×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Blocking Scheme - Generators

Blocking Scheme - Generators

Blocking Scheme - Generators

(OP)
I have a paralleling generator system in the USA at 12,470V, 5MVA (2 gens) and it parallels and terminates to the main service entrance bus.  There is a bus tap on the secondary of the main utility service entrance breaker that feeds a distribution board.

Q1) If there is a fault on my distribution board on the line side of the main breaker that would cause my main utility service entrance breaker to trip (tapped the bus on the secondary of the main).  That will in turn start the generators. Is it typical to wire some sort "blocking" scheme to prevent the generators from coming on the bus?  

Q2) If not as soon as my utility service entrance circuit breaker opens up and the generators come online they will imediatly see the fault and trip off as well? True?

RE: Blocking Scheme - Generators

As davidbeach would say for Q2, "It Depends".  What relays / protection do you use for the generators.  What are the settings and what is the logic programmed in these relays.  

RE: Blocking Scheme - Generators

I generally put the transfer switch ahead of the main breaker. If you must connect the transfer switch downstream of the main switch, then a blocking scheme may be considered.
I have seen a couple of installations such as you describe without blocking. Yes, if the main breaker trips (or is turned off manually) the generator will start. Fortunately main trips are very rare.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Blocking Scheme - Generators

I wouldn't say it's typical but have seen a number of systems recently with larger units that do have more complex control designs as compared to a simple ATS.

I recently finished a startup of a system similar to what you described. If the main protective relay saw a "serious fault" it rolled the 86 lockout, with the main breaker's lockout rolled only one generator would start and only close to the life safety feeder. The load shed load/add system opens the other feeders on loss of the main. If the main protective relay saw a loss of phase, low voltage or frequency, it provided the signal to start both generators and allow the load shed/load add to function normally, and when it determined the utility was "OK", it would send a "Utility OK" signal to the PLC which would do a closed transition return sequence.

I have done similar systems at a number of sites, one main concern with most customers is providing proper support to the life safety circuits. Sometimes we can do as described above, in other systems we have added another smaller generator in the LV part of the system close the the life safety circuits.

So back to "it depends", but it is done on many systems, just needs a little more thought and coordination than most other types of standby systems, at least in my experience.

Hope that helps,
Mike L.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources