1. Three-phase sinusoidal symmetrical alternating current, when the three-phase symmetrical load is balanced load, because the current vector is equal, the direction is 120 degrees different from each other, and the vector sum is zero, that is, the zero sequence current is zero.
2. Three-phase sinusoidal symmetrical alternating current, when the three-phase asymmetric load is unbalanced load, because the current vector is not equal, the direction is not all 120 degrees, and its vector sum is not zero, that is, its zero sequence current is unbalanced current, but it is less than any phase current.
3. Three-phase sinusoidal symmetrical AC power supply, due to the existence of nonlinear components such as diodes in the three-phase load, when there are DC components and three-phase integer multiples of the high-order harmonic in the three-phase load current, the zero-sequence current is its arithmetic sum, that is, the zero-sequence current may be greater than the phase line current, such as the three-phase half-wave rectifier circuit, any phase current is 1/3 of the load current, and the load current is the zero-sequence current.
4. However, in the three-phase bridge rectifier circuit, because there are currents in the positive and negative half-cycles of alternating current, and the positive and negative half-cycles are symmetrical in one week, and the symmetry between the three phases, that is to say, the higher harmonics of the integer multiples of the DC component and the three do not exist, so the vector sum of the three-phase current is zero, that is, the zero sequence current is zero.
5. In the single-phase bridge rectifier circuit, because there are currents in the positive and negative half-cycles of the alternating current, and the positive and negative half-cycles are symmetrical in one week, the DC component of the single-phase current and the higher harmonics of the three integer multiples do not exist.
6. If the three-phase load is such as a single-phase bridge rectifier circuit, although it is unbalanced, the vector sum of the three-phase current is not zero, that is, the zero sequence current is not zero, and the zero line current will not be greater than the phase line current.
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