CURRENT TRANSFORMER RATING FACTOR (RF)

In this post, I will try to review about one of the parameters that always appears in a current transformer nameplate, Rating Factor (RF) parameter.

As shown on below picture, rating factor parameter was placed on the bottom right corner of the nameplate.

The value of rating factor parameter is 4.0 at 30⁰C .

R.F. 30⁰C = 4.0

Understanding Of Rating Factor (RF)

Rating Factor or commonly abbreviated as RF is a number that define how large primary current of current transformer (CT) that can pass more than rated primary current value. Rating factor indicates ability level of a primary current flow in continuously without causing damage to the current transformer.

Rating Factor or RF parameters will not cause some of the following:

• The increase in the temperature of the current transformer temperature rise exceeds predetermined value.
• Changes in the accuracy class of the current transformer.

Value of Rating Factor (RF), commonly followed by ambient temperature conditions where the value is defined. Typically, the manufacturer defines the value of the ambient temperature at 30 degrees Celsius.

ANSI/IEEE standard have defined a rating factor (RF) of metering type current transformer in various numbers. Those numbers are : 1.0, 1.33, 1.5, 2.0, 3.0 and 4.0

Example # 1:

There is a current transformer metering type with the following specifications:
Ratio 200: 5A, RF 0.3BX.X 4.0 at a room temperature of 30 degrees Celsius.

Base on above specification, we can calculate that maximum primary current could flow continuously on the current transformer is 200A * 4 = 800A.

800A of primary current will not causing damage on the current transformer, exceed the temperature rise define by manufacturer and change of accuracy class as shown on below picture.

Ratio error of current transformer remains within +/- 0.3%, although 800A of primary current flowing continuously on the current transformer.

 

Effect Of Increasing Temperature On The Rating Factor.

In the previous example, Rating Factor defined at room temperature at 30 degrees Celsius. The question, what is the rating factor (RF) value of a current transformer if the room temperature is change from 30 degree Celsius to 50 degrees Celsius.

To determine the rating factor values ​​at different temperatures with the manufacturer's specifications, it can be used the following equation:

Example # 2:
A current transformer metering type with the following specifications:
Ratio 200: 5A, RF 0.3BX.X 4.0 at a temperature of 30 degrees Celsius.

What is the rating factor (RF) value at an ambient temperature of 55 degrees Celsius?

What is the maximum value of the primary current that can be passed at a temperature of 55 degrees Celsius without damage?
 

Answer:

note:
Manufacturer rating Factor (RF) value is 4 at a room temperature of 30 degrees Celsius.
Then the Rating Factor value at 55 degrees Celsius is:
New RF² / 4² = (85-55) / 55
New RF² = (30/55) * 4²
New RF² = √8.73 = 2.95
 

So, Rating factor (RF) value at 55 degrees Celsius is 2.95.

Thus, the maximum primary current that can be passed continuously at a temperature of 55 degrees Celsius is 2.95 * 200A = 590A.
 
From the example of above calculation, it appears that the value rating factor will change with changes in ambient temperature where the current transformer used.

References:
- ANSI / IEEE C57.13 2008.
- Instrument Transformer Basic technical information and application by Digital Energy.