ABB RELION REX640 Technical Manual page 817

1MRS759142 C
REX640
Technical Manual
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
Nominal phase current (I )
n
Nominal frequency (f )
n
Leakage reactance (X )
leak
Measured voltage and load currents of the machine
Phase A-to-phase B voltage (U
Phase A current (I )
A
Phase B current (I )
B
Measured frequency (f )
m
Voltage Max Cont
The setting
Voltage selection
The setting
Phase supervision
The setting
The pu leakage reactance X
U
n
X X
= ⋅ =
leak leakPU
Ω
I
3
⋅
n
GUID-54D7A902-FAC1-4988-B0B5-5283F5170187 V1 EN
The internal induced voltage E of the machine is calculated.
) 11000 V
n
7455 A
50 Hz
20% or 0.2 pu
) 11500∠0° V
AB
5600∠-63.57° A
5600∠176.42° A
49.98 Hz
100%
phase-to-phase
A or AB
is converted to ohms.
leakPU
11000
0 2 . 0 170378 .
⋅ =
7455 3
⋅
Section 4
Protection functions
Ohms
(Equation 205)
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