Function of stator winding RTD in a motor 7

[krisys]

The motor in question is 11kV, 3,300 kW Ex”d” 3-phase, squirrel cage induction motor. It has stator winding RTDs. Two RTDs per phase, thus total six (6) RTDs. Now one of the RTDs (say RTD#4) is found to be faulty. Presently the RTDs are connected to RTD module of the motor protection relay. This motor winding temperature is communicated to DCS and temperature is logged in the DCS. The trend can be plotted in graph.

When the RTD measured temperature reaches a set limit there will be an alarm. If the temperature exceeds higher value, the motor will trip instantaneously.

Now we have to disable RTD#4 (faulty) to run the motor. I want to know, what is the risk arising by keeping one of the RTDs (RTD#4) disabled. Do we need to replace the motor or rectify the faulty RTD to make it fit for running or it can be in service in this condition for the prolonged duration.

The following are my own views:
[ol ]
[li]RTD is basically a condition monitoring device. It detects the sample temperature of a winding. It cannot detect the hot spot temperature.[/li]

[li]One RTD per phase is technically sufficient to detect the winding temperature. So the second RTD in the same phase can be considered as spare. So as long as one RTD is available per phase, there is no problem.[/li]

[li]The motor temperature rise has a time dalay to develop. Hence in order to avoid any false tripping, a time delay, (say 5 to 10 minutes) can be given to the RTD trip. This will help the operator to take the necessary precautions before the motor trips.[/li]

[li]The motor trip by RTD temperature, is only a back up for the motor protection through the current based protection.[/li]

[li]Conventionally the motor RTD temperature was a good guidance for the operator to control the load. This was significant, when the motor was sized close to the load (no design margin). Hence controlling the load is important (specifically during the peak summer) to adjust the load to limit the motor temperature based on the ambient temperature.[/li]
[/ol]

Is there any problem, if we provide a time delay for the motor tripping? What can be the time delay? Is the time delay of say 5 minutes, will be risky?

Alternatively, can we keep only the alarm and disable the trip?

 

[waross]

If the motor is overloaded, the other 5 RTDs will alarm and trip.
If the motor is overloaded, the motor protection relay will trip.
If you have a supply balance issue, one phase may overheat and be detected by the RTD.
If you have a supply balance issue, the motor protection relay should detect and trip on differential protection.
If you have a failing stator winding, the RTD will hopefully detect and trip before there is arcing core damage.
This is more of an issue with lightly loaded motors.
RTDs are important, redundant protection to a good motor protection relay.
If your plant has down-time insurance, check the terms of the policy for the protection required.
When an RTD fails, motors are often run with one less RTD. However if the loss of an RTD voids your insurance policy you will need to get the motor fixed or replaced.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

Normally, winding RTD's are located in each winding circuit. A two parallel winding will have a RTD for each circuit i.e. 6 nos. in total. A three parallel circuit will have 9 RTD's in total. This is to ensure that if one circuit is lost (due to whatever reason like jumper cut/lead cut/ series connection cut etc. and which will be not be picked up the OL/OC protection), you will have enough warning to protect your winding.

I would take winding RTD's protection as seriously as any other protection.

Muthu

http://www.edison.co.in

 

[zeusfaber]

On the other hand, we specify motors with two thermistors (slightly different thing, I know) in each winding, then only connect half of them to the thermal protection unit - with the others available as spares.

A.

 

[dArsonval]

Perhaps someday, we will be able to track every electron to determine if it went where we wanted to go.

Resistive Temperature Devices have a history of being fragile items ever since their inclusion into motor windings.
Testing one, can result in ruining it... if not performed correctly.

The risk of running the motor with that lowly RTD disconnected is in the mind of the one thinking about it at night.

If that 3,300 Kilowatt Motor powered by 11,000 Volts fails because of a RTD not being hooked up or functioning properly...
I'd say the hierarchy of the reliability engineering was flawed from its beginning.

Put some black tape over the alarm, schedule the motor for refitting the failed 28 cent part, and get some sleep.

John

 

[krisys]

I would appreciate if someone can endorse or challenge my below understanding with good reasoning.


If the RTD is not for the detection of hot spot temperature, it shall be considered to be measuring the average temperature.


Technically one RTD per phase is sufficient to measure the average winding temperature of the phase, assuming it is not the split winding. So two RTDs are considered to be for redundancy only. It is assumed that the conductor is getting heated up uniformly over its length.


