ungrounded line protection relay

[powerjunx]

i got a problem on how to point out where the fault (overvoltage) lies on our system (embedded system), in which, when the directional relays ( 67 ) activated which trips our CB. A 59N (basler) relay will follow and trigger an alarm on ground overvoltage. when we energized the common bus, this alarm will disappear as noticed.
note, 59N is connected on ungrounded bus (common to 2 generator) after the generator CB, having a configuration of y-grounded primary/single phase secondary (PT's)
any suggestion is much appreciated!

 

[alehman]

Are the PT's rated for line-to-line voltage? If not, they may be saturating when the neutral displacement occurs (as evidenced by the 59N activation). This could cause improper operation of the 67 relay.

 

[mc5w]

You also need some phase to neutral ballast resistors to drain off any parasitic AC overvoltage. Ungrounded 3-wire 3-phase can have a phase to ground voltage 4 times the phase to phase voltage.

On a typical 480 volt ungrounded system you can have have 2,000 volts to ground. Typically, a ground detector that uses 200 watt 130 volt light bulbs connected to 500 VA transformers will drain off any parasitic overvoltage. Such a ground detector almost qualifies as resistance grounding. It would give you about 0.8 amps into a ground fault, not enough for protective relaying but enough to stabilize voltage to ground under normal conditions.

You could scale up these figures if your system is medium voltage.

 

[powerjunx]

alehman, about the PT's rating, we had resolved this few years ago upon re-configuring our export/import load unit.
moreover, this ALARM only comes out after our annual maintenance. were there other parameters in the system that requires to be check?

mc5w, the bus were these (59N) relay is connected in 13.8KV. "phase to neutral ballast resistors", where can i connect these units? since, our configuration of PT's is y-grounded primary/single phase (broken delta) secondary. and in secondary side, a resistor is already connected across the line. i assume, this resistor will avoid ferroresonance.. will this resistor you want to point out?