Directional Current Relay Trip

[gentech]

This is my first experience with this type of relay and its application. When attempting to parallel two 2500 kVa 9kV/380 V transformers via low voltage bus ties the 67 on the incoming bus operates approximately 1 second following breaker closure. Each bus operates under load when not paralleled. I only experienced this trip when I attempted to parallel. Both transformers are fed from the same 9 kv feeder and are the same specification, one digit apart in serial number. The control logic for the sequence of this operation is functioning correctly. The 67 has single CT and line (B to C) voltage inputs. The online transformer has 1100 kVa load and the incoming transformer 9 kv voltage breaker is closed. The system has been in operation for about 10 years. Could this be caused by sudden loading of the incoming transformer?

 

[powerjunx]

gentech, i bet you had a 3 units 67 (directional phase overcurrent) that resembles from single phase 67 connected in evry phase.Phase A (CT), Line VOltge B-C (PT) for 1st unit, Phase B (CT), Line voltage C-A (PT) for 2nd unit, Phase C, line A-B for 3rd unit. Else, it never applies to directional ground overcurrent, 67N.
gentech, since your protective relay is operational for over 10 years, i guess the factor that affects tripping was due to load growth expansion, or the abscence of regular calibration check.
In our case, we able to avoid such tripping from load growth through regular calibration of our 67 relay,and constant updates of load profile. We then therefore upgrade 67 unit since its tapping setting can't accomodate for a continuous load expansion.
Can you distinctively what really your 67 was? is it 67N or simply 67.

Regards,
bill









"Whenever you are asked if you can do a job, tell them, certainly I can! Then get busy and find out how to do it." Theodore Roosevelt.

 

[RRaghunath]

May be, you should check the tap positions.

The transformers under parallel operation should have the taps at the same position.

 

[DTR2011]

i'd agree with the circulating currents on due to tap mismatch on the paralleled transformers