U/f calculation
This module calculates the U/f ratio, that is, the excitation level from the internal
induced voltage (E) and frequency. The actual measured voltage (U
m
) deviates
from the internal induced voltage (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-earth"
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.
Table 638: Voltages and currents used for induced voltage (emf) E calculation
Voltage selection
setting
Phase supervision
setting
Calculation of internal induced voltage (emf) E
phase-to-earth
A or AB
E
U
I
j X
A
A
leak
=
⋅
+
⋅
⋅
(
)
3
(
)
phase-to-earth
B or BC
E
U
I
j X
B
B
leak
=
⋅
+
⋅
⋅
(
)
3
(
)
phase-to-earth
C or CA
E
U
I
j X
C
C
leak
=
⋅
+
⋅
⋅
(
)
3
(
)
phase-to-phase
A or AB
E
U
I
I
j X
AB
A
B
leak
=
+
−
⋅
⋅
(
)
(
) (
)
phase-to-phase
B or BC
E
U
I
I
j X
BC
B
C
leak
=
+
−
⋅
⋅
(
)
(
) (
)
phase-to-phase
C or CA
E
U
I
I
j X
CA
C
A
leak
=
+
−
⋅
⋅
(
)
(
) (
)
Pos sequence
N/A
E
U
I
j X
leak
=
⋅
+
⋅
⋅
(
)
3
1
1
(
)
If all three phase or phase-to-phase voltages and phase currents
are fed to the protection relay, the positive-sequence alternative is
recommended.
If the leakage reactance of the protected equipment is unknown or if the
measured voltage (U
m
) is to be used in the excitation level calculation,
then by setting the leakage reactance value to zero the calculated
induced voltage (E) is equal to the measured voltage.
The calculated U/f ratio is scaled to a value based on the nominal U
n
/f
n
ratio.
However, the highest allowed continuous voltage (in % U
n
) can be defined by
setting the parameter
Voltage Max Cont to change the basis of the voltage. The
measured voltage is compared to the new base value to obtain the excitation level.
The excitation level (M) can be calculated:
1
Voltages, currents and the leakage reactance X
leak
in the calculations are given in volts, amps
and ohms.
Protection functions
1MRS757644 H
674
620 series
Technical Manual
