Self calibration by signal injection

A system for self-calibration of a current transformer (CT) comprising:

• a ferromagnetic core adapted to accommodate a current conducting wire passed therethrough, wherein the current conducting wire acts as a primary coil of the current transformer;
• a secondary coil wound around the ferromagnetic core, the secondary coil having a first connection point and a second connection point;
• a reference coil wound around the ferromagnetic core, the reference coil having a first connection point and a second connection point;
• an analog-to-digital converter (ADC),
• an analog front end (AFE), the analog front end coupling the ADC to a representation of an analog signal appearing across the first and second connection points of the secondary coil and the ADC providing as an output a digital signal representative of the analog signal appearing across the first and second connection points of the secondary coil;
• a digital-to-analog converter (DAC) providing an analog signal as an output in response to a digital signal received as an input;
• and
• a filter electrically coupling the analog output signal of the DAC to the first and second connection points of the reference coil, wherein the analog signal provided by the DAC passes from the DAC through the filter and the resulting filtered version of the analog signal provided by the DAC is in turn supplied between the first connection point and the second connection point of the reference coil;
• wherein the system is configured to determine, based on the digital signal generated by the ADC and responsive at least in part to a signal generated by the DAC in response to a digital version of a reference signal, whether there is at least one CT calibration parameter that is different by a predetermined threshold value from a previously stored version of that CT calibration parameter and if so updating at least one CT calibration parameter to a value that will reduce a measurement error caused by the CT when measuring a current flowing through the primary coil of the CT.