What is a Current Transformer
A current transformer (CT) is an electrical component that measures alternating current (AC) in a power system, by utilizing a step-down process where high alternating current flowing through a conductor is reduced to a safer level. Depending on the application, a metering device, protective relay, data logger, or SCADA system can be paired to measure the output current, which can be used to calculate the input current for monitoring, billing, or equipment protection purposes.
This article decodes the step-down method and the principle behind how current transformers operate by deconstructing it down to fundamental parts.
How Does a Current Transformer Work
The primary function of a current transformer is to reduce alternating current to a more manageable level. While CTs may come in different form factors, it typically has a structure similar to the diagram below.
It consists of a primary and a secondary copper winding around a core made of ferromagnetic material such as silicon, steel, or iron. The primary winding is connected directly to an electric current-carrying conductor that flows through the primary winding, generating a magnetic flux in the core. The core efficiently concentrates the magnetic flux, inducing a current in the secondary winding, referred to as the secondary current.
This can be visualized more simply with two fans facing each other. By turning one of them on, it propels air current (magnetic flux), inducing the second fan to start spinning (creating a current). The speed at which the second fan spins depend on the strength of that breeze, which depend on the speed of the first fan (primary current). This is essentially what is happening inside a current transformer.
How Does Step-Down Method Work
The relationship between the primary and secondary current is an important factor to step-down current. According to Ampere’s law and Faraday’s law, the secondary current is directly proportional to the primary current.
This relation between the primary (Ip) and secondary current (Is) can be expressed with the following equation:
Where Np is the number of turns on the primary winding and Ns is the number of turns on the secondary winding. The ratio between Np and Ns is referred to as the turns ratio.
A typical current transformer has a turns ratio of less than 1, meaning the primary has fewer turns than the secondary winding so that the current, Is can be stepped down.
Three Types of Current Transformers
In order to suit specific applications, different variants of the CTs were created. These variants can be classified into three categories based on their form factors: wound current transformer, toroidal current transformer, and bar current transformer.
Wound Current Transformer
A wound current transformer has separate primary and secondary windings wrapped around a core. It is designed so the primary side consists of a winding with multiple turns. This design allows for precise control over the transformation ratio as both Ns and Np are customizable. It is also the standard form of a current transformer.
Toroidal Current Transformer
A toroidal current transformer is a variant of the wound current transformer with the primary side removed. This type of current transformer is shaped like a ring, allowing a current-carrying conductor to pass through. This conductor will act as the primary winding, and in this case, Np is fixed to 1.
Bar Current Transformer
The bar current transformer works just like a toroidal current transformer, except the current-carrying conductor passing through its secondary winding is always fixed, and not removable. The bar current transformer is the most ideal for high current measurement.
Split-Core CTs vs. Solid-Core CTs
In practice, toroidal current transformers are the most common, as they do not require a primary winding, which results in a much simpler installation process. However, as versatile as it is, it still requires the operator to terminate the live circuit to rewire the primary conductor through the CT core. This will disrupt the normal operation of the facility. Split-core current transformers were invented to solve this problem.
A split-core current transformer has its core made of two or more segments, which can be easily separated and reattached.
The split-core design brings many advantages. For example, this design approach allows the current transformer to be installed around existing live conductors without disrupting the system’s normal operation. This flexibility makes it suitable for a wide range of applications, especially where non-intrusive installations are required.
Accuenergy offers several models of split-core current transformers that are highly precise for revenue-grade measurement, while also flexible enough to be installed without interrupting the existing power system.
On the other hand, a more traditional solid-core current transformer uses a single, continuous piece of magnetic core, providing a major advantage in its accuracy and stability.
This occurrence derives from the design of split-core current transformers with a hinge mechanism to achieve a non-invasive installation form factor, leaving a slight air gap on the core, as shown in the diagram below:
The air gap has a different permeability compared to the core, which is a constant describing how easily a material can be magnetized. This difference in permeability will cause magnetic fringing, a phenomenon that causes the magnetic fields to deviate from its ideal pathway, leading to slightly lower accuracy.
A solid-core CT, on the other hand, has a single continuous core design, allowing no air gaps and no loss in accuracy.
Current Transformers Accuracy Classes
Current transformers can be divided into different classes based on their precision levels. According to the IEC 61869-2 standard, which is an international standard that provides detailed specifications for current transformers, the standard CT precision classes are 0.1, 0.2, 0.5, 1, 3, 5, 0.1S, 0.2S, and 0.5S.
The following plot illustrates the error curve for IEC 61869-2 Class 0.5:
The horizontal axis indicates the percentage of rated current, and the vertical axis represents the ratio error. Class 0.5 specifies that when the primary current is 100% of the rated current or above, the ratio error must be within 0.5%.
IEC 61869-2 Class 0.5S is a stricter version of Class 0.5 with higher requirements at the lower current levels.
When a current transformer is compliant with IEC 61869-2 Class 0.5 or above, it is referred to as a revenue-grade current transformer designated for billing purposes.
Another factor that affects the accuracy of a current transformer is phase displacement, which refers to the phase angle differences between the primary and secondary current.
The IEC 61869-2 standard also contains specifications for phase displacements. For instance, the above plot illustrates the phase displacement requirements for IEC 61869-2 Class 0.5.
Real-Life Applications of Current Transformers
Current transformers have a wide range of applications in electrical systems. For instance, they are often paired with metering devices to measure high alternating current, commonly used for energy and power monitoring, industrial automation, and renewable energy systems.
Traditionally, current transformers had a standard output of either 1A or 5A. With the emergence of solar and wind power in recent years, current transformer adoption with a 333mV output have increased, becoming a standard that offers a few advantages.
The 333mV current transformers are safer due to a built-in resistor that converts current signals into voltage signals. As a result, they don’t require the secondary to be grounded, making it much easier to install. Most importantly, they are more cost-effective while offering superior accuracy due to their unique internal components and windings.
Besides metering, current transformers are also used for protection by detecting unexpected current surges and allowing the system to step down to a safe level. The output is connected to protective relays that is responsible for disconnecting the downstream circuit. These CTs must handle at least 20 times of their rated current without malfunctioning.
When installing a current transformer, there are a few items electrical technicians need to be aware of. The most important is safety: make sure the CT can handle the input current, and if it needs to be grounded, the secondary must be connected to the ground.
There are a few other factors that may affect the performance of a current transformer. For instance, check its physical size and the rated current. The CT can only remain within the designed accuracy level when the input current is 5% to 120% of its rated current. Outside of this range, its performance cannot be guaranteed. Visit our other articles for more information and tips on how to properly install current transformers.