Currently we have Fir make electrostatic relay mp6 installed for short current, negative sequence and thermal overload protection. This relay uses current sensing from two current transformers on phases r & t. Now we are installing Siemens motor protection Siprotec relay 7sj622 to replace the PLD relay. Old electrostatic relay have a negative sequence setting of 2 amperes (these old relay do not use symmetrical component analysis for settings the pic up values).

Now with the same two phase CT we are installing this 72j622 relay, but the relay is not reading the sequence components as per the unbalance current injected. Can we use two phase CT to configure the negative sequence motor protection in this 7sj622 relay which picks up on set value defined by actual negative sequence component??? If this configuration can be used then what will be the relation ship between line unbalance, line currents, and the negative symmetrical component that relay reads?

 

Following is reply from Support services

Unfortunately I have to inform you that isn’t possible to use the Negative Sequence Protection just with to phase current transformer and one core balance current transformer. The calculation of I2 is just with three phase current transformer possible. This constellation is possible for voltage measurement but not for current.