Is my argument to provide a small time delay for tripping the motor justified?

 

[dpc]

Using only one RTD/phase was quite common in the past. RTD is mainly there to detect overheating of the winding due to overload or other condition. Response is too slow to be helpful in detecting motor faults. I see nothing wrong with ignoring a bad RTD. I certainly would not even consider replacing it assuming the motor is otherwise healthy.

There will always be a hot spot, but the settings for the RTD alarm and/or trip should account for this, as do the insulation temperature rise allowed for the motor.

You would not want that long of a time delay on an RTD trip, since they are inherent slow anyway. A few seconds perhaps.

The RTDs may also be providing a bias for the stator thermal model in the motor protection relay.

In 40 odd years, I never had an RTD motor trip occur that was NOT a bad RTD or bad connection. And that was not common either.

 

[Skogsgurra]

If I had the time. And if that motor was nearby. I would buy a nice modern impedance meter and arrange a circuit that cyclically connects to one winding at a time and checks the resistive part of the total impedance. There are meters that are good at that, using U and I and phase angle. I would then make a baseline measurement to see what the room temperature resistance is, compare to a DC ohms reading and then let the multiplexer check output from the CT:s and PT:s once every minute or every five minutes. There are CT:s with voltage outputs, so you don't have to worry about the possibility that the multiplexer opens the secondary. Most of those impedance meters have alarm outputs and if there is a higher-than-normal resistance, it issues a warning. That warning can turn into an alarm signal if a resistance corresponds to max temperature - and if ALL windings say about the same.
A sanity check, where all circuits supervised are compared and a temporary deviation from the majority is neglected, is quite easy to arrange.

This kind of measurement will give you the mean temperature and, as dpc said, the hot spot is included in the thermal limits. Also, the winding doesn't die if it reaches the hot spot temperature - all that happens is that the life of the windings get shorter. Like halving it for each 8 - 10 C temperature rise. So, there is no risk that you will have flames in the motor if your delay is five or ten minutes. Especially as the winding resistance reacts instantaneously to temperature. Much faster than an RTD does.

But that is me. I don't think anyone that isn't somewhat adventurous would go for it. But I can hardly wait to test that method. Or is it already available somewhere? It may be costly, but a rewind and change of a 3.3 MW 22 kV motor is is also quite costly.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[rhatcher]

I have a different take on this.
- RTDs are primarily meant to protect against inadequate cooling. This could be clogged air filters, inoperative blowers, clogged heat exchangers, clogged cooling fins, etc.
- RTDs can detect overloads but this is better accomplished by measuring current. A properly set overcurrent relay will trip before the motor reaches thermal limit from overload unless there is also improper cooling (see previous note).
- The idea that a particular RTD measures a particular phase is a fallacy. RTDs are placed between the top and bottom coils of a given slot. Many machines of this class have coil spans of between 4-7 coils and have pole groups of between 2-5 coils. Thus, the top and bottom coils in a given slot are usually different phases.
- The base choice of three RTDs is based on the minimum number that will effectively measure average stator temperature by placement 120 degrees apart.
- Critical applications may utilize 6 or 9 RTD’s. The additional RTD’s are usually considered by the manufacturer to be spares and, unless otherwise specified, may be placed in slots directly adjacent to the first three RTD’s or they may be ‘evenly spaced’, meaning 6 RTD’s distributed at 60 degrees apart or 9 distributed at 40 degrees apart, there is no industry standard for this that I am aware of. Also, keep in mind that there are a discrete number of evenly spaced slots so exact spacing of the RTD’s will not happen.

My recommendation is to disregard the bad RTD and to test it during the next scheduled shutdown. The RTD wires are typically very small and it would not be surprising to find that there is a broken wire or bad connection in the circuit.

The RTD can be tested with a multimeter based on resistance versus temperature. Note that the readings may not match your expectation, it takes a long time for a large motor to cool down and it may not cool uniformly. Also, if heaters are in use this will affect the readings of the adjacent RTD’s.

Finally, as previously described, the RTD’s are usually imbedded in the winding and cannot be replaced without a rewind. This being the case, in almost all cases of a bad RTD you run with it while monitoring the remaing RTD’s. The only industry that I can think of that would justify a MV rewind for an RTD alone is nuclear but there may be some oil/gas/chemical applications that are similarly treated.

