Understanding Split Core Current Transformer Design, Operation, & Applications

Introduction

Current transformers are fundamental components in power systems used for measuring current signals accurately. Split core current transformers are a notable variant that feature a design where the core is easily opened and closed around a conductor without the need for disconnecting the circuit. This article aims to provide a comprehensive understanding of split core current transformers and clearly explain their principles of operation, design considerations, comparative advantages and disadvantages, and applications.
 

What is a Current Transformer?

Current transformers (CTs) work on the principle of electromagnetic induction which involves the generation of a magnetic flux in the transformer’s core by a primary current. This magnetic flux then induces a proportional current in the secondary winding which enables safe measurement of the primary current. The core, typically made of high-permeability materials like silicon steel or ferrite, channels the magnetic flux efficiently, and the secondary winding connects to a power analyzer to measure current. The accuracy of a measurement depends on the core material, turns ratio, and the power analyzer.

Figure 2. Illustration of a solid core CT in operation
 

What are the Design Constraints of Solid Core Current Transfomers?

The primary constraint of a solid core CT is that its installation requires disconnecting the primary conductor to thread it through the CT’s core. Labor-intensive and time-consuming, this often requires a complete shutdown of the electrical system, which is impractical for continuous and/or critical operations. Additionally, the installation space demands ample room to accommodate the CT’s solid form, which is a challenge in tight environments. Despite their high accuracy and reliability, these constraints limit the use of solid core CTs in certain scenarios. For example, in electric vehicle testing facilities (where continuous operation is crucial for evaluating vehicle performance and safety), the need to disconnect primary conductors to install solid core CTs can lead to significant downtime. This interruption in testing processes disrupts research schedules and delays product development timelines. And because EVs often have complex electrical systems with limited access points, the rigid structure of solid core CTs makes finding suitable installation locations difficult and may necessitate modifications to the vehicle's wiring.
 

What is a Split Core Current Transformer?

Split core CTs incorporate a unique design that allows the core to be opened and closed around a conductor without interrupting the circuit. This hinged-core mechanism facilitates easy installation and removal and makes them ideal for retrofit applications and temporary setups. Due to the hinged mechanism, air gaps are introduced into the core. However, proper alignment and secure clamping minimize these air gaps, reducing disruptions to the magnetic path and their impact on accuracy. High-quality split core CTs feature precision clamps and latching mechanisms that address these issues.

Once the core is closed, the split core CT operates similarly to a solid core CT; the primary current induces a magnetic flux in the core, which in turn induces a proportional current in the secondary winding for measurement or monitoring. The secondary winding then connects to a power analyzer. Be sure the analyzer is properly connected and has the appropriate rating for the application, as these are essential for maintaining accuracy. Although air gaps with split core CTs can result in slightly lower accuracy compared to solid core CTs, regular calibration and maintenance helps ensure reliable performance.

Figure 2. Illustration of a split core CT (with characteristic air gaps) in operation
 

What is a Current Transducer?

The interaction between an alternating current and a magnetic field limits CTs to only capturing AC signals. To accommodate this, another current sensing product called a current transducer may incorporate fluxgate sensors that enable DC signal capture. Current transducers differ from CTs in that they can output voltage, current, frequency, or digital signals that correspond to the primary current instead of solely stepping the signal down.

Fluxgate sensors (which can integrate with a transformer to become a transducer) are composed of a magnetic core and a wound coil. These sensors are driven by an external periodic square wave signal that alternates the magnetic core through its B-H curve and causes it to enter and exit saturation and results in a symmetric current waveform under normal conditions. When an external magnetic field (like the primary current passing through the CT's aperture) is present, the flux density within the core changes. This alters the time required to reach saturation and causes the current waveform to become asymmetric. The asymmetry in the waveform can be measured by analyzing changes in the duty cycle or through harmonic analysis, which produces a linear output that corresponds to the primary DC current.

The sensors can be installed in a CT's air gap or constructed using a secondary magnetic core embedded inside the CT itself. This flexibility allows for a wide range of applications and makes fluxgate sensor-equipped current transducers a versatile choice for accurate current measurement in various electrical systems.

Figure 3. Fluxgate sensor
 

Table 1. Advantages and disadvantages of split current CTs

Considerations When Selecting a Current Transformer

1. Based on the application's precision requirements, does the CT provide the necessary accuracy?
2. Can the CT maintain linearity across the expected current range to provide consistent measurements?
3. Does the primary current rating match or exceed the maximum current in the circuit?
4. Is the frequency response suitable for the application, especially when dealing with harmonics or high-frequency components?
5. Is the CT’s aperture large enough to accommodate the primary conductor(s)?
6. Is it possible to disconnect and run the wire through a solid core CT or clamp on to an already configured system using a split core CT?
7. What space is available for mounting and what are the options for a secure and proper fit?
8. Does the secondary output match the requirements of connected measurement devices or protective relays?
9. To maintain accuracy, does the precision of the power analyzer in use align with that of the CT?
10. While higher accuracy and additional features can offer long-term benefits, will the initial increase in costs balance out with requirements?
11. What is the CT's availability and lead time, especially for large projects or critical installations?
     

Applications

Table 2. Example applications using split current CTs
 

Comparison of Current Sensors

Table 3. Comparison of different types of CTs
 

Conclusion

Split core current transformers offer a convenient and versatile solution for current measurement applications, with their unique design allowing for easy installation and retrofitting without interrupting the circuit. While they may have some limitations compared to other CT technologies, split core CTs are widely used across various industries for power monitoring, building automation, renewable energy, and equipment monitoring applications. Understanding the principles of operation, design considerations, advantages, disadvantages, and applications of split core CTs is essential for engineers and professionals involved in electrical engineering and power systems.