The CBF protective relay has connected in series to same current transformer core that connected to distance relay. Also because the extra high voltage power system stability requirements, the relays setting have done in tightly coordination systems. But the designed protection system sometimes couldn't operate correctly.
Sometimes all circuit breakers that connected to one of two substation bus bar (double bus bar system) are tripped due to one sever short circuit that occurred in one of output lines without any bus bar protection activity.
According to more investigation and data recording analyses, the line distance protection had operated correctly (less than 30 ms) and it had opened relevant circuit breaker in time. Also the CBF protective relay had operated too.
How can you explain the reason of that protection system mal-operation?