Siemens SIPROTEC 7UT613 series Manual page 240

2 Functions
Figure 2-99 Logic diagram of the overexcitation protection (simplified)
240
U/f >>) has been exceeded, another pickup indication is output and a timer T U/f
>> starts. A trip command is issued subsequent to the expiration of this timer.
The thermal replica is realised by a counter which is incremented in accordance with
the value U/f calculated from the measured voltages. A prerequisite is that the U/f
value has exceeded the pickup value U/f > of the warning stage. If the counter
reaches a level corresponding with the set trip characteristic, the trip command is
given.
The trip signal is cancelled as soon as the value falls below the pickup threshold and
the counter is decremented according to the set cooldown rate.
The thermal characteristic is specified by 8 value pairs concerning the U/f value (re-
ferred to nominal value) and the associated trip time T. In most cases, the default char-
acteristic for standard transformers provides for sufficient protection. If this character-
istic does not correspond to the actual thermal behaviour of the object to be protected,
any desired characteristic can be implemented by entering user-specific trip times for
the specified U/f overexcitation values. Intermediate values are determined by a linear
interpolation within the device.
The counter can be reset to zero by means of a blocking input or a reset input. The
internal upper limit of the thermal replica is 150 % of trip temperature rise.
7UT613/63x Manual
C53000-G1176-C160-2