Over Current protection time grading 2

[salem]

Is there is any reason to make a time grading between high voltage and low voltage over current protection of a power transformer .

 

[dpc]

Assuming both relays will cause the same outage if they operate, it probably is not critical that they coordinate. Operation of the primary device for secondary fault could complicate location of the fault and cast suspicion on the transformer itself on a through-fault.

Of course, you should set the instantaneous element of the primary protection to protect the transformer. The instantaneous element of secondary protection generally is not very useful.

 

[alehman]

If the primary is protected with fuses and the secondary with a circuit breaker, it may be desirable to set the secondary breaker so it trips before the fuses open (or are damaged) to reduce down time and cost of replacing fuses.

If both primary and secondary are protected with fuses, it is desirable to coordinate so you don't have to replace both sets of fuses after a fault. Secondary fuses should protect the primary fuse damage curve to avoid the case where the primary fuse links may be damaged, but not open.

If you have primary and secondary breakers, coordination may be desirable to allow easier determination of the cause of trip (internal to transformer or external).

Obviously there may be a trade-off with other overcurrent protection on the system.

 

[Bung]

The main problem with trying to get them to time grade is that you lose another grading step. WE generally set them pretty much the same, with the HV OC just being a backup to the transformer differential protection and the LV OC/EF.

Bung
Life is non-linear...

 

[busbar]

alehman’s hi-side “fuse saving” capability through lo-side breakers and relays is notable. A basic idea behind selective coordination is to minimize the area of outage. During restoration, being able to exclude a transfomer as one more item to carefully test often buys a lot of points with those suffering power loss. OTOH, “Burn and Learn” can be a dearly expensive lesson.

 

[reactive]

We generally set the pickup on the HV at 200-250% full load to avoid nuisance tripping due to inrush and the LV at 125% to provide some overload protection.

However, as Bung alluded to above, we find that one generally runs out of time when doing grading. Protection across transformers and cables is therefore generally a good place to gain some time and we typically end up with the curves fairly close together for faults approaching maximum.

 

[davidbeach]

If you have breakers on both sides of the transformer, the use of a differential relay at the transformer will allow setting the high side breaker to coordinate with the breakers downstream of the low side breaker, below the transformer through-fault damage curve. On through-fault you would clear sooner, particularly if the fault is between the transformer and the low side breaker, while operation of the differential relay would clearly locate the fault within the differential zone of protection. Good 87-T relays (with primary & secondary 50/51 in addition to 87) are available in the $2k range.

 

[SMB1]

Yes,
for POC you may have grading in time. Because any fault in the LV side or uncleard feeder fualt will cause tripping of the LV side breaker.
Your Xfmr will be still energized and you will know the location of the fault.

For (Dy) Xfmr there is no meaning of coordinate the ground fault on HV and LV side of the Xfmr.
set the GOC of the LV side as minimum as it is coordinated with the down stream relays.
set the GOC of HV to minimum setting.

 

[RRaghunath]

Petroleum Development Oman, a Shell joint venture doesn't provide O/C relay on low side of the transformer at all, meaning that the primary O/C relay is the back up protection for low side uncleared faults in addition to being back up to the transformer faults.

 

[uno]

True.

In fact I have seen low voltage switchboards which do not even have incoming circuit breakers.There is a direct connection of transfromer low voltage side to the switchgear busbars through a bus duct. Maybe if it is through a cable we might need to consider cable protection. Any further views on this?

 

[RRaghunath]

Uno,
It appears the transformer needs to be treated as part of the switchboard busbars in this case and it is not surprising.