In high-voltage networks, it is important that the high-quality components in the substation and other parts of the electricity grid are protected against overcurrent and short-circuit currents. In addition, it must be possible to settle the energy consumed (kWh) with the various customers with a high degree of accuracy. Both cases require measuring transformers. Measuring transformers convert the current and voltage in high voltage systems to values that can be measured by measuring instruments and protective equipment.
Measuring transformers is a collective name for current and voltage transformers while combinations of both types are also possible, the so-called combined measuring transformers. A current transformer transforms a large primary current into a galvanically isolated secondary, lower value and consists of one or more cores and corresponding secondary windings (active part). Different versions are available, depending on the application, but basically there are two versions, namely metering core and protection core. When protection cores are used, this is usually secondary with a 1A current because this results in lower losses in a cable than with a 5A core. The corresponding power and accuracy class is selected to suit the system. For instance, 5P20, which means that the current transformer has a maximum error of 5% at 20 times the rated current.
