1. switching mode of original current circuit

2. Improvement plan

In order to solve the problem of current circuit switching between this switch and bypass switch of transformer, it is necessary to avoid switching the current circuit by hard switching platen. The current circuit of this switch and bypass switch is directly connected to the transformer protection device, and the switching of current circuit is indirectly completed by software. Specifically, it can be realized through the following two schemes: scheme 1: the auxiliary contact of the bypass bus switch on the transformer, the bypass switch and the auxiliary contact of the two side switches are connected in series to the transformer protection device, and the input value is used to judge whether the transformer is the bypass supply. The advantage of this scheme is that the device can automatically identify the operation mode of the transformer; The disadvantages are that the switching circuit is complex, the contacts are stuck or poorly contacted, and the cables need to be laid, which increases the workload of on-site construction and maintenance.

Scheme 2: install a pressure plate or a switching handle on the transformer protection device, and judge whether the transformer is supplied by bypass through the switching operation of the pressure plate or the handle. The advantages of this scheme are that there is no need to lay cables, and there is no case of dead or poor contact; The disadvantage is that it can not automatically identify the operation mode of the transformer, which can only be realized by manually switching the pressing plate on or off or switching the operation handle.

After the identification of transformer operation mode is realized by external input input, the following modifications are made in the protection algorithm: if the sampling value of TA of this switch is i1j and the sampling value of bypass switch TA is i2j, the calculation formula of the switching current sampling value ij on the transformer side is as follows: ij = i1j si2j (J = a, B, C, n), where s is the operation mode to identify the switching quantity, The value of the switch is as follows: when the switch is running, when the s=0 bypass switch is running, the switch and retreat mode of s=1 generation supply panel (switch handle) is to switch the bypass switch into power supply transformer (s=1) before replacing the transformer. After the replacement supply is completed and the switch is restored, it will be cut off (s=0). The switch handle of this generation of supply plate is synchronized with the switch off operation of the pressure plate of the transformer protection exit bypass switch, which is not easy to cause misoperation. Therefore, the improved current circuit switching scheme greatly reduces the probability of manual misoperation.

Through the improvement of transformer protection algorithm, the switching function of current circuit is realized indirectly, and the hard switching of transformer switch and bypass switch current circuit is completely avoided, so that the transformer can be operated in the operation mode during switching operation, bypass switch for operation, or switching (parallel operation) of switch and bypass switch, The transformer protection can be adapted to solve the problems existing in the bypass power supply transformer.

3. Feasibility of improvement scheme

Compared with the current switching scheme of transformer realized by the traditional hard switching circuit, the main differences of hardware are as follows: 1) two-way switching capacity is needed to identify the operation mode of bypass switch instead of transformer, Through two spare pressing plates, the identification of high voltage side and medium voltage side of transformer can be realized by bypass switch. There is no problem in realizing the above functions by the protective device.

2) It is necessary to increase the input circuit of two analog quantities (respectively the current circuit of bypass switch on the high voltage side and the medium voltage side). Therefore, for the modern microcomputer transformer protection device, it will be further standardized to open and open the transformer protection device according to five groups of current circuits. In the case of no bypass switch, only the corresponding current circuit can be short circuited.

Through the comparison of the hard switching mode of conventional current circuit and the difference of the improved soft switching mode, it is feasible that the improved soft switching scheme has no problems in the realization of the protection device.

4 Conclusion

The current circuit switching scheme of transformer realized by the above soft switching mode completely avoids a series of problems existing through hard switching circuit, facilitates the operation on site, and eliminates the hidden danger caused by human factors. This improved scheme is not difficult for the transformer protection device of micro model, It can be used to make the operation on site convenient.