4.2.5.3
Functionality
The overexcitation protection function 24 is used to protect generators and power
transformers against an excessive flux density and saturation of the magnetic core.
The function calculates the V/f ratio (volts/hertz) proportional to the excitation level of the
generator or transformer and compares this value to the setting limit. The function picks
up when the excitation level exceeds the set limit and trips when the set tripping time has
elapsed. The tripping time characteristic can be selected to be either definite time (DT) or
overexcitation inverse definite minimum time (overexcitation type IDMT).
This function contains a blocking functionality. It is possible to block the function outputs,
reset timer or the function itself, if desired.
4.2.5.4
Operation principle
The function can be enabled and disabled with the
Operation
setting. The corresponding
parameter values are "Enable" and "Disable".
The operation of 24 can be described using a module diagram. All the modules in the
diagram are explained in the next sections.
GUID-93ECF983-CCBB-4277-8E7B-7EC43743A375 V1 EN
Figure 118:
Functional module diagram
V/f calculation
This module calculates the V/f ratio, that is, the excitation level from the internal induced
voltage (E) and frequency. The actual measured voltage (V
m
) deviates from the internal
induced voltage (emf) E, a value the equipment has to withstand. This voltage
compensation is based on the load current (I
L
) and the leakage reactance (X
leak
) of the
equipment. The leakage reactance of the transformer or generator is set through the
Leakage React
setting in percentage of the Z base.
The internal induced voltage (E) is calculated from the measured voltage. The settings
Voltage selection
and
Phase supervision
determine which voltages and currents are to be
1MRS240050-IB C
Section 4
Protection functions
REF615R
247
Technical Manual
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