ABB RELION 620 Series Technical Manual page 312

Section 4
Protection functions
306
excessive heating and severe damage to the insulation and adjacent parts in a relatively
short time.
Overvoltage, underfrequency or a combination of the two, results in an excessive flux
density level. Since the flux density is directly proportional to the voltage and inversely
proportional to the frequency, the overexcitation protection calculates the relative V/Hz
ratio instead of measuring the flux density directly. The nominal level (nominal voltage at
nominal frequency) is usually considered as the 100 percent level, which can be exceeded
slightly based on the design.
The greatest risk for overexcitation exists in a thermal power station when the generator-
transformer unit is disconnected from the rest of the network or in the network islands
where high voltages or low frequencies can occur.
Overexcitation can occur during the start-up and shutdown of the generator if the field
current is not properly adjusted. The loss-of-load or load shedding can also result in
overexcitation if the voltage control and frequency governor do not function properly. The
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
) 11000 V
n
7455 A
50 Hz
1MAC504801-IB E
620 series ANSI
Technical Manual