overcurrent coordination guidelines
overcurrent coordination guidelines
(OP)
I do not have culture of industrial power system. I have two problems about 13.8kV and 0.46kV system solidly grounding:
1- What is the priority:
For a normally closed tie breaker with two normally closed main breakers, the tie breaker coordinates with the main breaker or not ?
2-How do you set Residual relays (50/51N)on solidly grounded system?
3-How do you set ground relays (50/51G)of neutral transformers on solidly grounded system?
4-When I coordinate Transformer primary overcurrent device with the inrush current point , do I have to do same thing for secondary relays?.
1- What is the priority:
For a normally closed tie breaker with two normally closed main breakers, the tie breaker coordinates with the main breaker or not ?
2-How do you set Residual relays (50/51N)on solidly grounded system?
3-How do you set ground relays (50/51G)of neutral transformers on solidly grounded system?
4-When I coordinate Transformer primary overcurrent device with the inrush current point , do I have to do same thing for secondary relays?.






RE: overcurrent coordination guidelines
1. Coordination of tie is desirable but not always possible. How is your load set up? Does the each bus on either side of the tie (not just the main ratings) designed to carry 100% of facility, fully redundant ? or if you lose the tie you in fact lose 50% of load?
If fully redundant, tie coordination is more difficult but much less critical and not redundant, it is important to coordinate and probably easier as the tie is not carrying the full load.
2. and 3.: Refer to the relay manuals and some protection books.
4. No. Inrush is not seen by the secondary.
This forum cannot be a substitute for formal learning or training. May be you want to attend some classes offered by relay mfrs and switchgear mfrs.
Rafiq Bulsara
http://www.srengineersct.com
RE: overcurrent coordination guidelines
I make a mistake:
1- What is the priority:For a normally OPENED tie breaker with two normally closed main breakers, the tie breaker coordinates with the main breaker or not ?
The case mentioned is emergency: A CLOSED tie breaker with ONE normally closed main breaker.
3- Any suggest about guidelines instead of a whole book?
RE: overcurrent coordination guidelines
Therefore the answer is the same as before - it's desirable but not always possible. Use bus differential to make life easier.
RE: overcurrent coordination guidelines
I have two 13.8kV busbar auxiliary service system supplied by independent sources that cannot be paralelled.
If you lost one source , you close tie-breaker and providing source from the other busbar.
Question is:
There is a conflit in tie-braker coordination.
Should I coordinate tie-breaker in normally operation or not?
rbulsara
Question 3: - some people sets 51G for minimum tape and others setting it close to rated load.
RE: overcurrent coordination guidelines
The choice is yours. Compromise is often necessary in coordination. You might need to have slower main breaker relaying or faster feeder protection to provide coordination. This brings coordination with upstream and downstream relaying into consideration.
RE: overcurrent coordination guidelines
There was a thread on the same subject (51N/G) a few days ago here, you may want to search and read that up.
Coordination is as much an art as science. There usuyally are competing goals. The person setting the devices need to have a very good understanding of how the system is supposed to work? In some cases a slight loss of coordination may result in more availability of service and in some it may increase hazard. Many times it is just not possible to coordinate.
So there are no real guidelines or rules, the guideline is to achieve as much coordination as possible without sacrificing safety and availability of service.
Rafiq Bulsara
http://www.srengineersct.com
RE: overcurrent coordination guidelines
You can use not standard coordination in such cases.
You can use logical reverse protection with standard coordination.
What I meant.
Simple example:All feeders have 50 with 0.04s trip time
Main breakers 50 with 200 trip time.
You provide now logical reverse protection and set:
main breaker with 50-1 400ms trip time and 50-2 100ms trip time
tie you set with 50-1 200ms and 50-2 100ms too.
Bus 1 feeder start signal you connect to main 1 and tie, bus 2 feeders to main 2 and tie.
This is only scheme. Lot of application are used w/o protection on the tie.
Other possibility used 67 (directional overcurrent) on the tie.
Hope that help.
Best Regards.
Slava
RE: overcurrent coordination guidelines
If you don't coordinate the tie breaker with the main, you risk loosing all feeders for a bus fault instead of just the feeders on the faulted bus IN an emergency operation.
But, you have faster trip for busbar fault and faster backup for feeders IN a normal operation.
... If you don't coordinate the feeders with the tie breaker, you risk loosing half of the feeders for a feeder fault....I did not undertand this comment! I do not coordinate feeders with tie-breaker.My criteria is Mains coordinate with feeders. Tie breaker will have same setting as mains.
rbulsara,
I found discussion about 50N,50G , 50NS.
What I do not understand is the discrepancy of criteria for coordination:
Question 3: - some people sets 51G for minimum tape for pickup(above neutral unbalance) while others setting it close to rated transsformer load.
slavag,
Logical reverse protection is interesting. I need relay has 2 option of setting. I am go to check if digital Siemens relay 7SJ602 has more than 1 level of settings.
