The causes of zero-sequence current and the selection of treatment equipment

In the power system, zero-sequence current is an important and common phenomenon. Excessive zero-sequence current can cause additional heating of transformers and lines, deterioration of power quality, misoperation or malfunction of protection devices, and even lead to safety accidents. Therefore, a thorough understanding of its causes and the correct selection of control equipment are of vital importance for ensuring the safe, stable and efficient operation of the power grid.

I. Causes of Zero Sequence Current

1. Asymmetry of line parameters: Incomplete symmetry in the arrangement of overhead lines, manufacturing or laying errors of cables, will cause the capacitance to ground of the three-phase lines to be not exactly equal, thereby generating zero-sequence current.

2. Transformer excitation current: The excitation current of the transformer inherently contains a small amount of third harmonic components. When these components are superimposed at the neutral point, a zero-sequence current will be formed.

3. Equipment aging or failure: For instance, aging of cable insulation leads to an increase in leakage current to ground, or in the case of a single-phase grounding fault, a very large zero-sequence current will be generated (this is the signal that the protection device needs to act and remove the fault).

4. Excessive use of single-phase loads: In low-voltage power distribution systems, loads such as lighting, air conditioning, computers, and charging stations are all connected in single-phase. If the single-phase loads on the three phases are not evenly distributed, it will lead to an imbalance in the three-phase current, thereby generating zero-sequence current.

II. Selection of Governance Equipment

1. Zero-sequence current filter

Working principle: Usually composed of one or more reactors, they are connected in series between the phase line and the neutral line. It has a very low impedance to the power frequency (50Hz) current, but presents a high impedance to high-frequency zero-sequence harmonic currents (such as 150Hz, 250Hz), thereby preventing these harmonic currents from flowing to the transformer and the upstream power grid, and limiting them to the local load side.

2. Active Power Filter (APF)

Working principle: It can real-time detect the harmonic and reactive components (including zero-sequence components) in the load current, and then generate a compensating current of the same magnitude but opposite direction as these harmful currents through the power electronic converter and inject it into the system, thereby canceling them out. Modern active power filters generally have the function of simultaneously compensating for harmonics (2-25th and above), reactive power, and unbalanced currents.