Functions
2.8 Overexcit. Protection (Volt/Hertz) 24
SIPROTEC, 7RW80, Manual
C53000-G1140-C233-1, Release date 10.2010
102
2.8 Overexcit. Protection (Volt/Hertz) 24
Overexcitation protection is used to detect inadmissibly high induction in generators and transformers, espe-
cially in power station unit transformers. The protection must intervene when the limit value for the protected
object (e.g. unit transformer) is exceeded. The transformer is endangered, for example, if the power station
block is disconnected from the system from full-load, and if the voltage regulator either does not operate or
does not operate sufficiently fast to control the associated voltage rise. Similarly a decrease in frequency
(speed), e.g. in island systems, can lead to an inadmissible increase in induction.
An increase in induction above the rated value quickly saturates the iron core and causes large eddy current
losses.
2.8.1 Functional Description
Measurement Method
The overexcitation protection feature servers to measure the voltage V/frequency f, ratio f, which is proportional
to the B induction and puts it in relation to the B
N
nominal induction. In this context, both voltage and frequency
are related to nominal values of the object to be protected (generator, transformer).
The calculation is based on the maximum of the three phase-to-phase voltages. The frequency range moni-
tored extends from 25 Hz to 70 Hz.
Voltage Transformer Adaptation
Any deviation between the primary nominal voltage of the voltage transformers and of the protected object is
compensated by an internal correction factor (V
Nom prim
/V
Nom Mach
). For this reason pickup values and charac-
teristic do not need to be converted 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 Sections 2.1.3 and
2.1.6).
Characteristics
Overexcitation protection includes two time graded characteristics and one thermal characteristic for approxi-
mate modeling of the heating of the protection object due to overexcitation. As soon as a first pickup threshold
(warning element 4302 24-1 PICKUP) has been exceeded, a 4303 24-1 DELAY time element is started. On
its expiry a warning message is transmitted. At the same time a counter switching is activated when the pickup
threshold is exceeded. This weighted counter is incremented in accordance with the current V/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.
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