Line to Ground Fault on Motor Running Using VFD

Sounds like a lightning strike...you need to check all the wiring for damage, and associated electrical devices....

1. No lighting strikes as only some machine are efftcted and some remain healthy
2. We are very much away from our transformer

A qualified electrician needs to examine your system. It is impossible for us online strangers to diagnose without even seeing it.

Thanks Rixter

i have prepared a SLD but image not getting upload

Please go through link if it can help

//drive.google.com/file/d/1-2ZsmoSzpfUKH5TZRl7g8t0KBayhCnhz/view?usp=drivesdk upload

Not relevant. See #2⇑.

When chasing a fault in your system, I suggest you disconnect the system parts as many ways as needed so you can measure the resistance to ground for each component separately. You might find the fault is in some location other than where you expect it.

Do you have access to a megohmmeter ("megger"), that can put 250 or 500 DC volts onto the motor's leads to (one tester's lead attached to the motor leads and the other to ground/earth)? This will give a real-world measure of any earthing fault for the motor. Using an ordinary multimeter will not do this. Important--DO NOT attach any megger to the VFD's output because that will destroy it; disconnect the VFD before doing any megger tests.

If you can't make and send a sketch or drawing of your system, try to describe these components and their connections in words. Your original post is very unclear. I presume English is not your primary language, so you are doing well---continue.

JMM

Megger was already done and found Y phase sort with motor body

We have replaced motor .But the concern is that why all other system ( mostly on single phase) has been efftcted?

Please go through the link

https://drive.google.com/file/d/1-2ZsmoSzpfUKH5TZRl7g8t0KBayhCnhz/view?usp=drivesdk

Please expand the question <...why all other system ( mostly on single phase) has been efftcted?...> with more information, otherwise all the replies will be little more than educated guesswork. Remember, no-one else in this forum can see the installation!

If the fault was on the LOAD SIDE of a VFD, it has no effect on the line side circuit other than an increase in the VFD input current (unless it causes the entire VFD to melt, which is highly unlikely). A VFD is only using the line connection as “raw material” for creating a DC power source, then it uses that DC to create a NEW 3 phase pseudo AC output for the motor. The only real risk of a down stream grounded fault is in the possibility of damaging the output transistors in the VFD.

Tyagimanoj90, Thanks for the drawing. It does not tell the connections to the other single-phase systems. Knowing this would help us to think.

You say the earth connection to the motor is good, but I wonder about that.

During the moment of a fault, while the motor is running you get a large current flow in the earthing connection--this comes from the output power of the drive as well as the rotational energy stored in the motor being dumped to the earth. It is possible that this brief current flow in the earth connection was more than the connection could handle without an excessive voltage rise. If your single-phase components shared this earth connection or were close to it, this would create a voltage spike to the single-phase components you mention. This would be a good possibility if the earth connection was inadequate when loaded with a fault current but still testing OK when you measure its no-load resistance.

Do I assume that your single-phase loads that have a fault are all physically close to the location where your motor faulted on the Y phase? Do I assume that these single-phase loads were at a lower voltage than that for the faulted motor? If both of these assumptions are true then I would investigate the quality of your earthing connection to the equipment.

In many installations, the conduit or pipe that encloses the wires going to the equipment is used as the earthing connection, without any separate earthing wire in it. If this has a poor coupling or rust, then your earthing connection is inadequate.

Let us know!

--JMM

I would be VERY VERY surprised if the ground fault on the load side persisted for more than the few milliseconds it would have taken for a VFD to shut down to try to protect its transistors. So if the transistors themselves are not damaged, there is no way that enough energy made it to the ground plane to do any collateral damage elsewhere.

VFD tripped several times but machine operator tried seven to eight times before informing to maintenance

So there are <...seven to eight times...> the normal reasoning for investigating and clearing the fault. Go for it. Let the forum know how it went.

Do I assume that your single-phase loads that have a fault are all physically close to the location where your motor faulted on the Y phase? -Yes

I assume that these single-phase loads were at a lower voltage than that for the faulted motor?-May Be

If both of these assumptions are true then I would investigate the quality of your earthing connection to the equipment.

We have no of earthing pits for each machine but having doubts that they are too close to each other .

Also having doubt that we have mixed neutral and earthing somewhere on this feeder .

Now my question is how to proceed further to know the exact reason of the cause and how to avoid such mishaps in future ?

<...Megger was already done and found Y phase sort with motor body...>

The cause of the fault was a loose wire either in the motor or in the cable leading to it from the VFD causing a <...Line to Ground Fault...>.

