Differential Protection
Differential Protection
(OP)
hi folks,
what are people opnions on differential protection on large motors (6.6kV motors 1MW +)? is it necessary anymore. old company policy here states it is, my opnion is its not.
as we have s/c protection.
many thanks
Flashover





RE: Differential Protection
RE: Differential Protection
RE: Differential Protection
RE: Differential Protection
RE: Differential Protection
The differential relays will only see internal machine faults, or at least only faults within its zone. Your overcurrent relays will see everything and must be set to coordinate with other OC relays.
An internal stator fault can cause major damage to the stator core (iron) in addition to the obvious failure in the winding itself. The more rapidly the fault can be cleared, the less damage to the iron. This can save weeks or months of time in repairs.
The downside is additional cost and complexity. Nuisance tripping can be an issue when starting large motors due to unequal CT saturation and other transient effects.
RE: Differential Protection
RE: Differential Protection
hi folks,
what are people opnions on differential protection on large motors (6.6kV motors 1MW +)? is it necessary anymore. old company policy here states it is, my opnion is its not.
as we have s/c protection.
many thanks
Flashover
///If you are thinking in terms of:
http://www.geindustrial.com/products/brochures/469.pdf
469 SR Motor Management Relay, then it comes in the package, so that your questioning of potentially physically discrete differential relay is correct. It would not be necessary as a separate entity since it is part of the package. The above postings compare the differential relay 87 to short circuit relay only, not to the modern integrated management relays where the 87 relay is built in. As it is seen in the circuit, the motor has to have six leads available to have 87 relay functional. This is in agreement with the above postings. It may be viewed as a disadvantage at motors that do not have six leads available at the motor terminal box.\\\
RE: Differential Protection
I know companies who do not specify differential protection for motors of rating upto 3.5MW.
The practice is governed by the quality of the motors available in the market and the users' own experience of motor failures, which must be the case even for other equipment.
RE: Differential Protection
my next question to you guys is what protection on these motors would ye advise? taking into account no redundancy and some are critical drives!
thanks
Flashover
RE: Differential Protection
"Floating" or ungrounded systems can have destructive transient-overvoltage problems, with poor sensitivity to stator faults for differential protection. Retrofitting a system with at least high-resistance grounding to allow for minimum differential relay sensitivity should be considered.
IEEE C37.96 §2.4.7
RE: Differential Protection
Flashover, I mean no offense, but your 20-Nov and 22-Nov postings seem a bit contradictory—id est, "old policy" versus "critical/non-redundant." Differential protection will likely be more sensitive with lower ground-fault currents.