ABB RELION REX640, Техническое руководство, Страница: 894 / 1960

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
Ohms
leak leakPU
n
n
Ω
= ⋅
⋅
= ⋅
⋅
=
3
0 2
11000
7455 3
0 170378 . .
(Equation 251)
The internal induced voltage E of the machine is calculated.
E U I I jX
AB A B leak
= + − ⋅
( ) ( )
(Equation 252)
E = 11500∠0°+ (5600∠-63.57°- 5600∠176.42°) · (0.170378∠90°) = 12490 V
The excitation level M of the machine is calculated.
Protection functions 1MRS759142 F
894 REX640
Technical Manual