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Is it safe to run the motor by disabling the RTD for one of the phases?
3

Is it safe to run the motor by disabling the RTD for one of the phases?

Is it safe to run the motor by disabling the RTD for one of the phases?

(OP)
One of the motor winding temperature RTD Loop is malfunctioning and tripping. The motor is 11kV, 3300 kW, fed from 11kV switchboard.
Historically, two (2) RTDs (one from Y-phase and other from B-phase) of the same motor are out of service since commissioning. The present problem is with the second RTD of Y-phase. So now there is no RTD for Y-phase winding.

The RTD’s are connected to Areva MiCom P225 motor protection relay. The trip command is issued from MiCom relay. When the temperature trend was taken out from the DCS, most of the time the temperature is normal. However, intermittently the temperature shoots up abruptly then comes back to normal. It appears that there is something wrong with the RTD or the related circuit. Because motor temperature cannot shoot up in one winding such abruptly. Also all the other five RTDs are showing the normal temperature.

I suspect the following:

  • EMC issues, due to which some surge is getting generated due to induction from 11kV winding
  • Varistor (connected across the RTD for surge protection) could be malfunctioning
  • RTD module in the motor protection relay may be having some problem

  • The motor is driving a very critical pump. Presently it is kept off, as Pump-B is running. But we want it to be in stand by. We are proposing to disable the RTD which is causing the motor trip and take the motor into service.

    My question: Is there any risk for the motor or the pump, by disabling one RTD trip? Can other RTDs can do the job?
    I would appreciate, if someone from the fraternity could share their experience.

    RE: Is it safe to run the motor by disabling the RTD for one of the phases?

    First, roll the connections on the protection relay.
    You may consider reviewing the settings of the other parameters to be sure that the motor is adequately protected.
    Does the plant carry downtime insurance? If so there may be an insurance requirement for RTD protection.
    The Areva MiCom P225 motor protection relay provides a lot of protection in addition to the optional RTD protection.

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

    RE: Is it safe to run the motor by disabling the RTD for one of the phases?

    If it is critical (like in nuclear plant cooling and such), there are lots of legal things to consider.

    If it is not, then the protection may be adequate with two RTD:s - heat is usually spread quite evenly throughout the motor so the two remaining should be OK.

    Did you measure the three RTD's resistance? If so, what were the results?

    Also, as Bill says - the P225 will protect the motor against overload. But if cooling is bad, it is good to have RTD:s.

    Gunnar Englund
    www.gke.org
    --------------------------------------
    Half full - Half empty? I don't mind. It's what in it that counts.

    RE: Is it safe to run the motor by disabling the RTD for one of the phases?

    How many RTD's are there? Six @ two per phase is the industry norm. Have you checked the 'failed' RTD's resistance at room temperature? It should be around 112 ohms at 30 deg C for PT100 element.

    As a rewinder, we find the RTD's are the earliest failures. That's why we always provide 100% additional RTD's for each circuit in each phase when we do the rewinds. These RTD's are not wired permanently to the board. They are used when the in-service RTD's fail. Surges do not bring about the RTD failures. It's just a reliability issue.

    Never seen "Varistor (connected across the RTD for surge protection)".

    Winding RTD's are usually located in between the top and bottom layers of winding, which is 'theoretically' the hot spot of the winding. As a quick and dirty solution (for no rewinds), we provide new RTD's just below the slot wedges and tell the client to lower the alarm and trip levels for those RTD's.

    Yes, RTD's are a very critical protection for the motors and generators. Overcurrent/Overload relays do not sense loss of cooling.

    Muthu
    www.edison.co.in

    RE: Is it safe to run the motor by disabling the RTD for one of the phases?

    (OP)
    Thanks for your valuable inputs. The motor is installed in one of the offshore platforms, pumping the oil.

    A little more clarifications on the varistor connection as below:
    The RTDs are three wire type. In the RTD terminal box, the varistors are provided. Three varistors per RTDs are provided. All the varisotrs are from the RTD terminals to ground connected.

    After tripping, they have tested the RTD and it is found healthy. Even, when the motor is standstill, we have taken the trend for 24 hours. We have observed one spike of temperature shooting up to 160 deg C. The spike duration may be 5-10 minutes, as the sheet is covering 24 hours trend. Now I will also get the time scale zoomed in to analyse the shape of the spike.

    As we have a spare motor in onshore, replacement is an option. But replacement is a challenge, as the motor is Exd type, weighing 24 tons and huge in size. Hence the replacement is the last option.

    I like your advice of checking the cooling tubes frequently! It can be one of the additional measures to cover ourselves.

    Also you have reminded me the insurance aspect. Probably I will check that aspect as well.

