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. Total resistance combines secondary winding resistance and leads and relay resistance. 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 is the 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, a voltage drop across the secondary impedance is created due to the secondary current flowing through it.

This voltage drop can introduce errors in the secondary current measurement, 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

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

Substituting the values into the burden formula

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 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 is the effect of saturation on CT(current transformer)?

When a CT becomes saturated, its core cannot amplify the primary current accurately, 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

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

Using the turns ratio to find the corresponding primary current

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. For every 1000 amps on the primary side, the CT produces 5 amps on the secondary side. Therefore,

Step-3: Rated Secondary Current (Is)

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

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

where

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

Substituting the values, we get

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

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, below the saturation current. Therefore, the CT will not saturate under normal operating conditions.

Frequently Asked Questions – FAQs

What is the burden of a current transformer?

The burden of a current transformer is the total impedance connected to the secondary winding of the CT.

How is the burden measured?

The burden is measured in volt-amperes (VA) at a specified current.

Why is the burden important for CT operation?

The burden impacts the accuracy and performance of the CT. If the burden exceeds the CT's rated burden.

What are the burdens for different applications?

Burden ratings can vary based on application. For protective relays, burdens can range from a few VA to 30 VA or more. For metering applications, the burdens are generally lower below 10 VA.