ABB RELION REX640 Technical Manual page 894

Protection functions
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
Nominal phase current (I
Nominal frequency (f
Leakage reactance (X
Measured voltage and load currents of the machine
Phase A-to-phase B voltage (U
Phase A current (I
Phase B current (I
Measured frequency (f
The setting
The setting
The setting
The pu leakage reactance X
X
leak
The internal induced voltage E of the machine is calculated.
E U
=
E = 11500∠0°+ (5600∠-63.57°- 5600∠176.42°) · (0.170378∠90°) = 12490 V
The excitation level M of the machine is calculated.
894
)
n
)
n
)
leak
)
A
)
B
)
m
Voltage Max Cont
Voltage selection
Phase supervision
U
n
X 0 2 .
= ⋅ =
leakPU
Ω
I
⋅ 3
n
( I I ) ( jX )
+ − ⋅
AB A B leak
)
n
)
AB
is converted to ohms.
leakPU
11000
0 170378 . Ohms
⋅ =
7455 ⋅ 3
1MRS759142 F
11000 V
7455 A
50 Hz
20% or 0.2 pu
11500∠0° V
5600∠-63.57° A
5600∠176.42° A
49.98 Hz
100%
phase-to-phase
A or AB
(Equation 251)
(Equation 252)
Technical Manual
REX640