What is positive sequence current and negative sequence current and zero sequence current? - 推酷

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The positive sequence, negative sequence and zero sequence appear to analyze the asymmetric components of the three phases into symmetrical components (positive and negative) and zero-sequence components in the same direction when the system voltage and current are asymmetrical. As long as it is a three-phase system, the above three components can be decomposed (a little like the synthesis and decomposition of the force, but in many cases the value of a component is zero). For an ideal power system, the values ​​of the negative sequence and the zero sequence component are zero due to the three-phase symmetry (this is why we often say that there is only a positive sequence component in the normal state). When the system fails, the three phases become asymmetrical, and then the negative sequence and zero sequence components of the amplitude can be decomposed (sometimes only one of them), so by detecting these two should not be normal. The component that appears can be known to have a problem with the system (especially the zero-sequence component of single-phase grounding). The method for simply obtaining the amplitude and phase angle of each component is described below. The prerequisite is that the voltage or current (vector value) of the three phases is known, and of course, the components are directly measured in practice. Since you can't get the picture, please draw on the paper according to the text.
A vector diagram of the system's three-phase current (using current as an example, the voltage is the same) from known conditions (for clarity, don't draw too extreme).
1) Find the zero sequence component: add and sum the three vectors. That is, phase A does not move, and the origin of phase B is translated to the top of phase A (arrow). Note that phase B is only translational and cannot be rotated. The same method translates phase C to the top of phase B. At this point, the vector from the A-phase origin to the top of the C-phase (sometimes the arrow-to-arrow) is the sum of the three-phase vectors. Finally take the one-third of the magnitude of this vector, which is the magnitude of the zero-sequence component, the direction is the same as this vector.
2) Find the positive sequence component: The original three-phase vector diagram is first processed as follows: phase A does not move, phase B rotates counterclockwise by 120 degrees, and phase C rotates clockwise by 120 degrees, thus obtaining a new vector diagram. According to the above method, the vector diagram is three-phase summed and taken in one-third, which results in a positive-order A phase, and the two phases of B and C are respectively drawn by the amplitude of the phase-A vector by 120 degrees. This leads to the positive sequence component.
) Find the negative sequence component: Note that the processing method of the original vector graph is different from the normal sequence. The A phase does not move, the B phase rotates 120 degrees clockwise, and the C phase rotates 120 degrees counterclockwise, thus obtaining a new vector diagram. The following method is the same as the positive sequence.
Through the above methods, we can analyze the general situation of various system faults, such as why the zero-sequence protection will operate when single-phase grounding occurs, and there is basically no zero-sequence current when the two-phase short-circuit occurs.
Here again talk about the relationship between each component and harmonics. Since the harmonic has a special relationship with the frequency of the fundamental wave, it will exhibit positive sequence, negative sequence and zero sequence characteristics when synthesized with the fundamental wave. But we can't equate harmonics with these components. From the above, the reason why the fundamental wave is decomposed into three components is to facilitate the analysis and state discrimination of the system. In the case of zero sequence, many cases are single-phase grounding. These analyses are based on the fundamental wave. It is the harmonics superimposed on the fundamental wave that produces an error in the measurement. Therefore, the harmonic is an external interference quantity, and its value is not what we want in the analysis, just like the interference of the third harmonic to the zero sequence component.