 

[ScottyUK]

Good summary rhatcher.

The other reason why RTD's might be critical is in variable speed applications, where they are often required as part of the motor certification.

In my experience O&G is less protective of its electrical machines than power generation.

 

[krisys]

dpc, Skogsgurra, rhatcher, ScottyUK all of your responses are excellent. rhatcher’s response truly stands out.

The blockage of cooling circuit is where the RTDs would prove their worth. Especially, the High Voltage motors, where the cooling circuits of wide variety is used.

The blockage of cooling circuit is normally a slowly evolving problem due to deposition of dirt, scale etc. Now to re-iterate on my earlier argument of providing some time delay (say 5 minutes), should not matter in this case. In such evolving problems, a small time delay should not have any compromise in the protection.

Also, if there is such cooling circuit blockages, generally all the RTDs would shows the increase in temperature. So by taking one RTD out of service should not matter much.

 

[Karuppu. Bala]

We all know the purpose of RTD.
Normally winding RTD will be installed closely connection-side winding (Inside).
In-case, if anything not working, we can able to set a new RTD on the same phase at STATOR winding overhang area by dismantling Motor.

 

[electricpete]

if anything not working, we can able to set a new RTD on the same phase at STATOR winding overhang area by dismantling Motor.

I've heard that. I'm a little curious about hazards that might arise in an 11kv motor like op or 13.2kv motors like at our plant. When inserted into the slot, the RTD is adjacent to the grounded semicon tape of the coil. On the endwindings it's a different story.

I assume the RTD installed in the endwinding beyond the grading area (not in the transition area just outside the slot), correct?

Is the RTD circuit grounded? If yes, it seems it brings ground voltage and possibly a stress riser geometry to the endwinding. If no, it seems we might induce high voltage on the instrument wires.


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(2B)+(2B)' ?

 

[ScottyUK]

Excellent points ePete. I'd be very hesitant of fitting anything in this area for those reasons - anything which significantly modifies the electrical field contours in the endwinding area, where stresses are generally high to start with, is likely to prove troublesome in the long term with partial discharge an ever-present risk.

 

[edison123]

pete, Scotty

Semicon treatments are applied only for windings 5 KV and above and RTD's are embedded for these <5 KV stators with no special grounding without any safety issues reported. I don't believe RTD's located in the slots or in end windings will have any induced high voltages.

Muthu

http://www.edison.co.in

 

[ScottyUK]

Hi Muthu,

It's an 11kV machine, and only 3.3 MW so I expect the endwinding geometry will be quite tight.

 

[electricpete]

I guess I'm weak on the instrumentation side. I do know that any shield terminates in the aux junction box and does not go into the winding. I don't know if the three RTD wires themselves (in a 3-wire rtd) are in a grounded circuit.

=====================================
(2B)+(2B)' ?

 

[edison123]

pete

RTD wires are not shielded. They are just PTFE insulated wires.

Hi Scotty - Not getting your point.

Muthu

http://www.edison.co.in

 

[electricpete]

I'm a little curious about hazards that might arise in an 11kv motor like op or 13.2kv motors like at our plant. When inserted into the slot, the RTD is adjacent to the grounded semicon tape of the coil. On the endwindings it's a different story.

Semicon treatments are applied only for windings 5 KV and above and RTD's are embedded for these <5 KV stators with no special grounding without any safety issues reported. I don't believe RTD's located in the slots or in end windings will have any induced high voltages.

I think you're making the point that successful application of RTD's in the slot section of lower voltage machines with no semicon should reduce concerns about appplication of RTD's in endwindings of higher voltage machines where there is also no semicon. It's a good point. If the instrumentation circuit is ungrounded and the concern is inducing high voltage onto the instrumentation wires, then I agree with the logic it's an analogous situation. If the instrumentation circuit is grounded and the concern is the potential to affect the stress distribution windings themselves, then I don't agree because 1 - the higher voltage machine has more voltage stresses; 2 - the endwinding insulation is not as robust as the slot insulation.

=====================================
(2B)+(2B)' ?

 

[ScottyUK]

Hi Muthu,

Just your comment about <5kV stators puzzled me a little with this being an 11kV machine - I would expect there to be some sort of stress control at that voltage. Maybe I misunderstood your comment? It's been a day for me doing that, ha-ha!