<...have no of earthing pits for each machine but having doubts that they are too close to each other...> If the earthing arrangements have a satisfactorily low impedance to operate the circuit protective devices, as will have been shown by the testing that took place when the installation was completed, then this is irrelevant. A simple re-test will alleviate any concerns. Make sure that there is a record of the test for future reference.

<...Also having...mixed neutral and earthing somewhere on this feeder...> This is a serious safety concern, for in the event of a neutral becoming disconnected upstream, there is a risk to personnel from electric shock. Isolate the power, investigate and correct without any further delay.

<...have replaced motor...> Learn from it and move on. It's over.

Tyagimanoj90,

We appreciate your replies to our questions. Too many times a person who posts a question here never replies as we talk about the question. You sent earlier a drawing of the feeder to the VFD and the faulted motor. But you have not given us a drawing of the feeder details to each of the 1-phase loads that are also having problems. Knowing this will help.

Some different questions--How old was the motor that faulted Y-earth? Was this motor designed and built for use on a VFD? What cable type and what length was the run between the drive and the motor? Older motors and motors not designed for use with VFD's frequently fail because of the very high voltage imposed on the first turns of their windings by the high-frequency "noise" from the drive reflecting at the connection between the cable and the motor. This voltage often is twice the output voltage of the drive.

--JMM

How old was the motor that faulted Y-earth?

08 yrs

Was this motor designed and built for use on a VFD?

No

Whatcable type and what length was the run between the drive and the motor?

Flexible cu cable 4C*16 Sq. mm

Length approx 25 Mtr

https://drive.google.com/file/d/1-2zlUjkcKIRMchjkLfgPvXXFpj59cOA6/view?usp=drivesdk

I have tried to make sld for 1-phase supply but it is not standard

So i am describing here in words

On same machine where fault occurred all indications bulb got fuse (Powered with neutral and phase )

On another Machine NBL 01 on same feeder all indication bulb got fuse (Powered with neutral and phase )

On another Machine IBT-01 on same feeder smps supply got burnt (input powered to smps was given using phase and earth ).

<...On same machine where fault occurred all indications bulb got fuse (Powered with neutral and phase )

On another Machine NBL 01 on same feeder all indication bulb got fuse (Powered with neutral and phase )

On another Machine IBT-01 on same feeder smps supply got burnt (input powered to smps was given using phase and earth )....>

The cause of that is a loose neutral connection upstream; nothing to do with the motor/cable fault.

It sounds as though an assessment of the installation for safety is well overdue. Isolation, investigation and rectification is strongly recommended before someone gets hurt/killed.

I agree with PWSlack. If PE is connected with Neutral wire but both are not connected continuously with the supply transformer neutral point PE [ and neutral] conductor get phase Y potential through the faulted motor and then the single phase consumers get 400 V[intermittent] instead of 230 V [as if the neutral is connected with the transformer neutral]

If this would be the case all the consumers supplied by this transformer would have suffered but in this case only we have suffered .

Tyagimanoj90,

I see four issues that you need to look at and correct during the shutdown you plan in the coming week:

1. The use of a standard motor on a VFD (as compared to a motor specifically designed for use with a VFD) puts high voltage spikes on the motor winding insulation and leads to premature failure to ground. This is the fault you had, and will be seen when the motor is taken apart and the stator windings are examined.
2. You most likely have a poor connection between the earthing point at your motor or its controller and the earthing point at your power source (a stepdown transformer or the service entrance). The high resistance on this connection will cause excessive voltages on the earthing conductor at the equipment during a line to earth fault. Consider that a 10-amp fault onto a 10-ohm earthing conductor's resistance can cause to a 100-volt rise in that conductor's local voltage.
3. The connection of your failed DC power supplies' input to earth is usually considered a code violation because this uses the earthing conductor as a current-carrying conductor. In your case, the probable earthing conductor's momentary voltage spike caused the failure.
4. You may have an extra connection between your neutral and earth conductors. The National Electrical Code (used in the USA and some other countries) requires them to be separate except at the power source, with the earth conductor being used only for safety.

Regarding #2 above, I am unsure what you mean by the term "earthing pit", but I believe you are describing the placement of an earthing electrode into a hole or pit designed to provide a low resistance path for electricity into the earth. In your case the resistance between the pit and your power source is probably too high to safely conduct fault current from the location of an earth fault back to the power source. You would need a dedicated conductor between these.

A standard multi-meter will not accurately measure the resistance of the path between the power source and your earthing pit. However a low-resistance ohmmeter will--it injects a DC voltage onto this path and measures the current flow through it, using two leads between the meter and each test point. One pair inject the voltage and measure the current flow and the other pair separately measure the actual voltage injected (without any errors caused by lead resistance. If you don't have access to such a meter you can design your own with the DC voltage coming from a DC power supply capable of operating into a short circuit, suitable wire leads, a DC ammeter, and a DC voltmeter. Then calculate the resistance by applying Ohm's law R = E/I.

