Issues and Solutions for Installation of Zero-Sequence Current Transformers

Zero-sequence current transformer is a single-turn core-type current transformer, typically used in conjunction with power system protection devices such as small current grounding selection devices and computerized anti-harmonic devices.

Working principle of zero-sequence current transformers

The principle of applying zero-sequence current transformers to line protection is Kirchhoff's current law: the sum of the algebraic currents entering any node in a circuit is equal to zero.

During a single-phase ground fault, capacitive current is generated, which will produce a magnetic flux in the ring-shaped iron core of the zero-sequence CT current transformer, and reflect to the secondary side through the protection device to generate a tripping signal, cutting off the fault and protecting the line.

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

The installation position of the zero-sequence current transformers and the grounding wire of the cable termination head is incorrect, mainly whether the steel armored shielding grounding wire of the 10kV cable passes through the zero-sequence current transformers.

After inspection, it was found that some metal shield grounding wires of the cables did not pass through the zero-sequence CT current transformer when they should, and some that should not pass through the zero-sequence CT current transformer actually did, causing the zero-sequence protection to fail to operate correctly during ground fault.

There are two ways to install zero-sequence current transformers: one is to position the three-finger sleeve of the cable termination head on top of the window of the zero-sequence current transformers; the other is to position the three-finger sleeve of the cable termination head below the window of the zero-sequence current transformers.

When the three-finger sleeve of the cable termination head is positioned on top of the window of the zero-sequence current transformers, the grounding point of the cable termination head is at the upper end of the zero-sequence current transformers, and the grounding current of the cable casing has passed through the zero-sequence current transformers, and the zero-sequence CT current transformer has already produced induced current, so the grounding wire should be returned, passed through the window of the zero-sequence CT current transformer, and grounded.

When the three-finger sleeve of the cable termination head is positioned below the window of the zero-sequence current transformers, the grounding current of the cable shield has not passed through the zero-sequence current transformers, so it will not affect the zero-sequence current transformers, and the cable grounding wire should not pass through the window of the zero-sequence current transformers, but be grounded directly.

The zero-sequence CT current transformer is open-circuited, and the magnetic circuit is not closed. The power cables used in substations are mostly three-core cross-linked polyethylene cables, with a cross-sectional area of mostly 240mm2 and 300mm2. If the zero-sequence current transformer is not inserted before the cable termination head is made, due to the larger outer diameter of the cable, the CT current transformer can only be disassembled and installed during construction. Once the connection piece is not restored properly, the secondary circuit of the current transformer will be open-circuited, and the magnetic circuit of the primary circuit will not be closed, and it cannot work properly.

Therefore, zero-sequence current transformers should be inserted as early as possible before making cable termination heads, and if it is necessary to disassemble the connection piece, it is necessary to ensure good restoration to ensure the normal operation of the zero-sequence current transformers.

There are individual substations that tie the zero-sequence current transformers to the cable, and the switchgear is not tightly sealed, making it easy for dust and animals to enter, posing hidden dangers during operation. Fixed supports should be used to fix the zero-sequence current transformers on the switchgear and seal the switchgear completely.

The grounding of the cable metal shield is poor. In actual work, it has been found that the metal shield grounding wire of the cable is not crimped with a copper connector or not connected to the grounding copper busbar in the switchgear. The cable metal grounding wire should use copper twisted wire or tin-plated copper braided wire, and the cross-sectional area should not be less than 25mm2. The grounding end should be crimped with a copper wire ear and reliably connected to the grounding copper busbar. It is also important to ensure that the zero-sequence CT current transformer body and secondary winding are reliably grounded.

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