I did not understand 67 in tie breaker. Tie breaker in emergency operate in both directions of current.
RE: overcurrent coordination guidelines
If the main is the only breaker with GFP, you need to make sure that it still coordinates with your feeder breakers. If you have a second tier of GFP in feeder breakers, you would have more flexibility in GF coordination.
Rafiq Bulsara
http://www.srengineersct.com
RE: overcurrent coordination guidelines
No, you don't need two settings, both setting is work always. In case of trip on the bus, reley will trip with t=100ms ( in my example). in case of fault on the feeder, feeder relay will BLOCK 100ms stage.
Usually in case of logical protection recommended use 3 stages of overcurrent, first is IDMT protection with standard coordination.
Scheme is-- feeder on bus 1 BLOCK tie and tie is BLOCK main 2. ( or vise versa)
Directional used in special ceses, forgot about it.
BTW, used newer 7SJ80x relay instead 7SJ60x.
Best Regards.
Slava
RE: overcurrent coordination guidelines
Why did you ask the original question about coordinating the tie breaker with the mains?
It would be better to use logical reverse protection per slavag (also known as zone interlock or fast bus trip) or use bus differential relays. If you use logical reverse protection, you can have the tie breaker block the mains also. That way, during emergency operation with one main, a fault on the bus downstream from the tie breaker will not trip the main. Feeders on the upstream bus will not be interrupted.
RE: overcurrent coordination guidelines
Thank you.
slavag,
I think I will do:
1- coordinate IDMT relays of mains with IDMT relays of feeders. Tie breakers IDMT relay will be same as mains.
2- 50> of all feeders with 0.04s for trip time.
3- 50> of mains with 100ms if tie breaker is open.
4- 50>> of mains with 200ms if tie breaker is closed.
5- 50> of tie breaker with 100ms.
Thank you about 7SJ80x. I hope I can change aquisition of equipments.
jghrist,
Some designers adopt only coordinate the tie breaker with the mains in any operational condition. I do not like this criteria. Because of this I submit question to forum.
RE: overcurrent coordination guidelines
1. Why???
2-5 NO!!!
You must have standard grading +150/200ms
150ms is minimum of minimum, better 200ms, that meant:
outgoing feeder 0.04s-tie 250ms ( but possible set 200ms, you use newewr relay and I suggest newer CB)
Trip timee of main always 0.4s, isn't importmat tie CB is open or not. Of course possible do it with change setting, but I don't like this option.
IN additional main breaker and tie protective relays are include 100ms blocked stage.
If you go use 7SJ80x, you can do something more intresting.
This relay is include 4 stage of overcurrent, see example:
Main breaker:
51. IDMT
50-1. 0.5s, always in operation
50-2. 0.25s, blocked by tie CB closed position.
50-3. 0.1s, logical reverse protection, always in operation
Tie breaker
51. IDMT
50-1. 0.25s, always in operation
50-2. 0.1s, logical reverse protection
Feeders
51. IDMT
50-1. 0.04s
For my pinion , now you closed all options. BUt do not desugn change sewttings, it is last option.
Best Regards.
Slava
RE: overcurrent coordination guidelines
Imagine a fault at the end of feeder cable. We have two fault current path:
a)Normal condition( tie breaker is normally opened.):
1- trip CB feeder(40ms) 2-trip CB main (100ms)
b)Emergency condition(tie breaker is normally closed):
1- trip CB feeder (40ms) 2-trip CB tie breaker(100ms) 3- trip CB main(200ms)
For IDMT I do not have 2 curves avaiable to make these coordination.
There is an other problem . How do you coordinate 50 feeder (40ms) with the protection of load at the end of the feeder cable?
How to know if a fault ocurred was at the load or feeder cable connection?
RE: overcurrent coordination guidelines
I give only example.
40ms of feeder it's for maximal 3-ph fault, not for overload or 2-ph minimal current in end of line or LV side of transformer.
You need additional 51 IDMT or DT for minimal fault current and 49 for overload ( for example). And of course ground fault protection.
Second, it's not grading 40ms-100ms between feeder and main, it's logical protection, standard grading is 40ms feeder and 200-250ms main.
IDMT you don't need two stage, only coordinate feeder-tie and main.
I recommend to you read some documents about relay coordination, possible in the Siemens manual too.
Best Regards.
Slava
RE: overcurrent coordination guidelines
I only explained half of problem:- At the end of feeder cable is 13.8/0.46 kV transformer with inrush current of 8.7 au to 12.5In. So, 50 is delaying 100ms to avoid maloperation and IDMT is the mainly protection in normal condition; Coordination with tie breaker will result in more delay.
We do not use to install dedicated bus protection at auxiliary system. I read Siemens reverse interlock scheme and I understood what you were talking.
I have to do a reformulation in my concepts.
Thank you!