480 Y Generator grounding 1

[SergioM]

Hi everybody,

In our company we have low voltage generators diesel Caterpillar of 550 KW , 480Y/277 volts, wye, 480 Vll connected to a MCC (3-phase 4-wire system).

The MCC distribute to a motors with DD connection, 3-phase, 480 VAC, grounded (bonding).

The last supervisor didnt have a grounding standard, for that reason some generators are solidly grounded (neutral) and others no.

I'm making new standarts for the company based on NEC, my question is.. Is a mistake ensure that a Solid grounding in this generators facilitates the automatic clearing of
ground faults by circuit protective equipment (fuses and
circuit breakers thermo magnetic)?

Do I have to use a deferencial protection or other protections?


Best Regards

Sergio

 

[iceworm]

First, I have no idea what the application entails - commercial offices, military barracks sy the South Pole, Off grid industrial, oil field temporary camps. They all need something different.

So my responses are general and likely don't fit your application.

Here are two good references. they are a bit old, but the information is still good.
ieee 141 Electric Power Distribution
ieee 142 Grounding of Industial and Commercial Power Systems

Grounding/not grounding is a design decision - not an NEC regulatory decision.. You get to do a bunch of research. Here are a few of the issues:

If the choice is made for un-grounded 480V, the NEC required ground detectors. If the existing system directly feeds 277 lighting, the system is required to be grounded.

Next, are you running generators in parallel? If so, there is sometimes a problem where multiple paralleled generators can deliver sufficient current to an internally faulted gen to where the unit is damaged such that it is no longer rewindable. Years back that was solved with a small reactor between the neutral and ground. One could do that and still use 277V lighting. No more.

Again considering paralleled generation, solidly grounded: Some systems ere set up such the N-G bond was in one of the generators - and all the rest did not have an N-G bond. Not a good idea. Makes pulling maintenance on the one with the N-G bond difficult.

Keep in mind that 550kw gens is 860 FLA. And may produce less than 4000A short circuit. Solidly grounded systems usually fault to ground, relatively low level. Current is not high enough to quickly trip. Eventually the fault will go phase-to-phase, current increases quickly, OCP trips quickly. But by then a lot of damage has been done. Ground fault protection of selected feeders can definitely reduce the damage. As for differential protection, depends on the zone. But still, not usually applied to smaller generators. Depending on OCP to quickly GF is not a good idea.

Un-grounded is not so good either. System is subject to arcing, restriking ground faults. The voltage can quickly climb to where equipment is destroyed from over voltage. Unless there is a special reason, last on my list.

My preference is HRG 480Y. This is based on most of my career being industrial, plenty self generated, with no grid. Ground faults only carry 5A - 10A. No damage. For phase-to-phase faults, currently climbs quickly, trips quickly. For paralleled generation, maintenance issues are solved by adding a neutral disconnect at each generator. 277v lighting requires a 480D/480Y xfm, ususlly small, often 100kva.

First thing I would do is to get a good, accurate one-line.

ice

Harmless flakes working together can unleash an avalanche of destruction

 

[electricpete]

Un-grounded is not so good either. System is subject to arcing, restriking ground faults. The voltage can quickly climb to where equipment is destroyed from over voltage. Unless there is a special reason, last on my list.

I don't disagree. But I have to mention from time to time that we have hundreds (maybe a thousand) motors on ungrounded 480vac systems at my plant and I've never seen any evidence of this phenomenon during my two decades here. If I'm not mistaken the US Navy uses similar system and also doesn't see this. I'd suspect there are system design factors that render some facilities susceptible and others not... but I don't know exactly what those factors are.

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(2B)+(2B)' ?

 

[waross]

Pete: A couple of my instructors experienced this when they were in the field.
The problem is rare, but when it does manifest, you may lose a great many motors plant wide. If an arcing ground fault develops high voltages, all the motors and equipment on that supply transformer secondary are at risk, including the supply transformer.
The worst case is a an arcing ground fault internal to a motor winding or a transformer winding. The faulted winding may act as an autotransformer and step up the voltage of the high frequency current produced. System capacitance to ground and part of the faulted winding act as a spark gap transmitter and the high impedance that the motor and transformers present to the high frequency isolate the high frequency and high voltage. The high voltage (it may be several thousand volts) sits on one phase and stresses the coil end winding turns.

Bill
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"Why not the best?"
Jimmy Carter

 

[electricpete]

Thanks Bill. I've heard that.

I agree your characterization of rare. Whether the rareness is purely time-dependent or also facility-dependent (subject to design factors) remains somewhat unproven but I have a suspicion stated above. I offered my own experience and discussion into the mix for others to do with as they see fit. It was somewhat of an irrelevant tangent to op - my bad. And I'm certainly not advocating to ignore code or standard proven approaches for new construction.


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(2B)+(2B)' ?