One of the chief requirements, for operation of power system is to
ensure acceptable quality of system frequency and to protect the equipments from
abnormal supply frequency. Specifically an abnormal frequency in generator can
result in shaft fatigue and failure of the shaft. The generator shaft is not a
uniform mechanical shaft but has different masses on the same shaft that has
different time response for any changes in the input mechanical energy and output electrical
energy. Consequent to different time responses of the masses on the shaft, the shaft
is subjected to torsional fatigues.
An industrial plant may employ its own captive power plant to partly
or wholly meet its plant load demand. Most industrial units operate in parallel
with the grid and may import/export power from/to the grid. The following
scenarios may arise
-
Loss of part of the in plant generation due to some fault
conditions, resulting in excessive load demand on the remaining generators
within the plant
-
Loss of generation in the external grid, which diverts
excessive electrical demand on the industrial plant
Either of the above scenarios will result in fall in industrial
plant frequency, thus affecting the quality of power supply and subjecting the
remaining generators to torsional fatigue/stress.
Typically generator manufacturers specify the band width of the
frequency within which their units should function. They also state the limits
on acceptable rate of change of frequency, which when exceeded, results in generator shaft
fatigue. The control and protection of the generator must take these limits into
account, while designing the suitable protection. Depending on the situation,
the following protection functions may be considered for an industrial plant
-
Load Shedding
-
Isolation from The grid
-
Isolation of the generation plant itself from the network.
The frequency response of the generator unit is dependent on the following
factors
-
Type of disturbances
-
Electrical power output from the units under consideration
-
The response of the generator prime-mover governor system
-
The response of the generator excitation system that
influences the electrical power output
-
The power swings between the various units and the grid
Evidently the frequency response is not identical in all the cases and needs to
be studied in detail for various operational and disturbance scenarios. Such analysis can be performed using
a transient stability analysis tool such as the one available with the PowerApps
product. The importance of the
transient stability analysis of the system is as follows
- To identify the critical operating and disturbing scenarios
which result in unacceptable generator frequency and thereby taking appropriate
control and protection measures. The most important thing is that the control
and protection scenario may not be necessary for all operating conditions and
disturbance scenarios and may be necessary for specific conditions only.
- To recommend the sequence of control and protection
scenarios under specific operating condition and disturbance scenarios and to
suggest the necessary measurements and specifications to be implemented for a
PLC based control and protection scheme.
Thus a more precise and dependable solution for the protection and control of
generator units can be provided by a detailed transient stability analysis,
considering the characteristics of the generators, excitation system,
governor/prime mover systems, external grid, plant operating conditions and so
on.
The simulation from transient stability analysis can also be used for setting
conventional relays such as underfrequency and under voltage based relay
settings.
PLC based Load Shedding Scheme
