Qualitrol 509ITM Series, Manual

Page: 174 / 189

174
www.qualitrolcorp.com
Frequently Asked Questions About the 509ITM
Q:
In the Simple CT Temperature Scaling Setup window, what is the
“Oil Temperature
Adder
”? And where do I get the values for the other three variables?
A:
The Simple Calculation Setup Method , which uses the Simple CT Temperature
Scaling Setup window, calculates the winding temperature of the transformer by
using the oil temperature of the transformer and the load or current running through
the windings. The RTD input module that measures this oil temperature is the Oil
Temperature Adder. The other three variables in this section of the window are:
•
Winding Rise Temperature
•
Current Factor
•
Winding Time Constant
You can determine the values for these variables from the Transformer Test Report
that is included with the transformer. The temperature rise (or delta T) is the hot
spot temperature of the winding over the oil temperature at a given load. You can
calculate the current by using these steps:
1. Divide the secondary kVA by the secondary voltage.
2. Divide by the instrumentation CT ratio that is being used to give you the load
current for that delta T rise.
Example: The transformer is a 14000 kVA with a winding of 34400 V. The
instrumentation CT is 1200 : 5.
•
14000 kVA divided by 34400 V equals 407 Amp
•
407 Amp divided by 1200/5 (or 240) equals 1.69 Amp
The same report indicates that the hot spot rise over fluid is 12.7º C. Therefore, in
the
Simple CT Temperature Scaling Setup window, enter:
•
12.7º C in the Winding Rise Temperature field
•
1.69 Amp in the Current Factor field
If no value for the Winding Time Constant field is provided on the Transformer
Heat Run Test Report and the manufacturer cannot supply it, then the IEEE Std
C57.91 recommends using a number between 5 and 8 minutes.
Q: What is a time constant?
A: The time constant is the time that it takes for the winding temperature to reach
63.2% of its full value for a given current load. That means for a given load on a
transformer, it will take approximately 5 time constants for the winding temperature
to reach its final value. So if the time constant is 6 minutes, it will take the winding
temperature 30 minutes (5 time constants × 6 minutes) to reach full temperature for
a given load.