A current transformer measures alternating current and converts it into a smaller signal that can be evaluated by an energy meter or measuring device. The term current clamp is generally used for clamp-on current sensors. In this article, it primarily refers to a clamp-on current transformer for AC measurements, also known as a split-core current transformer.

Current Transformers at a Glance

• Current transformers enable the measurement of high alternating currents that cannot be processed directly by an energy meter or measuring device.
• A current clamp can normally be installed around an existing conductor without disconnecting it.
• For complete phase-specific measurements, one current transformer is usually installed per phase.
• Calculating active power and energy additionally requires voltage measurements, correct phase assignment, and the phase angle.

How Does a Current Transformer Work?

A conventional current transformer operates according to the transformer principle. The conductor carrying the current forms the primary side. The magnetic field generated by the alternating current induces a smaller, proportional current signal on the secondary side. Industrial current transformers commonly provide a standardized secondary current of 1 A or 5 A.

The transformation ratio describes the relationship between the primary and secondary current. With a 500 A/5 A current transformer, for example, a primary current of 500 A corresponds to a secondary current of 5 A. The connected measuring device must be configured with this ratio to calculate the actual primary current correctly.

More compact current sensors may instead provide a mA or voltage signal. The signal type and measuring range must be compatible with the input of the energy meter or measuring device being used.

A current clamp has a split core and can be retrofitted around an existing conductor. Split-core current transformers therefore simplify installation in existing systems. Depending on their design and accuracy class, solid-core current transformers may offer greater measurement accuracy. However, installing them normally requires the conductor to be disconnected.

Rogowski coils are flexible current sensors for high or rapidly changing alternating currents. Their output signal depends on the rate of change of the current. They therefore require additional electronic signal processing or an integrator.

Conventional current transformers based on the transformer principle cannot measure constant direct current. Other sensor technologies, such as Hall-effect sensors, are required for DC or combined AC/DC measurements.

Current measurement alone is not sufficient to determine active power and energy. The measuring system must also measure the voltage and the phase angle between current and voltage. In multiphase systems, the correct assignment of current and voltage phases is also essential.

Where Are Current Transformers and Current Clamps Used?

Current transformers are used wherever electrical currents must be monitored or power and energy values calculated. Typical applications include:

• At the grid connection point, current transformers support the measurement of grid consumption and export.
• In sub-distribution boards or on individual machines, they provide transparency into energy distribution and the consumption of specific system areas.
• In PV systems and BESS, they are used to determine generation, charging, discharging, and grid power.
• In EMS applications, the calculated measurements provide feedback values for self-consumption optimization, peak shaving, or export limitation.

Measurement Accuracy, Installation, and Technical Limitations

The measuring range and accuracy class must be suitable for the specific application. A significantly oversized current transformer may measure low currents inaccurately. At the same time, the expected maximum current must not exceed the intended measuring range, as this may result in measurement deviations or transformer saturation.

For complete phase-specific measurements, one current transformer is usually installed per phase. Certain measurement methods in three-wire systems may deviate from this approach and operate with two current transformers.

For operating-current and power measurements, the clamp may only enclose the individual conductor being measured. If the outgoing and return conductors are measured together, their magnetic fields may cancel each other out. For residual current measurements, by contrast, several live conductors are deliberately routed through a single sensor to detect possible fault or leakage currents.

The installation direction is also important. Many current transformers have an arrow or markings such as P1/P2 or K/L to indicate the intended direction. Reversing the installation direction can produce incorrect signs and consequently interchange grid consumption and export.

For conventional current transformers with a current output, the secondary circuit must not be improperly opened while primary current is flowing. This can generate dangerously high voltages. Installation, testing, and modifications must therefore be performed by qualified electrical professionals. The manufacturer’s instructions for the current transformer and measuring device must also be followed.

Processing Measurement Data with EcoPhi

EcoPhi can collect, store, and visualize measurement data from compatible energy meters and measuring devices. Current transformers are normally connected to the respective measuring device. The device processes the input signals and provides the calculated current, power, and energy values through a communication interface.

In an EMS, these measurements can serve as feedback values for control algorithms. For example, the controller can compare the measured power at the grid connection point with a defined setpoint or limit and adjust connected PV inverters, BESS, or other controllable assets accordingly.

The specific implementation depends on the measuring hardware, available interfaces, update rate, and required measurement quality. Depending on the project, specific configuration or additional integration services may be required.

Current Transformers in Energy Management

Current transformers make high alternating currents measurable for monitoring systems. Clamp-on versions can often be retrofitted without disconnecting the conductor. Reliable power and energy measurements require appropriate dimensioning, correct installation, and additional voltage measurement.

Frequently Asked Questions About Current Transformers

What Is the Difference Between a Current Transformer and a Current Clamp?

A current transformer is a measuring device that converts alternating current into a smaller proportional signal. Current clamp is a broader English term for clamp-on current sensors. These include split-core current transformers and, depending on the design, sensors based on other measurement principles.

Can Current Transformers Measure Direct Current?

Conventional current transformers based on the transformer principle cannot measure constant direct current. DC measurements require Hall-effect sensors or other suitable measurement technologies.

Can a Current Transformer Measure Active Power Directly?

No. A current transformer initially provides a current-dependent signal. To calculate active power, the measuring device also requires voltage measurements, correct phase assignment, and the phase angle.

What Does the Rating 500 A/5 A Mean?

This rating specifies the transformation ratio. At a primary current of 500 A, the current transformer provides a secondary current of 5 A under its specified operating conditions.