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2.8
24 Overexcit. Protection (Volt/Hertz)
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.
Functional Description
2.8.1
Measurement Method
The overexcitation protection feature servers to measure the voltage V/frequency f, ratio f, which is propor-
tional to the B induction and puts it in relation to the B
frequency are related to nominal values of the object to be protected (generator, transformer).
[uebereregungsschutz-020827-ho, 1, en_US]
[uebereregungsschutz2-020827-ho, 1, en_US]
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
istic 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 Power System Data 1
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 decre-
mented in accordance with a parametrizable cool-down time.
The thermal characteristic is specified by 8 value pairs for overexcitation V/f (related to nominal values) and
trip time t. In most cases, the specified characteristic for standard transformers provides sufficient protection.
If this characteristic does not correspond to the actual thermal behavior of the object to be protected, any
desired characteristic can be implemented by entering customer-specific trip times for the specified V/f overex-
citation values. Intermediate values are determined by a linear interpolation within the device.
SIPROTEC 4, 7RW80, Manual
C53000-G1140-C233-4, Edition 07.2018
N
Nom prim
and
2.1.6 Power System Data
2.8 24 Overexcit. Protection (Volt/Hertz)
nominal induction. In this context, both voltage and
/V
). For this reason pickup values and character-
Nom Mach
2).
Functions
91
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