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Burden and saturation of CT(Current Transformer)-Electronicsinfos

What is a Current Transformer?

Burden and saturation of CT(Current Transformer)-Electronicsinfos

A Current Transformer (CT) is an instrument transformer used to measure alternating current (AC) in electrical systems. It consists of a primary winding, which carries the current to be measured, and a secondary winding which generates a proportionally reduced current.

Burden Vs Saturation

Current transformers (CTs) are used to measure the electrical currents which provide accurate secondary current output. In this article, we discuss the Burden, Saturation and curve of transformer saturation. 

Burden of CT(Current Transformer)

The burden of a current transformer is the impedance to its secondary winding. A Burden is also defined as the total secondary resistance of the current transformer. The Total resistance is a combination of resistance of secondary winding and resistance of leads and relays. The burden is expressed in ohms or volt-amperes (VA).A Maximum Burden of C.T Depending on the Turn Ratio, the Current rating of C.T 

What Effect of Burden on CT (current Transformer)?

The burden affects the performance of the CT and its accuracy. When a CT is connected to a load, it creates a voltage drop across the secondary impedance due to the secondary current flowing through it. This voltage drop can introduce errors in the measurement of the secondary current, mainly if the burden impedance is not within the specified limits of the CT. It is important to ensure that the burden impedance is within the acceptable range specified by the CT manufacturer. If the burden impedance is too low, it can result in excessive secondary current and potentially cause CT saturation. On the other hand, if the burden impedance is too high, it can cause excessive voltage drop and result in inaccurate measurements.

Burden Calculation of Current Transformer

Let's consider a current transformer with the following details

  • Primary Current Rating (Ip): 1000A
  • Turns Ratio (N): 1000:5 (Primary: Secondary)
  •  Burden (B): 5 VA (Volt-Amps)

   The burden of a CT is calculated using the formula

  

Burden Calculation of Current Transformer

 where

  •    (Is) is the secondary current in amps.
  •    (Vs) is the secondary voltage in volts.

 Using the turns ratio, we can find the secondary current

 

Burden Calculation of Current Transformer

Substituting the values into the burden formula

Burden Calculation of Current Transformer

Therefore, if the secondary current is 1A, the secondary voltage ( Vs ) required to achieve a 5 VA burden is 5000 volts.

Cautions

Properly matching the burden impedance to the CT's specifications is crucial for accurate current measurement. It ensures that the CT operates within its optimal range and minimizes errors caused by burden-induced voltage drops.

Burden ratings for IEC CTs 

  • 1.5 VA
  • 3 VA
  • 5 VA
  • 10 VA
  • 15 VA
  • 20 VA
  • 30 VA
  • 45 VA
  • 60 VA

The burden rating of ANSI/IEEE C.T

  • B-0.1 
  • B-0.2 
  • B-0.5 
  • B-1.0 
  • B-2.0  
  • B-4.0

Saturation of Current Transformer(C.T)

Saturation is a phenomenon that occurs when the primary current of a current transformer exceeds its rated limit which causes distortion in the secondary current waveform. Saturation occurs when the magnetic core of the CT reaches its magnetic saturation point. When a CT is saturated, its core is unable to magnetically amplify the primary current accurately, leading to distorted secondary current readings. Saturation can occur due to high fault currents, transient overloads, or improper selection of CTs for the specific application.

What Effect of Saturation on CT(current Transformer)?

When a CT becomes saturated, its core cannot accurately amplify the primary current, resulting in distorted secondary current readings. Saturation can be triggered by high fault currents, transient overloads, or improper selection of CTs. Saturation affects the accuracy and performance of the CT.

Transformer saturation Curve of Current Transformer

Transformer saturation Curve of Current Transformer

Saturation Calculation of Current Transformer

Saturation in a CT occurs when the core becomes magnetically saturated due to excessive primary current, leading to inaccurate readings. 

Suppose we have a CT with the following details

  • Saturation Current (Isat): 1200A (maximum current before saturation)
  • Primary Current Rating (Ip): 1000A
  • Turns Ratio (N): 1000:5 (Primary: Secondary)

To calculate the primary current that would cause saturation, we use the turns ratio

Saturation Calculation of Current Transformer

Using the turns ratio to find the corresponding primary current

Saturation Calculation of Current Transformer

Therefore, when the primary current reaches 1200A, the CT will saturate and may no longer provide accurate measurements.

How to find Current Transformer Performance Calculation?

Let's work through an example to calculate the performance of a current transformer (CT) using the provided parameters

Step-1 : Assumptions

  • Turns Ratio (N): 1000:5 (Primary: Secondary)
  • Rated Primary Current (Ip): 1000A
  • Burden (B): 5 VA
  • Saturation Current (Isat): 1200A
  • Accuracy Class: Class 1

Step-2: Turns Ratio Calculation

The turns ratio ( N ) is given as 1000:5. This means that for every 1000 amps on the primary side, the CT produces 5 amps on the secondary side. Therefore, 

How to find Current Transformer Performance Calculation?

Step-3: Rated Secondary Current (Is)

Since ( N = 200 ), the rated secondary current ( Is ) is calculated as 


How to find Current Transformer Performance Calculation?

Step-4: Burden Calculation

The burden ( B ) is given as 5 VA. This represents the maximum load impedance the CT can handle without affecting accuracy. The burden formula is

How to find Current Transformer Performance Calculation?

 where 

  • ( Is ) is the secondary current and 
  • ( Vs ) is the secondary voltage. 

Substituting the values, we get 


How to find Current Transformer Performance Calculation?

Solving for ( Vs ), we find ( Vs = 1000V ).

Step-5: Accuracy Calculation

The accuracy class is Class 1, which means the CT provides an accuracy of ±1%. For a rated secondary current of 5A, the accuracy can be calculated as 

How to find Current Transformer Performance Calculation?

Step-6: Saturation Analysis

The saturation current ( Isat ) is given as 1200A. This represents the maximum primary current before the CT becomes saturated.

In our case, the rated primary current ( Ip ) is 1000A, which is below the saturation current. Therefore, the CT will not saturate under normal operating conditions.

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EMF Equation Of Electrical Transformer

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