Answer:

A balanced 3-phase system consists of positive sequence currents only. This means that the three phases of the system have equal magnitudes and are 120 degrees apart from each other in terms of phase angle. The positive sequence currents represent the normal operating conditions of a 3-phase system.

Explanation:

To understand why a balanced 3-phase system consists of positive sequence currents only, let's first understand what positive, negative, and zero sequence currents are.

Positive Sequence Currents:
- In a balanced 3-phase system, the positive sequence currents are the currents that flow in the same sequence and magnitude in all three phases.
- These currents have a phase angle of 120 degrees between each other.
- The positive sequence currents represent the normal operating conditions of a 3-phase system.

Negative Sequence Currents:
- The negative sequence currents are the currents that flow in the opposite sequence as the positive sequence currents.
- These currents have a phase angle of -120 degrees between each other.
- Negative sequence currents can occur due to imbalances in the system, such as unbalanced loads or faults.

Zero Sequence Currents:
- The zero sequence currents are the currents that flow in all three phases in the same direction and at the same magnitude.
- These currents have a phase angle of 0 degrees between each other.
- Zero sequence currents can occur due to imbalances in the system, such as unbalanced loads or faults.

Why a balanced 3-phase system consists of positive sequence currents only:
- A balanced 3-phase system is designed to have equal magnitudes and 120 degrees phase angle separation between the three phases.
- Under normal operating conditions, the loads in a balanced 3-phase system are also balanced, meaning they draw equal currents from each phase.
- When the loads are balanced and the system is operating normally, there are no imbalances that would cause the flow of negative or zero sequence currents.
- Therefore, in a balanced 3-phase system, only positive sequence currents are present.

In conclusion, a balanced 3-phase system consists of positive sequence currents only because under normal operating conditions, the loads are balanced, and there are no imbalances that would cause the flow of negative or zero sequence currents.