(1) In the non-selective zero-sequence voltage protection device and zero-sequence power direction protection device, the primary and secondary neutral points of the voltage transformer must be reliably grounded, and the primary winding neutral point grounding is not only safety grounding but also working grounding. If the neutral grounding is not reliable, the secondary system cannot correctly reflect the unbalanced voltage zero-sequence power direction when the primary system has a grounding fault, so the voltage polarity of the opening triangle must be correct.
(2) In the ground protection device composed of zero-sequence current transformer (mostly cable outgoing), when the grounding fault occurs in the power grid, the fault current may flow not only through the earth, but also through the conductive sheath and armor of the cable. Therefore, the security ground wire of the cable head above the zero-sequence current transformer must be grounded along the cable direction through the LH line side (see Figure 1).
Figure 1 Cable head security ground diagram
The grounding of the cable skin below the zero-sequence ct does not need to pass through the zero-sequence CT to avoid the formation of a short-circuit ring. The fixed collet of the cable and the cable shell, the insulation of the ground wire, the ratio of the zero-sequence ct and the polarity error should be adjusted in a consistent and correct way to reduce the mutual inductance error.
(3) The fifth harmonic component protection of the cable capacitance and the transient current protection are usually used in the single-phase grounding protection of the power network through the arc suppression coil grounding, which is difficult to achieve the selectivity. The fault can be selectively removed by means of zero-sequence current protection and directional protection by using effective resistance to increase the active component of fault current when grounding fault is found.
(4) In the capacitor switching system, the compensation capacitor should be connected to neutral point ungrounded Y or D connection. After grounding, the three-phase loads operate symmetrically, so that the zero-sequence current is not affected, ensuring the sensitivity and correctness of grounding protection.
(5) In the cable line of the same system and the overhead line out of the cable line, there are differences in their single-phase grounding capacitance current, so the fixed value of zero-sequence current protection should be fully considered.
(6) Zero-sequence current filter composed of three current transformers must overcome the influence of unbalanced current.
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