Monitoring; Transformer; Harmonic Derating - GE 845 Instruction Manual

MONITORING

Monitoring

Transformer

Harmonic Derating

NOTE:
4–248
Figure 4-100: Monitoring Display Hierarchy
The presence of harmonic currents in transformers increases the I
the core, clamps and structural parts, and eddy current losses in the windings. Of these
losses, the eddy current losses are of most concern since they tend to be proportional to
the square of the load current and approximately proportional to the square of frequency
which, in turn can cause excessive winding loss and abnormal winding temperature rise in
transformers when harmonics are present.
The Harmonic Derating element makes use of the harmonic derating factor (HDF)
computed by the Series 8 relay, using the harmonic content of the current signals and the
transformer data (refer to IEEE C57.110-2008 for the computation method). Once the
derating factor falls below a set value, the relay operates accordingly. The derating factor
is used to evaluate the load capability of the installed transformer under the non-
sinusoidal load currents. Once harmonics are present, the derating factor will decrease
from the ideal value of one per unit, which means the load capability is reduced due to
harmonics.
The Load-Limit Reduced LED indicator is ON and an operand of 'Load Limit Reduced' is
asserted if the HDF of any winding is less than 0.96, regardless of whether the element is
disabled or enabled.
The following three settings are used in this element:
• SETPOINTS > SYSTEM > TRANSFORMER > TRANSFORMER SETUP > LOAD LOSS AT RATED
LOAD
• SETPOINTS > SYSTEM > TRANSFORMER > TRANSFORMER SETUP > RATED WNDG TEMP
RISE, and
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS
2
R losses, stray losses in