    As such the RTD in discussion has not even failed. The readings go suddenly erratic for a brief period. When it goes crazy, there is no time delay between the alarm level and trip level. Hence trips the motor. Alarm and trip levels occur simultaneously.

    So in order to avoid tripping, I am thinking to disable the trip to either delayed trip or alarm only. The delay should be about say, 30 minutes. As the station is attended, the operator can take appropriate action before tripping the motor manually within 30 minutes.

    Previously, I have come across the facilities, where the RTD temperatures are given only for alarms. Specifically, if the facility is critical and attended.

    RE: Is it safe to run the motor by disabling the RTD for one of the phases?

    Does your relay incorporate a “voting” feature for the RTD inputs? That’s where at least two of the RTDs must agree in order to initiate a shutdown because as Skogsgurra said, heat in a motor is rarely localized in a narrow enough area to only affect one device. I almost always enable this, but I’m not familiar with that relay.


    " We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

    RE: Is it safe to run the motor by disabling the RTD for one of the phases?

    It may be an issue with the varistors, with the wiring or with the protection relay.
    Given phases A, B and C and an issue on B phase:
    Disconnect the A phase and the B phase wiring at the relay end and at the motor end.
    At the relay, interchange the leads from A phase and B phase.
    Swap the B phase RTD with C phase RTD.
    Swap the B phase varistors with A phase varistors.
    Now a wiring issue or a varistor issue will show up on A phase.
    An RTD issue will show up on C phase.
    A relay issue will still show up on B phase.

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

    RE: Is it safe to run the motor by disabling the RTD for one of the phases?

    krisys; You have precisely described a failing RTD. A slowly failing sense lead or the element will cause radical increases in the temperature readings. Remember this is a Resistance device! Small increases in resistance will cause pretty large temp reading changes. These excursions will get larger, and more frequent, up till it becomes permanent.

    Keith Cress
    kcress - http://www.flaminsystems.com

    RE: Is it safe to run the motor by disabling the RTD for one of the phases?

    (OP)
    waross,

    Thanks for giving that clue for trouble shooting. Very useful. Yes, I can certainly interchange (swap) the RTD and the circuit beyond RTD, to narrow down the problem.

    So far it was truly a puzzle for me.

    RE: Is it safe to run the motor by disabling the RTD for one of the phases?

    (OP)
    Muthu,
    There are two RTDs per phase.
    RTD # 1, 2 & 3 for R, Y & B phase windings respectively.
    Again, RTD # 4, 5 & 6 for R, Y & B phase windings respectively.
    RTDs are PT100 type.

    One each RTDs not in service from the beginning. i.e. one each from Y and B phase. The RTD in question is belonging to B phase. Thus for B-phase now no RTD protection.

    Is it prudent to get the motor re-wound, just because some of the RTDs are not working?
    At least to my mind, it is not prudent to rewind it. Because, the motor is just three years old, and working fine. But, someone in our team is favouring to replace the motor with spare motor and get this motor rewound.

    RE: Is it safe to run the motor by disabling the RTD for one of the phases?

    krisys

    3 out of 6 failed? The RTD's seem to be of really bad quality.

    No need to rewind the stator. As I said before, install new RTD's under the slot wedges and reduce the alarm & trip levels. Of course, the motor has to be dismantled to do this work.

    Muthu
    www.edison.co.in

    RE: Is it safe to run the motor by disabling the RTD for one of the phases?

    Is the motor running at sea-floor level? If so, there is filled with oil under pressure to avoid water coming in. That will distribute heat quite well and I would NOT change the motor. Temperature will be the same in all the windings and any overtemperature will be detected by the remaining RTD:s.

    Gunnar Englund
    www.gke.org
    --------------------------------------
    Half full - Half empty? I don't mind. It's what in it that counts.

    RE: Is it safe to run the motor by disabling the RTD for one of the phases?

    (OP)
    The motor is well above the sea water splash and the high tide levels. May be about 10 to 15 meters higher than the sea water splash level.

    It is like an outdoor onshore motor, exposed to sunlight and rain. Here the difference is the humidity level which will be quite high; as high as 90 - 98%.

    Otherwise it is an Ex"d" motor with standard Foot mounting (IEC type B3)

    RE: Is it safe to run the motor by disabling the RTD for one of the phases?

    Quote (Edison123)

    Winding RTD's are usually located in between the top and bottom layers of winding
    Is that between two phase windings? If so then each RTD would be influenced by two phase windings.
    Losing one position would take away some redundancy but would still leave all phases monitored.
    I'm asking, not suggesting.

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

    RE: Is it safe to run the motor by disabling the RTD for one of the phases?

    Bill

    RTD's are placed in between top & bottom layers in the same phase coils/bars. That's how you get temperatures for each phase. If it is between two phases, then which phase is running hotter becomes debatable.

    Muthu
    www.edison.co.in

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