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Load Shedding using Multilin 750 relay
2

Load Shedding using Multilin 750 relay

Load Shedding using Multilin 750 relay

(OP)

I a Multilin 750 relay used on the incoming breaker of a main switchear lineup which has a tie-breaker feeding another plant so essentialy this 750 relay is being used on a breaker now feeding two plants.  We are temporarily running two plants due to remodification of one of our utility transformers.

One of these plants is now in operation and cannot be taken off-line.  The second plant is being commisioned and we are slowly adding load.  If we get to the point where we are putting to much load on the plant we are commisioning, I want to trip this plant off line, rather than trip the 750 relay and drop out existing plant.  This cannot be done from a corridation issue becuase the main breaer being used to feed both plants cannot handle both plants at full load, so downstream breakers could potentially let through enough current from both plants to trip relay.

I thought about possibly using the 750 relay to set up some kind of load shedding feature to where if the total current on the 750 approached its tripping value, then it would trip a feeder breaker to our new plant rather than trip both plants.  There appears to be a monitoring function in this relay to monitor current, or demand where you can set pickup points for tripping an aux contact which I could use to then trip the feeder breaker on the new plant.

The 750 relay is set up with a pickup of 3000A.  So If I set the current or demand monitoring feature to pickup at 2950A then I could trip the new plant offline before I reached the pickup on the relay.  I'm not sure if I would make the time delay for this instantaneous or some sort of delay.

Mo other concern is for transient or starting currents on the new plant that might cause the relay to go over 3000A temporarily.  Should I set a time delay on the monitoring function to deal with these transients.  Should I make sure this time delay is less than the delay set up on the 750 relay.  

I wanted to see if anyone had any experience in using this relay for such a load shedding application?  The only other solution I could think of was to dial down the settings on the new plant feeder breaker to limit the current that the plant could draw.  This would be a guessing game however because how much current was left avaliable on the 750 relay would be a function of what the existing palnt was operating at.

RE: Load Shedding using Multilin 750 relay

You should be able to configure multiple setpoint groups to allow the 750 to trip another breaker.

The time delay question would take some analysis of the system and loads.

Alan
----
"It's always fun to do the impossible." - Walt Disney

RE: Load Shedding using Multilin 750 relay

2
For what you are describing, you may be able to program Phase OC2 (second Phase OC setting) for alarm only and activate an aux relay output and use that trip the feeder breaker. This will be totally independent of the main OC setting.

 

Rafiq Bulsara
http://www.srengineersct.com

RE: Load Shedding using Multilin 750 relay

(OP)
rbulsara

That is what I was thinking.  I was thinking that I could program a second curve just to the left of the existing 750 main breaker curve and then use this curve to trip feeder breaker to open breaker before we bumped into main curve.  This second curve should cover all aspects including time delay issues.

I looked through the manual but it was not obvioulsy clear how to implement this second curve as an auxilary output.  I'm thinking it may have something to do with the setpooint groups.   

RE: Load Shedding using Multilin 750 relay

Look up Output Relay Setpoints in the manual, Not the setpoint groups. You can assign relays 3-7 to any protection element such as OC2, if that is what you choose. In fact you can activate a relay on multiple conditions, so make sure one you choose is not already been selected.

 

Rafiq Bulsara
http://www.srengineersct.com

RE: Load Shedding using Multilin 750 relay

(OP)
Attached is the second OC2 curve I plan on setting in the relay to handle the load shed.  Basically I just created a new curve (51-MC1 Load Shed) similar to the main breaker curve (51-MC1) and shifted it to just below the main breaker curve.  The main breaker curve has a pickup of 1xCT and my load shed curve has a pickup of .98 x CT.  I believe this new curve should be able to trip intended feeder breaker before tripping main breaker.  Does the spacing look adequate, or should I try and put it tighter / looser?

It looks like then all I have to do is go into the relay and program this OC2 curve.  I would set the "Phase Time Overcurrent 2 function" to "control" rather than "trip" and then would set the "Phase Time Overcurrent 2: Relay 3" to "Operate".  I would then use the N.O. contact on the relay inserted in the feeder breaker trip circuit.  

Can I make all of these settings with the relay in service?

RE: Load Shedding using Multilin 750 relay

I would check the time margin at available short circuit current and would keep it to at least 0.15 seconds or more if possible.

You should check with GE Multilin tech support if they advise changing the program while in service. I personally would not recommend it. Even if it is doable, I would put everyone on notice while doing this and pick the least risky time, if there is one.



 

Rafiq Bulsara
http://www.srengineersct.com

RE: Load Shedding using Multilin 750 relay

You can change a setting on the 750 while it is in use.  You can not, however, reflash with a new program file while it is in use unless you want to trip.

Rockman,
I used to use Multilin 750s for load shedding in my last job with the methodology you are describing... except that I was using underfrequency rather than overcurrent to trip the appropriate breaker.  It works like a champ.   

RE: Load Shedding using Multilin 750 relay

antigfk:

Thanks. Under-frequency is for generators. This is different as the system capacity is not overloaded but just the rating of the main breaker is not to be exceeded.

Rafiq Bulsara
http://www.srengineersct.com

RE: Load Shedding using Multilin 750 relay

(OP)
Rbulsara

I spoke with Multilin and they informed me that I could make these changes with the relay in operation.  I held my breath and went ahead an programmed in OC2 and set it for an output relay used to trip the respective feeder breaker.  I added these settings and made the wiring changes with no problems.

Thanks for all the help

RE: Load Shedding using Multilin 750 relay

Rockman7892:
You are quite welcome! Thank you for the feedback and I am very happy for you with the positive outcome. Hope all works out for you.

Rafiq.

Rafiq Bulsara
http://www.srengineersct.com

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