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Fachmagazin der Fertigungs-und Prozesstechnik |
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| 7th sense for fault analysis |
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The gas/steam turbine power plant in Budong, South Korea detects faults up to an hour in advance and
provides advice on how they can be avoided. This has been made possible by the retrospective installation
of diagnostic and simulation software which predicts alarms and shutdowns on the basis of current data. |
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Diversity is a proven concept in safety
engineering: a second functionally
though differently structured surveillance
system sits alongside the actual automation
system which it monitors and checks; if
faults are identified, it intervenes to correct
then in order to avoid any adverse effects.
infoteam Software in conjunction with its
Korean partner BNF Breakthrough and
Fusion took up this concept and
implemented it in a software architecture
for the monitoring and surveillance of
complex systems.
The software architecture was intended to
emulate the functionality of a complete
distributed control system (DCS)on a
single server. However, the
attempt to use
commercially available products failed at
the trial stage. Chief engineer Jong-Hun
Lee from Korea South East Power: ,,All
the products we tried ended in failure.
It
took much too long to trace the cause of a
fault,with the result that it was usually too
late for countermeasures. It was also
impossible to recreate the internal
conditions of the control logic, so software
faults simply could not be identified using
such systems.“
It became apparent that both multi-core
PCs with RAID architecture and, above
all, operationally proven and extremely
powerful software had to be used. |
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| Requirements |
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The requirements for the new functions
were determined as part of a research
project in 2005. They are:
-> Synchronous recording of process data
as with a transient recorder, but also of all
relevant internal program data from the
DCS system and the control actions
-> At least two hours’ recording at a
resolution of 20 ms
-> Simultaneous display of the data as a
control display screen, continuous
function chart and trend chart
-> Free choice of the moment of observation
for all historic data with a playback
function as on a video recorder
-> Early detection of faults based on
heuristic rules by means of automated
database monitoring and alerting of the
operators.
-> Automated analysis of the cause of the
fault and output to the continuous
function chart
-> Recommended action by the operators
for timely fault resolution
-> Validation of changes to the process and
control logic with the aid of historic data
to avoid similar faults in future
The,, Trip Information System (TIS)“
architecture is the result of the
specification profile. |
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| TIS architecture and mode of operation |
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The plant in Budong is based on a DCS
system by ABB with continuous function
chart programming in Progress2 which
supplies data for the higher-order control
system via a field bus. This distributed
control system has now been upgraded to
include a central data server in TIS which
stores all the I/O data from the field bus in a
historic database. The related logic system
for the DCS system -more than 700 pages -
is implemented as an IEC 61131-3-
compliant continuous function chart on the
data server. |
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The TIS system benefits from the power of
a modern soft PLC which enables the
functionality of around a dozen distributed
control systems in total to be implemented
on a single PC. The original application is
divided into 35 tasks in order to enable a
more detailed assignment of the function
unit to the application program. This is one
of the prerequisites for being able to carry
out specific, speedy diagnostic perations at
a later stage.
This mirror application is performed by a
soft PLC which receives the I/O data
cyclically from the database and saves all
the application’s local readings cyclically
every 20 ms to the database. The database
therefore contains all the data from the plant
covering a time window of 1 to 2 hours.
Clients from higher-order engineering
stations can also access this database and
output process displays and trend charts for
the recorded readings to the continuous
function chart, in addition to the program
and variable status. |
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| Operating modes |
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The complexity of the remit requires the
coordinated interaction of very many
system components,on both the server and
multiple clients. Operators are overburdened
by such complexity when faced by a fault
and when under stress.
The TIS functionality was therefore adapted
to the various issues which occur during a
shift by introducing the Monitor, Al-arm,
History and Simulation modes: |
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-> Monitor : The normal operating mode in
which the process data is recorded on the
server, and the control stations permit the
staff different views of the process being
controlled -ranging from standard operation
and monitoring, to trend displays to
continuous function chart details whose
logic is animated using real-time data from
the plant.
