<i>Moderator note: This is an unedited or as some would say uncensored post</i>

To all:

Of late, my posts have attracted a number of Cowardly "Neighsayers" which have far exceeded the number of Thankful "Yeasayers", and I have had enough! My reasons(s) follow:

In the 12 years (ca) I’ve been associated with this forum, I presented more than 1,800 posts as well as responses to more than 2,000 off-forum requests for help or information. Even though there were the expected crazies, the to and fro was mostly professional. Furthermore, differences of opinion were plentiful and varied, but handled with common courtesy! But, in recent times there has been a preponderance of Kindargarten-like features to "reward" (for a better word) posters! And, at my age I don’t need more grandkids, than the 12 I have.

I will repeat an earlier Caveat, "I won’t accept anonymous requests for information or help... something to do with the fact that anonymity breeds boorishness!" Therefore, anyone seeking advice, help, or even simple information, must go the extra mile, i.e., provide me with some personal information. All I ask is that you provide your full name, your affiliation with a company (or school), and your location! In return, if wanted, I will send you a resume covering some 56 years experience... an abridged or complete version or both!

In closing, I would like to quote the late Bob Dylan's Lyrics, "The line is drawn, the curse is cast, the slow one now will later be past, as the present now, later will be past. The order is fadin', and the first one now will later be last. For the times they are a changin'."

Finally, "Chickens by any other name are still chickens!"

Regards, Phil Corso (cepsicon [at] aol [dot] com)

 

Hi all, your friendly local moderator here,

>Of late, my posts have attracted a
>number of Cowardly "Neighsayers" which
>have far exceeded the number of Thankful
>"Yeasayers", and I have had enough!

Just to let people know, Mr. Corso has the second highest number of yea or yes this was helpful posts on the site.

Regards,
Peg Ferraro, Control.com moderator

 

Phil Corso,

As I said earlier (more than six months earlier), I think the reason people don't understand the Neutral Disconnect switch (89ND) is because they don't understand why the neutral needs to be disconnected from earth during motoring (starting) operation, and then re-connected during generator operation. It's as simple as that, really.

I would add that most AC induction motors do not have their stator windings connected to ground. The wye- or delta-connections of AC induction motors are not always grounded.

However, as you have said, the Neutral Grounding Transformer (NGT) is used to provide some impedance between the wye-connected neutral of the AC synchronous machine when being operated as a generator. If the wye-connected neutral were directly connected to ground then certain faults could be very disastrous to plant equipment. And, protective relays monitor the voltage across/through the NGT to detect problems before they get too disastrous (or at least that's the intent).

So, on these machines, when being started the generator breaker is open and a "switch" (89SS, Static Starter switch) is closed to connect the variable frequency drive (LCI, Static Starter) output to the generator terminals, and the wye-connection of the generator neutral is disrupted by opening 89ND (Neutral Disconnect switch). This occurs during starting, when the AC synchronous machine is being used as a motor.

Once the unit reaches approximately 95% of rated speed, the variable frequency drive (LCI, Static Starter) is shut down and 89SS is opened, and then 89ND is closed to tie the wye connection of the generator terminals to ground through the NGT for operation of the AC synchronous machine as a generator.

I was only asking if you could provide some explanation/insight into the specific reasoning for this switching operation. It has to happen; the system doesn't work if it doesn't. People are just trying to understand why it needs to do what it does--and has to.

As I said, also, I've never given it a lot of thought. I just know that it needs to do that to work correctly. (My bad; I hate it when people tell me, "Because that's the way it works!" but sometimes that's the answer--until we learn otherwise.)

And speaking of bad, sorry to hear about your situation.

 

CSA... Did I not adequately answer your 13-May-11 (13:23) post? Specifically addressed to me, you stated:

"Because this thread started out asking why the neutral must be grounded if the transformer is already grounded. You said in your Item 5, they're not related. But it's still not clear why the generator neutral must be grounded(?)"

A verbal 'Yes' or 'No' will suffice!

Phil

 

Phil Corso,

Yes!

 

CSA... I must admit your behavior is consistent!

So, then, you still don't know why the alternator which is connected to the Delta-side winding of the GSU must be grounded (NGR, NGT, or other, even though the GSU Y-side winding is already grounded?

Perhaps, some one else can enlighten you!

Phil (always there for you!)

 

Hi everyone,

Just been looking back and reading this entire thread, and it reminds when I posed exactly the same question when starting my employment at a 9FA site, using an LCI to start the machine.

I have been told that during initial starting of the LCI, when rolling off from barring gear, that the LCI output energises two of the three (phase) stator windings at the same time, in rotation. I'm not entirely clear on the reason - maybe something to do with limiting flux, slip or reducing current draw on the LCI incomer / transformer. At the instance that two of the phase windings are energised, and the other is not, effectively you have an unbalanced three-phase load, which would result in a neutral current if the 89ND switch was closed from the generator star point (or neutral) to earth. This was explained to me on electrical switching courses, and also by a GE LCI / EX2000 TA when I grabbed 5 minutes of his time. Take my information with a pinch of salt though please, although I have a basic understanding of three-phase electrical theory, my field of expertise is controls and instrumentation. Maybe Mr Corso could expand on the theory if correct? Also CSA I would be keen to hear you thoughts on this too.

Thanks.

 

(Sorry; I didn't mean to shout.)

 

> CSA... I must admit your behavior is consistent!

Thanks, Phil Corso. I do pride myself on being consistent, as most engineers (oddly enough) are not.

But, I did/do understand your explanation.

So, consistently, I am confused by your post.

I think it best if we just agree to disagree (I think others have said that to you before, perhaps even myself--if I'm being consistent) and discontinue any further exchanges.

How about that?

A thumbs-up from me!

(Sorry, again, for shouting.)

 

Mellberg... normally a thread covers one topic. Unfortunately, this thread addresses three:

o Connection of the ANSI Device # 89.

o Explanation of how the LCI system is used to convert the alternator into a motor for accelerating the GTG machnery-train from standstill to near operating speed.

o Elimination of the misunderstanding by some, that because the GSU is grounded/earthed, then, the alternator need not be!

If you need additional detail contact me off-forum, keeping in mind the Caveat in my 18-Oct-11 (7:01pm) post to this thread.

Regards, Phil Corso

 

Mellberg... Mellberg... a 4th topic I overlooked was "Ungrounded Systems."

Phil Corso

 

OK Phil. It seems on reflection I have misinterpreted the OPs question. I will contact you off forum regarding my previous post as you suggest.

Thanks.

 

Phil and I have had some discussion off-forum regarding the reason for having the 89-ND switch in the first place, here I'll share his explanation.

"Your concern that a winding-current unbalance will cause a problem with the Neutral-Grounding (NGT) system is unfounded:

o ignoring harmonics, the sum of the 3 winding-currents is zero, hence no neutral spill-over.

o the source is delta-connected (even if there are intervening transformers, one between the source and the LCI front-end, the other between the LCI back-end and the alternator!)"

"In conclusion, the neutral is isolated during start-up to prevent a DC-Link ground-fault from saturating the NGT!"

"NGT" - Neutral Ground Transformer - the transformer which helps restrict the maximum fault current by having a nominal resistance connected across its secondary winding with the primary winding in series with the neutral ground connection. If there was a DC fault current, from the LCI DC Bus, the secondary winding would have little or no effect because of the absence of changing flux associated with AC in the primary winding.

 

Mellberg,

Thanks for the information!