Role of Current Transformers

Current transformers (CTs) are widely used in power systems. Their role is to induce a proportional smaller current from high-current cables for metering and relay protection purposes.

Upon opening a distribution panel, one can observe multiple current transformers inside. Some panels may contain only one CT, while others might have five or six. They all function by sensing current signals from the busbar; however, the applications of the measured currents vary. What roles do these CTs play inside the distribution panel?

Single Current Transformer

When there is only one CT, it is typically used to sense the current from a single line. Applications include extending the range of a single-phase meter, measuring single-phase current, measuring balanced three-phase current, and sampling for reactive power controllers.

1. Extending the range of a single-phase meter involves connecting one CT to a single-phase meter.

2. Measuring single-phase current involves connecting one CT to an ammeter to measure the current in a single-phase line.

3. Measuring balanced three-phase current is similar to measuring single-phase current since the currents in a balanced three-phase line are nearly equal.

4. Sampling for reactive power controllers aims to obtain current magnitude and phase information from the busbar through the CT.

Two Current Transformers

If two CTs are installed on the same phase busbar, their function is similar to that of a single CT. If two CTs are installed on different phase busbars, their primary purpose is to measure the currents in a balanced or unbalanced three-phase system.

When the three-phase system is unbalanced, the sum of the three-phase current vectors is zero. Knowing the currents in two phases allows calculating the current in the third phase.

Three Current Transformers

Using three CTs is mainly for measuring balanced and unbalanced three-phase currents. They can also be connected to three-phase metering systems to measure consumed energy or to thermal relays to extend their operating range.

Multiple Current Transformers

For systems with multiple CTs, a comprehensive analysis corresponding to the above scenarios will suffice.