Cable tail End protection MFAC34 voltage setting calculation 1

[zertic1985]

Dear all

I need some experts help for my following issue in my site

In our substation we are going to use cable TEE protection by MFAC34 and MVTP31.

For that the final setting coming from client is following
CT Ratio:1200/5A
Rct:0.4ohms
Rloop:1.0ohms
Setting voltage:225V
Effective primary operating current:115A
But the problem is we have the relay MFAC34 with setting range 25-175V only.So now my question is if i change the setting voltage 175 instead of 225 what will be the problem?
It affcet the system?or i need to know how to calculate the voltage setting?
From the manula i got the following formulae for the voltage setting calculation
Vs=If/N*(Rct+2Rl)

If-Maximum through fault current when the relay should maintain stability
N:CT ratio
Rct:CT resistance
Rl:Loop resistance

I want to know how to calculate the If?


Now we are working shutdown project if somebody give some advise it will be very grateful for me

 

[ScottyUK]

If is determined by the characteristics of the installation - the impedances of the source(s), the cable(s), line(s) and any infeed from rotating plant. You need to model the system to determine the value of If. In most cases the highest fault current will be phase-phase, unless you have a lot of local generation in which case phase-neutral will be higher.

 

[zertic1985]

Dear Mr.Scott,

First of all i wolud like to thank for your immediate reply.

Can you please advise whether i can use the setting 175V in the MFAC34 relay

Voltage level:132Kv
Fault current:45KA

 

[ScottyUK]

At 45kA I think you need a setting voltage greater than 262.5V, calculated from Vs = (45000 / 240) * (0.4 + 1.0) The nearest plug setting would be 275V on the relay with a range 25V - 325V. Your relay does not have a sufficiently high range.

The CT lead resistance Rl referenced in the manual is multiplied by two to give the loop resistance. You already seem to have the loop resistance, so you do not need the x2 multiplier.

The manual is attached.

 

[zertic1985]

Thank you Mr.Scotty,

If i use the setting 175 V with the adjustment of stabilizing resister it will affect the system?