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# Zero sequence current relay related standards

Zero sequence current relay related standards

Two conditions that produce a zero-sequence current:

1. Whether it is a vertical fault, a horizontal fault, or an asymmetry in normal and abnormal conditions, as long as a zero sequence voltage is generated;

2. There is a path for zero sequence current.

These two conditions are indispensable. Because there is no one, there is no source; the second is the question of whether there is voltage or current.

Zero sequence formula: 3U0 = UA + UB + UC, 3I0 = IA + IB + IC

The emergence of positive sequence, negative sequence, and zero sequence is to analyze the asymmetry of three phases into symmetrical components (positive and negative sequence) and the same zero sequence component when the system voltage and current are asymmetric. As long as it is a three-phase system, the above three components can be decomposed (a bit like the synthesis and decomposition of force, but in many cases the value of a component is zero). For an ideal power system, due to the three-phase symmetry, the values ​​of the negative sequence and zero sequence components are both zero (this is why we often say that there is only a positive sequence component under normal conditions). When the system fails, the three phases become asymmetric. At this time, the negative sequence and zero sequence components with amplitude can be resolved (sometimes only one of them), so it should not be normal by detecting these two. The presence of components, we can know that the system is faulty (especially the zero-sequence component when single-phase grounding). The following introduces the method used to obtain the amplitude and phase angle of each component by using the diagram method. The prerequisite is that the three-phase voltage or current (vector value) is known. Of course, in actual engineering, each component is directly measured. Since the picture cannot be drawn, please draw the picture on paper according to the text description.

Draw a vector diagram of the three-phase current of the system (using the current as an example, the voltage is the same) from the known conditions (for clarity, do not draw too extreme).

(1) Find the zero-sequence component: add three vectors and sum them. That is, phase A does not move, and the origin of phase B is translated to the top (arrow) of phase A. Note that phase B is only translated and cannot be rotated. In the same way, the phase C is translated to the top of the phase B. At this time, the vector (the arrow-to-arrow) of the phase A origin to the top of the phase C is made, and this vector is the sum of the three-phase vectors. Then take one third of the magnitude of this vector, which is the magnitude of the zero-sequence component, and the direction is the same as this vector.

(2) Finding the positive sequence component: The following three-phase vector diagrams are processed first: Phase A is stationary, Phase B is rotated 120 degrees counterclockwise, and Phase C is rotated 120 degrees clockwise, so a new vector diagram is obtained. Add the three phases of this vector diagram and take one third according to the above method. This will get the positive sequence A phase. Use the amplitude of the A phase vector to draw the two phases B and C respectively by 120 degrees. This gives a positive sequence component.

(3) Find negative sequence components: Note that the processing method of the original vector graph is different from that of seeking positive sequence. Phase A does not move, Phase B rotates 120 degrees clockwise, and Phase C rotates 120 degrees counterclockwise, so a new vector diagram is obtained. The following method is the same as the normal sequence.

Through the above methods, we can analyze the general situation of various system failures, such as why the zero-sequence protection works when a single-phase ground occurs, and there is basically no zero-sequence current when the two-phase is short-circuited.

Let me talk about the relationship between each component and harmonics. Because the harmonic has a special relationship with the frequency of the fundamental wave, it will show positive sequence, negative sequence, and zero sequence characteristics when synthesized with the fundamental wave. But we cannot equate harmonics with these components. From the above, the reason why the fundamental wave is decomposed into three components is to facilitate the analysis of the system and the discrimination of the state. If there are many cases of zero sequence, single-phase grounding occurs. These analyses are based on the fundamental wave. It is the harmonic that is superimposed on the fundamental wave that causes an error in the measurement. Therefore, the harmonic is an external interference quantity, and its value is not what we want when analyzing, such as the third harmonic interference on the zero-sequence component. 