Brief Analysis on the Wiring Method of Current Transformer for Transformer Differential Protection

Differential protection is the main protection of the transformer, and its working condition is very important to the normal operation of the transformer. In order to ensure that the differential protection is reliable during normal operation of the transformer or when there is an external fault on the transformer, it is necessary to try to make the phase difference of the CT secondary line current on the power supply side and the load side of the transformer, and the action ampere-turns generated by the current equal. As long as these two conditions are met, the differential protection of the transformer will not operate when the transformer is normal. In order to make the phase difference of the CT secondary current on the power supply side and the load side of the transformer, the following wiring methods are introduced:

The first wiring method: Take the 1# main transformer of the 110kV substation in our county as an example. Its capacity is 20,000 kVA. The wiring group is Yao/Yao/A-12-11. The 110kV side is the power supply side, and the high-voltage side and the low-voltage side are the load side. The wiring diagram is shown below. Because the wiring group of the transformer is Yao/Yao/A-12-11, its low-voltage measuring line currents Ia, Ib, and Ic are respectively ahead of the high-voltage side line currents. The secondary phase current of the high-voltage side CT is in phase with the primary current when the polarity is reduced. In order to make the phase difference of the secondary line current of the CT on the power supply side and the load side of the transformer. Try to make the CT secondary line current on the low-voltage side of the transformer lag behind the phase current, so that the connection order of the low-voltage side CT is the head of phase a connected to the tail of phase C; the head of phase b connected to phase a.

The second wiring method: We also use the 12 hours of the clock to represent the wiring group of the CT. In the first wiring method, the CT on the high voltage side is connected at 6 points, the medium voltage side is connected at 12 points, and the low voltage side is connected at 1 point. The second wiring method is to connect the CT on the high voltage side to 12 points, the medium voltage side to 6 points, and the low voltage side to 7 points.

The third wiring method: connect the CT secondary of the high-voltage side to 11 points, the medium-voltage side to 5 points, and the low-voltage side to 6 points.

The fourth wiring method is to connect the CT secondary to 5 points on the high-voltage side, 11 points on the medium-voltage side, and 12 points on the low-voltage side.

There are four wiring methods for transformer differential protection. When selecting the CT ratio, there are two methods for each side; one is star wiring and the other is delta wiring. If the first wiring method is used, for a three-winding transformer, two CTs on the three sides of high, medium and low are connected in star, and only one side is connected in delta. The wiring is relatively simple. Under certain conditions, this wiring method can solve the unbalanced current that cannot be solved in the differential current circuit. Of course, no matter which wiring method is used, the effect is the same, but due to the different technical levels in different regions, in order to prevent the differential protection from causing protection cross-action due to CT wiring errors, it is best to choose one of the wiring methods as a typical setting.

The second wiring method: We also use the 12 hours of the clock to represent the wiring group of the CT. In the first wiring method, the CT on the high voltage side is connected at 6 points, the medium voltage side is connected at 12 points, and the low voltage side is connected at 1 point. The second wiring method is to connect the CT on the high voltage side to 12 points, the medium voltage side to 6 points, and the low voltage side to 7 points.

The third wiring method: connect the CT secondary of the high-voltage side to 11 points, the medium-voltage side to 5 points, and the low-voltage side to 6 points.

The fourth wiring method is to connect the CT secondary to 5 points on the high-voltage side, 11 points on the medium-voltage side, and 12 points on the low-voltage side.

There are four wiring methods for transformer differential protection. When selecting the CT ratio, there are two methods for each side; one is star wiring and the other is delta wiring. If the first wiring method is used, for a three-winding transformer, two CTs on the three sides of high, medium and low are connected in star, and only one side is connected in delta. The wiring is relatively simple. Under certain conditions, this wiring method can solve the unbalanced current that cannot be solved in the differential current circuit. Of course, no matter which wiring method is used, the effect is the same, but due to the different technical levels in different regions, in order to prevent the differential protection from causing protection cross-action due to CT wiring errors, it is best to choose one of the wiring methods as a typical setting.