ABB REG670 Applications Manual page 426

Section 10
Voltage protection
420
loaded, the induced voltage and hence the flux density in the core can not be read off
directly from the transformer terminal voltage. Normally, the leakage reactance of
each separate winding is not known and the flux density in the transformer core can
then not be calculated. In two-winding transformers, the low voltage winding is
normally located close to the core and the voltage across this winding reflects the flux
density in the core. However, depending on the design, the flux flowing in the yoke
may be critical for the ability of the transformer to handle excess flux.
The Overexcitation protection (OEXPVPH) has current inputs to allow calculation of
the load influence on the induced voltage. This gives a more exact measurement of the
magnetizing flow. For power transformers with unidirectional load flow, the voltage
to OEXPVPH should therefore be taken from the feeder side.
Heat accumulated in critical parts during a period of overexcitation will be reduced
gradually when the excitation returns to the normal value. If a new period of
overexcitation occurs after a short time interval, the heating will start from a higher
level, therefore, OEXPVPH must have thermal memory. A fixed cooling time
constant is settable within a wide range.
The general experience is that the overexcitation characteristics for a number of power
transformers are not in accordance with standard inverse time curves. In order to make
optimal settings possible, a transformer adapted characteristic is available in the IED.
The operate characteristic of the protection function can be set to correspond quite
well with any characteristic by setting the operate time for six different figures of
overexcitation in the range from 100% to 180% of rated V/Hz.
When configured to a single phase-to-phase voltage input, a corresponding phase-to-
phase current is calculated which has the same phase angle relative the phase-to-phase
voltage as the phase currents have relative the phase voltages in a symmetrical system.
The function should preferably be configured to use a three-phase voltage input if
available. It then uses the positive sequence quantities of voltages and currents.
Analog measurements shall not be taken from any winding where a
load tap changer is located.
Some different connection alternatives are shown in figure 208.
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
1MRK 502 071-UEN -
Application manual