could someone please explain why GE turbine generator needs an 89nd switch when it appears that gsu transformer is grounded. Thanks for any reply

 

Don't understand your question.
ANSI 89 is a line switch...is this what you mean?
Is the gsu solidly grounded or impedance/resistance grounded? Sometimes the net-gnd bond is opended when parallelling with grid and closed in island/standby mode. Also effects the type of speed (gov) regulator and voltage (VAR) regulator.

 

I believe the 89ND is the generator neutral disconnect switch, not the GSU neutral disconnect switch.

The originator needs to look at the One-Line Diagram of the unit power distribution to confirm this.

I believe that a unit with an 89ND would be a GE-design heavy duty F-class gas turbine-generator, which means the torque required for accelerating the unit during starting is obtained from the synchronous generator which is temporarily "converted" to a motor and driven by the "static starter"/LCI (Load Commutated Inverter). During this phase, I believe the generator neutral is disconnected from earth, and once the acceleration is complete and the synchronous machine is to be "converted" back to a generator, the 89ND is closed to ground the generator the neutral.

The unit One-Line Diagram should reflect this, or something similar.

 

Thanks for replying..

Yes an 89ss is used with LCI for starting, this opens at predeterminded speed (95% ?) and then 89nd closes. Our big electric motor now becomes a generator. I don't really understand why the 89nd has to ground the center tap of the y windings of the generator output when it appears that the stepup transformer is grounded on the primary side.

Thanks for any reply

 

Okay; I think I understand the question now. You're not asking why the generator neutral is disconnected during starting; you're asking why the generator neutral is grounded at all.

I've never considered this question. I'm so accustomed to seeing the neutral of a wye-connected synchronous generator connected to ground that I've never stopped to ask why it needs to be.

I've looked through some of my reference texts, and I don't see any real explanation of why the neutral is or must be grounded. There are just diagrams showing that wye-connected machines are grounded and delta-connected machines are not grounded. One of the texts actually states the neutral connection to a wye configuration is not always provided, but it doesn't state when it is used and when it is not (it must be one of those intuitively obvious things that isn't intuitively obvious to everyone, or to me anyway).

Perhaps we could ask Mr. Phil Corso to provide some insight to this question? Is it safety? It is just a convention? Is it for reference?

 

Thanks CSA. Randy, one picture is worth a thousand words! Send a simple Single-Line-Diagram, even a pencil sketch, to me!

Regards, Phil Corso ([email protected])

 

Sorry for the delay Mr. Phil. I've been out of town for awhile. I will send you a sketch because I am interested in learning. Thanks

 

I'm really trying to understand system better..Thanks for replying

 

Randy, thanks for the sketch. Recapping:

1) A wye-connected 18kV Generator is connected to a 500kV grid through a Yd step-up transformer ('Y' represents the HV winding and 'd' the MV winding.)

2) An ANSI Device #89 Line-Switch is located between the generator's neutral and ground (earth), while the Y-side of the step-up xfmr is solidly-grounded (earthed.)

3) The generator is started as an induction-motor via, as CSA puts it, a LCI (Load Commutated Inverter), although I would have said a Natural Comutation Converter.

4) During the start-phase, the 'motor' windings can't be grounded because of operation of the LCI, hence the need for an 'open' neutral connection which is the sole reason for the '89' switch! The '89' has nothing to do with GOV or AVR action, or sytem protection.

5) Also, the fact that the Y-side of the step-up transformer is solidly grounded plays no-part in the operation of the motor-generator transition.

Unrelated Topic:
Randy, the following has nothing to do with your original question. I also understand your sketch is rudimentary. But, to satisfy my own curiosity, can you find out if the generator's neutral is solidly-grounded, or is there some sort of current-limiting. Seems like the Generator and Grid-Transformer are very costly assets to expose to to high ground-fault currents associated with a solidly-grounded 18kV system.

Regards, Phil Corso ([email protected])

 

Mr. Phil,

I was re-reading your reply and just wondering about a few things. The 89SS(in my situation) does tie a LCI to the generator to "motor" it during startup. At the same time an 89ND switch is open. When 89ss opens(motor becomes generator) 89nd closes. This is where I'm kinda confused. Don't really understand the 89nd switch like I should.

Thanks for any reply...R.barrett

 

R. Barrett...

My comment was based on the sketch Randy1588 provided that showed just one 89nd switch used to connect the generator's neutral to ground.

I would appreciate a sketch illustrating the 89ss to connect the LCI in your situation.

Regards, Phil ([email protected])

 

Phil,

The 89SS is just a big disconnect switch between the LCI and the generator stator, to isolate the LCI from the stator during generator operation and to connect the LCI to the stator during motor operation.

You have basically answered the question in your earlier response:

4) During the start-phase, the 'motor' windings can't be grounded because of operation of the LCI, hence the need for an 'open' neutral connection which is the sole reason for the '89' switch! The '89' has nothing to do with GOV or AVR action, or sytem protection.

5) Also, the fact that the Y-side of the step-up transformer is solidly grounded plays no-part in the operation of the motor-generator transition.

