Technical description
2 Protection functions
2.5 Volts/hertz over-excitation
protection Uf> (24)
VM55.EN005
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2.5.
Volts/hertz over-excitation protection
U
f
> (24)
The saturation of any inductive network components like
transformers, inductors, motors and generators, depend on the
voltage and frequency. The lower the frequency, the lower is
the voltage at which the saturation begins.
The volts/hertz over-excitation protection stage is sensitive to
the voltage/frequency ratio instead of voltage only. Figure 2.5-1
shows the difference between volts/hertz and a standard
overvoltage function. The maximum of the three line-to-line
voltage is used regardless of the voltage measurement mode
(chapter 4.3). By using line-to-line voltages any phase-to-
ground over-voltages during earth faults have no effect. (The
earth fault protection functions will take care of earth faults.)
The used net frequency is automatically adopted according the
local network frequency.
Overexcitation protection is needed for generators, which are
excitated even during start up and shut down. If such a
generator is connected to a unit transformer, also the unit
transformer needs volts/hertz over-excitation protection.
Another application is sensitive overvoltage protection of
modern transformers with no flux density margin in networks
with unstable frequency.
Setting groups
There are two settings groups available. Switching between
setting groups can be controlled by digital inputs, virtual
inputs (mimic display, communication, logic) and manually.
Figure 2.5-1 This figure shows the difference between volts/hertz and
normal overvoltage protection. The volts/hertz characteristics on the left
depend on the frequency while the standard overvoltage function on the
right is insensitive to frequency. The network frequency, 50 Hz or 60 Hz, is
automatically adopted by the relay.
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