Direct connection of generators to a Utility Grid

[p5t1]

I work for a utility company where there are 12 kV feeders. The question is: Can a 12 kV, 4 MVA generator be connected directly to a 12 kV feeder? Is it necessary to install a 12/12 kV transformer to isolate the generator from the 12 kV electric grid? As aditional information, the feeders are supplied by a 110/12 kV, 25 MVA power transformer, where 110 kV side is delta connected and 12 kV side is solidly grounded wye connected. The feeders are three phase lines without neutral conductor. Connected to the feeders are stepdown transformers 12/0.4 kV where high voltage side is delta connected and low voltage side is solidly grounded wye connected.

best regards,
p5t1

 

[AAK]

Hi p5t1,

Before answering your question, can you calrify the following :

1. What's the purpose of the generator ? Is it for emergency back up only ?

2. Are you allowed to parallel with the normal supply ?

But I think in all cases, the isolating transformer is not required. However, depending on your answers to the above, If paralling is done, then you may need to use a reactor if the 12Kv busbar short circuit is exceeded due to additional source (generator).
And in general, connecting a comparably small sized generator to a grid may lead to an undesirable active and reactive power control.




Best regards,
AAK

 

[ERaySir]

Hello p5t1,

I am assuming you want to synchronize with the grid and produce power. One concern is the available fault current level and whether the connected or (close proximity) industries' switchgear motors etc. Can take the extra fault current available. There is a liability issue. Power Stations usually have more destructive faults for this reason.

Have a good day.

Ray Micallef, P. Eng.
Power Generation/Utility Industry

 

[dpc]

The short answer is that it is technically feasible to connect the generator directly to the 12kV grid, but there are a lot of good reasons to install an isolation transformer.

A transformer will allow the generator to be better isolated from the frequent line-to-ground faults on a typical utility distribution feeder, and will allow use of a high-impedance grounding method on the generator. As well as reducing fault currents as mentioned previously.

 

[busbar]

There are three applicable standards that do not conveniently mesh.
IEEE C37.95-2002 Protective Relaying of Utility-Consumer Interconnections
IEEE C62.92.2-1989 Neutral Grounding in Electrical Utility Systems, Part II-Grounding of Synchronous Generator Systems
IEEE C62.92.4-1991 Neutral Grounding in Electric Utility Systems, Part IV-Distribution

Operational safety impacts also bear careful review.

 

[briand2]

I ran an installation like this for a prestige building in the UK a couple of years ago. We had installed a 5 MWe gas turbine alternator (11 kV) which we parallelled directly with the local utility's 11 kV network. We were served by a utility substation that had four 15 MW transformers, of which two were usually on the bars that served us. From time to time the utility had to put another transformer on those bars, at which time we had to get off to keep their fault level down (or extra 5 MW was the straw that could break the camel's back!). They usually rang us if they had to do this, but if they forgot then the interlock kicked us off anyway!!

Brian

 

[p5t1]

AAK:

The generator is always running in paralel with the grid. It´s not used for backup.

You answered that the generator can be installed directly to the grid, but what about the stress imposed on the generator due to single line to ground short circuits ocurring on the 12 kV feeder?


DPC:

I think the same as you. If you remember, the 12 kV grid is solidly gounded on the power substation, so if I want to install a generator directly to the grid, I would necessary have to ground the neutral point of the generator solidly. This manner I would maintain low overvoltages levels upon single line to ground faults. However, this single line to ground fault currents could stress the generator.

 

[ohhn]

You can install the generator directly to the 12 KV feeder, but first you have to check the fault current on the location with your generator added.
If you're worried by the impact of the line to ground faults on the feeder, ground the generator trough an impedance.

 

[hibb]

It's been my experience with distributed generation that the smaller units are usually not rated to handle the line to ground short circuit currents that they can deliver, so their neutral needs to be impedance grounded. We set the 51N relays on the generator sensitive enough to trip first on line to ground faults before the utility feed trips.
We've done several 2-6MW diesel sites directly connected at 4kV to the power system without any problems. Isolation transformers are not normally considered for this type of installation unless you have a high fault level you're trying to knock down.

 

[Rocketir]

How would the protection requirements change at the power tranformer, especially under light loading situations?

Jasjit Khangura P.E.
Electric Distribution Engineer

 

[cuky2000]

Beware that most protective scheme published by the utilities for interconnection requirement even with GSU or isolation transformer were developed to protect the system mainly on the power grid side after the point of common coupling (PCC). It is responsibility of the generator owner/engineer to verify if the unit is fully protected.

Several critical scenarios should be considered even if a GSU or isolation transformer is present as follow:
o Mis-synchronization after utility reclosing after initial feeder clearing.
o Out of phase closing
o Pumping current into utility fault.
o Detection of abnormal utility operation _{(Over/under voltage, over/under freq, excitation, unbalance voltage, single phasing.}

The enclose site and other similar site could address some of the issues related with DG interconnections.

http://www.cpuc.ca.gov/published/Final_decision/4117-13.htm#TopOfPage

 

[jbartos]

Suggestions:
1. High-resitance grounding or medium-resistance grounding is recommended, based on system grounding calculations.
2. Reactors, if needed, might be preferred to transformers since they are simpler, provide lower voltage drop, and designed for current limiting.
3. The protection may need to be more thorough in terms of various protective relays that would cover reverse power flow, harmonics, negative sequence, distance relay, etc.
See

http://www.pge.com/002_biz_svc/gen/pdf/wi_handbook/app_r.pdf

4. Stability issues may be investigated over the computer modeling
5. Surge arrestors will be needed.
6. Proper information about grid interface parameters