18-12-2012, 06:19 PM
Functional and Operational Control of Thermal Power Plants
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Operation control
Since safe and economical operation is carried out at thermal power stations while carefully
checking environmental problems, there are many points that operators must judge to take
appropriate measures. Therefore, a large load is applied to operators in case of an emergency.
Therefore, it is necessary to automate emergency manual operations to be taken against faults, as
well as to automate normal manual operations in order to minimize operators’ judgments. To keep
the final protection of the plant, it is absolutely required to take appropriate measures for the plant
facilities.
A unit protection device is installed to protect each unit if a fault occurs in any unit and it
becomes difficult to continue safe operation of the unit. This unit protection device is called the
“unit trip interlock.” Basically, the unit trip interlock is classified into the boiler protection
interlock (MFT), turbine protection interlock (MTS), and generator protection interlock (86G).
These interlock systems may vary depending on the manufacturer’s design. In principle,
however, the once-through unit boiler, turbine, and generator are mutually interlocked. Figure 35
shows an example of the trip interlock system.
Boiler protection interlock (MFT)
This boiler protection interlock is intended to shut down the fuel supply to stop the boiler if it
becomes difficult to continue stable combustion of the boiler. The conditions for tripping of this
interlock may vary slightly depending on the type of boiler, that is, whether it is drum boiler or a
once-through unit boiler. Generally, these conditions are fuel pressure drop, high furnace pressure,
stopping of two ventilating fans, protection of the reheating unit, supply water flow rate drop, and
drum level drop. In addition to these conditions, unit emergency stop and turbine/generator trip
conditions are interlocked. According to the boiler model, further conditions are interlocked.
Turbine protection interlock (MTS)
If it becomes difficult to continue stable operation of the turbine, the solenoid is operated to stop
the turbine. The conditions for tripping of this interlock are turbine overspeed, thrust error,
bearing hydraulic pressure drop, and degree of vacuum drop, etc. In addition to these conditions,
the unit emergency stop, turbine manual stop, and generator trip conditions are interlocked.
Generator protection interlock (86G)
A status where stable operation of the generator or transformer is difficult is detected by the
protective device or protective relay. After this, the generator is disconnected from the system and
the turbine is tripped to stop the generator at the same time. The conditions for detection of the
protection are ratio differentiation of the generator, loss of excitation, ratio differentiation of the
ground fault or transformer, impulse hydraulic pressure, overexcitation, etc. In addition to these
conditions, the high/low frequency of the system and the protection of the bus-bar are interlocked.
Protection device tests during operation
The important point during plant operation is that the plant can be stopped safely in case of an
emergency. To maintain this safety, it is necessary to periodically check the operation status of
various safety prevention apparatus installed for protection of the plant. Table 3 shows examples
of the protection device tests.
Boiler operation control during normal operation
It must be strongly attempted to find the error status early and to prevent problems during
normal unit operation in order to maintain stable operation status.
The actions to be actually taken are basically classified into the inspection at the work field, and the sampling and
evaluation of the operation records. It is important to take these actions daily in order to check status change in
the early phase, and this leads to appropriate actions and measures being taken in a timely manner.
Inspection at the work field
As a rule, the inspection interval must be every work shift. Walkaround inspection of the boiler
main unit parts and boiler auxiliary devices is carried out. The inspection results must be kept.
If any problem symptom is observed, it is necessary to grasp any status change as time elapses.
Generally, walkaround inspection is carried out according to the checklist. In addition to this
inspection, further inspection points, such as unusual noise, unusual odor, or discoloration must
also be inspected.
The combustion status inside the furnace must also be checked during walkaround inspection.
However, if the type of coal to be used is changed, the inspection must be carried out with special
attention.