Disconnecting Switch Generator Neutral Grounding Equipment


Thread Starter

ann jano

I am installing a disconnect switch between the wye connection of a synchronous generator and neutral grounding equipment (i.e. transformer with resistor in the secondary). The purpose of the switch is for isolation the neutral grounding equipment only (when maintenance or testing is needed).

The switch will be rated for 600 A continuous current (even though the line-ground fault will only be ~30 A), and for 240 V line-to-line.

Am I supposed to specify additional ratings such as:
- load making current
- fault making current

I don't think these are necessary because this switch is for the neutral of a generator. If I am wrong, then why are these necessary? And how do I choose these parameters?

Also, how does one know what the maximum amount of current that will flow through the neutral will be in any given scenario? Is a single line to ground fault the worst case? What about sever phase imbalancE?

AJ... the system you describe is called an NGT or Neutral-Grounding-Transformer (NGT) ystem. I suggest you search the Control.com archives and acquaint yourself with the basics.

Regarding specifics; does the 240 V value represent the generator line-line or phase-neutral voltage?

Please forward the generator nameplate data.

Answers to your questions will be addressed once it is clear what the design objectives are!

Regards, Phil Corso
Hello Phil:

I looked through the archives and didnt' find anything about a disconnect switch.

I am familiar with neutral grounding to an extent, and followed IEEE C69.92.2 to size a high resistance grounding system and know what size of NGT & NGR I need. Now I need to size a disconnect switch and what I need to confirm is whether specifying the voltage and continuous current is good enough, or wehther I have to ALSO specify a short time circuit rating (please ignore my original request... i just realized load making is unnecessary, fault making is unnecessary).

Design spec:

NGT primary voltage : 16 kV
NGT secondary "": 240/120 V
NGT will have 10 min overload
NGR: 0.07 ohms with 2000 A rating
Single line-ground fault current flowing through neutral: ~48 A

The disconnect switch I need is 16 kV and 600 A continuous. I am choosing 600 A because that is the size of disconnect switch that is more typically available. Also, the cable from teh disconnect to the neutral of the generator may be large.

FYI This is for a 577 MVA/16 kV/0.90 pf hydro generator.

Also are you experienced with sizing the cable that would connect from the neutral wye of a generator to the disconnect switch? Would it be based on simply the single line-ground fault current flowing through the neutral (in my case, 48 A) + 20% margin..... or are there other worse case scenarios to consider (such as a short circuit in the generator + internal fault in the transformer)?

IEEE C69.92.2 is easy to follow for the NGT and NGR, but there is no guideline for connection leads (cables) or disconnect switches ! Where I work, there is no strong design basis for these.

Thanks for any advice.
Allow me a small question.

Why complicate matters and insert a disconnecting switch in the star-point circuit of your generator, rather than remove a link in that same circuit, if the disconnection is required solely for testing/maintenance purposes?

An appropriately located disconnectable link would be more that fit for purpose.

On the other hand a disconnecting switch, especially if it being contemplated to have this remotely controlled, will then require interlocking with the generator control systems to be sure that it is in the proper position before the generator is started.

Mind you, the disconnecting link would need to be 'controlled' by appropriate work permits to prevent mishaps.

Actually disconnecting the links (i.e. between the generator neutral terminals and the star point) is more complicated. The generator has 6 circuits per phase, and there are connections with 6 bolts (and other links) per circuit. That's 108 bolts to remove and install and check torques on every time... A disconnect downstream of the star point is much simpler, trust me.

The disconnect switch would be manual (disconnect with a hook stick). It is also local (not remote). We have lockout procedures to prevent leaving it open, grounded, etc, when starting up.
I seem to be missing out something.

From what I am understanding, by installing a neutral disconnection switch you will still have all your six circuits per phase connected to each other on the common star point busbar, so that will not help you much under certain test conditions.

In addition, if your grounding circuit is designed for ~48A (according to your previous posts), then surely between the busbar mentioned above and your final ground point, there must be a simple two bolt disconnection link somewhere, which you can easily remove, and this will have exactly the same function as your disconnection switch. The switch you intend installing will not be connected to the disconnecting links you mentioned. It will be installed downstream somewhere along the grounding circuit.

Sorry for stating all the above, but sometimes we tend to over complicate things, when much simpler solutions will be more than fit for purpose.
Jojo... I suggest you run the numbers to determine the physical size associated with a dual-winding alternator’s phase leads, and its neutral connection!

AJ... more later. In the interim:

(Q1) Can you send info on the distribution xfmr capacity.

(Q2) How much of the winding (in %) do do you expect to achieve?

(Q3) Can you confirm that the ground-fault current exceeds the maximum capacitive current in the 16 kV system, including surge suppressors, and lightning arresters?

Regards, Phil Corso