In practical production, issues arising from the polarity and wiring of current transformers have led to malfunctions and failures in protection devices, resulting in power outages. This problem has occurred frequently in the Karamay grid, particularly in main transformer differential protection, 110 kV line protection, and busbar protection. For instance, at the Luliang 110 kV substation and the Mobei 35 kV substation in the Shixi area, repeated station power losses were caused by incorrect polarity and wiring of the current transformers used in the main transformer differential protection of No. 1 and No. 2 transformers. Therefore, accurately determining the polarity of current transformers and ensuring correct secondary wiring is crucial for system reliability.
To illustrate, let’s take the example of double-winding transformer differential protection. The polarity of the current transformer defines the relationship between the primary and secondary windings. As shown in Figure 1, L1 and K1 are same-polarity terminals, as are L2 and K2. Polarity is marked with a “*†on the same-polarity terminal. When the primary current flows into the L1 terminal, the induced current in the secondary winding should exit through the K1 terminal.
For correct secondary wiring of the current transformer, when the transformer is connected in Y/Δ-11 configuration, there is a 30° phase difference between the currents on both sides. To eliminate unbalanced current, the secondary side of the current transformer used in differential protection should be connected in Δ/Y configuration. On the low-voltage side, the secondary windings are connected in Δ, while the corresponding current transformer secondaries are connected in Y. If the current transformer is polarized, the positive terminals are connected together to form a neutral line, and the secondary leads are connected to the negative terminals of each phase (a, b, c).
On the high-voltage side, the secondary wiring of the current transformer should be connected in Δ. The negative terminal of the A-phase CT connects to the positive terminal of the B-phase CT, and so on, to extract the phase currents. A vector diagram is then created based on the phase relationships. Under ideal conditions, the secondary currents from both sets of CTs are in phase, so the current entering the differential relay should be zero.
In addition, overcurrent protection typically relies on time delays for selectivity, but this is insufficient for double-ended power lines or ring networks. Adding a directional component to the protection creates direction overcurrent protection, which can detect the direction of power flow. When power flows from the bus to the line (e.g., at point D1), the direction is “positive,†and the protection operates. When power flows from the line back to the bus (e.g., at point D2), the direction is “negative,†and the protection does not operate.
For 110 kV line protection, including zero-sequence and distance protection, the polarity of the current transformer plays a vital role in ensuring the device functions correctly. During equipment commissioning, test reports often contain complete technical data and pass all tests, but they may lack records of CT polarity and wiring due to careless acceptance processes. These errors can remain undetected until the equipment is in operation, leading to misoperations or failures.
To prevent such issues, several measures should be taken:
1. Experimenters should study theoretical knowledge thoroughly and understand the operating principles of various protections. They must also pay close attention to the importance of CT polarity and wiring, and strictly follow design drawings.
2. Protection setting personnel should clearly specify the polarity of current transformers for specific lines in the setting list. For example, if the fault current flows from the bus to the line, it should be considered “positive,†and the device should operate reliably. Conversely, if the current flows from the line to the bus, it should be “negative,†and the device should not operate.
3. The test method, results, and wiring method of the same-polarity CTs should be clearly documented in the test report.
4. Equipment acceptance forms should include additional items that are often overlooked but essential, such as the testing method for CT polarity, the test results, and the correct wiring method. This ensures a more comprehensive and reliable commissioning process.
medical transformer,adt transformers,lighting transformer,amp transformer
IHUA INDUSTRIES CO.,LTD. , https://www.ihuagroup.com