ASEA Overcurrent Relay Current Rating
ASEA Overcurrent Relay Current Rating
(OP)
Hi all, we have an ASEA vintage relay: RXIDF 2H, Very Inverse, RK 473 005-AA.
The relay current rating (In) can be set to either 0.5A, 1A, 2.5A and 5A (reconnectible). This relay is connected to a CT rated at 600/5A.
My question is:
Is the relay current rating (In)dependent to the rated CT secondary (in our case is 5A)?
Or is this a setting where I can calibrate In..say if I choose 2.5A, then the CT secondary current of 5A becomes proportional to 2.5A in the relay?
I will appreciate any experience on this relay since this model is way out of my generation.
Thank you in advance
The relay current rating (In) can be set to either 0.5A, 1A, 2.5A and 5A (reconnectible). This relay is connected to a CT rated at 600/5A.
My question is:
Is the relay current rating (In)dependent to the rated CT secondary (in our case is 5A)?
Or is this a setting where I can calibrate In..say if I choose 2.5A, then the CT secondary current of 5A becomes proportional to 2.5A in the relay?
I will appreciate any experience on this relay since this model is way out of my generation.
Thank you in advance






RE: ASEA Overcurrent Relay Current Rating
I commissioned dozens of these RXIDF relays years ago. I do not know if they are still being made.
You can have a look at the ABB library site, where a lot of the old RK data sheets of ASEA can be downloaded.
regards, rasevskii
RE: ASEA Overcurrent Relay Current Rating
Im curious, if say I set it at In=2.5A, will this going to change the calibration to a 5A CT secondary as 2.5A/5A.
Wherein a pickup setting of 1.2 multiples would be:
Is = 1.2 x (2.5/5)x 600A
Any comments? Hopefully there are ABB old timers here.
RE: ASEA Overcurrent Relay Current Rating
Rafiq Bulsara
http://www.srengineersct.com
RE: ASEA Overcurrent Relay Current Rating
If it is indeed the relay current rating, at 2.5A rating, the 100% will be 300A, using a 600/5A CT.
So if the pick up is set at 80%, the actual primary pick up current would be 240A.
0.8 x 300=240A
or 0.8*2.5= 2A, which is 2A * 600/5= 240A.
Rafiq Bulsara
http://www.srengineersct.com
RE: ASEA Overcurrent Relay Current Rating
In a certain large city several years ago there was a major blackout, where, on a 400KV system, someone had connected a 5A CT to a relay set and calibrated for a 1A CT. When the load increased due a parallel incomer being out of service, it tripped its own (the remaining) 400KV incomer..
rasevskii
RE: ASEA Overcurrent Relay Current Rating
Presently, the equivalent primary current where the relay is et to pickup is 720A. This relay is protecting a 10/13.33MVA, 25-4.16kV transformer with an ONAF primary rated current of 308A ONLY. My aim is to set the relay pickup close to 308A but not beyond it. This is an industrial applications so I want to limit operational loading to ONAF rating only.
The relay publication RK 47-11 E states that:
"The relay can be reconnected for different current ratings. To change the current rating of the relay, a screw connection is moved between four alternative terminals visible on the front of the relay."
This is all what the publication discuss about this screw settings. I am looking at this like a tap range selection similar to the CO-9.
RE: ASEA Overcurrent Relay Current Rating
It would be perhaps better to protect the transformer by oil or winding temperature rather than a low overcurrent setting. There can be problems with large motors (if any) starting, etc.
rasevskii
RE: ASEA Overcurrent Relay Current Rating
regards, rasevskii
RE: ASEA Overcurrent Relay Current Rating
Furthermore, the transformer magnetizing inrush (approx. 12 times ONAN primary current rating) is well below and away the relay curve.
Having that 720A is still a question to me.
RE: ASEA Overcurrent Relay Current Rating
In any case you can protect the transformer against overload with the relay at the secondary side set at about 110-115% In.
720 A is for my feeling probably a bit too much
RE: ASEA Overcurrent Relay Current Rating
With this expensive transformer, an investment in a modern protection system is likely advisable.
regards, rasevskii
RE: ASEA Overcurrent Relay Current Rating
Still not the best way to do this...see all the comments above.
regards, rasevskii
RE: ASEA Overcurrent Relay Current Rating
The transformer may have 49 devices to protect it from abnormal temperatures due to overloading, but the cable only relies on the relay pickup for overload protection.
I will check the secondary (4.16kV) relay pickup equivalent amps and reflect that to the 25kV primary and ensure that the value is less than this de-rated primary cable ampacity...any comments?
RE: ASEA Overcurrent Relay Current Rating
Also when setting relay amp pick up, compare them to a comparable MV Fuse curve. For example, 200E MV fuse is in fact equivalent of a 400A relay setting. You also need to leave room for coordination with downstream devices.
As for transformers, applying temperature monitoring for thermal damage of transformers is a better option.
Rafiq Bulsara
http://www.srengineersct.com
RE: ASEA Overcurrent Relay Current Rating
Rafiq, what do you mean when you say "At MV, cable overload protection is protected by DESIGN".
Regards...
RE: ASEA Overcurrent Relay Current Rating
I can't explain it over this forum (too much to type) but refer to some good reference books on MV distribution or even IEEE buff book.
One of the reasons, the MV fuse characteristics are what they are, that is they do not even open below 200% of their rated current or takes hours to open if ever.
Rafiq Bulsara
http://www.srengineersct.com
RE: ASEA Overcurrent Relay Current Rating
please help.