
turbine in present-day electricity generating plant presents
a complex design problem. A multi-stage turbine, for example,
may have four inlet valves toa high pressure stage and a
similar,-or greater, number of intercept valves for intermediate
pressure stage admission all requiring to be separately driven,
whilst multiple sources of control parameters are involved.
The latter may comprise transducers for steam inlet temperature,
boiler pressure and condenser vacuum in addition to speed
detectors and controls derived from the electricity ge~erating
portion of the plant.
The desirability of a control system which is
substantially fail-safe for turbine control leads from primary
considerations to a system having a high number of information
carrying channels. A lesser number of control channels is
adopted in practice, however, since the multiplicity of
similar control channels in a complex system results in a
eorrespondingly high number of faults within the control system
itself which may lead to partial load operation of the turbine
under control. This is made possible by the fact that during
operation of a eontrol system incorporating a number of parallel
ehannels eon~eying similar signal information, some channels
are effeetively redundant and circuitry may be devised to
reduee the number of ehannels required taking advantage of this
redundaney.
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'1073~80
~he object of the present invention is to provide
improved circuits in control systems of the tvpe described
hereinabove for facilitating the adoption of redundancy
techniques whilst incorporating a minimal number of circuit
componen-ts of such a nature as to enhance the ability of the
system to provide full continuous operation of the controlled
elements whilst minimising the cost of the system.
The invention consists in an electrical logic circuit
arrangement comprising a direct-coupled operational amplifier
having an input and an output, at least two control signal
circuit inputs each connected to the amplifier input by way of
a signal current path including a resistor, the values o:F the
resistors being substantially equal and at least one current
path including a current limiting arrangement between the
resistor in that path and the amplifier input, the circuit
further comprising a number of feedback paths equal to the nu~ber
of signal current paths each feedback path leading separately
from the amplifier output.to an individual signal current path,
there being one feedback path to each signal current path, each
feedback path being connected to the respective input signal
path at a point between the resistor of that path and the
amplifier input, this point further being located intermediatethe resistor and the current limiting device of the signal input
path for a path including such a device, and each feedback path
including a resistor of substantially the same value as that of
each signal input path resistor.
~he invention also consists in an automat;ic electrical
control system incorporating at least one logic circuit arrange-
ment as set forth in the preceding paragraph and comprising
~ ~073980
a plurality of input controlling parameters and at least one
controlled output parameter, in which system at least two input
- controlling parameters are applied to a logical circuit
arrangement, which parameters are capable of two modes of
variation with respect to each other in accordance with the
presence or absence of a fault in the control system affecting
a controlling parameter and the logic circuit arrangement
accordingly being capable of operation in two modes corresponding
to the modes of variation of the input parameters whereby
control of the controlled output parameter is achieved in
accordance with values of non-faulty input controlling parameters
for both operational modes of any logical circult arrangement.
In the preferred form of the invention the control system
according to the preceding paragraph has a logic circuit which
operates in a first mode of an averaging circuit and in a second
mode as a majority-voting circuit. In such a system the circuit
may have a number of control signal inputs equal to the number of
information channels in the system and each input signal current
path includes a current limiting device. The general principle
of operation of a majority-voting circuit and the particular form
and application of such circuits according to the present
- invention will be described hereinafter. A turbine control
system requiring only three information channels whilst control-
ling operation of a steam turbine with a figure for occurrence
of faults requiring partial load reduction six times better than
in previous systems using four information channels may be
provided using majority-voting circuits according to the
invention.
~073980
Accordingly, the invention further consists ir,
an electrical control system for governing steam turbine
operation employing at least one majority-voting circuit
incorporating an electrical logic circuit as described
hereinafter.
~ he invention will now be further described with
reference to the accompanying drawings in whick :
Figure 1 shows a simplified circuit diagram
of a conventional signal averaging circuit
using a direct-coupled amplifier;
~ igure 2 shows a circuit diagram similar to
~igure 1 but incorporating a circuit
modification according to the present
. invention;
~igure 3 shows a circuit diagram of a majority-
voting circuit accordi.ng to the present
invention;
~igure 4 is a graph indicating the operating
characteristics of the circuit shown
- 20 in ~igure 3,
: and
1~)739~30
Figure 5 is a block diagram showing a steam turbine
governing electrical control system in which circuits as shown
in Figure 3 are incorporated.
Referring first to Figure 1, the circuit shown is
commonly used for obtaining at the output terminal O of a
direct-coupled amplifier 1 a voltage equal to the average value
of voltages applied to terminals A, B and C. Equal resistors
of value R are included in each input line leading to a common
input terminal 2 to the amplifier, as shown, whilst a resistor
of value 3 is connected as a feedback path from the amplifier
output terminal O to the amplifier input terminal 3.
Referring now to Figure 2, the circuit of Figure 1
is modified in accordance with the invention to enable it to
give an output equal to the average of the input voltages to
A, B and C for situations where the values of these voltages do
not differ from each other to a marked extent (predetermined by
circuit parameters), or an output signal voltage equal to the
average of two of the voltages, say the voltages at B and C,
plus or minus a predetermined voltage when the voltage on A
deviates to a substantial (predetermined) degree from the
average of voltages B and C. The ~.agnitude of the predetermined
voltage added to or subtracted from the average of voltages
B and C in the latter condition is determined by the limiting
value of current in the line from input terminal A to the
amplifier imposed by a current limiting device Lt of which there
is one in each input signal current path.
