Basic principle of transformer measurement

The use of energy meters in photovoltaic systems for dynamic in-feed control or for a battery storage system is meanwhile nothing new. In larger systems, however, it is often not possible to connect the normal meters directly, as these could be destroyed by the sometimes very high currents.

Basic principle of current transformer

The so-called measuring conversion is utilised here. In a measuring transformer meter, the current flow is not recorded directly by the meter but instead via an external current transformer (CT).

The mode of operation here is really simple:

An electric circuit flowed through with current always generates a magnetic field. In current transformers, a so-called “secondary current flow” is induced by this magnetic field, which – depending on the design – is smaller by a specified multiple than the output current (conversion or transformation ratio). The smart meter only records the significantly smaller secondary current and then converts this to the original primary current via the conversion ratio. The simple structure of the transformers is illustrated clearly below.

An example:

Current transformer from ABB
Primary current: 200A
Secondary current: 5A
Conversion ratio: 200:5 or 40:1

Would typically mean:
If the smart meter records 2A as secondary current, 80A must flow in the primary current circuit.

This therefore makes it possible to measure very high currents without any danger.

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The designation of the CTs indicates the amperage for which the measuring transformer is approved. This value should not be exceeded, as the measuring results would otherwise be increasingly inaccurate.