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2.24
Overexcitation (Volt/Hertz) Protection (ANSI 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.
2.24.1
Functional Description
Measurement Method
The overexcitation protection feature servers to measure the voltageU/frequency ratio f, which is proportional
to the B induction and puts it in relation to the B
are related to nominal values of the object to be protected (generator, transformer).
The calculation is based on the maximum voltage of the three phase-to-phase voltages. The frequency range
monitored extends from 10 Hz to 70 Hz.
Voltage Transformer Adaptation
Any deviation between primary nominal voltage of the voltage transformers and of the object to be protected is
compensated by an internal correction factor (U
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 2.5 and 2.7).
Characteristic Curves
Overexcitation protection includes two staged characteristics and one thermal characteristic for approximate
modeling of the heating of the protection object due to overexcitation. As soon as a first pickup threshold (warn-
ing 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 threshold is exceed-
ed. 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 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 U/f over-
excitation values. Intermediate values are determined by a linear interpolation within the device.
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
2.24 Overexcitation (Volt/Hertz) Protection (ANSI 24)
nominal induction. In this context, both voltage and frequency
N
/U
). For this reason pickup values and characteristic
N prim
N Mach
Functions
185
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