Calculating the thermal overload relay setting for a distribution transformer involves evaluating numerous elements, including the transformer rating, load current, and ambient temperature.
Here’s a simple calculation:
1. Determine the transformer’s Full Load Current (FLC):
FLC = transformer rating (kVA) / rated voltage (kV)
2. Identify the Overload Setting (OL):
OL = FLC X Overload Setting (represented as a percentage of FLC)
3. Adjust for Ambient Temperature:
If the transformer runs at temperatures above the normal rating (about 40°C), derating may be required. For ambient temperature-dependent derating factors, consult manufacturer specifications or industry standards.
4. Set the Thermal Overload Relay:
Set the thermal overload relay to trip at the calculated overload (OL). Check that the relay’s time-current characteristic curve satisfies the transformer and application specifications.
5. Test and Verify:
After establishing the thermal overload relay, run tests to ensure proper operation under different load situations and ambient temperatures. Check that it provides appropriate protection without causing annoyance tripping.
It is important to note that actual calculations and settings may differ depending on transformer characteristics, application requirements, & applicable standards or regulations.