ABB RELION 670 SERIES Applications Manual page 587

1MRK 506 369-UUS -
Line distance protection REL670 2.2 ANSI
Application manual
occuring at disturbance where high voltages and/or low frequencies can occur.
Overexcitation can occur during start-up and shut-down of the generator if the field
current is not properly adjusted. Loss-of load or load-shedding can also result in
overexcitation if the voltage control and frequency governor is not functioning properly.
Loss of load or load-shedding at a transformer substation can result in overexcitation if the
voltage control function is insufficient or out of order. Low frequency in a system isolated
from the main network can result in overexcitation if the voltage regulating system
maintains normal voltage.
According to the IEC standards, the power transformers shall be capable of delivering
rated load current continuously at an applied voltage of 105% of rated value (at rated
frequency). For special cases, the purchaser may specify that the transformer shall be
capable of operating continuously at an applied voltage 110% of rated value at no load,
reduced to 105% at rated secondary load current.
According to ANSI/IEEE standards, the transformers shall be capable of delivering rated
load current continuously at an output voltage of 105% of rated value (at rated frequency)
and operate continuously with output voltage equal to 110% of rated value at no load.
The capability of a transformer (or generator) to withstand overexcitation can be
illustrated in the form of a thermal capability curve, that is, a diagram which shows the
permissible time as a function of the level of over-excitation. When the transformer is
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, 24) 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 (24) 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 (24) 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
Section 10
Voltage protection
581