How is the polarity of instrument transformers classified?

Polarity of instrument transformers is classified as subtractive when referring to how voltage and current measurements are related and how the transformers are connected within an electrical system. In instrument transformers, particularly current transformers and voltage transformers, the concept of polarity is crucial for ensuring accurate readings and proper functioning of protective devices.

Subtractive polarity means that the phase angle of the output signal is such that if two transformers are connected together, their outputs subtract from one another to maintain proper phase relationships. This is important to ensure that, for instance, in a metering application or protective relay setup, the readings are correct and reflect the true values of the system they are measuring.

The other classifications of polarities, such as positive and negative, do exist but they do not define the relationship of measuring phases between those transformations clearly concerning how they are used in practice. Therefore, the concept of subtractive polarity provides a clear understanding in contexts where accurate measurement, and proper phase alignment or connection is key to functionality within electrical systems.