Grounding system cause motor problems? 6

[electricpete]

We have a group of outdoor motors located outdoors and remote from our main plant.

We have noticed a dramatically higher winding failure rate on these remote outdoor motors than on our remaining motors which are indoors and at our plant. Many of the failures occur during rain and lightning storms. Most of the failures were phase to ground and a few phase to phase. None showed evidence of turn to turn damage except for phase failures where damage could not be distinguished Some line-end coils, some middle coils, some end-turn, some slot. (total of about 12 failures over 12 years.... sorry I can't describe them all).

We have focused on what appears to be the obvious cause: outdoor environment and possible moisture intrusion.

Someone raised another possible consideration. There is a strongly interconnected ground grid below the main plant. All transformers and generators are tied directly to this grid. However the remote outdoor area with the high-failure-rate motors is expected to have a weaker connection to the main ground grid. (we haven’t done any ground system measurements).

The motors are ug-wye connected. The systems are 13.2kv low-resistance grounded w surge caps at motors and 4kv high-resistance grounded (no surge caps at motors). This is the same system that feeds the plant motors (low-failure rate). Switchgear is all located in the plant and cables feeding these outdoor motors are ~200 meters.

Would you think based on the above there might be some considerations associated with the grounding system (or power system) that could contribute to the higher failure rate of the remote motors?

 

[electricpete]

I guess I should say that these remote motors are located in a structure rising above a large reservoir... a good target for lightning. On at least two of the occasions people reported seeing lightning in the area of the structure. Although we never saw any dramatic structure or motor frame damage we would expect to accompany a lightning strike.

 

[busbar]

There is the possibility of transient potential differences from inductance in local-versus-remote ground-grid sections. Aside: are there surge capacitors or arrestors close to the exposed motors?

Although this does not parallel your apllication, I learned early on that an industrial-setting combination of outdoor rotating machines, fog {not necessarily rain} with very-high-resistance-grounded LV distribution can cause repeated, almost pathetic ground-insulation-failure disasters.

 

[GordS]

Pete...I think you were probably on the right track originally. I would not suspect lightning because that kind of damage should show up at the entrance point to the winding and your failures are distributed throughout the motor. (Lightning impulses have a rapid rise time which means the resulting voltage distribution is over the first few turns of the winding)

 

[Shortstub]

Kabos,
1) hat is cable size and length between motor and supply?

2) What is grounding current magnitude at upstream system neutral grounding element?

 

[electricpete]

-cables approx 200m from switchgear to motor
-13.2kv system low-R grounded thru 20-Ohm (400A).
-13.2kv motors have surge caps in term box
-4kv system hi-R grounded thru 240-ohm (10A)
-4kv motors have no surge caps
- surge suppressors also on 25kv isophase bus which feeds 13.2kv thru xfmr which feeds 4kv thru xfmr.

Hi failure rate occurs on remote outdoor motors fed by both 13.2kv and 4kv.

 

[Aquarius]

There are two issues which may cause/contribute to the problem you've described;
1) system grounding - the medium voltage systems (2.4 to 15 kV) should not be solidly grounded because high e/fault currents may damage the rotating machinery; Also, It should not be ungrounded because it would be subjected to high o/voltages. You have a mixture, a low and a high resistance grounded systems ... without looking at your SLD it is difficult to know if this is correct; have a study being performed to check currents and voltages ?

2)equipment grounding - multiple bonding/connection of ground return paths is not necesserily all good, it has disadventagesuneven distribution of fault voltage, multiple paths via the parts of plant which are not intended to be conductive, path of the return current)

Because your motors fail mainly during the lightning I would suggest you check the connections of surge protective equipment. Capacitors should be connected with direct, short leads this way the rate of voltage rise at the motor terminals will be slow, then the leads to arrester can be longer so their jX will soften the suddent V drop at the terminals when the arrester sparks over.

Your motors may by damaged due to the high e/f current and/or e/f transient o/voltages..
or lightning but then as GordS pointed damage you've desribed doesn't realy support this ...
or simply motors are not of adequate enclosure protection for outdoor use and ingrees of water causes ph/earth faults - I think most likely cause..

 

[edison123]

pete,

it looks to me environment problem (moisture entry, condensation) given your configuration (grounded surge protection et. al.). don't look for devils in heaven.

 

[Shortstub]

Electricpete,

Sorry for initial name error.

Is primary of supply transformer fed from overhead line?

 

[DanDel]

'pete, are there any differences in routine maintenance megger or hipot insulation test results between the inside and outside motors? I would bet that the outside motors give consistently lower insulation results, which should indicate moisture is your problem.

 

[electricpete]

I guess I am not really looking for any further vote on whether it is more likely moisture or grounding system. As I said in my original post, moisture appears to be the obvious cause. I myself have seen strong evidence of the role of moisture. In one failed vertical motor we saw horizontal water streak marks trailing through soot on the rotor fan blades. Soot could only have occured after motor failure. Water droplets flowing horizontal after the soot deposited could only have occured while motor was spinning down after the trip... i.e. excessive water in the motor at time of the trip. We are looking closely at our air flow paths, gaskets/fit-ups, and we are rewinding motors with sealed epoxy vpi.

Still, a knowledgeable motor engineer has suggested that grounding system problems can cause motor failures. My objective in this thread is to understand that scenario better...

Let's assume for the sake of argument that we did have a relatively poor ground system at the motor location. How could that make the motor susceptible to failure? Has anyone ever heard of bad ground system contributing to motor failure?

 

[jbartos]

Question: What kind of enclosure motors have? Is it waterproof? Weatherproof?

 

[DanDel]

The only way it could cause motor failure is when the voltage between the windings and ground rises to a value above the insulation rating. Possibly this could happen if there is not a solid reference to ground(such as an ungrounded system), but I would expect the insulation rating to ground would be equal to the phase-to-phase rating.

 

[Shortstub]

Electricpete,

My query was to establish some statistical info that could help in developing a probable scenario:

How large was the population of remote motors in which 12 failures occured?

Are you able to provide some sort of historical time-related failure data?

Were any of the failures repeats of the same motor?

How many "like" motors were in the general population, indoor + remote?

Were more failures associated with a particular bus? Or a particular rated voltage?

 

[jbartos]

Suggestion: If the motor enclosure is not hermetically sealed to be waterproof, then a combination of moisture and surges will cause the motor failures.