Precautions and Control Points for Current Transformer Replacement

The specific process of current transformer (CT) replacement includes the removal of old transformers and related secondary cables, the installation of new transformers and the connection of secondary cables, etc. Due to the change of the secondary circuit of the current involved in the replacement process, there is a possibility that the protection device may send a letter or operate by mistake.

Precautions and Control Points for Current Transformer Replacement

What are the danger points in the process of replacing the transformer?

1. The current secondary circuit is open

When the current transformer is in normal operation, the magnetomotive force generated by the secondary current will demagnetize the magnetomotive force generated by the primary current. If the secondary side is open, the demagnetization effect of the secondary side current will disappear, and the primary current will completely become the excitation current, which will cause a sharp increase in the magnetic flux in the iron core, making the iron core in a highly saturated state, resulting in a high voltage generated at both ends of the secondary winding. If the voltage is too high, it will not only damage the insulation of the secondary winding, but also seriously endanger personal safety.

The replacement of the current transformer cannot leave the secondary circuit in an open state. Then during the replacement of the current transformer, the secondary cable is falsely connected, the terminal connection of the current terminal box is not connected in place, and the operating current circuit in the CT terminal box is removed by mistake. It is the factor that causes the open circuit of the secondary circuit of the current.

2. The secondary circuit of the current is not grounded

For the safety of personal and secondary equipment, the secondary circuit of the current transformer must be reliably grounded at one point. If the secondary circuit loses the protective grounding point, the high voltage on the primary side of the current transformer will form a partial pressure through the distributed capacitance between the primary and secondary coils of the current transformer and the ground capacitance of the secondary circuit, and introduce the high voltage into the secondary circuit. There will be a threat to people and equipment. During the replacement of the current transformer, the insulation test of the current secondary circuit will remove and restore the corresponding grounding point. It is easy to cause the grounding point to be unreliably restored or the wiring is wrong due to work negligence, and the secondary circuit will lose the grounding point.

3. Two or more points of the current secondary circuit are grounded

The grounding network of 500kV substation is not the actual equipotential surface, and there is a certain potential difference between different grounding points. If the current secondary loop is grounded at different points in the substation at the same time, the potential difference on the grounding network will cause the current secondary loop to generate additional secondary current, resulting in false signaling or even malfunction of the protection device.

4. Wrong ratio setting

The CT transformation ratio is used to represent the multiple relationship between the primary and secondary currents. Its setting has a direct impact on the sampling of protection, measurement and other devices. The wrong setting of the transformation ratio may cause the protection device to refuse to operate or malfunction, and the measurement and metering device to sample. Occurrence of errors, etc. Since the CT selected in the 500kV substation has the characteristics of adjustable primary winding and multi-tap secondary winding, in the CT replacement work, the series-parallel connection of the CT primary winding and the selection of secondary winding taps are important factors affecting the CT transformation ratio. An error in any one of them will cause an error in the transformation ratio of the CT.

5. Reverse polarity

CT polarity refers to the relationship of the direction of current flow between the primary and secondary windings. The first end of the CT primary winding is marked as P1, and the tail end is marked as P2; the first end of the secondary winding is marked as S1, and the tail end is marked as S2. Under normal circumstances, CTs are labeled with decreasing polarity, that is, the primary current flows in from P1, and the secondary current flows out from S1, so as to ensure that the current connected to the secondary device conforms to that when the primary current is in the positive direction, the secondary current flows in from S1. secondary device. The determination of CT polarity is usually verified by the primary maintenance personnel, and the secondary maintenance personnel mainly ensure the correctness of the polarity of the secondary wiring between the CT junction box and the CT terminal box. If the wiring is reversed, the phase of the secondary current will change by 180°, and the secondary circuit will not be able to correctly reflect the flow direction of the primary system, which will lead to sampling errors in measurement and metering devices, misoperation or refusal of protection devices, and affect the safe and stable operation of the system. The reverse polarity is mainly caused by the reverse connection of the junction box or terminal box circuit wiring during the replacement process. 6. Accuracy level error Electromagnetic current transformers with different types of windings have different error characteristics, accuracy, steady-state saturation characteristics, and transient saturation characteristics. Secondary equipment should choose secondary windings with appropriate accuracy levels, otherwise it may cause Incorrect operating behavior of protective devices in the event of a fault. The accuracy level of the secondary winding is indicated by the TPY level, the circuit breaker protection generally uses the 5P level with a faster secondary current decay speed, and the measurement and metering devices usually use the 0.2S level to ensure the sampling accuracy requirements.

avoidance measures

1. Before the replacement of CT, the secondary maintenance personnel should use the secondary safety measures ticket, correctly implement the secondary safety measures and make a construction disclosure to ensure effective isolation from the operating protection equipment, and then make a safety disclosure with the construction personnel and sign the recording. construction is allowed.

2. When the old CT is dismantled, take pictures of the internal wiring and nameplate of the secondary junction box of the old CT, and confirm the transformation ratio, polarity and accuracy level of each winding of the old CT; Record work. When the new CT is installed, also take pictures to record the internal wiring and nameplate of the secondary junction box of the new CT, and check with the transformation ratio, polarity and accuracy level of the old CT, secondary cable core wire and other information to ensure correctness.

3. In order to avoid false alarms and malfunctions of the protection device caused by the two-point grounding of the current secondary circuit due to CT replacement work, the secondary maintenance personnel not only need to carefully check the appearance process after construction wiring during the acceptance stage, but also need to inspect the new CT. After the junction box cover is closed, check the cable insulation and grounding to ensure the correctness of the grounding; in order to avoid the open circuit of the current loop endangering the safety of the person and the equipment, the maintenance personnel need to measure the current loop of each winding between the CT secondary junction box and the CT terminal box. Direct resistance, to ensure that the resistance of each loop presents a low resistance value and the three-phase balance of the single-phase and inter-phase DC resistance values. At the CT replacement site with the condition of one-time up-flow, the correctness of the transformation ratio, polarity and secondary circuit of CT can be verified again by means of one-time up-flow test.

4. After the acceptance inspection, the maintenance personnel should carry out the secondary safety measure recovery work according to the secondary safety measure ticket, fasten all the current circuit connections and terminals involved, and measure the direct resistance of the entire current secondary circuit at the same time to ensure that there is no open circuit problem . 5. When starting the loading stage, the data should be recorded and compared. The maintenance personnel test phase by phase, compare the data size and phase angle of the theoretical calculation value, the measurement value of the clamp meter behind the screen and the actual sampling value of the screen cabinet to ensure the consistency of the three. 6. In order to make the above control measures better applied on site, technical forms such as new and old CT ratio polarity sheets, insulation data sheets, circuit direct resistance tables, and start-up load record sheets can be compiled to assist on-site implementation .

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