Discussion on the open phase protection scheme for standby transformer in large scale generator units

When the breaker of the starting-standby transformer runs with open-phase, it will produce negative sequence current that will damage the loads which connect to the transformer to different degree depending on its magnitude, at the same time it will influence the power system, which should be promptly removed. This paper analyses the relationship between the phase sequence current and voltage in open-phase condition and the normal load current. When the standby transformer is in small load, the negative sequence current is so small that the traditional protection relay can't identify it easily. New scheme of the negative sequence proportion increment and the diagram of open-phase protection are presented. It analyses the problem of current open-phase protection of the standby transformer through the actual example. Detailed configuration scheme and the starting of the breaker malfunction in different cases are described.