-> Alarm : Using a defined rule, the automated
fault detection system has detected
that at least one of the monitored signals is
approaching a critical limit. The cause of the
TRIP signal triggering the alarm is
determined and displayed in colour-coded
form on the continuous function chart so
that the operator can instigate countermeasures
immediately. Where there are a
number of signals,the sequence of events
[SoE ] is recorded.
-> History : For the purposes of accurate
causal analysis, the moment of observation
for all analyses can be moved to anywhere
within a window of two hours before the
alarm to half an hour after it. As with a
video recorder, the operator can then
automatically reconstruct the sequence of
events in 20 ms increments.
-> Simulation : To validate planned modifications,
the user can change the settings of
signals in accordance with the KKS power
station designation system so that they are
different from the historic value and
therefore test the impact on the control
program. However,he can also make
changes to the continuous function chart
and verify correct functioning with historic
data. |
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| Data processing |
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The data from the DCS system is directly
recorded in the switchgear cabinet via
Architecture: the Trip Information System (TIS) enables
the timely elimination of the cause of the fault before it
can impair safety or operational readiness
The software components of the TIS
Trip-cause Information System
communication cards which monitor all
readings and control signals and transmit
them to the TIS server via a 1 GHz Fast
Ethernet connection. The real-time database
(1 -please see Fig.4 for explanations)
archives the data once this has been
converted from a manufacturerspecific to a
standard format and has been date-stamped.
Since the data in a distributed system comes
from a variety of sources, it is cyclically and
synchronously transmitted from the
database interface (2) to the operating time
system for processing via an OPC server
(3). |
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The soft PLC (4) processes the continuous
function charts which were previously
drawn up using the OpenPCS IEC 61131-3-
compliant programming environment in
several dozen processes.
These accurately reproduce the function of
the automation software in the DCS system
and contain functions such as the monitoring
of vibrations, oil pressure and temperature,
speed and temperature of the gas turbine and
also the pressure, throughput and temperature
in the steam turbine. But auxiliary systems
such as the condenser, boiler,and water and
steam circuit are also fully simulated.
,,Simulated“ because the calculated control
signals are not transmitted to the process but
are stored in the database together with the
internal conditions of the program logic,
also date-stamped (5).The database therefore
contains a complete, synchronous copy of
all readings. |
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During the normal Monitor mode,
operating staff use various client stations to
monitor and control the plant. Up till now
these display options were assigned to a
range of different software tools, and the
simultaneous output of a continuous
function chart (6), a user interface (7) and a
trend chart (8) of important readings in a
framework application was not possible.
Only with the advent of synchronised
animation (9) has it become possible to rule
out misinterpretations arising from an
inconsistent display. |
The current readings can be compared with
their permissible limit values by the
operator comparing, for example, the oil
temperature trend with the current turbine
speed displayed on the control panel.
A glance at the control logic’s continuous
function chart shows him the identical
figures together with the oil cooling
program and the overspeed protection
system. The operator can rely on an accurate
match between the representation of the
plant displayed on the continuous function
chart and the actual control algorithm!
Hardcopy documentation which is generally
used,on the other hand,is usually out of date. |
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| Alarm: the sooner, the better! |
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Every DCS system has alarm functions
when limit values are reached and a
recording function which logs the time of
the alarm and its clearing by the operator.
What is usually lacking, however, is a
component to identify the cause and
suggestions to the operator for proven
recommended action to restore the
exceptional situation that has occurred to its
normal status. This is where TIS comes in
with its heuristic control system:first the
chief engineer specifies which signals in
accordance with the KKS power station
designation system are relevant for alarms.
He selects these from the TIS signal table
(10)and then uses the TRIP Rule Editor
(11)to specify thresholds and evaluation
rules that are to initiate a causal analysis.
Selection is not limited to values from the
process but contains all the database entries
on the server. The current values are
displayed for the operator relative to their
limit value. This enables warning stages to
be defined before the DCS system itself
would trip an alarm. |
While the system is in its normal data
recording and processing mode, the rule
monitoring system (12) automatically
continuously evaluates the rules for tripping
an alarm. Since these also permit fuzzy
conditions such as combinations of
rules,some of the rules may be only partially
complied with. However, as soon as a rule is
fully complied with, TIS automatically
moves to ,,Alarm“ mode.
