HT Motor Tripped Without Any Fault Indication

" Carried out solo run after decoupling motor runs smoothly" i think you have already found-out the reason as to why your motor OLR Trips. try measuring the line current drawn by your 500 Hp motor without load. it should at least be 30% of your Rated current (Nameplate Current) else if its higher your motor have some problems with the windings (this is for re-wounded motors). if your motor have been in operation prior to the troubles occurence then i suggest your better check the alignment of the motor viz-a-viz with the coupled load.

Without any knowledge of what protections your motor has, a remote person has an impossible task.

I would start by injecting each trip device at sufficient level to initiate a trip and checking it does this at the correct level/delay time & actually does give the expected flag, lamp or display indication as a memory of the trip.

Causes could range from a "joker" hitting a remote stop button, which does not latch down, or resetting it to hide the cause: to a failure of a memory device or even a design fault which means a self-resetting device e.g. overcurrent can give a trip faster than "work-time" of memory device with ageing, temperature or supply voltage change. It has even been known that "open" relay contacts have touched due to shock or a defect like loose contact clamping.

Did anything happen in plant ten minutes after motor start? Which could be a cause of EM interference or supply disturbance or......

One possibility is that the trip operated because the motor is being asked to do more then <...500HP...> of work.

So if this is an important motor for your refinery operation, why is there no protective system for it that gives you any indication as the the cause of a trip? Your methods and materials of controlling and protecting this motor need some SERIOUS reconsideration if your best hope for resolution is in asking a group of anonymous forum members for help without providing any useful information.

VCBs are NOT motor starters by the way, especially for something as small as 500HP where there are better alternatives. They are USED that way by people too cheap to do it right, but you are throwing good money after bad if the motor must start and stop more than about once per year. The contacts inside of the vacuum bottles used in vacuum motor starters are designed differently than those used in VCBs, because in a VCB, the contacts are only expected to clear a fault, which is not expected to be a common and repeating event like starting a motor. What will happen inside of a VCB used as a motor starter is that the repeated opening and closing of the contacts will eventually deposit contact material on the inside of the bottle, leading to a dielectric breakdown, even in the presence of a vacuum.

No indication doesn't mean no protection...

VCBs used for 400hp 4160V motors for pulper motors with lots of starts per day, I installed GE Vacuum Limitamp starters in 1987, still going. The air-mag starters needed rebuilding 4 times a year, got to be tiresome. Surge arresters to take care of the chop seemed to work just fine.

3.3kV vacuum contactors are quite normal up to 500HP.

Above that 11kV VCB’s are used as starters. Admittedly one did go haywire, a 6000HP motor single phasing sounds horrendous.

Are people using 11kV for 500HP motors? Seems excessive...

On the flip side of that, I just quoted a VFD for a 1600HP 480V motor! I shudder to think of what the conductor arrangement will look like...

Since it is in a refinery, I suppose the motor may have process trips such as process safeguard trip wired to the motor breaker. Possibly, there is some thing to check with process team.

Also, check whether motor breaker is with No-volt release!!

Please do post your findings for the benefit of every one.

Any poor contact in the magnet coil circuit, probably on the ac side. Did run across something very similar to this in a Freightliner assembly plant in Mexico, where the contactor would open at odd times for no reason. Turned out the air gap of the armature was just a little wider than tolerance. Any small hiccup, the thing would drop out. It had no protection attached to it, so no red herrings wondering if the protection didn't set flags for an operation.

All other times traced to bad wires or contacts on the Stop button, disgruntled employee, loose wire on terminal block which would self weld from time to time, then break loose. All these were found by hand over hand inspection of every wiring termination. Sometimes traced these to fatigued stab connections on separable terminal blocks, where a small structure shift maybe due to a dropped bale of pulp 20 feet away was enough to shut it down. These stabs are hidden, and nearly impossible to trace unless you directly inspect.

Whether it tripped on starting or while running.Please check thoroughly your VCB.

There might also be case of induction voltage in one of the trip command wires directly connected to the trip coil and not taken though motor relay .