Special Motor Insulation for ungrounded system? 2

[electricpete]

NEMA MG-1 states the following:
14.31 MACHINES OPERATING ON AN UNGROUNDED SYSTEM
Alternating-current machines are intended for continuous operation with the neutral at or near ground
potential. Operation on ungrounded systems with one line at ground potential should be done only for
infrequent periods of short duration, for example as required for normal fault clearance. If it is intended to
operate the machine continuously or for prolonged periods in such conditions, a special machine with a
level of insulation suitable for such operation is required. The motor manufacturer should be consulted
before selecting a motor for such an application.
Grounding of the interconnection of the machine neutral points should not be undertaken without
consulting the System Designer because of the danger of zero-sequence components of currents of all
frequencies under some operating conditions and the possible mechanical damage to the winding under
line-to-neutral fault conditions.
Other auxiliary equipment connected to the motor such as, but not limited to, surge capacitors, power
factor correction capacitors, or lightning arresters, may not be suitable for use on an ungrounded system
and should be evaluated independently.

Please consider each of the following questions individually:1 - Would/should a motor supplier give you any different insulation if you tell him the motor operates on 480vac ungrounded system than if you tell him the motor operates on 480vac grounded system? Would the answer to this question have been any different in early 1980's?
2 - Shouldn't a standard 480vac motor should be adequate even for mild transient overvoltages expected in ungrounded system. Factory hi-pot is 2kv which is around 7 times normal line to ground (~260vac). Maintenance hi-pot levels are in the range of 1.5kvdc.
3 - If standard 480v motor is adequate/reliable and is all that an OEM would provide for this specificaiton, then what is the purpose of the NEMA statement quoted above. Perhaps it is intended towards higher voltage systems where the margin between hi-pot voltage and normal line-to ground becomes much smaller (using the 2*kv+1 rule the 1 plays huge role for 480v motor but diminishing role for higher voltage motors).

 

[electricpete]

By the way, are ungrounded systems used above 480vac?

 

[electricpete]

typo correction: maintenance hi-pot levels are in the range of 1.5kvac[/] (not dc).

 

[jbartos]

Suggestions marked ///\\\:
Please consider each of the following questions individually:1 - Would/should a motor supplier give you any different insulation if you tell him the motor operates on 480vac ungrounded system than if you tell him the motor operates on 480vac grounded system?
///Please notice that custom-built motor may be built to various specifications. The motor manufacturer would probably cover the product by some limits on the voltage transients due to arcing on the ungrounded system.\\ Would the answer to this question have been any different in early 1980's?
///It would probably be the same, since there is one electrical engineering theory only.\\2 - Shouldn't a standard 480vac motor should be adequate even for mild transient overvoltages expected in ungrounded system.
///It would depend on a definition of "mild".\\ Factory hi-pot is 2kv which is around 7 times normal line to ground (~260vac). Maintenance hi-pot levels are in the range of 1.5kvdc.
///What is the question?\\3 - If standard 480v motor is adequate/reliable and is all that an OEM would provide for this specificaiton, then what is the purpose of the NEMA statement quoted above. Perhaps it is intended towards higher voltage systems where the margin between hi-pot voltage and normal line-to ground becomes much smaller (using the 2*kv+1 rule the 1 plays huge role for 480v motor but diminishing role for higher voltage motors).
///If the manufacturer states that the product meets MG-1, then the product should meet all adverse conditions in the MG-1 including the above ones.\\

 

[GordS]

Pete
1. Can't respond to question #1 as I don't supply motors.
2. I think the stress of a DC hi-pot is much less than the stress due to an arcing fault on an ungrounded system. Such a fault results in a high frequency transient that is related to inductance and capacitance in the ground circuit. Depending on the rise time of this transient, the resulting voltage may no longer distribute itself along the entire winding. High frequency transients tend to stress the beginning of windings.
3. I don't have any ungrounded systems but I have had high resistance grounded systems improperly designed that resulted, by calculation, in overvoltages approaching those that can be expected on ungrounded systems (i.e. approx 6x).
During some ground faults, we seemed to also suffer a rash of motor failures occurring within seconds to days of the fault. I think this indicates motor insulation is not designed for the transients due to arcing faults on ungrounded systems. I also read a paper entitled "Ground Fault Protection for Ungrounded Distribution systems" that mentions a plant on Oklahoma that lost 6-600 hp 2400 v motors at one time from this kind of overvoltage. I think this is why NEMA is suggesting specific motor construction standards for upgrounded applications.

 

[electricpete]

Thx Gord. Lots of good info to think about.

One ironic aspect: NEMA is actually not giving any specific guidance. There is no different specified hi-pot or surge test requirement for this ungrounded motor.. only consult the oem... apparently up to their discretion. Our local oem-affiliated repair shop motor insulation guru tells me he would give me the same rewind whether it was 480VAC grounded or ungrounded system. (600v standard insulaton system.)

 

[electricpete]

Kumar (Edison) - I see your comments have disappeared. I'm not sure what happened there. A key point I was trying to make is that my question applies to grounding of the power system (transformer), not grounding of the motor itself. I'm interested to hear your thoughts.

 

[edison123]

pete,

This has happened before.

As regards to your original question, I agree with your local motor repair shop. I have rewound both grounded (solid/resistance) system and ungrounded system fed MV motors with the same insulation system / HV test levels and they are all working fine. As for LV ungrounded systems, we do not have any in our country and so I cannot comment on that.

 

[electricpete]

Thanks kumar, that is interesting.

How high voltage do ungrounded systems go in your country?
(I was under the impression they were limited to <=575v here).

 

[edison123]

pete,

The maximum motor capacity we had rewound was 6.6 KV, 3.5 MW (Boiler Feed Pump) Motors which are fed from ungrounded unit auxiliary trafo (DD0). This was one of earliest power stations (50's origin) in our state. The later power stations use 6.6 KV grounded supply systems. In both systems, we have rewound MV motors with the same winding technology and they are all working well for the past 25 (earliest) to 5 years (latest) without any problem in spite of hostile working environments (frequent voltage dips, unit pull outs, over voltage, under voltage, frequency ranging from 47.5 to 51.5 Hz) etc.

 

[jbartos]

Suggestion: The voltage transients within the ungrounded power distribution system may become very high and repetitious due to arcing. Therefore, the motor manufacturer has to set a limit on the overvoltages while engineering and designing the motor, since the motor winding insulation could become excessively bulky.
This is where, the manufacturer statement &quot;motor meets NEMA MG-1,&quot; has to be further documented by the motor manufacturer. However, this is a typical manufacturing issue pertaining to other equipment, e.g. Solenoids, etc., not only to motors.