Considerations When Selecting Current Transformers for Energy Monitoring

Selecting the appropriate current transformers (CTs) for energy monitoring is a crucial step in ensuring accurate data collection and effective energy management. This article delves into the considerations necessary for monitoring single and three-phase AC loads, exploring the different types of CTs available, their features, benefits, and potential drawbacks. This guide aims to assist professionals in making informed decisions regarding CT selection and installation.

Basic Principles of Operation

Current transformers (CTs) are devices used to measure alternating current (AC) by producing a reduced current proportional to the current in the circuit. This reduced current can be monitored and recorded, allowing for precise energy management and monitoring. CTs play a vital role in energy management systems (EMS), building energy management systems (BEMS), demand response (DR), distributed energy resources (DER), distributed energy resource management systems (DERMS), virtual power plants (VPP), and battery energy storage systems (BESS).

Types of CTs

1. Solid Core CTs: These CTs are known for their high accuracy and reliability. They consist of a solid ring of magnetic material through which the conductor is threaded. Due to their construction, they require the circuit to be disconnected for installation, which can be inconvenient in live systems.
2. Split Core CTs: These CTs can be opened and clamped around the conductor without the need to disconnect the circuit, making installation easier and less disruptive. However, they may offer slightly lower accuracy compared to solid core CTs.
3. Rogowski Coils: These are flexible coils that can be easily installed around large or irregular conductors. They require an integrator to produce an output similar to conventional CTs. Rogowski coils are ideal for high-current measurements but require careful consideration of shielding and calibration.

Physical Size and Maximum Rated Current

The physical size of the CT must be adequate to encompass the conductors without being excessively large, as this could affect measurement accuracy. Selecting a CT with an appropriate maximum rated current is crucial to ensure it can handle the load without saturation, which would distort measurements. The CT should fit snugly around the conductors (and in the case of Rogowski coils, the conductor should be centred in the coil) to avoid inaccuracies caused by excessive air gaps and orientation.

Installation Considerations

Selecting the appropriate CT involves balancing several factors:

• Installation Difficulty: Solid core CTs require circuit disconnection, while split core CTs and Rogowski coils offer easier installation without disrupting the circuit.
• Accuracy: Solid core CTs generally provide higher accuracy. However, high-quality split core CTs and Rogowski coils can also achieve acceptable accuracy levels for many applications.
• Physical Size: The CT should fit the available space without being too large, which could lead to installation challenges and potential inaccuracies.
• Cost: While solid core CTs are generally more accurate, they can be more expensive and difficult to install. Split core CTs and Rogowski coils offer a cost-effective and easier-to-install alternative.
• Maximum Rated Current: Ensure the CT can handle the maximum expected current without saturating.

How Many CTs Are Needed?

For single-phase loads, one CT is sufficient. For three-phase loads, the number of CTs needed depends on the connection method of the load. If the load is connected in a star (wye) configuration with a neutral wire, typically three CTs are required—one for each phase. For delta configurations without a neutral, only two CTs may be necessary to measure the phase currents, using vector summation to determine the third phase.

Surveying the Electrical Distribution Board

Before installing CTs, conduct a thorough survey of the electrical distribution board. Watch for:

1. Multiple Conductors per Phase: This is sometimes done to facilitate routing heavy cables and requires careful handling during installation.
2. Incorrect Marking/Insulation Colours: Particularly in older systems, phase markings might not be reliable. Use a phase rotation meter and multimeter to confirm phase identification and connections.

Voltage Considerations

Ensure that the CTs are rated for the voltage present in the metered location. Incorrect voltage ratings can lead to inaccurate readings and potential equipment damage. Verify that the CT insulation is adequate for the system voltage to ensure safety and reliability.

Shutdown Procedures

Plan for necessary shutdown procedures to install CTs correctly, considering potential disruption to site operations and following local electrical regulations, using only qualified personnel. Safety should be a priority, ensuring that installation follows proper electrical safety protocols while keeping downtime to a minimum.

Mounting and Stability

Proper mounting of CTs is essential to ensure they remain in the desired location on conductors and do not slip due to vibrations. Secure mounting ensures consistent and accurate readings over time. Use appropriate mounting hardware (e.g. cable ties) and techniques suitable for the specific type of CT and installation environment.

CT Output and Cabling Requirements

Installers should understand the CT ratio. For example, a 3000:1 ratio split core CT monitoring 100A on the primary will produce an output of 0.033A (33mA) on the secondary. The CT’s selected should be compatible with the metering hardware they’re connected to, and the appropriate CT ratio may need to be configured on the meter. Consider the cabling requirements, including the need for shielding with Rogowski coils, and the impact that extending or shortening CT cables can have on accuracy. Adjusting the CT cable to make them longer or shorter than the length they were calibrated at can introduce errors, so it’s essential to use cables as supplied or recalibrate if modifications are necessary. If threading CT cables through a gland or bushing, check that they can be disconnected from terminals at the CT side, and be careful not to mix up the CT’s when reconnecting.

Safety and Built-In Protection

Certain CTs can be hazardous when connected to a supply and left open circuit. Look for CTs with built-in protection to mitigate these risks and ensure safe operation. Alternatively, use shorting links and always follow the manufacturers recommendations for safe installation. CTs with voltage-limiting features or integrated protection circuits can prevent dangerous open-circuit conditions.

EpiSensor Solutions

If all of the above sounds too complicated, consider EpiSensor’s ZEM and ZDR wireless 3-phase electricity monitors. They’re designed to be really easy to install, while maintaining the highest standards of accuracy, reliability and security. Factory-calibrated to a system-level accuracy class of 0.5S, these systems include all necessary accessories and feature safe split core or Rogowski coils to accommodate various currents, voltages, and connection types in three-phase systems.

For a detailed guide on EpiSensor products and their applications, visit Our Product Page.