GE P645

2 OVERFLUXING PROTECTION
If a power transformer is connected to an active load such as a generator, it is possible that the ratio of voltage to
fr
equency exceeds certain limits. This will result in Overfluxing, sometimes known as overexcitation. An excessively
high voltage or excessively low frequency causes the V/f ratio to rise, producing high flux densities in the magnetic
core of the transformer. Overfluxing causes the transformer core to saturate resulting in stray flux in non-
laminated components not designed to carry flux. This in turn causes eddy currents in solid components (e.g. core
bolts and clamps) and end-of-core laminations causing rapid overheating and damage. Transformer
manufacturers provide information about the V/f capability as a function of time. The limit is either in the form of a
curve or a set-point with a time delay.
Transformer overfluxing might arise for the following reasons:
● High system voltage
● Generator full load rejection
● Increased voltage due to light loading of transmission lines (Ferranti effect)
● Low system frequency
● Generator excitation at low speed with Automatic Voltage Regulator (AVR) in service
● Geomagnetic disturbance (effects of solar radiation)
● Low frequency earth current circulation through a transmission system
The initial effect of overfluxing is to increase the magnetising current for a transformer. This current will be seen as
a differential current, which could cause the device to maloperate, therefore some sort of restraint is needed. The
fifth harmonic component of the current is used to block the differential element during mild or short term
overfluxing conditions.
Persistent overfluxing however, may result in thermal damage or degradation of a transformer as a result of
overheating.
The following protection strategy is therefore advisable to address potential overfluxing conditions:
● Maintain protection stability during transient overfluxing by blocking the differential protection
● Ensure tripping for persistent overfluxing by applying the overfluxing protection. It is common practice to
use the overfluxing element to protect the transformer during system disturbance, especially on large
network transformers.
2.1 OVERFLUXING PROTECTION IMPLEMENTATION
The Overfluxing settings are in the O
VERFLUXING column of the relevant settings group. Depending on your device
model, it provides one or two overfluxing elements with four stages of overfluxing protection plus an additional
alarm stage). The P642 has a single-phase VT only, therefore only one overfluxing element is provided. The P643
and P645 can have a single-phase VT and a 3-phase VT, and therefore provides a single-phase overfluxing
element and a three-phase overfluxing element. Both elements are similar in functionality and follow the same
logical principles.
You enable or disable each stage of the overfluxing protection for each element by the relevant Status cells V/
Hz>(n) Status and V/HZ Alm Status.
The first stage can be set to operate with a definite time or inverse time delay (IDMT). This stage can be used to
provide the protection trip output. The other three stages are all definite time stages. These can be combined with
the first stage inverse time characteristic to create a combined multi-stage overfluxing trip operating
characteristic using PSL.
An inhibit signal is provided for the first stage 1 only. This allows a definite time stage to override a section of the
inverse time characteristic if required. The inhibit signal has the effect of resetting the timer, the start signal and
the trip signal.
Chapter 12 - Frequency Protection Functions P64x
254 P64x-TM-EN-1.3