The operation of the circuit shown in Figure 2 for the
averaging regime for all applied voltages to the three terminals
A, B and C when these are close in value is readily explicable.
In this mode it is apparent that the current in the three input
slgnal paths balance and as a result there is virtually zero
current through the limiting devices Lt, which are effectively
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.. .
.()739~30
of low resistance. Feedback paths through resistors Rf thereforc
terminate on the signal input paths at points of substantially
the same potential, this being the zero potential input to the
direct-coupled amplifier. Resistors RF, which are of substan-
tially equal value to resistors R are therefore effectively in
parallel, giving an effective feedback path resistance of
RF R
If the signal voltage on terminal A deviates to any
degree from those on terminals B and C, circulating currents are
set up between the signal input paths from terminals A to B
and from A to C, through junction 3. When the deviation becomes
sufficiently large the current limiting device Lt in the signal
input path from terminal A comes into effect, limiting the
current flowing through this path and isolating the resistor R
in this path and the resistor RF in the feedback path leading
to it from the virtual earth point 3 at the amplifier input.
This second mode of operation is that in which the circuit
operates as a majority-voting circuit since, of three input
; voltages, it is giving an output which is the average for two
closely similar input voltages whilst effectively declaring the
third voltage redundant and discounting it, since the constant
voltage contributed at the output by the deviant input signal
is relatively small.
Figure 3 shows a practical majority-voting circuit as
described in which the current limiting device in each input
signal path comprises a diode bridge network operating between
+15V and -15V supply rails. The circuit is capable of handling
three input voltages in the range -+10 volts. When the input
voltages lie within -1 volt of each other, an average voltage
is produced. In the majority-voting mode, when one input
voltage lies outside a band of ~1 volt to -1 volt occupied by
the other two voltages in the range +lOV to -lOV, the voltage
1073980
produced is the average of the two similar voltages - ~ volt.
The range!of voltage in which the averaging mode operates is
dependent upon the value chosen for resistors RL in the limiting
circuits comprising these resistors and diode bridges sA, Bs
and BC.
The above figures are illustrated in the graph shown
in Figure 4 which plots the output voltage of the circuit
against the input voltage on terminal A when the input voltages
B and C are maintained at zero volts.
The graph shows that for values of voltage on terminal
A between -1 volt and +1 volt the output voltage always lies
between - ~ volt, this being the averaging range of operation
of the circuit. For voltages on terminal A greater in
magnitude than -1 volt (which corresponds to the limiting
current being reached as determined by the current limiter),
~he output voltage is held constant at - 1 volt. Thus the
output voltage never differs from the value of the equal voltage
at terminals s and C by greater than 1 volt.
The control system shown in Figure 5 is applied to a
steam turbine having four electrically controlled primary
governor valves designated GOV.l - GOV. 4, and four electrically
controlled intercept valves designated INT.l - INT.4. The
latter valves are located at the inlet to the intermediate
pressure turbine downstream of a reheater in a three-stage
turbine system comprising a high pressure, intermediate pressure
and low pressure stage as described in our British Patents Nos.
1,241,415 and 1,185,630.
Three fundamental signals for governing the turbine
are derived from three main probes Pl, P2, P3 spaced at 120~
intervals on a wheel rotating with the turbine shaft. These
signals, operating by way of a speed detector system, form the
basis of a three channel control system operating the eight
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~073980
valves of the turbine. Various other parameters, as indicated
on the block schematic of Figure 5, are also accommodated in
the system.
Each valve of the turbine is controlled by way of a
single majority-voting circuit of the type shown in Figure 4.
In addition, three sets of triple majority-voting circuits are
shown in connection with transducers for steam inlet temperature,
boiler pressure and condenser vacuum. Each triple circuit
- shown as a single block in Figure 5 comprises three separate
circuits of the type shown in Figure 4.
Statistical estimates of the frequency with which
faults necessitating partial load reduction on the turbine
would occur using the system shown in Figure 5 give a figure
of one fault in six years. This figure compares with
estimates of one fault per annum in previous systems employing
four channels and more elaborate voting circuits, the total
number of faults ~that is, including faults not requiring
partial load reduction) per annum, however, being approximately
four in each case.
~hilst Figure 5 shows majority-voting circuits
according to the invention applied in a three channel control
system it should be noted that they can equally well be employed
in a four channel system.
Furthermore, whilst the logic circuit shown in Figure
4 makes use of diode bridges as current limiters the invention
is not limited to the use of any particular form of current
limiting arrangement or device.
It should also be noted that the majority-voting
circuit application of the logic circuit according to the
inyention is not the sole use of the circuit. An alternative
form of the circuit, for example, may readily be used as a
limited range trim control in servomechanisms employing position
feedback transducers inorder tominimise deadband effects.
