Have you used SEL 710 Motor Protection Relays? If yes, I have a query on thermal element protection that is specific to SEL 710 relays.

What is your testing issue ?

Please pose relay settings, test currents (and note if it is single phase or three phase balanced) and your relay test time.

What was your calculated test time and which formula did you use ?

it is mentioned in SEL710's instruction manuals, that for the calculation of (locked rotor) trip time due to rotor thermal element "12 >= I >= 2.5" for the case to qualify as rotor element trip, where I is current drawn by the motor, in multiples of its Full Load Current (FLA). I would like to know why a lower limit of 2.5 has been chosen.

When I injected a current of 2 x FLA, the Stator TCU reached 100 %, but the relay displayed "LOCKED ROTOR TRIP" after the triggering a trip command when the calculated (stator) thermal overload tripping time was reached. Per instruction manual, it should have tripped on Stator thermal overload and displayed "OVERLOAD TRIP". During this test, the Rotor TCU % was at 47.7% at the instant the trip occured.

If you need SEL's literature on thermal element for more clarity, let me know. I'll arrange for it.

Under 2.5FLA it is still considered an Overload and not a Locked Rotor condition.
Traditionally a Locked Rotor condition for 3 phase motors is around 6FLA (6.3 for many good size motors).

The pkp for the OC curve starts at 1.2-1.45FLA and you have to consider and stay above imbalance issues (~1.7 times the current), so, 1.45 x 1.7=~2.5 as the min.

Hope it helped,

Eric Stark

Yes, Mr. Stark, the conditions for locked rotor and thermal overload are mostly met by the conditions that you stated above. My query was more about how the relay - SEL 710 - detected and differentiated the two. SEL710 uses a thermal capacity unit (TCU), as a thermal element to sense/detect if the motor has to trip in locked rotor protection or in thermal overload. It is designed such that if the rotor component reaches "100% Rotor TCU", the locked rotor protection acts. Of course, this condition occurs in an actual locked rotor, where ~ 6 times FLA is drawn by the motor during starts for a certain length of time, say T.

And, the motor would trip on thermal overload when the stator component reaches "100% Stator TCU". This would usually occur when the motor is drawing current more than Service Factor times its FLA over a long duration of time.

But, like I mentioned, SEL had mentioned explicitly that locked rotor trip at I > 2.5 FLA. Just for academic purposes, during the testing of the relay, it was noticed that it tripped on locked rotor even when 2xFLA current was injected.

I just don't want the relay to display a wrong message on its display when either a commissioning or an operating engineer needs to troubleshoot a load trip (touchwood), and hence my concern/query.

I haven't worked on other relays yet, I'm just starting out my career, and SEL is the first protection relay that I am working on. I have been told by seniors that SEL's thermal element is something of its own and it has its own unique approach to it.

There is nothing unique about SEL's thermal element.
It is common to advanced relays (like GE SR469 and M60).
The thermal element is NOT linear in its formulation and way of jumping to conclusions. It is rather a commutative, an integral of past events and several variables, transparent to you. Traditionally the full formulation is a company secret.

No manual will tell you more in depth about the formulation.

Having said that, regardless of the above, it should NOT give you a Locked Rotor flag under 2FLA.
CHECK THE PROGRAMMING!!! It might be programed to do exactly that (without being documented anywhere. A common designer's malady).

Clean the relay and revert to company default state, and test again. (of course after saving the setting file ahead of doing so)

Hope it helped, 

Extract

"The SEL-710 Motor Protection Relay takes the next logical step in motor
monitoring and control. While other motor relays assume a constant
value for rotor resistance, the SEL-710 dynamically calculates motor
slip and uses this information to precisely track motor temperature
using the AccuTrack Thermal Model. Rotor resistance changes
depending on slip and generates heat, especially during starting,
when current and slip are highest. If your motor protection uses a
constant rotor resistance for thermal protection, it could be off by a
factor of three or more. By correctly calculating rotor temperature,
the AccuTrack Thermal Model reduces the time between starts. It also
gives the motor more time to reach its rated speed before tripping."

I think this extract is very clear to your point of query