Is the zero-sequence current related to the grounding method of the neutral point?

In power system analysis and protection, zero sequence current is a core concept that is closely related to the grounding method of the system. A clear answer is: Zero sequence current is not only related to the grounding method of the neutral point, but also its magnitude, path, and even its existence are all completely determined by the grounding method. Understanding the relationship between the two is the key to mastering fault analysis in power systems, relay protection configuration, and power quality management.

I. Core Concept: What is Zero Sequence Current?

Zero sequence current is not an ordinary load current; rather, it is a special current component generated when the system experiences an asymmetric fault (such as single-phase grounding) or three-phase imbalance. Using the symmetrical component method, any three-phase asymmetric system can be decomposed into three symmetrical systems: positive sequence, negative sequence, and zero sequence.

II. How is the neutral point grounding method determined for the zero sequence current?

The neutral point grounding method directly defines whether the zero-sequence current can flow smoothly and the magnitude of the path impedance, thereby determining the amplitude of the zero-sequence current. It can be mainly divided into two categories:

⑴ Neutral point effectively grounded system (high grounding current system)

Method: It includes direct grounding of the neutral point or grounding through a small impedance.

Zero sequence path characteristics: It provides a low-impedance path for zero sequence current (to flow back to the transformer neutral point through the earth).

Impact on zero-sequence current:

When a single-phase grounding fault occurs, the faulty phase will generate a huge zero-sequence current.

Due to the small loop impedance, the current amplitude is very large (up to several thousand amperes), which is of the same order of magnitude as the phase short-circuit current.

(2) Neutral point non-effectively grounded system (small grounding current system)

Method: It includes neutral point ungrounded or grounded through an arc suppression coil.

Zero sequence path characteristics: It provides a high-impedance path or an incomplete path for zero sequence current to flow through.

Impact on zero-sequence current:

- Neutral point not grounded: There is no direct electrical connection between the system and the ground. When a single-phase grounding occurs, the grounding capacitance current is very small (only the algebraic sum of the system's ground capacitance currents), with a very small value (usually from a few amperes to several tens of amperes), and no large arc is generated at the fault point. The system's line voltage remains symmetrical and the system can operate with faults for 1-2 hours.

- Grounded through an arc suppression coil: The arc suppression coil is an inductive coil. When a single-phase grounding occurs, the inductive current generated by it compensates (neutralizes) with the capacitive current of the system to ground, making the residual current at the grounding point very small. The arc can extinguish by itself, and the system can also operate with a fault for a short period of time.