While data is still being written to the
server,the client freezes the display of the
current status.The trip engine’s most important task now is to determine the
cause of the alarm.This involves
backtracking along the signal path in the
function block program using the
database.
Starting from the alarm signal, the
system determines which input signal in
the function block changed last. Since
function blocks can require a number
of cycles to calculate relatively
complex algorithms, the database is
rolled back in 20 ms increments until
a change in the input is identified.
This analysis requires close
cooperation between various software
components: |
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1.The rule engine identifies the alarm
signal based on the violated rule.
2.The trip engine requests the signal
names of the inputs of the last
function block which output the
alarm signal from the continuous
function chart editor.
3.In response, the database supplies the
signal name which last changed.
4.This whole process is repeated until a
reading from the process has been
identified as the cause in the left
margin of the continuous function
chart. |
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This process typically takes about a
minute compared with hours or days for
manual evaluation. Once the path
information is available, the continuous
function chart automatically displays
the right program at the relevant
location with the KKS signals’
identified path in red (see Fig.3). Since
the cause of the fault generally lies
some time in the past, it is extremely
beneficial for the operator to have an
automatic display of all the information
such as the HMI and trend chart at the
time the fault was caused.
The relevant signal which caused the
fault has already been filtered out of the
flood of many thousands of signals by
TIS. |
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| However, additional information on
eliminating the possible cause of the
fault is also stored with each rule. For
example: |
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Alarm,pressure increase in the
condenser, triggered by the rule,, Pressure
is less than 10%from the limit value of
0.2 bar“.
Depending on whether one of the
vacuum pumps is also indicating a fault
and how quickly the pressure change is
taking place, the recommendation is
automatically made either to switch on
a backup pump or to check the filter.
This not only saves valuable time, it
also ensures that less experienced shift
staff take the right action without having
to wait for the specialists to analyse the
fault. |
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| Accelerated fault analysis |
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Since data continues to be recorded for
a certain time when an alarm is tripped,
the History mode can show not only a
detailed causal analysis but also a study
of the effect of the action taken.The
complete recording of all readings and
internal data means that a fault and the
process leading up to it can be
comprehensively analysed, as with an
aircraft's flight data recorder. Archived
data from previous faults can also be
used,however,tocheck the effectiveness
of rules and measures.The development
of the knowledge base is validated by
historic data.
As with a video recorder, the moment of
observation can be forwarded or
rewound at high speed or in slow
motion using the Data Manager (9).
Every condition in the plant can
therefore be reproduced and replicated
in the HMI, on the trend chart or on the
continuous function chart with historic
data. The step-by-step execution of each
individual cycle also helps in debugging
the control logic. Whereas in the past
the operator did not have access to
software faults, the entire operation is
now transparent. |
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| Pilot-Installation |
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The power plant constructed in Budong in
1998 supplies the metropolitan region with
electricity and also supplies heating energy
from waste heat to 170,000 households.
The gas/steam turbine power plant built by
ABB combines electricity generation by eight
gas turbines each with an output of 75 MW
with two additional steam turbines each
with an output of 150 MW. The main causes of faults are the vibration of
measuring instruments, pressure rises in the
vacuum area, inexplicable changes in the oil
pressure in the lubricant circuit and sudden
temperature changes in the combustion
chamber.
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Initial indications of such faults often turn
out to have been recorded hours in advance
by the instrumentation but either they
were ignored or the control room staff were
unable to locate the cause and failed to
instigate any appropriate countermeasures.
The TIS project’s approach is to combine
time-correlated external metered values with
the control system’s continuous function
chart which provides an insight into the
internal logical connections and the cause
and effect of faults. This enables the timely
implementation of appropriate countermeasures
and increasingly permits faults to
be avoided in the first place.
Following the successful trials in the summer
of 2007 KOSEP is planning to install TIS
next year in eight gas turbines, eight steam
generators and two district heating systems.
Further power generators in Korea are
expected to retrofit TIS in their power plants
in the foreseeable future. |
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