delay time
, the OPERATE output is activated. In a drop-off situation, that is, if the
underpower condition disappears before the operation delay is exceeded, the timer
reset state is activated. If the reset timer reaches the value set by
Reset delay time
,
the operation timer resets and the START output is deactivated.
The Timer calculates the START_DUR value which indicates the percentage of the
time elapsed since the activation of the START output with respect to
Operate delay
time
. The value is available in the Monitored data.
The DISABLE input can be used to coordinate the correct operation during the
generator start-up situation. By activating the DISABLE signal, both the START and
OPERATE outputs are blocked. Once the DISABLE signal is deactivated, the Timer
remains blocked for an additional time duration as set through the setting
Disable
time
.
Blocking logic
There are three operation modes in the blocking functionality. The operation
modes are controlled by the BLOCK input and the global setting Configuration >
System > Blocking mode which selects the blocking mode. The BLOCK input can be
controlled by a binary input, a horizontal communication input or an internal signal
of the protection relay's program. The influence of the BLOCK signal activation is
preselected with the global setting
Blocking mode
.
The
Blocking mode
setting has three blocking methods. In the "Freeze timers"
mode, the operation timer is frozen to the prevailing value. In the "Block all" mode,
the whole function is blocked and the timers are reset. In the "Block OPERATE
output" mode, the function operates normally but the OPERATE output is not
activated.
4.8.1.5 Application
The task of a generator in a power plant is to convert mechanical energy into
electrical energy. Sometimes the mechanical power from the prime mover may
decrease so much that it does not cover the internal losses. The task of an
underpower protection is to protect the generator from very low power output
conditions.
Steam turbines become easily overheated if the steam flow becomes too low or if
the steam ceases to flow through the turbine. Hydro turbine of the Kaplan type may
be damaged due to the fact that the turbine blade surfs on the water and sets up
axial pressure on the bearing. Diesel engines may be damaged due to insufficient
lubrication.
If the generator size is very large, it is uneconomical to continue running it with
low generated power. In the reverse power condition, large generators draw a
considerable amount of power from the rest of the system to feed their internal
losses. Hence, it is desirable to disconnect the generator in such situations.
In case of the parallel-connected generators, for example, the load of one generator
may be so low that it is better to disconnect it and let the remaining generators
feed the network.
Where a low value of power setting is required, for example less than
2%, the correction parameters should be used to compensate for the
measuring errors. The manufacturer of the measuring devices is to be
contacted for information on the measuring errors.
Protection functions 1MRS757644 H
732 620 series
Technical Manual
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