SHORT LINE PROTECTION WITH DISTANCE RELAY

[azadani]

Hi.
Although relay manufactuerers claim their distance relays are capable of protecting short lines ( less than 10 Km) effectively provided SIR is less than 30 .
It is also claimed that CVT transients have no major effect on the operation of relay.
Meanwhile most of electric design engineers and consultant engineers beleive the reliability of protection is improved and gauranteed by differential protection.
There should be a compromise between these two types of prot ie 87 and 21 in HV and EHV short trans.lines.
I would appreciate it if you could clarify the issue based on your experience and technical knowledge.
Thanks in advance.

 

[cranky108]

Although the 87 relay for line protection is better, it has some limitations, mainly the communications needs.
It is therefore a good idea to keep the 21 relays for short line protection.

The microprocessor technology has decreased the minumim settings for distance relays, to the point of less than line of sight.

 

[xxjohnh]

Easy to use a differential with a fiber optic pilot. Go SEL website.

 

[cranky108]

Also easy for mouse to eat into fiber pilot. Use a 21 backup.

 

[slavag]

Hi.
For the HV and EHV ( long, short) used both of function: 87L and 21. Each one have advantages and disadvanteges. Possible write here a long list .
Toaday possible use one relay is include both of functions.
Recommended use 67N ( directional earh fault) or depened..21 grounding.
From my point of view a good practic is two relay:
1. 87L, 21(option , recommended for EHV) functionality
2. 21,67N, (50,51,50N, 51N as buck up functions)functionality.
Used one or two communication links to other side:
1. via 87L
2. additional link.

Possible also use only two relays with 21 functionality with one communication link, with POTT or PUTT communication logic is "same" as 87L.

Best Regards.
Slava