# Relay types - basic description and uses?



## ElecPwrPEOct11 (Oct 11, 2011)

Hi all,

I know what relays are, but am not familiar with the different types or their common uses. This was brought to light in a Complex Imaginary question (variation 1, question #68) that mentions the following types: impedance, differential, overcurrent, and voltage. It says that impedance relays are useful in setting up zones of protection in transmission applications. I had incorrectly guessed that overcurrent relays were the correct device to use in this setting.

I searched online but couldn't find a good summary of these types of relays and their common uses. Wikipedia doesn't have good info and Blackburn is way too detailed. Any help is very appreciated. Thanks!


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## Dark Knight (Oct 11, 2011)

Relays are categorized according to their use and have an IEEE number to help with the id process. Any relay with a 50 or 51 ID is an overcurrent relay. These types of relays are mostly used for distribution applications, i.e. feeder breaker protection in a substation.

Not trying to teach since I am no expert then we have the differential relays. These types of relays are used to protect against internal faults in power equipment, i.e. transformers. The IEEE number is the 87. You can also use them for bus protection. If you see a 87B designation that would be a bus differential scheme. 87T is transformer differential scheme.

Another relay you will find in protection schemes is the distance relay, or 21. These relays use the impedance of a line and the voltage. Since the impedance of the transmission line is known, and you also know the voltage in that line the settings engineer can set the relay to issue a trip order if the current exceeds a predetermined value. It is not a simple thing to do but the concept cannot be easier. Ohm's Law. That was probably what the problem was stating when mentioned the impedance.

I think Blackburn, as all the Protection books, has a table with the IEEE relays designation.

I do not see the PE Test going deeper than this but feel free to post here if you have more questions. Ahh...Do not take my word for all this. As I mentioned, not an expert. Most likely someone else will jump in and answer your questions.


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## Jonjo (Oct 12, 2011)

Hi all , I have info for ElecPwrPEOct11 but with this new form, I don't know how upload , need help !


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## Jonjo (Oct 12, 2011)

I copy and paste and did not work !! , still need help for upload


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## ElecPwrPEOct11 (Oct 12, 2011)

Dark Knight- thanks for the help! That is the level of detail I am looking for. The distance relay certainly sounds like a good device for locating faults along a transmission line. I am still trying to see how utilities would use it to create zones of protection (per the problem statement). Why wouldn't you simply use overcurrent relays?

Do you have some guidance for voltage or impedance relays? I do have the ANSI table of device numbers so that will be of some help.

Jonjo- Click 'more reply options', then there is an 'attach files' section towards the bottom. Hope this helps.


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## Dark Knight (Oct 12, 2011)

ElecPwrPEOct11 said:


> Dark Knight- thanks for the help! That is the level of detail I am looking for. The distance relay certainly sounds like a good device for locating faults along a transmission line. I am still trying to see how utilities would use it to create zones of protection (per the problem statement). Why wouldn't you simply use overcurrent relays?
> 
> Do you have some guidance for voltage or impedance relays? I do have the ANSI table of device numbers so that will be of some help.
> 
> Jonjo- Click 'more reply options', then there is an 'attach files' section towards the bottom. Hope this helps.


The protection zones are not determined by the relays. Protection zones is a fundamental concept in protective relays. The theory behind it is that you do not leave, never ever, an unprotected portion of a power system or else. Bad things can happen.

The relays you use in each protection zone will depend on the application and/or the philosophy in the company. What was mentioned in my first post is just a very basic part of the puzzle.

I would suggest you not to dig too deep into it because it can be confusing. Just learn the basics. For example, an overlooked component of a protection scheme is the DC volt. The relay will use DC to operate. Another concept is overlapping. Meaning that usually you will have areas in protection zones that will overlap. That will increase reliability. If one relay does not pick up the other, hopefully, will.

I must have an old example on how to set protection zones. Just have to find it. I will try to and will post it here. Stay tuned.


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## Jonjo (Oct 12, 2011)

Here it is the info

Reles codes.pdf


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