Circular or quadrilateral impedance characteristic?

[521AB]

What is the reason for the US to use the circular (MHO) in impedance protection relays, when all the research is in the direction of the quadrilateral characteristic?
Is there any technical advantage in using MHO compared to quadrilateral?
I am confused. I see now a lot of numerical relays offering MHO characteristic. If we go back to the 90's, everybody wanted to have quadrilateral.
It is just culture or there are some technical benefits?

 

[DiscoP]

From what I understand, the Mho is a simpler calculation for a relay to perform so the relay operating time is faster.

The downside is less fault coverage for high resistance faults.

Mhos are also easier to set and test.

 

[521AB]

From my uderstanding:
the fault resistance coverage close to the reactive reach setting (80% of line for instance) is almost zero... If you want to increase it, you have to íncrease the reactive reach (go to 90% of the line), risking your relay to overreach...
Is that worth a faster trip time?
By the way, how fast can be a numerical mho relay (with deírectional detection)? I have no idea...

 

[DiscoP]

You are correct - it will be zero at the reach point. A fault at this point is either covered by Zone 2, or some sort of channel aided scheme or DEF scheme.
You could increase the zone 1 reach, however this means you have no resistive reach at 90% of the line - as well as having the issues with possible overreach you mentioned.
Sub-cycle operating times are possible with numerical distance protection relays. Off the top of my head, the LFRZ and P443 relays from Areva offer this, and they are both Mho.
Our organisation uses mho - mainly for historical reasons and because they are easier to set and test, rather than from any operation time perspective.
I have just heard from manufacturers that the only way to get the fast times is to use mho.
Whether or not it is worth it is a question you are best to answer yourself.

 

[DiscoP]

PS - please note there is no directional decision required with a mho - the characteristic is inherently directional (one of those speed advantages I guess !!)

 

[521AB]

I have received the following answers from some colleagues:

1) the small resistive reach at the end of the settings is not an issue. High resistive faults are covered by blocking scheme or permissive overreaching scheme based on directional earth-fault protection (this is what DiscoP said by the way...).

2) directionality is needed also for mho characeristic (directionality is a difficult item: if you have aclose-in fault the fault voltages drop and it's difficult to understand the fault direction. Digital relays use memory voltage for that.

I have also been told that US market is looking with interest into the quadrilateral characteristic...

 

[DiscoP]

If you need some further information on how the Mho is inherently directional (ie a separate directional decision is not required), there are a number of protection text books that discuss this.

Two on my bookshelf that are also available on the web are

GE's "The Art & Science of Protective Relaying" by C Russell Mason" - see Chapter 4 'Mho Type Distance Relay'

also

Areva's "Network Protection & Automation Guide" - see chapter 11 section 11.7.3

Both books also deal with how relays cope with close-up faults.