Relay SEL 751

[unit]

I am trying to find how can I reprogram database so that I can select setting groups via front panel of SEL 751. Any input will be appreciated.

 

[jghrist]

Use of logic equations and programming the pushbuttons is explained in the relay instruction manual, which is available on the SEL website. You can also download AcSELerator Quickset to make setting changes. The 751 also provides for graphic development of logic with AcSELerator Quickset. SEL support is also very good. They can help you if you get stuck with the programming.

 

[unit]

Use of logic equations and programming the pushbuttons is explained in the relay instruction manual, which is available on the SEL website. You can also download AcSELerator Quickset to make setting changes. The 751 also provides for graphic development of logic with AcSELerator Quickset. SEL support is also very good. They can help you if you get stuck with the programming.

Thank you for your answer, so far I wasn't able to get SEL support personel to call me back. I guess I will have to search manual. I have another question: My client is asking for more agressive settings to decrease Arc Flash Hazard. How far I can go from the stand point of selective coordination?

 

[rcwilson]

I assume you are thinking of using the front panel pushbuttons to switch setting groups to lower trip settings and reduce arc flash hazards during maintenance or switching.

If that is the case, the objective is to get the lowest pickup setting and the fastest trip that will not false trip on normal load current or motor starting inrush. That fast setting will probably not coordinate with any downstream breaker or fuse during an actual fault. The client has to be aware that to reduce arc flash hazard, he has to accept the possibility of a larger power outage when a downstream fault occurs during maintenance.

Program the relay to also operate an alarm output or flash a front panel LED or message whenever the maintenance mode is selected as a reminder to return the setting to normal.

 

[jdsmith]

I assume you are thinking of using the front panel pushbuttons to switch setting groups to lower trip settings and reduce arc flash hazards during maintenance or switching.

If that is the case, the objective is to get the lowest pickup setting and the fastest trip that will not false trip on normal load current or motor starting inrush. That fast setting will probably not coordinate with any downstream breaker or fuse during an actual fault. The client has to be aware that to reduce arc flash hazard, he has to accept the possibility of a larger power outage when a downstream fault occurs during maintenance.

Program the relay to also operate an alarm output or flash a front panel LED or message whenever the maintenance mode is selected as a reminder to return the setting to normal.

I agree with Ron's comments. The relay settings themselves are as much an art as a science. Without getting into too much detail, there are three general categories of coordination:
- coordinated protection with sufficient security to avoid false tripping and to ensure selectivity for all available fault currents
- protection that is optimized to trip as fast as possible for faults but is set to be secure for all load currents, including motor inrush
- protection that is somewhat coordinated and secure with regards to load current, but may contain various compromises to minimize clearing time, or maximize upstream coordination with sketchy downstream coordination

The first two options are preferable to the third. Either optimize selectivity and security, or minimize clearing time and sacrifice coordination. Of course in the real world compromises are required, although on new projects with modern microprocessor relays I would argue that very few compromises are necessary and little additional cost is required to avoid those compromises.

Having multiple setting groups available in the relay is a double-edged sword. I do not recommend changing relay settings groups just to enable a faster clearing time for arc flash reduction because it introduces a lot of complication and it's easy to get mixed up when programming multiple settings groups with similar but slightly different settings. For enabling/disabling one protection element, like a high speed trip for arc flash reduction, use logic in the relay and keep everything in one settings group. If you are changing (rather than enabling/disabling) functions or altering a lot of functions it may be OK to use different settings groups. Some people recommend putting identical settings in all available settings groups in case the active settings group is inadvertently changed. I personally (in an industrial plant and on insulated cables) implement all functionality in settings group 1 and disable the other groups to stay far away from the possible confusion for protection engineers, field engineers, and relay techs.

You do need to spend some quality time with the SEL 751 manual to become familiar with the SEL relay word bits (digital points) that are available in the relay, as well as the function of latches and SV timers. The logic is tough to learn from scratch, so here is some off the top of my head that you may find helpful. Try to figure out what this does, how it works, and respond back here and we can help you work through the initial learning curve.

50P1P = 8.0
50P1D = 0.2 sec
TRIP = 51P1T OR (50P1T AND LT01)
OUT103 = TRIP
SET01 = PB01 AND NOT LT01
RST01 = PB01 AND LT01
SV32 = NOT SV32T
SV32PU = 0
SV32DO = 0.3 sec
PB1A_LED = LT01 AND SV32T
OUT102 = SALARM OR HALARM OR LT01
DP01 = 1, RID
DP02 = 1, TID
DP03 = LT01, "MAINTENANCE"
DP04 = LT01, "MODE ENABLED"
SER1 = 50P1P 50P1T 51P1P 51P1T PB01 LT01 OUT102 OUT103 TRIP SALARM HALARM

 

[unit]

I thank you both very much for your time. Great recommendations. I am going through 751 manual right now, on Monday I will get the relay and will start settings upload and testing.