Also, you asked about the neutral grounding connection. If I recall correctly, most of these applications have a large resistor in the neutral grounding circuits. (Which is probably going to generate another question, so if you could explain that in anticipation it would probably help.)

It seems the difficulty here is explaining why the generator neutral needs to be grounded when the synchronous machine is being operated as a generator. As you have stated, the neutral connection must be isolated from ground when the machine is being operated as a motor to protect the LCI output. That seems pretty straightforward.

But, the problem seems to be understanding why the machine's neutral must be connected when being operated as a generator, since if it didn't have to be connected to ground as a generator it wouldn't have to be isolated when the machine is a motor.

If you can explain why a generator neutral is grounded (either directly or through a ground a resistor). There must be some physical reason which could help the understanding. Whether it's for machine protection, or reference (phase to ground), personnel protection--whatever the reason(s). I think if the reason(s) the neutral of the generator must be grounded can be defined that will answer the question.

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.

 

Mr.Phil,

Sorry I haven't responded sooner....Been working tons of ot..I thank you for your response..I now understand that the only reason for the 89ND (Taking center tap of Y generator output) is for monitoring. The generator really doesn't need this to run ...right?

As you suggested 89nd is tied to a PT..I'm assuming for indication...

Thanks..Randy

 

I took a look on the SLD and noted that there is a NGT that seems like the key to 89ND myth. It is kind of powering Generator Management Relay. Not sure why must it taps from there.

 

More specifically, the center tapped NGT is for ANSI 59Aux, 27TN, 27Aux and 64TN protection relays to function.

 

>>Ok I just got a lesson from a GE Control Room operator;

My understanding of the 89ND is that it is for protection of the Generator during normal operation. The LCI 89SS is the protection, by way of 41A, during start up. So during normal operation the 89ND is closed and is the indirect protection for the generator during normal operation. (I'm only talking about a 19.5kV Gen). Say there is a phase to ground fault; with the 89ND closed, the fault sends energy to the NGT, which sends energy to the NGR, which sends energy to the G60 relay, which sends a signal to the 52G to trip. I don't know if I explained it right, but that's how I understand it.

 

The generator neutral needs to be grounded to sense earth faults. Suppose if the generator neutral is ungrounded and one of the phase windings get grounded, this fault will go unnoticed until a second fault in the same phase or a different phase winding occurs (Line to line) and to trip the machine on over current/differential. Grounding the generator neutral through a NGT (neutral grounding transformer)or NGR (neutral grounding resistor) will provide a high resistance/impedance to ground fault current limiting its value, but sufficient enough for the CTs of protection relays to sense and trip the machine without causing a catastrophic damage to generator windings. I hope this helps. To know more, the advantages of grounded systems over ungrounded system is readily available in World wide web

 

CSA... Reviewing this thread after Anthony's 16-Oct-11 (02:28) post, revealed that I failed to reply to your 22-May-09 (13:23) query. I sincerely apologize for not answering!

You asked, paraphrasing, "Why is the alternator-neutral grounded if the transformer-neutral is already grounded?"

The alternator is connected to the transformer's low-voltage winding which is delta-connected. The transformer's high-voltage winding is wye-connected and its neutral is solidly-grounded.

If the alternator-side is left ungrounded, it and every electrical apparatus connected to it would be considered to be an "Ungrounded System," meaning without an intentional connection to ground except for voltage measuring or indicating devices. In reality, the alternator-side equipment, i.e., alternator-winding, exciter xfmr, station xfmr, cables, bus, surge suppressors, lightning arestors, etc, are "capacitively grounded" by the fact that all electrical components have distributed-capacitance to ground.

This capacitance, then, makes it possible for very destructive transient over voltages to occur for phase to ground-faults, simple-switching operations, and arcing-phenomena that occurs during fault-clearing.

The transformer's high-side neutral ground has little influence ameliorating voltage transients on the alternator-side, because it does not materially alter the distributed-capacitance effect on the alternator-side. (It some cases it actually aggravates the situation!)

Let me know if additional explanation is required?

Regards, Phil Corso

 

GTG... "Ungrounded Systems" (admittedly low-voltage) can be found throughout various industries, especially the marine industry, including the US Navy. The only argument in favor of "Ungrounded Systems" is the one you cited; a ground-fault will not trip the affected circuit and plant operation can continue until the fault is found; hopefully, before a second-fault occurs. A single ground-fault is easily detected. In fact ground-fault detection devices are readily available.

Now some technical information: a ground-fault on any phase will cause full line-to-line voltage to appear between ground and the two unfaulted lines. These voltages are only 73% of rated voltage. Insulation levels are certainly adequate, unless the fault persists for long periods, or if the fault produces an arc. Then, the resultant over voltage can rise to infinity (theoretically) but more like 2 to 4 times (depending on the magnitude of the distributed capacitance), thereby causing insulation breakdowns throughout the system.

For proof of the latter you don't have to go to the World Wide Web, instead I urge you search the Control.Com Archive for "Ungrounded System" horror stories presented by Forum users.

(BTW, often electrical equipment for "Ungrounded Systems" is specified with upgraded insulation levels.)

Regards, Phil Corso

 

CSA... Doesn't my belated answer to your specific question merit any comment at all?"

Regards, Phil Corso