Grid connection and fault current problem

[ivanngwh]

I have to connect a LV renewable generator to the utility's LV grid.
The LV renewable generator is 380V, TPN, 50Hz, 1500kVA, 12.3MVA fault level contribution.
The utility transformer LV side is 380V, TPN, 50Hz, 2000kVA, 24MVA fault level.
When coupled, the total fault level is about 12.3+24=36.3MVA thus the fault current is about 55kA.
The utility company said that his transformer LV terminal will damage when the fault current is >40kA and therefore the grid connection scheme is rejected.
Somebody said that the simplest solution to the present problem is to add a high breaking capacity fuse (say 2500A 40kA) on the LV renewable generator side so that its fault current will not flow to the utility transformer's LV terminal.
Then, the utility transformer LV terminal will never experience a fault current of >40kA in all situations.
Is this simple scheme feasible?
If yes, what kind of fuse is suitable for this application?
I have no experience in buying fuse of such large rating or even special characteristic fuse (if required).
Thanks
Ivan Ng

 

[Palletjack]

I’m still trying to figure out what type of fault would present the added values to the LV terminal…

 

[jghrist]

A fault on the LV system would produce 24 MVA (36.4 kA) through the transformer terminal if there is an infinite bus (zero source impedance) on the HV side. Fault contribution from the generator would not flow through the transformer terminals. There may be equipment on the LV system that would see the entire 36.3 MVA fault, but not the transformer terminals.

 

[cranky108]

It also depends on the type of generator, and type of fault. Most inverters will clamp off at 1.1 to 1.3 PU, and most will not produce any negative or zero sequence current.
Also fault currents don't exactly add like that. Fault currents add vectorially, and will depend on the source impedance.
And, are you talking symmetrical, or asymmetrical fault current?

And as stated above the utility transformer terminals will not see more than 40KA.

 

[FacEngrPE]

Recommend you run hire an engineer familiar with protection coordination. Your utility probably will require some sort of pedigree (in the us it would be a licensed professional engineer stamp).

Easy power can do this sort of calculation but you need some experience to get the inputs correct. In this case the utility constraint of not exceeding fault current 40kA will be an input. If the answer is something other than a fuse (possibly an inductor) simulation in software first is very cost effective.

Protective Device Coordination the Easy Way Webinar Series is a four-part series that covers the definition and focus of selective device coordination, TCC plots for comparing performance loads and OCPD, fuses and breakers with NEC 2020 requirements, protective relays, and ground fault protection.

If you need an arc flash report for your system, that is an output from the same software, so some economy can be obtained by doing both investigations at the same time.

 

[FreddyNurk]

I'd like to know what kind of renewable generator presents those fault values as well...

 

[ivanngwh]

Thanks all,
Well noted. Now I also believe the fault current from the renewable generator will not flow to the utility transformer LV terminal in all cases. Only an outgoing feeder may have the chance to experience the sum of fault currents from both the renewable generator and the utility transformer if the fault occurs at downstream side of the feeder breaker. As such, I will install a 65kA ACB for a feeder circuit.
My current task is to explain to the utility company the above. Hope successful.

My renewable generator is a steam turbine generator. It is driven by the waste steam from an incinerator. The generator itself is no different from the alternator of a diesel generator.
Thanks for all your technical knowledges provided.

 

[ivanngwh]

My explanation to the utility company failed.
The utility company said that his transformer supplier declared that the transformer LV terminal is not able withstand a fault current of >40kA and without any duration data. i.e. >40kA for 0 second is also not allowed.
He said that if the fault occurs exactly at the transformer LV terminal, 55kA will be experienced and thus not accepted.
He said if <40kA fault current occurs at the transformer LV terminal, the terminal will damage but will not cause explosion.
He said if >40kA fault current occurs at the transformer LV terminal, explosion may occur.
Any idea?
The utility company insists the grid connection shall be on the 11kV side.
It should be electrically feasible but the impacts to the Project is large.

 

[iop95]

A fault at LV terminals mean a fault current in transformer windings from transformer itself only.
Generator fault current is in LV terminals not in transformer windings (external of transformer)... so no explosion risk.

 

[jghrist]

The utility is being unreasonable. What kind of explosion will occur with a fault on the terminal but not flowing inside the transformer?

What is the utility's proposed solution? They should furnish a transformer suitable for the application, including terminals that in their opinion are suitable. Are they saying that it is not possible to provide an interface with customer owned connected generation?

 

[cranky108]

"The utility is being unreasonable." More likely you are just talking with the wrong person.
Ask for one of there engineering staff.

In our company there is a position called a 'field engineer'. Most of them are not engineers. They all do have high school educations, and maybe some college, but of the ones that are engineers, they are just out of school.
Ask for one of the older engineering staff. They go by different titles in different companies.
If the title is 'Chief engineer', go to someone else.

 

[ivanngwh]

The utility transformer is 11kV/380V.
The utility company asks for 11kV grid connection.
It is electrically possible but impacts to my project is very big.

 

[bacon4life]

Perhaps the utility is concerned about tank rupture of the transformer for internal transformer faults. At the terminals inside the transformer the arc inside the transformer would be 55 kA. Having the transformer tank rupture would be much worse that just having the fuse/breaker open.

I have looked, but have not been able to find any information about testing the fault current withstand capability of padmounted transformers. Overhead transformers are tested at an 8 kA fault per ANSI C57.12.20. Cubish padmounted transformer do have more room to deform than cylindrical overhead transformers.

 

[ivanngwh]

Dear All,
Dear bacon4life,
Understand this may be the utility's concern and such requirement may not likely be relaxed.
Would you please suggest any method that I can overcome this <40kA criteria? as my client still wants to do LV grid connection but not HV.
The 3.5kA remaining fault current criteria (generator contribution) is too stringent.
Is reactor / isolating transformer or even inverter technically feasible and practical?

 

[stevenal]

Correct me if I'm wrong, but I understand the principal connections are utility grid to HV bushings to transformer winding to LV bushings to generator. Based on the values provided, there is no fault location in this path that will result in 36.3 MVA through either set of bushings. The worst case would be 24 MVA through the bushings when the fault was located between the LV bushings and the generator. This is the same level as any other non-generating customer with a similar connection. A fault between the LV bushing and winding would result in 12.3 MVA through the LV bushing supplied by the generation. 55 kA might be the total, but it's being supplied from two directions. Try explaining this to a degreed engineer at the utility, and not the technician you've been dealing with.

There should be a bushing standard (IEC?) involved that provides a time period for that 40 kA number, but you are well under anyway.