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.

Burden Understanding Of Current Transformer.

BURDEN

The term of burden is always raised on the instrumentation transformers, both current transformer and voltage transformer. Sometime, term of burden is also equated with the term of load. Since the understanding of burden are very important and significant to an accuracy class of current transformer and voltage transformer, we try to present it in a comprehensive manner to assist practitioners in the field of medium voltage control to have better understanding of the burden.
As started, we will explain the meaning of burden for current transformer.

BURDEN DEFINITION

Definition of burden base on IEC 60044-1 standard.

The impedance of the secondary circuit in ohms and power-factor.

The burden is usually expressed as the apparent power in voltamperes absorbed at a specified power-factor and at the rated secondary current.

Thus, a burden can be expressed in two ways, namely:
Firstly, as the total impedance of the secondary circuit in Ohms. This definition is commonly used in ANSI / IEEE standard.

The following table of burden base on ANSI/IEEE Standard.

Below are sample of Metering Current transformer technical spec base on ANSI/IEEE standard.

0.3B0.2

0.3 is indicating class accuracy of current transformer, with limit error between +/- 0.3%  and B0.2 is indicating a burden, with impedance value of 0.2 Ohm.

0.6B0.9

0.6 is indicating class accuracy of current transformer, with limit error between +/- 0.6%  and B0.9 is indicating a burden, with impedance value of 0.9 Ohm.

Secondly, burden can be expressed as form of apparent power VA at certain power factor in the rated secondary current.

This definition commonly used in IED60044-1 standard. In this standard, manufacture has to perform class accuracy using burden at certain value with power factor 0.8 lagging as shown of below statement:

For testing purposes when determining current error and phase displacement, the burden shall have a power-factor of 0.8 inductive except that, where the burden is less than 5 VA, a power factor of 1.0 is permissible.

Note:

Burden and accuracy class is 2 things are interrelated, so it is always defined simultaneously. The value of error ratio and phase displacement will shift from the pre-defined value that is used when the burden is larger or smaller than a predetermined limit burden.

As we know, the definition of the secondary circuit is a circuit which is connected to the secondary coil of the current transformer. Secondary circuit may include a cable, or measuring and protection equipment.
If illustrated in graphic form, the understanding of burden is shown in the below figure.

Thus, burden of current transformer are includes all components connected to the secondary coil of the current transformer such as meters or protective equipment, connectors, cable, indicator lights, and so forth. In the above example, the burden including the connecting cable impedance and measuring or protection equipment.

Since the resistance of connecting cable between the current transformer’ secondary terminal and measuring equipment or protective equipment greatly impact the performance of a current transformer, then some of the following should be noted.

• When measuring or protective equipment is located far away from the secondary terminal of current transformer, The secondary current should be used lower value, such as 1 ampere. Using a current transformer with a 5 ampere output rating for measuring devices located far away from the secondary terminals, will lead to a bigger  VA that is absorbed by the cable.
• To ensure accuracy in the range of values ​​corresponding to the results of the manufacturer, the value of the burden should be in the range of 25% to 100% of rated burden (for Current Transformer with standard IEC60044-1) or in the value of rated burden(ANSI / IEEE C57. 13).
• Use of a larger or smaller burden than the burden identifier (rated burden), will lead to the increasing of error ratio (ratio error and phase shift).
• Use of burden that is not suitable, it will also result a shifted  value of FS or ALF, which can cause damage to the measuring equipment or the protective current transformer will easily saturated.