3.3kv substation 2

[paliha]

I have a few MCC's each driving a few 3.3kV motors. (total 1200kW load) A 11kV /3.3kV substation is to be constructed a 3.3kV distribution panel is to be designed for this. There is a choise to be made for the substation transformer secondary whether to go for
(1) star with neutral solidly grounded or
(2) delta with ungrounded arrangement?
If i go for ungrounded arrangement, what type of protections to be provided on the incoming vacuum circuit breaker? What are the advantages of each option and what is the standard practice?

 

[slavag]

Hi Paliha.
From my point of view, best solution is:
Low impedance grounding of 3.3kV with 300-500A ground/earth fault level.
Solidly grounding = high level of ground/earth fault current.
Ungrounded system= needs a special protection for ground/earth fault selectivity. If you will protected only infeed VCB, possible add VT with broken-delta connection and used function 59N only. But in this case you tripped whole bus. Another case, used 59N for signal only and in case of second fault ( cross-country)system will tripped on the phase to phase fault.
Regards.
Slava

 

[slavag]

BTW.
On this moment, I write SW for trnsformer protection y/d (ungrounded system) that feed aux. bus of power plant and according to customer 59N used for signal only.
Regards.
Slava

 

[ScottyUK]

I agree with Slava - probably best to use a Dyn* transformer with a low impedance earth as this will restrain phase-earth faults (by far the most common) to a few hundred amps which minimises the resultant damage.


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If we learn from our mistakes I'm getting a great education!

 

[dpc]

+1 for low resistance grounding. With today's relays, you can limit fault current to 50 to 100 A and still have good selectivity.

 

[slavag]

Hello.
To DPC's good point, I would like add:
In this case for the ground/earth fault protection I would recommend use ring/toroid/window CT ( around whole 3 phases).
Regards.
Slava

 

[ScottyUK]

Hi Slava,

What would you do where the circuit is too large to use a single cable and requires multiple single-core conductors? In
this case it isn't easy to fit a CBCT.

I'm curious because I have an installation like this which uses residual connected CTs but coordination with other relays requires such a low plug setting that the scheme is prone to misoperation during switching.


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If we learn from our mistakes I'm getting a great education!

 

[slavag]

Hi Scotty.
We have two solutions:
1. We have small local CT factory, that provide for all local industry CBCT according to request window size, problem it isn't type tested CT, but work fine.

2. Second please see attached catalog of ABB Poland factory,
are include several types of split ring core.

3. And small tip, that I recommend to all my customers:
always use CT with minimum range 100/1A 10P10 it's better solution for the SEF protection.

4. As I remeber, before some time you asked about option connect to CBCT in parallel. I found before 2-3 weeks some application where is work, isn't exactly your case, but like to it. 3 CBCT's connected to parallel, each one on grounding conductor of single phase AT 400/161kV 167MVA. In operation about 12years.
Best Regards.
Slava.
Have a nice vacation.
Happy New Year

 

[jghrist]

There is good information on the advantages/disadvantages of different grounding systems at

http://www.i-gard.com/publications.htm

 

[ScottyUK]

Thanks Slava,


Those rectangular window CTs (KOLA 06 series) look like they are worth a closer look when I get back to work. I don't have the dimensions of the problem switchgear cubicle at home.

Happy New Year



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If we learn from our mistakes I'm getting a great education!