Providing a neutral grounding point for 11kV generators
Providing a neutral grounding point for 11kV generators
(OP)
Hello,
I have an offshore installation with an 11kV bus, like this:
- Five 11kV generators connected to the 11kV switchboard, 4nos 2.7MW and 1no 4.2MW, installed at different time.
- All generator neutrals are brought to a switching (contactor) panel so that only one neutral is grounded.
- The NGR at the neutral switching panel is 31.75ohm, limiting ground fault currents to 200A.
Now we have a plan to add 3 more generators. Unfortunately I am coming to a problem with the neutral switching panel because:
- there is no isolation switch to isolate half of the switching panel for extension to add more generator neutrals.
- I need to shutdown all the generators in order to make the neutral switching panel safe for extension, which I need to avoid.
I am considering 2 options to overcome this:
(1) Install a new neutral switching panel so that the existing neutrals can be transferred over to the new panel in stages.
(2) Eliminate all the generator neutral groundings and replace by two nos of earthing transformers that can be used to supply power generator auxiliaries. This means that the 2nos existing delta-star transformers to aux switchboard are to be replace by 2nos zig-zag transformers. The neutral grounding for the 11kV system will be provided by the 2nos zig-zag transformers.
Option (1) is possible.
Option (2) is my idea of avoiding Option (1) in order to avoid searching for new spare spaces for the new switching panel. Moreover, I won't have to worry about how many new generators to be added in the future.
I am not sure whether Option (2) is a good idea and I would like to have your views.
I have an offshore installation with an 11kV bus, like this:
- Five 11kV generators connected to the 11kV switchboard, 4nos 2.7MW and 1no 4.2MW, installed at different time.
- All generator neutrals are brought to a switching (contactor) panel so that only one neutral is grounded.
- The NGR at the neutral switching panel is 31.75ohm, limiting ground fault currents to 200A.
Now we have a plan to add 3 more generators. Unfortunately I am coming to a problem with the neutral switching panel because:
- there is no isolation switch to isolate half of the switching panel for extension to add more generator neutrals.
- I need to shutdown all the generators in order to make the neutral switching panel safe for extension, which I need to avoid.
I am considering 2 options to overcome this:
(1) Install a new neutral switching panel so that the existing neutrals can be transferred over to the new panel in stages.
(2) Eliminate all the generator neutral groundings and replace by two nos of earthing transformers that can be used to supply power generator auxiliaries. This means that the 2nos existing delta-star transformers to aux switchboard are to be replace by 2nos zig-zag transformers. The neutral grounding for the 11kV system will be provided by the 2nos zig-zag transformers.
Option (1) is possible.
Option (2) is my idea of avoiding Option (1) in order to avoid searching for new spare spaces for the new switching panel. Moreover, I won't have to worry about how many new generators to be added in the future.
I am not sure whether Option (2) is a good idea and I would like to have your views.






RE: Providing a neutral grounding point for 11kV generators
RE: Providing a neutral grounding point for 11kV generators
RE: Providing a neutral grounding point for 11kV generators
Actually it ( used one resistor)reduce GF level.
In lot of applications ( specialy offshore generators) is used only one resistor/grounding-neutral point per system.
digitrex , is very difficult say something.
Zig-zag is good idea too.
You can isnstall new generators with switch (grounding switch)and resistor, for every one and add some logic for rgulation of number of connected resistors in the system.
For my pinion, it's more simple, then change now exsisting installation.
Good Luck.
Slava
RE: Providing a neutral grounding point for 11kV generators
"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)
RE: Providing a neutral grounding point for 11kV generators
RE: Providing a neutral grounding point for 11kV generators
RE: Providing a neutral grounding point for 11kV generators
I was away for a few days and only able to respond today.
I am weighing the different options to see which is the better for the existing site, bearing in mind that spaces at offshore location is scarce and precious.
I was hoping for more comments from you folks on my Option 2.
Now I have 3 options to be considered:
(1) Installing a new neutral switching panel and transfer existing generators' neutral to the new panel. This will need to be done at stages and each generator will be shutdown for the transfer work.
(2) Install zig-zag transformers to provide fixed neutral groundings. Existing generators' ground fault protection relay settings will be adjusted/reduced to the lowest possible. Each of the 2 existing distribution transformers will be shutdown for changeout to zig-zag transformers.
(3) Install individual NGR for each generator, including the existing units. Each of the existing generators will be shutdown for the NGR installation. Ground fault protection relays of the existing generators and feeders will be reviewed to accommodate the variable ground fault levels depending on no. of online generators.
Of the 3 options above, I think Option 2 has less interfacing works with the existing running plant, eventhough it would involve lifting/installation of heavier equipment.
These are my views, I will be glad if you can give me advises/comments on Option 2, something which I need to consider.
RE: Providing a neutral grounding point for 11kV generators
You can go to option2 w/o any problems.
Standard configuration. Don't worry.
BTW, option3, I meant other, only NEW additional generators with individual NGR for each generator, exsisting installation will no change.
Good Luck.
Slava
RE: Providing a neutral grounding point for 11kV generators
RE: Providing a neutral grounding point for 11kV generators