What is current transformer – Definition, working principle, how to choose

Current Transformer Definition

A current transformer (CT) is a type of instrument transformer that is used to measure the electric current flowing through a conductor. It is commonly used in power systems and electrical equipment to monitor and measure the current flowing through a circuit without du physically disconnecting.

A CT operates on the principle of electromagnetic induction. It consists of a primary winding, which is connected in series with the conductor carrying the current to be measured, and a secondary winding. The secondary winding is wound on a magnetic core and is connected to a measuring device, such as a meter or a protective relay.

When current flows through the primary winding, it creates a magnetic field around the conductor, which induces a proportional current in the secondary winding. The secondary winding current is then measured by the connected measuring device, which provides a scaled current representation of the the primary conductor.

CTs are designed to be highly accurate, with errors typically less than 1%. They are widely used for a variety of applications, including power system protection, metering, and monitoring of power quality.

Current Transformer Working Principle

A current transformer (CT) is an instrument transformer that works based on the principle of electromagnetic induction. It is used to measure the current flowing in a power system circuit or an electrical device without disconnecting the conductor.

A CT typically consists of a primary winding, a secondary winding, and a magnetic core. The primary winding is connected in series with the conductor carrying the current to be measured, while the secondary winding is connected to a measuring device or protective relay.

When current flows through the primary winding, it creates a magnetic field around the conductor. This magnetic field induces a current in the secondary winding according to Faraday's law of electromagnetic induction. flowing through the primary winding and the turns ratio of the transformer.

The turns ratio of a CT is defined as the ratio of the number of turns in the secondary winding to the number of turns in the primary winding. The secondary winding of a CT typically has a large number of turns compared to the primary winding, which enables the CT to step down the current to a level that can be safely measured by a measuring device or protective relay.

The output of a CT is typically connected to a measuring device or protective relay that is calibrated to provide an accurate reading of the current flowing through the primary conductor. The measuring device or protective relay uses the output signal from the CT to trigger alarms, control switches, or display the current value.

CT is a critical component of many power system protection and control schemes, providing accurate current measurement and enabling effective protection of power system equipment.

What is burden in current transformer?

Burden in a current transformer (CT) refers to the electrical load that is connected to the secondary winding of the CT. The burden includes any external resistance, capacity, or inductance connected to the secondary winding. The burden is important because it can affect the The accuracy and performance of the CT, particularly at low currents.

The burden of a CT is typically expressed in terms of its impedance, which is the complex ratio of the voltage across the secondary winding to the current flowing through the burden. The impedance of the burden can be measured using a variety of techniques, including the voltage method and the current injection method.

It's important to note that the burden of a CT should be carefully selected to ensure that it is compatible with the CT's characteristics, particularly its saturation and accuracy limits. If the burden is too large, it can cause the CT to become saturated, leading to distorted current measurements. If the burden is too small, it can cause the output voltage of the CT to rise excessively, leading to inaccurate current measurements.

The manufacturer's specifications for a CT will typically provide information on the maximum burden that can be connected to the CT without affecting its accuracy or performance. It's important to follow these guidelines when selecting a burden for a CT to ensure that the CT operates correctly and provided accurate current measurements.

How to select current transformer?

To select a current transformer, you need to consider several factors, including the primary current range, the secondary current rating, accuracy class, burden, and application requirements. Here are the general steps to follow when selecting a current transformer:

Determine the maximum primary current: The first step in selecting a current transformer is to determine the maximum primary current that the transformer will be expected to measure. This value should be based on the maximum expected current in the circuit, plus a safety margin.

Select a suitable transformation ratio: Once you know the maximum primary current, you can select a suitable transformation ratio that will provide an appropriate secondary current rating. The transformation ratio is the ratio of primary current to secondary current, and it determines the current that the transformer will output.

Choose an accuracy class: Current transformers are available in different accuracy classes, ranging from 0.1 to 5. The accuracy class determines the maximum allowable error in the output current compared to the actual current. The choice of accuracy class depends on the application requirements level of accuracy required.

Consider the burden: The burden is the total load that the current transformer must drive, including the connected metering or protection devices. It's important to select a current transformer that can handle the expected burden without causing excessive voltage drops or other issues.

Consider the application requirements: Finally, you need to consider any other application requirements, such as the operating environment, mounting options, and electrical connections.

It's important to note that selecting the right current transformer is critical for accurate and reliable measurement or protection. If you are unsure about the selection process or have specific application requirements, it's recommended to consult with a qualified Delixi electrical engineer for assist.

LB6-33(W) Oil-immersed Type Current Potential Transformer  

LB6-110kV Oil-immersed Type Current Transformer