ABB RELION RET670, Applications Manual

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Planned loading beyond nameplate rating
Hours of the day
12 2 4 6 8 12
10
2 4 6 8 10
12 2 4 6 8 12
10
2 4 6 8 10
12 2 4 6 8 12
10
2 4 6 8 10
12 2 4
6 8
12 10
2 4
6 8
10
Hours of the day
Hours of the day
Hours of the day
80°C
90°C
100°C
110°C
120°C
130°C
140°C
80°C
90°C
100°C
110°C
120°C
130°C
140°C
80°C
90°C
100°C
110°C
120°C
130°C
140°C
100°C
110°C
120°C
130°C
140°C
150°C
160°C
170°C
90°C
Normal life expectancy loading
Short time energy loading Long time energy loading
Hot spot temperature
Hot spot temperature
Hot spot temperature
Hot spot temperature
IEC15000441-1-en.vsdx
IEC15000441 V1 EN-US
Figure 412: Typical load cycles for example
Impact of the increased currents and temperature leads to premature transformer failure
and this may have an immediate short term effect or a cumulative long term effect.
• Short-term effect : Reduction in dielectric strength due to the possible presence of
gas bubbles which leads to super saturation of the oil and reduction in short circuit
strength.
• Long-term effect : Continuous deterioration of the conductor reduces the
transformer life.
Deterioration of insulation reduces mechanical strength and dielectric strength. Heating
from heavy overload or large electromagnetic force resulting from short circuit causes
expansion and unusual movement of conductors and it leads to turn-to-turn fault.
1MRK 504 163-UUS A
Section 18
Monitoring
Transformer protection RET670 2.2 ANSI 903
Application manual