Let us know how things work out. --JMM

In this case the wrong connection of neutral/PE it is located close to your installation site.

Thanks

Could you send this image on my personal email as image is not clear

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• A fault in a home, the symptoms of which were the tripping-out of the whole-house 100mA earth trip on a TT when the third ring of the electric oven was turned on, turned out to be an unintended chance connection between neutral and earth on a lighting circuit elsewhere in the building. It took a week of testing to find it and about 10 minutes to fix once found.

No-one other than the original poster is in a position to carry out that style of investigation, and it certainly cannot be found by repeated questioning over the internet.

The problem is best addressed by following the advice in #2⇑.

"Time is money" _{- Anonymous Poster #0}

Clue: <...line to ground fault...> involves the symptoms FLASH! and BANG! and the circuit protective device operates to disconnect the fault. In order to investigate, the personnel in the area need to be questioned as to what they saw, and what they heard. No-one other than the original poster is in a position to do this.

An isolation transformer for the 3 phase VFD delta wye would uncouple the single phase drives from the 3 phase drive and motor

It is unlikely in this case that the single phase drives were harmed, to them, it looked like a loss of power at the input when the 3 phase drive went to ground.

It is common these days to make the secondary of the drive isolation transformer high resistance grounded, to limit voltage excursions for an arcing or momentary contact ground fault. A meter across the resistor lets you know if there is a ground fault, so you can get the fault fixed before it devolves into a double phase to ground fault, which will trip your circuit protection.

“It is unlikely in this case that the single phase drives were harmed, to them, it looked like a loss of power at the input when the 3 phase drive went to ground.”

Again, NO.

When there is a ground fault on the OUTPUT side of a VFD, it has ZERO EFFECT on anything connected to the line side of the VFD. It ONLY AFFECTS the VFD itself in that it might damage the transistors if it cannot turn off fast enough. The fact that it was reset 7 times and attempted again meant that the VFD WAS turning itself off before the transistors were damaged.

So so there is ZERO CHANCE THAT THIS CAUSED ANY HARM ANYWHERE ELSE IN YOUR FACILITY!

Move on you solving your procedural problems wherein your operator ignored a major fault on the VFD and kept resetting it 7 times before calling someone, that’s a MUCH worse issue! You almost lost a very expensive piece of equipment; the VFD itself. It’s also possible that this has stressed the transistors and the next issue could cause them to fail, so be prepared for that by having a spare VFD on hand ready to drop in its place to avoid costly down time waiting for a replacement, because that is the kind of failure that will not be repairable.

As JRaef already said, in my opinion also, the supply line system is grounded [that means the transformer neutral is grounded] but the output system [vfd output] is not. Since you have not a separating transformer with ungrounded neutral a circulating current is created from the supply transformer up to the faulted place-in inner side of the motor or on the connecting cable.

If the VFD protection is correct settled the fault will be cleared in time.

A high level voltage drop from the supply transformer up to VFD input will feel the other single phase on all three phase but only up to fault clearing time.

If you delete the grounding connection at motor side you keep the motor functioning but the induction motor outside body will remain energized and someone could get electric shock.

You have made a right diagram of our system

But with one correction motor was earth with separate earthing

During fault why fault current not discharge through earth ?

If there is circulating current through faulted phase earth-transformer neutral why not other system were effected supplied by same transformer ?

If the other machines-single phase-was affected that means one of the phase or neutral got an overvoltage.

If it happened on all phases then neutral was overvoltage.

The high voltage potential could be transferred to low voltage system through PE wire in a case of high voltage to earth fault.

The utility company would check if the substation earthing grid comply with BS EN 50522.

A lightning stroke on the high voltage installation could rise the earth potential at the transformer site and then could be transferred through PE.

If in doubt as to the safety of personnel and the robustness of the equipment, then isolate it from the supply and have a qualified Electrician go through the installation with a view to correcting the several faults present. One doesn't want to appear in court defending a case of Criminal Negligence or Corporate Manslaughter!

It's a simple as that. Please now leave this forum and do what is so urgently needed. There is no time to waste.

<unsubscribes>

Thanks

As we are approaching our planned shut down in coming week

I want to know all the possibilities so that same can be corrected in one go

The earth fault setting is very precise in the VFD. If the drive trips with earth fault there is 100 % chance that something is wrong with the motor or the cable. The cable must be checked along with the motor.

In most of the ground fault cases when an induction motor runs on VFD, you will find the cable defective.

The earth fault can not travel at the supply side.