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2.18
Overexcitation (Volt/Hertz) Protection (ANSI 24)
Overexcitation protection is used to detect inadmissibly high induction in generators
and transformers, especially in power station unit transformers. Overexcitation protec-
tion must intervene when the admissible induction level for the protected object (e.g.
unit transformer) is exceeded. The transformer is endangered, for example, if the
power station block is disconnected from full-load operation and the voltage regulator
does not operate sufficiently fast, or at all, to control the resulting voltage increase.
Similarly, a decrease in frequency (speed), e.g. in island systems, can lead to in-
creased induction.
An increase in induction above the rated value very quickly saturates the iron core and
causes large eddy current losses.
2.18.1 Function Description
Measurement
The overexcitation protection measures the voltage/frequency ratio proportional to the
Method
induction B and compares it with rated induction B
ferred to rated values of the protection object (generator, transformer).
The calculation is based on the maximum voltage of the three phase-to-phase voltag-
es. The frequency range monitored extends from 10 Hz to 70 Hz.
Transformer Adap-
Any deviation between the primary nominal voltage of the voltage transformers and
tation
the object to be protected is compensated by an internal correction factor (U
Gen prim
to secondary values. However the system primary nominal transformer voltage and
the nominal voltage of the object to be protected' must be entered correctly (see Sec-
tions 2.3 and 2.5.
Characteristic
Overexcitation protection includes two staged characteristics and one thermal charac-
Curves
teristic for approximate modeling of the heating of the protection object due to overex-
citation. As soon as a first pickup threshold (warning stage 4302 U/f >) has been
exceeded, a 4303 T U/f > time stage is started. On its expiry a warning message is
transmitted. At the same time a counter switching is activated when the pickup thresh-
old is exceeded. This weighted counter is incremented in accordance with the current
U/f value resulting in the trip time for the parametrized characteristic. A trip signal is
transmitted as soon as the trip counter state has been reached.
The trip signal is retracted as soon as the value falls below the pickup threshold and
the counter is decremented in accordance with a parametrizable cool-down time.
The thermal characteristic is specified by 8 value pairs for overexcitation U/f (related
to nominal values) and trip time t. In most cases, the specified characteristic for stan-
dard transformers provides sufficient protection. If this characteristic does not corre-
7UM61 Manual
C53000-G1176-C127-3
). For this reason pickup values and characteristic do not need to be converted
2.18 Overexcitation (Volt/Hertz) Protection (ANSI 24)
. Voltage and frequency are re-
N
/U
N VT prim
N
113
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