Differential Protection 1

[Flashover]

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

 

[peterb]

Your short circuit protection will operate after the fact when a heavy fault occurs, with the possibilty of widespread equipment damage. Differential protection is much more sensitive and can operate for lower level faults before they evolve into something far worse. Motor size is one criterion, importance to the particular process is another.

 

[peebee]

Hmmm. . . . I've heard of differential protection on transformers and switchgear, where you're checking to see if power in = power out. But I've never heard of this on a motor, and checking for power in = power out seems impossible. Is this actually zero-sequence ground-fault sensing?

 

[peterb]

Motor differential requires 6 leads brought out of the motor. Install CTs on each lead and sum the result per phase, as is done for generator protection. Alternative is to install a zero sequence CT per phase, with both line and neutral end leads passed through it. In either case, current in = current out for normal operation.

 

[peebee]

Gotcha. Thanks.

 

[dpc]

I agree with peterb that differential protection on large rotating machines can provide much better protection against damage from internal faults than standard overcurrent relaying. I have seen (mis-wired) motor differential relays trip a breaker before the motor starts rotating.

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.

 

[BJC]

Have you considered a self ballancing scheme. Ypu only need three CTs. Check IEEE Standard 242 for the details. I expect that with some the solid state relays available this scheme would be cheap and effective.

 

[jbartos]

Suggestion to Flashover (Electrical) Nov 20, 2002 marked ///\\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.\\

 

[RRaghunath]

Flashover,

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.

 

[Flashover]

the motors are 6.6kV and most above 2MW. our problem is we do not have redundancy on most of these machines so protection and early warning is crucial. the 6.6kV system is floating. i am led to believe that earth fault detection is still possible due to the capacitance in the cable, some current willl flow to earth hence earth fault can be picked up.
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

 

[busbar]

"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

 

[busbar]

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.