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1MAC050144-MB C
Section 4
Protection functions
615 series ANSI
249
Technical Manual
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.
4.2.6.4
Operation Principle
The function can be enabled and disabled with the
Operation
setting. The corresponding
parameter values are Enable and Disable.
The operation of the overexcitation protection can be described with a module diagram.
All the modules in the diagram are explained in the next sections.
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
used. If the
Voltage selection
setting is set to "phase-to-ground" or "phase-to-phase", the
Phase supervision
setting is used for determining which phases or phase-to-phase voltages
("A or AB", "B or BC" and "C or CA") and currents are to be used for the calculation of
the induced voltage.
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