Question regarding steam turbine Generator Current transformers (multi core vs dedicated) 1

[Kash2013]

For a large Steam turbine Generator (300 MW class), any preference for dedicated current transformers for each phase vs multi core current transformers?
i.e. One multi core CT on line and One multi core on Neutral side vs 9 on line side and 9 on Neutral side.

Any suggestion?
Thanks in advance !

 

[cranky108]

To keep costs down most installs I have seen use a fixed ratio. This also reduces the size of the CT.
Because most of these installations don't change during there life, just install the single ratio that you need.

 

[wcaseyharman]

Only downside I can think of is a CT failure on your multi core CT would be quite difficult to replace and very heavy. Spares would be more expensive too, larger and harder to store. Probably harder to source too, which may make it more likely to create costly outages in the future, although CT failures aren’t common, at least at my utility.

I’d probably go with multiple single CTs for the reasons above.

 

[stevenal]

Please explain why 9. Seems excessive. Is that per phase? Can you share a one-line?

 

[edison123]

How would you even install one multicore CT for such large machines?

Normal set up is one line CT and two split neutral CT's (since 2 pole windings are invariably two circuit windings) per phase making it total 9 CT's for a machine.

Muthu

http://www.edison.co.in

 

[RRaghunath]

Generally speaking, multi core CTs are the norm as these reduce space requirement and cheaper too.
How many cores in one CT and further details are subject to engineering consultant design for the requirements and the CT supplier for practicability of CT design.

R Raghunath

 

[Kash2013]

cranky108 (Electrical
Thanks.

wcaseyharman (Electrical)
Thanks.

stevenal (Electrical)
Thanks. 3 CTs per each phase so total 9 for line side. Same way 3CTs per each phase on Neutral side so total 9 for Neutral side.

edison123 (Electrical).
Thanks. You replied as "Normal set up is one line CT and two split neutral CT's (since 2 pole windings are invariably two circuit windings) per phase making it total 9 CT's for a machine."
As per your reply one CT on line side per phase. Actually my question is Total 3CTs per phase on Line side so Total 9 on Line side and same way Total 9 on Neutral side. Is that too much?

 

[wcaseyharman]

Has the protection and metering been designed?already? That would normally dictate how many CTs are needed.

Sometimes extra CTs are needed in the neutral end for a Unit Differential (extended transformer differential) in addition to the two sets for generator differential, and the metering and excitation that often come off the line side CTs sectionals are separated.

I don’t think I’ve seen many generators with 18 CTs though, 9 or 12 is more common.

 

[Kash2013]

wcaseyharman (Electrical)
Thanks.
We are in the early design phase with the Generator supplier so protection and metering design is not done.

I am not Generator/Elect. expert.
So can anyone advise me best way to proceed, i.e. how many Total CTs on line side(per phase) and how many Total CTs on Neutral side (per phase), if multicore is not reliable/redundant.

 

[wcaseyharman]

I just looked at a relatively new gas turbine (GE 7FA 170MW) and a recent Hydro generator (Black and Veach electrical design, 30MW) as examples of more recent units (often I’m doing work on much older machines and older machines seemed to have fewer CTs)
It seems common to have 3 per phase on the neutral side (two for differential, at least one for metering/excitation control)

The GE turbine had 3 per phase on the line side, but one was spare and one was for relaying, which is unusual.

Without knowing anything about your installation, I’d go for 3 in the neutral and one on the line side per phase. That would provide two for redundant relaying, one dedicated for metering and one dedicated for excitation.

Both of these machines appear to have used single core CTs, and so almost all the other units in my company (except for a 65 year old hydro generator), so I’d probably stick with with single core.

 

[Kash2013]

wcaseyharman (Electrical)
Thank you for your helpful feedback and sharing knowledge.

 

[waross]

It comes down to a compromise;
Price
Availability
Size, shape, physical restraints
Characteristics
Personal preference

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Kash2013]

wcaseyharman (Electrical)
Further to your reply in above thread.
for 300 MW Steam turbine Generator;
We plan for
3 CTs for each phase on line side (total 9 CTs)
3 CTS for each phase on neutral side (total 9 CTs)

Now customer is asking to keep
3 CTs for each phase on line side (total 9 CTs)
4 CTS for each phase on neutral side (total 12 CTs). The reason for extra CT on each phase of neutral side is to protect the grounding transformer from failing. If the grounding transformer fails, it places the entire system at higher risk for damage.
Does this reason make sense to provide 4CTs on each phase of neutral side?

Thanks in advance,

 

[cranky108]

The grounding transformer is a form of protection, and therefore maybe subject to damage to protect the generator.
Then again, if you don't have any faults on the generator, you should not fail the grounding transformer.

My recollection of the several large generators was that the copper bus bars off the unit have the CT's on the bus bars. And while the intent is to keep the run short between the generator and the grounding transformer, it is usually a few meters long, which is enough to add another CT. That said, how the CT is mounted maybe an issue, as it should not be touching the bus bar, and suspended from above.

Also the bus bar from the generator to the iso-bus is usually just as short, because the iso-bus is to ensure any bus fault is a ground fault, which the magnitude is kept low by the impedance on the grounding transformer.
The iso-bus then runs to the auxiliary transformer and the GSU.
There may be one or more auxiliary transformers, and an exicator transformer.

 

[wcaseyharman]

If the machine is conventionally grounded with high resistance there is insufficient fault current near the neutral point to pickup the differential anyway. The main protection of your neutral transformer and resistor is your neutral overvoltage/overcurrent relay 59N or 50N (mostly have seen the 59N applied, but occasionally a 50N).
Failures very close to the neutral, since there’s insufficient voltage to trip the 59N, are either protected by a 3rd harmonic undervoltage, 3rd harmonic ratio, or a frequency injection product that provides 100% stator ground protection.
Setting the 3rd harmonic element can be tricky and requires hours of testing different load points, and some machines, like the GE 7FH2 generators, have insufficient 3rd harmonic voltage to use this element.

If you’re really worried about faults around the neutral transformer invest in a 100% stator ground protection system like the SEL 2664S (my company is an SEL shop, I think most relay manufacturers make one).

 

[cranky108]

Agreed with the external third harmonic device. Using the third harmonic protection in most generator protection relays will not work.