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Functions
2.24 Overexcitation (Volt/Hertz) Protection (ANSI 24)
Thermal Characteristic
A thermal characteristic is superimposed on the trip stage characteristic. For this purpose, the temperature rise
created by the overexcitation is approximately modeled. Not only the already mentioned pickup signal is gen-
erated on transgression of the U/f induction limit set at address 4302, but in addition a counter is activated ad-
ditionally which causes the tripping after a length of time corresponding to the set characteristic.
Figure 2-78
The characteristic of a Siemens standard transformer was selected as a default setting for the parameters
4306 to 4313. If the protection object manufacturer did not provide any information, the preset standard char-
acteristic should be used. Otherwise, any trip characteristic can be specified entering parameters point-bypoint
over a maximum of 7 straight lengths. To do this, the trip times t of the overexcitation values U/f = 1.05; 1.10;
1.15; 1.20; 1.25; 1.30; 1.35 and 1.40 are read out from the predefined characteristic and entered at the address-
es4306 t(U/f=1.05) to 4313 t(U/f=1.40). The protection device interpolates linearly between the points.
Limitation
The heating model of the object to be protected is limited to a 150 % overshoot of the trip temperature.
Cooling Time
Tripping by the thermal image drops out by the time of the pickup threshold dropout. However, the counter
content is counted down to zero with the cooldown time parametrized at address 4314 T COOL DOWN. In this
context this parameter is defined as the time required by the thermal image to cool down from 100 % to 0 %.
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 (U
eters 221 Unom PRIMARY and 251 UN GEN/MOTOR were properly entered in accordance with Section 2.5.
188
Thermal tripping time characteristic (with default settings)
/U
). For this it is necessary that the relevant param-
N prim
N Mach
C53000-G1176-C149-7, Release date 03.2010
SIPROTEC, 7UM62, Manual
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