Vibration spikes on 8.5 MW motor in start or during large load switching on same Bus

[Yasit]

On 8.5MW motor vibration spikes are observing on both drive and non-drive end bearings . During spikes the motor DE/NDE bearing vibrations value exceed more than 6.3mm/s rms for very short duration and then become normal. However, the coupled compressor and gear box vibration during this period don’t show any change in vibration.

It is to note that the vibration spikes are coming during switching of loads on the same 6.6KV bus from where NGC-1 motor power is coming. During the switching of HP FWP Motor (rating 1000kW) or at the time of energization the transformer (rating: 800KVA) at no load vibration spikes are observing. The duration of vibration spikes is less than a seconds during transformer energization and in 3 seconds during HP FWP Motor Startup. Moreover, such behavior is not observing on remaining 8.5MW motors (of same manufacturer) connected on buses IB and IC.

Once same 8.5MW Motor was started but when motor started it was observed that motor was taking unbalance current . There was a difference of 60A between Phases as shown in below mention picture. The motor was stopped and inspection of motor was performed by removing all insulators as shown in pictures.

During inspection, 1 stud was found damaged which was repaired and after repaired following tests were performed in order to verify the health of motor (Report attached for reference).

1- Surge Comparison test
2- Winding resistance test
3- Insulation Resistance test.

The motor was put into operation after ensuring health of motor but again continuously vibration spikes are coming. Therefore, support is required in understanding possible reasons of stud damage and possible causes of vibration spikes on motor.

 

[Drivesrock]

Hi Yasit.

The insulated stud looks like it has suffered heat damage - slightly loose connection? Incorrect torquing of the bolts?

I have been investigating a vibration spike issue recently on a large gas compressor - electrical/power side - at start of one other equipment, every time. But in this case one of the compressor sensors was 'spiking' and occasionally tripping the machine. Very interesting project!

Is this increased vibration tripping the motor or just some spike or spikes but below the trip level?

Some points to consider that were looked at here:

Do none of the other vibration sensors detect any increase in vibration at the same time - even if at a much lower level?

1. Mechanical source?
a) Are the HP FWP and transformer mounted close to or on the same structure as the NGC motor? Consider too, heat exchanger air or water conduits / piping, cable trays...
b) Are these other equipment controlling anything in the NGC process that could be disturbing the medium? (I would expect the compressor vibration sensors to be detecting or alarming / tripping the machine).
c) Any process valves operating during the other equipment starting?

2. Electrical Source?
a) Electrical interference issue with the vibration sensor cabling.
b) Do the HP FWP and transformer cabling run together with the NGC motor cable?
c) In the HV switchgear / HV bus, where are the feeders physically, relative to the generator(s)? And compared to the buses IB and IC.
d) Does this always happen irregardless of the online generator configuration?
e) Could the 'high' inrush currents and subsequent (albeit maybe very small) voltage drop / waveform disturbance during switching on be affecting the NGC motor stator voltage and therefore the motor flux and torque for a few cycles and revolutions of the rotor?

There were other mechanical and electrical items that were considered / checked too.

The jury is still out on my case based on my findings and of others and a plant shutdown. Some mitigating steps have been taken to avoid trips to keep the process going.

I look forward to learning more about your plant.