Control system for controlling at least one variable of a...

Power plants – Combustion products used as motive fluid

Utility Patent

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C060S039270

Utility Patent

active

06167690

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the field of control technology. It relates to a control system for controlling at least one variable of a process, the at least one variable being calculated from a multiplicity of measured process variables, the measured process variables in each case being measured via associated measuring sections, the transfer functions of which have a different time response.
2. Discussion of Background
In the control and monitoring of gas turbines (GTs), information on the multiplicity of thermodynamic variables, such as, for example, pressures, temperatures and mass flows, is required. In this case, however, there are important process variables which cannot be measured directly with sufficient accuracy and reliability at reasonable costs. Such process variables therefore have to be determined on the basis of auxiliary measurements and known relationships (on the basis of physical laws and experimental data) which link the auxiliary measurements to the desired process variables.
A typical example of such an indirectly determined process variable is the turbine inlet temperature (TIT) of a gas turbine. The conditions at the turbine inlet do not permit an efficient temperature measurement at this location. The local temperatures are too high and lead to a very short service life and high failure rates. Direct measurements of the TIT are therefore avoided. Instead, the TIT is calculated from the (lower) turbine outlet temperature (TAT) and the pressure at the compressor outlet (p
C
), both of which are comparatively easy to measure. A typical closed-loop control circuit for the TIT of a gas turbine with the associated measuring sections is reproduced schematically in FIG.
1
. The gas turbine
10
comprises a compressor
12
, a burner
13
and a following turbine
14
. Connected upstream of the compressor are adjustable inlet guide vanes
11
. A temperature controller
15
controls the turbine inlet temperature TIT according to a preset desired value TIT
c
. It influences the process in the gas turbine
10
by virtue of the fact that it adjusts the inlet guide vanes
11
via a first control line
16
and thus controls the combustion air, and sets the mass flow of the fuel to the burner
13
via a second control line
17
. The turbine outlet temperature TAT is measured via a first measuring section
18
(transfer function G
1
(s)), and the pressure p
C
at the compressor outlet is measured via a second measuring section
19
(transfer function G
2
(s)). The measured values p
Cm
and TAT
m
are transmitted to a computing unit
20
, which, with the aid of a relationship, expressed as a formula, between p
C
, TAT and TIT, calculates the (indirectly) measured value TIT
m
of the turbine inlet temperature and feeds it back to the temperature controller
15
for comparison with the desired value TIT
c
.
The following general statements can be applied to the relationship expressed as a formula and used in the computing unit
20
: if y designates the process variable which cannot be measured directly, and if x=x
1
, x
2
, . . . denotes a set of measurable process variables (auxiliary variables) from which y can be calculated, an algebraic expression of the following form applies in the simplest case between the variables:
y=f
(
X
).  (1.1)
For the turbine inlet temperature TIT, this algebraic expression has the form of the so-called TIT formula, which in the basic form is:
TIT=f
(
TAT,p
C
).  (1.2)
In practice, the formula is additionally dependent on ambient conditions and other variables, which in the present discussion are considered to be constant and are therefore not explicitly specified.
The evaluation of the formula (1.1) is based on measurements X
m
=x
1m
, x
2m
, . . . and leads to a value Y
m
. The variable
y
m
=f
(
x
1m
,x
2m
, . . .)  (1.3)
is designated as pseudo measurement of y, because it is not directly measured but is calculated from auxiliary measurements X
m
. As an example, the pseudo measurement TIT
m
of the turbine inlet temperature TIT may be mentioned here and this is expressed according to the TIT formula (1.2) as follows:
TIT
m
=f
(
TAT
m
,p
Cm
)  (1.4)
where TAT
m
and p
Cm
designate the corresponding measurements of TAT and p
C
.
The accuracy of the pseudo measurement y
m
is determined by the measuring error:
e
y
=y
m
−y.
  (1.5)
This measuring error depends on the accuracy of the measurements X
m
. In order to obtain an acceptable accuracy of y
m
, the relationship (1.3) is normally calibrated at various steady-state working points of the gas turbine
10
. Unfortunately, even a perfect steady-state calibration cannot prevent a poor control quality from resulting when pseudo measurements y
m
are used in dynamic closed loops (as that represented in
FIG. 1
) during transient actions. Precisely in the case of the control or monitoring of the turbine inlet temperature TIT, this may result in risks for the gas turbine, since thermal overloading may occur.
The main reason for this problem may be traced to the different dynamic properties of the measuring sections (
18
,
19
in
FIG. 1
) which are linked to the auxiliary variables (TAT, p
C
in FIG.
1
). In the case of the TIT formula (1.2), the dynamics of the thermocouples which are normally used to measure the turbine outlet temperature TAT are generally very much lower than those of the pressure sensors which measure the pressure PC at the compressor outlet. As a result, the measurements TAT
m
and p
Cm
are not synchronous, and a pseudo-measured TIT
m
transient according to the formula (1.4) may then deviate considerably from the actual relationship according to formula (1.2), even in the event of the steady-state accuracy being high (for this see as an example the representation in
FIG. 2
, from which it can be seen that, as a consequence of non-synchronized measurements TAT
m
and p
Cm
, the pseudo measurement TIT
m
calculated therefrom may have a considerable phase error (circled) compared with the actual value TIT).
It goes without saying that the problems of the pseudo measurement which are discussed here using a gas turbine as an example may also occur during other measurements, processes or process control actions and may lead to adverse consequences.
SUMMARY OF THE INVENTION
Accordingly, one object of the invention is to provide a novel control system in which the different dynamic or time response of the measuring sections used has virtually no adverse effect on the stability and accuracy of the control during both steady-state and transient actions.
The object is achieved in the case of a control system of the type mentioned at the beginning in that, to avoid instability, correction means which equalize the different time responses of the individual measuring sections are provided. The essence of the invention, therefore, consists in correcting the unequal time responses of the individual measuring sections in such a way that the corrected measuring sections have the same time response.
A first preferred embodiment of the control system according to the invention is defined in that the correction means comprise one or more correction elements which are arranged downstream of the measuring sections and correct the time response of the individual measuring sections. In this way, it becomes possible to specifically correct the time response of each individual measuring section in such a way that virtually synchronous provision of the measured values is obtained overall for the calculation of the control variable.
This correction is preferably carried out in such a way that each of the correction elements has a transfer function which, together with the transfer function of the associated measuring section, produces an overall transfer function which is at least approximately the same for all the process variables to be measured.
Furthermore, according to the invention, such a control system is used for controlling the tu

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