ABB 615 series Technical Manual page 658

Section 4
Protection functions
652
low frequency in a system isolated from the main network 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-ground voltage does not mean overexcitation. For example,
in an ungrounded power system, a single-phase-to-ground fault means high voltages of
the healthy two phases to ground 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 (V
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 (V
Phase A current (I )
A
Phase B current (I )
B
Measured frequency (f )
m
The setting Voltage Max Cont
Voltage selection
The setting
Phase supervision
The setting
) 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
1MAC059074-MB A
615 series ANSI
Technical Manual