Zero sequence current, known as residual current or triplen harmonic current, is a phenomenon that occurs in electrical systems due to the presence of unbalanced loads or nonlinear loads.

The primary reason for the presence of zero current is unbalanced loads. In a three-phase power system, the loads are ideally balanced, meaning that the three phases draw equal currents. However, in practical scenarios, the loads may be unbalanced due to variations in impedance, different connected loads, or asymmetrical distribution of single-phase loads. When such unbalanced loads are present, a zero sequence current flows in the system.

Additionally, nonlinear loads such as rectifiers, adjustable-speed drives, electronic equipment, or certain types of lighting can also contribute to the zero sequence current. These loads draw non-sinusoidal currents, and the harmonic components in these currents can interact with the system's impedance to create zero sequence currents.

Zero sequence currents can have several negative effects on power systems, including:

Increased neutral current: The presence of zero sequence current increases the current flowing in the neutral conductor, which can lead to overheating and increased losses in the neutral conductor.

Voltage distortion: Zero sequence currents can cause voltage distortion in the system, leading to harmonic voltage drops and potential issues with sensitive equipment.

Unbalanced magnetic fields: Zero sequence currents can create unbalanced magnetic fields in transformers and rotating machinery, causing increased losses, mechanical vibrations, and reduced equipment lifespan.

To mitigate the negative effects of zero sequence currents, measures such as proper load balancing, installation of harmonic filters, and employing symmetrical and balanced system designs are often implemented.