ABB 615 Series ANSI, Техническое руководство, Страница: 264 / 713

1MAC050144-MB C
Section 4
Protection functions
615 series ANSI 257
Technical Manual
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 equipments.
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
Consider the nominal values of the machine as:
Nominal phase-to-phase voltage (V
n
) = 11000 V
Nominal phase current (I
n
) = 7455 A
Nominal frequency (f
n
) = 60 Hz
Leakage reactance (X
leak
) = 20% or 0.2 pu
Assume that the measured voltage and load currents of the machine as:
Phase A-to-phase B voltage (V
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
) = 59.98 Hz.
The setting Volt Max continuous = 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 as per
Equation 11
.