Problems in the installation of zero-sequence current transformers and their solutions

1) The installation position of the zero-sequence current transformer and the grounding wire of the cable terminal is wrong, mainly due to the problem of whether the 10 kV cable steel shield grounding wire passes through the zero-sequence current transformer. After inspection, it was found that some cable metal shielded ground wires should not pass through the zero-sequence current transformer, and some cables that should not pass through the zero-sequence current transformer were reversed, causing the zero-sequence protection to fail to operate correctly during accidental grounding. There are two ways to install zero-sequence current transformers: one is that the three-finger sleeve of the cable terminal is located above the window of the zero-sequence current transformer; the other is that the three-finger sleeve of the cable terminal is located under the window of the zero-sequence current transformer.

When the three-finger sleeve of the cable terminal is located above the zero-sequence current transformer window, the grounding point of the cable terminal is located at the upper end of the zero-sequence current transformer, and the ground current of the cable shell has passed through the zero-sequence current transformer, zero-sequence current transformer When the induced current has been generated, it is necessary to return the ground wire, pass through the zero-sequence current transformer window, and be grounded.

When the position of the three-finger sleeve of the cable terminal is under the window of the zero-sequence current transformer, the ground current of the cable shielding layer has not passed through the zero-sequence current transformer, so it will not affect the zero-sequence current transformer. The cable grounding wire should not Through the zero sequence current transformer window, but directly grounded.

(2) The zero sequence current transformer is open and the magnetic circuit is not closed. The power cables used in substations are mostly three-core XLPE cables with cross-sectional areas of 240 mm2 and 300 mm2. If the zero-sequence current transformer is not penetrated before the cable terminal is made, due to the large outer diameter of the cable, the current transformer can only be disassembled and installed during the construction. Once the connection piece is poorly restored and the interface is not properly restored, current will be caused. The secondary circuit of the transformer is open, and the magnetic circuit of the primary circuit is not closed and cannot work normally. In this regard, it is necessary to avoid disassembling the zero-sequence current transformer connecting piece, and insert the zero-sequence current transformer before the cable terminal is made. If you have to disassemble the connecting piece, it must be restored well to ensure the normal operation of the zero-sequence current transformer.

(3) In some substations, the zero-sequence current transformer is tied to the cable. The switchgear is not tightly closed, and it is easy to enter dust and animals, and there is a hidden operation hazard. The zero-sequence current transformer should be fixed on the switch cabinet with a fixed bracket to seal the switch cabinet intact.

(4) The grounding wire of the metal shield of the cable is poorly grounded. In actual work, it was found that the metal shielded ground wire of the cable was not crimped with the copper terminal, or was not connected to the grounding copper bar in the switch cabinet. The metal grounding wire of the cable should use copper stranded wire or tinned copper braided wire, and the cross-sectional area should not be less than 25 mm2. The grounding end should be crimped with copper wire lugs and reliably connected to the grounding copper bar. It is also necessary to ensure that the body of the zero-sequence current transformer and the secondary winding are reliably grounded.