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2 Functions
2.27
Inadvertent Energization (ANSI 50, 27)
The inadvertent energizing protection serves to limit damage by accidental connection
of the stationary or already started, but not yet synchronized generator, by fast actua-
tion of the mains breaker. A connection to a stationary machine is equivalent to con-
nection to a low-ohmic resistor. Due to the nominal voltage impressed by the power
system, the generator starts up with a high slip as an asynchronous machine. Thereby
inadmissibly high currents are induced in the rotor which could destroy it.
2.27.1 Functional Description
Criteria
The inadvertent energizing protection only intervenes if measured quantities do not
yet exist in the valid frequency working area (operational condition 0, with a stationary
machine) or if an undervoltage below the nominal frequency is present (machine
already started up, but not yet synchronized). The inadvertent energizing protection is
blocked by a voltage criterion on transgression of a minimum voltage, to prevent it
picking up during normal operation. This blocking is delayed to avoid protection being
blocked immediately in the event of an unintended connection. Another pickup delay
is necessary to avoid an unwanted operation during high-current faults with heavy
voltage dip. A dropout time delay allows for a measurement limited in time.
As the inadvertent energizing protection must intervene very rapidly, the instanta-
neous current values are monitored over a large frequency range already in operation-
al condition 0. If valid measured quantities exist (operational condition 1), the positive
phase-sequence voltage, the frequency for blocking inadvertent energizing protection
as well as the instantaneous current values are evaluated as tripping criterion.
The following figure shows the logic diagram for inadvertent energizing protection.
This function can be blocked via a binary input. For example the existence of the ex-
citation voltage can be used here as an addition criterion. As the voltage is a neces-
sary criterion for enabling the inadvertent energizing protection, the voltage transform-
ers must be monitored. This is done by the Fuse–Failure–Monitor (FFM). If it detects
a voltage transformer fault, the voltage criterion of the inadvertent energizing protec-
tion is deactivated.
162
7UM61 Manual
C53000-G1176-C127-3
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