Import Underpower

[pithpal]

Hi
I am an engg. trainee trying to write a protection philosophy for a distributed generation projetc and came across an article by beckwith electric
on "Use of Import Underpower as a means of protecting Against parallel operation in DG Interconnection Applications ". The article talks about Peak Shaving application. Could you help me getting a bit on highlight on what a peak shaving application means and Should I consider it a Active Protection or Passive Protection.
Thanks

 

[Electic]

Peak shaving is when Utility Demand is limited generally to reduce the power bill. This can be accomplished by adding power to the system with an on-site generator or by diversifying loads etc.

You question seems to infer the use of on site generation to eliminate a portion of the utility demand.

Not sure what you mean by the other statements.

 

[rcw retired EE]

For Peak Shaving applications, the generator parallels the utility power service and supplies the peaks in the facility's power use This levelizes total power use below some maximum to reduce excess demand billing charges.

A directional power relay on the utility feed can monitor the facility's power use and alarm or trip if utility power consumption drops below some arbitrary value. The alarm or trip could be used to turn off the peak shaving generator since the peak load has passed. The same setting would also trip the generator if it started delivering power to the utility.

This would be an active control.

 

[pithpal]

Thanks everybody for the answer,
Just wondering, a simple doubt not related to earlier question though, Is it true, the CB rating is always selected on basis of sub transient reactance irrespective on the basis of size of motor or generator.
Thanks

 

[davidbeach]

Fault calculations always use subtransient reactance values for motors and generators, so the fault duty of a breaker will be influenced by the subtransient reactance, but may be more strongly influenced by other portions of the system. So, I guess the answer to your question is "It depends"; there just isn't enough information in the question to provide more of an answer.

 

[pithpal]

Thanks david for such a prompt reply, I will explain you what I know and then you can add, if I am missing some thing here. we have a 400 KW generator, a small community in country side wants to develop their site for commericial use, as it is viable from the point of financial returns. Now the simple single line is a
Ind. generator + CB+ Transformer. Now for setting the rating of CB near Generator, only cable length up to CB and the Ind. generator reactance will come in to picture. Now the manufacturer has given me the short circuit reactane Xd". and the locked rotor current. So that means the three phs short circuit current will be I=1/(Xd"+Xcable). Now as the generation volatge is 600 V, so probably, It will be a MCCB and as far as my knowledge is concerned, Low voltage CB's has only one rating and HV rating has two ratings. Now I have to make sure that this CB is tested on X/R,which is equal to the generator X/R as that will contributed to Asymmetrical Component. Please advise if I am missing some thing here.
Thanks

 

[davidbeach]

pithpal, are you buying UL (or ANSI) breakers or are you buying IEC breakers? The fault ratings are calculated differently and I can't speak to the IEC breakers. In either case though, my guess is that the generator isn't what matters; the fault current from the utility is likely higher than the fault current from the generator and would be the determining factor.

 

[pithpal]

Thanks david,I am located in Canada so it is UL, When you talked about utility current, if a sckt occurs on gen terminals occurs, then ofcourse there will be an inflow from utility also but I am not sure if that would make a difference as the current flow will be in opposite direction for the CB when compared to utility.So, I am setting up the rating of Ckt breaker on the rating of CB on basis of fault current from generator. Plz let me know if I am fine with it.
Thanks

 

[davidbeach]

pithpal, you need to look at fault current both directions, one will be higher than the other. The UL standards define the X/R test values based on breaker interrupting rating. If the prospective fault has an X/R not more than the test value, the interrupting rating applies directly. If the prospective fault is more than the test value, you have to derate the breaker (or increase the prospective fault current) and compare using the adjusted values. The IEEE Blue Book covers this, as does the general material at the beginning of the Cutler-Hammer (Eaton) Consultant Guide. There's a rather nasty formula and my references are nearly 2000 miles away at the moment so I can't provide the formula at this point.