AC Induction HV Motor Differential protection

[slavag]

Hi.
What is your excpireince with 87M protection for AC Induction HV Motor? Size of Motors is from 1.5MW up to 6MW,
Un 3.3kV or 6.6kV , direct connection.
It's new project that include 18 motors. Customer also request functionality 27, 59, 81 for the motors ( for each one). Up today, we puted one relay 27/59 on the BB and was send common trip to motors.
What is your opinion?
Thanks for replys.
Regards.
Slava

 

[dpc]

87M is common for motors of that size, at least in the US. Of course, the motor has to specified to have the back end leads brought out and CTs provided. The 87 will save the motor stator core when the stator winding fails. I would also recommend surge arrester and surge capacitor at the motor.

In theory, use of a common 27/59/81 device to protect all motors on a bus will work fine. In practice, most digital motor relays will provide these functions, so I would use that instead.

 

[jraef]

I agree. Most protection schemes for motors over 2500HP here in the US will include differential protection. high-end digital motor protection relays now usually include that capability. But as dpc said, the trick is to make sure the motor is specified correctly to facilitate it, which by the way also includes a junction box big enough for the extra CTs.

 

[slavag]

Dpc and Jraef.
Thanks a lot for your information.
Dpc, yes, most of new digital relays are included 27/59 functionality. But in all cases I prefer put one common 27/59 relay per BB, if I need change settings, add some logic, I change it in the one relay only ( I test this functions in one relay only). But actually it's not issue, add and test 27/59 in each motor protection, is not problem.

What about 81 function, it's also common in US?
I mean as motor protective function, usually we use it as load shedding function and as protective function only for syn. generator and motors.

What is a type of 87M are common :
1. Flux balanced ( with additional ring CT on in/out of stator)
2. High impedance ( with varistor and damping resistor)
3. Simple sum. wiring of 6 CT to 3 special diff. current inputs.

Regards.
Slava
PS
Dpc, Thanks for recommendation: add surge arrester and surge capacitor at the motor. It's not my part of project, but I forward it to designer and customer ( of course if we win this project ).

 

[oldfieldguy]

slava--

I had a synchronous motor with an 81 function trip on it. Our situation was that this motor was fed from a tap on a 230-kv transmission line through a step-down transformer.

If the remote end of the line opened, the motor would act as a generator for several cycles, back-feeding the transmission line, until its inertial load allowed the voltage to decay far enough to trip the motor feeder breaker on undervoltage.

We added the 81 relay to the circuit and set it to trip at a slightly lower frequency. Under normal conditions, the line stayed at 60 Hz, but if the line opened, the motor's backfeed was a rapidly decaying frequency, and this would trip the breaker in a short time.

Most motor diff. circuits I see are the core balance type, but I have seen the summation type also.

old field guy

 

[slavag]

Dear Oldfieldguy.
Thank you for reply.
81 functionality for the syn. motor it's must, w/o any Q's.
What about induction motor?
"Most motor diff. circuits I see are the core balance type"
It's very important for me. Core balance, that mean relay is protected only motor, nod feeding cable.
Thanks a lot again.
Best Regards.
Slava

 

[dpc]

I don't think I ever saw an 81 device on motor protection until the advent of digital multi-function relays. Since it is "free" in the relay, the function is often used. I'd be more inclined to use a V/Hz protection, if available.

 

[jraef]

On motors I have never seen anything other than the 6 CT method used.

And yes, that has only been something I noticed being specified since the advent of digital protection relays, i.e. Multilin and SEL. I wasn't working with MV motors in the days of separate electro-mechanical relays (luckily).