can result in overexcitation if the voltage-regulating system maintains a normal
voltage.
Overexcitation protection for the transformer is generally provided by the
generator overexcitation protection, which uses the VTs connected to the
generator terminals. The curves that define the generator and transformer V/Hz
limits must be coordinated properly to protect both equipment.
If the generator can be operated with a leading power factor, the high-side voltage
of the transformer can have a higher pu V/Hz than the generator V/Hz. This needs
to be considered in a proper overexcitation protection of the transformer. Also,
measurement for the voltage must not be taken from any winding where OLTC is
located.
It is assumed that overexcitation is a symmetrical phenomenon caused by events
such as loss-of-load. A high phase-to-earth voltage does not mean overexcitation.
For example, in an unearthed power system, a single phase-to-earth fault means
high voltages of the healthy two phases to earth but no overexcitation on any
winding. The phase-to-phase voltages remain essentially unchanged. An important
voltage to be considered for the overexcitation is the voltage between the two ends
of each winding.
Example calculations for overexcitation protection
Example 1
Nominal values of the machine
Nominal phase-to-phase voltage (U
n
)
11000 V
Nominal phase current (I
n
)
7455 A
Nominal frequency (f
n
)
50 Hz
Leakage reactance (X
leak
)
20% or 0.2 pu
Measured voltage and load currents of the machine
Phase A-to-phase B voltage (U
AB
)
11500∠0° V
Phase A current (I
A
)
5600∠-63.57° A
Phase B current (I
B
)
5600∠176.42° A
Measured frequency (f
m
)
49.98 Hz
The setting
Voltage Max Cont
100%
The setting
Voltage selection
phase-to-phase
The setting
Phase supervision
A or AB
The pu leakage reactance X
leakPU
is converted to ohms.
X
X
U
I
leak
leakPU
n
n
Ω
=
⋅
⋅
(
)
=
⋅
⋅
(
)
=
3
0 2
11000
7455
3
.
0
0 170378
.
Ohms
(Equation 138)
The internal induced voltage E of the machine is calculated.
1MRS757644 H
Protection functions
620 series
Technical Manual
681
