In practical production, issues arising from the polarity and wiring of current transformers have led to malfunctions or failures in protection devices, resulting in power outages. These problems are frequently observed 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 Mobei 35 kV substation in the Shixi area, repeated station power losses occurred due to incorrect polarity and wiring of the current transformers used in the main transformer differential protection of No. 1 and No. 2 units. Therefore, correctly determining the polarity of current transformers and ensuring accurate secondary wiring is crucial for reliable system operation.
To address this, let's take the example of a double-winding transformer’s differential protection configuration and briefly explain how to determine the polarity of current transformers and ensure correct secondary wiring.
The polarity of a current transformer refers to the relationship between its primary and secondary windings. It should be marked accordingly. As shown in Figure 1, L1 and K1 are same-polarity terminals (L2 and K2 are also same-polarity). The standard method for marking polarity is to use an asterisk (*) on the same-polarity terminal. When the primary current flows into the L1 terminal, the induced secondary current should flow out from the K1 terminal.
For the correct secondary wiring of current transformers, when a transformer is connected in Y/△-11 configuration, there is a 30° phase difference between the currents on both sides. Specifically, the low-voltage side current leads the high-voltage side current by 30°. To eliminate this unbalanced current, the secondary windings of the differential protection current transformers should be wired in △/Y configuration, as illustrated in Figure 2.
On the low-voltage side of the transformer, the secondary winding is connected in â–³, so the corresponding secondary wiring of the current transformer should be connected in Y. If the current transformer is polarized, and assuming the busbar side is positive, the positive terminals of the current transformers are connected together to form a neutral line. The secondary leads are then connected to the negative terminals of each a, b, and c phase.
On the high-voltage side, the primary coil is connected, and the corresponding secondary wiring of the current transformer should be in â–³. The negative terminal of the A-phase current transformer connects to the positive terminal of the B-phase current transformer, drawing the A-phase current. Similarly, the B-phase negative terminal connects to the C-phase positive terminal to draw the B-phase current, and the C-phase negative terminal connects to the A-phase positive terminal to draw the C-phase current. Based on the phase relationships, the vector diagram shows that the secondary currents from both sets of current transformers are in phase. Assuming no other influencing factors, the current flowing into the differential relay should be zero.
In addition, general overcurrent protection relies solely on time delay for selectivity, which may not be sufficient for double-ended power lines or ring networks. To achieve proper selectivity, directional elements are added to each current protection, forming directional overcurrent protection. These directional components detect the direction of power flow. When power flows from the bus to the line (e.g., at D1), the direction is “positive,†and the protection operates. Conversely, if power flows from the line back to the bus (e.g., at D2), the direction is “negative,†and the protection does not act.
For 110 kV line protection involving zero-sequence directional protection and distance protection, the polarity of the current transformer is directly related to the correct functioning of the device after it is put into service.
In many cases, experimental reports for newly installed equipment contain complete technical data and pass all tests, but often lack records of current transformer polarity and wiring. This is usually due to insufficient attention during acceptance. Errors in polarity and wiring can go unnoticed but may cause protection malfunctions or failures once the system is operational.
To prevent such issues, several measures should be taken:
1. Experimenters should thoroughly study theoretical knowledge and understand the operating principles of various protection systems. They must recognize the importance of current transformer polarity and wiring and strictly follow design drawings.
2. Protection setting calculation personnel should clearly specify the polarity of current transformers for specific lines in the setting list. For example, based on the busbar, if the fault current flows from the busbar to the line, the device should operate reliably; if the current flows from the line to the bus, the device should not operate.
3. The test methods, results, and wiring configurations of the same-polarity current transformers should be clearly documented in the experimental report.
4. According to quality management requirements, the equipment acceptance form should include items that are often overlooked but critical, such as the test method for current transformer polarity, test results, and correct wiring procedures.
encapsulated power transformer,EI low frequency transformer,Industrial transformer,power supply transformer,alarm system transformer
IHUA INDUSTRIES CO.,LTD. , https://www.ihuagroup.com