Over Current protection time grading
Over Current protection time grading
(OP)
Is there is any reason to make a time grading between high voltage and low voltage over current protection of a power transformer .
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Over Current protection time grading
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Over Current protection time gradingOver Current protection time grading(OP)
Is there is any reason to make a time grading between high voltage and low voltage over current protection of a power transformer .
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RE: Over Current protection time grading
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.
RE: Over Current protection time grading
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.
RE: Over Current protection time grading
Bung
Life is non-linear...
RE: Over Current protection time grading
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.
RE: Over Current protection time grading
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.
RE: Over Current protection time grading
RE: Over Current protection time grading
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.
RE: Over Current protection time grading
RE: Over Current protection time grading
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?
RE: Over Current protection time grading
It appears the transformer needs to be treated as part of the switchboard busbars in this case and it is not surprising.