ABB RELION 620 Series Technical Manual page 686

Section 4
Protection functions
680
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
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
The setting Voltage selection
Phase supervision
The setting
The pu leakage reactance X

U
n
X X

= ⋅
leak leakPU
Ω
(
I


GUID-D143EE29-4A67-4B5E-9C1A-87D60F10CFCB V1 EN
) 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
 
0 2 11000
.
 
= ⋅
) (
3 7455
 
⋅
n
 
1MRS757644 F

0 0 170378 . Ohms

=
)
3

⋅

(Equation 137)
Technical Manual
620 series