The busbar protection still needs to meet the requirements of quick-action and selectivity. The busbar differential protection is based on the principle of current differential protection. Because the bus can still be regarded as a centralized parameter layout, it is in this area, and the distance is very close, hundreds of meters, so we take its current from the distance is not far, unlike the same transmission line hundreds of kilometers, which is relatively easy. Generally speaking, the principle of current differential protection is very simple, practical and reliable, Generally, we still use the principle of current differential protection to form the bus differential protection.

       Principles can be divided into two categories:

       Full current differential protection. Full current differential protection is full current differential protection. We have talked about transformer protection, generator protection and line protection.

       Current phase differential protection. We also talked about the current phase differential protection of transmission lines

       When we use differential protection for transmission lines, his contradiction is how to get the current at the far end. Although the distance between the two ends of the transformer differential protection is very close, it is not a voltage grade and its transformer model is not enough. Secondly, it is through a ferromagnetic element. There is a reverse magnetic current in the middle. The current on both sides is not exactly the same in theory. In the case of generators, If we take out the current of each molecular winding inside the neutral point as the differential protection, it is difficult to install the current transformer. At this time, we should try to install as few transformers as possible and get some incomplete current differential. Then we still use the principle of differential protection for the bus. Its main contradiction is that there are too many current circuits. Our high and low voltage distribution system, There are more than 40 outgoing lines, and generally there are more than 20 outgoing lines whose current is all led through a device for synthesis and comparison, which requires great caution in practical operation. At this time, our relay protection maintenance personnel are most worried about the circuit circuit break. If a line is not connected, the transformer can explode during operation. If the connection is reversed, the protection may malfunction, so he is not willing to move the current circuit as much as possible.

       As for the bus, the main reason is that there are many loops of introduced current. This is the main contradiction when the bus uses differential protection. Its reliability requirements are high, and the bus protection has been misoperated, so the misoperation of several line protections is not as harmful as it is, because if there are more than 20 lines on the bus, it will be cut off at the same time, so its position and importance can be imagined, The busbar differential protection is also caused by unbalanced current, which is a common feature of differential protection. The reason is that the unbalanced current is caused by unbalanced current after the CT comes out. The generation of unbalanced current is mainly caused by the difference of current transformer transmission characteristics. In addition, there is the influence of some fault components during the fault. The second is the CT protection problem caused by external fault, After the protection, he can't measure the fault current correctly, so the unbalanced current generated when the unbalance is taken again will be very large.