Calculating differential over current settings

[mbrooke]

Ok, say I have 2000/5 C800 CTs, and I know the bus fault current min/max and what elements are connected to that bus. My question is how do I determine when a CT will saturate, and once I do how do I derive a number for my over current relay?

For example, in a none saturating configuration I would use about a 12 amp definite time pickup because in theory current will always be zero, and 12 amps will cover the minimum bus fault current of 8000 amps.

 

[Phil Corso]

The formal procedure is to calculate Vs... the RMS symmetrical secondary induced voltage, using CT specs, burden, and maximum secondary current!

Then, using Vs, calculate flux density, Bmax! Thus, the CT will not saturate if its spec flux density is below the calculated value!

Phil

 

[mbrooke]

The formal procedure is to calculate Vs... the RMS symmetrical secondary induced voltage, using CT specs, burden, and maximum secondary current!

Then, using Vs, calculate flux density, Bmax! Thus, the CT will not saturate if its spec flux density is below the calculated value!

Phil

I follow you, but break it down for me just a tad further. Vs? Burden is low and I know you should keep the secondary circuit I2R as low as possible.

 

[topgone]

I follow you, but break it down for me just a tad further. Vs? Burden is low and I know you should keep the secondary circuit I2R as low as possible.

Vs, the saturation voltage-> the level at which further increase in current doesn't effect an increase in voltage.

 

[mbrooke]

Vs, the saturation voltage-> the level at which further increase in current doesn't effect an increase in voltage.

And that is effected by the connected burden plus CT to relay resistance?

 

[Phil Corso]

MBrooke...

I can send the appropriate section from my EE-4231/2 course text, "Silent Sentinels" taught by Prof. J.L. Blackburn, my teacher and mentor!

BTW, in countries using SI Units, Vs is known as "Knee-point"!

Phil

 

[mbrooke]

MBrooke...

I can send the appropriate section from my EE-4231/2 course text, "Silent Sentinels" taught by Prof. J.L. Blackburn, my teacher and mentor!

BTW, in countries using SI Units, Vs is known as "Knee-point"!

Phil

That would be of huge help, send me a PM I will read through it and ask more questions.


I know CT saturation is dependent on line angle, but when connected to a bus, I have no idea what that line angle would be since multiple elements including transformers are involved.

 

[mbrooke]

Also leaving this here in regard to the subject, but still have questions:

https://cdn.selinc.com/assets/Liter...mize_JR-SZ_20110627_Web.pdf?v=20150812-085652

 

[Phil Corso]

... I know CT saturation is dependent on line angle...

[
What do you mean by ‘line-angle’? I use the tangent of a line for Vs located on the CT’s Excitation Volts vs Sec'y-Excitation Amps curve! Are we talking about the same line?

Phil

 

[mbrooke]

[
What do you mean by ‘line-angle’? I use the tangent of a line for Vs located on the CT’s Excitation Volts vs Sec'y-Excitation Amps curve! Are we talking about the same line?

Phil

System X/R ratio.

 

[mbrooke]

[
What do you mean by ‘line-angle’? I use the tangent of a line for Vs located on the CT’s Excitation Volts vs Sec'y-Excitation Amps curve! Are we talking about the same line?

Phil

This to be exact:

 

[Phil Corso]

MBrooke,

Do it your way!

Phil

 

[mbrooke]

MBrooke,

Do it your way!

Phil

:?:?

 

[Phil Corso]

MBrooke,
It's the A.H. Knable (of AEP fame) way! it will work!
Phil

 

[junkhound]

...Thus, the CT will not saturate if its spec flux density is below the calculated value!

Phil

I know what you meant, but the way the text reads I sounds backwards? Just to not confuse some folk....

e.g CT will not be saturated as long as the calculated flux density is BELOW its specified saturation level.


interesting discussion on method, never have used the 'line' type methods, simply use the basic integral of Vs*dt = N*B*A
That there line method (either yours or mbrooke's) sounds like the old A, B, C, D parameters they tried to teach for linear circuit analysis 55 years ago, never have used that either after that one class in the early 1960's! Think the prof wrote the book and was proud of the method?

 

[Phil Corso]

Junkhound…
You’re right! Easier for me to do than say… like describing “Ratchet” w/o using your hands! Initially, the simplest procedure to determine Vs was construction of a tangent-line using a 45⁰ isosceles triangle template on a plot of the CT’s characteristic curves (usually plotted on log-log paper). Later, the curve was digitized using first, Fortran, then still later, Excel!

MBrooke…
The first “Silent Sentinels” was published in 1924! My course text in 1964. In 1979, a book, titled, “Applied Protective Relaying”, was published by Westinghouse Electric Corp, Relay Instrument Division. All are available on the Web!

Phil

 

[mbrooke]

Junkhound…
You’re right! Easier for me to do than say… like describing “Ratchet” w/o using your hands! Initially, the simplest procedure to determine Vs was construction of a tangent-line using a 45⁰ isosceles triangle template on a plot of the CT’s characteristic curves (usually plotted on log-log paper). Later, the curve was digitized using first, Fortran, then still later, Excel!

MBrooke…
The first “Silent Sentinels” was published in 1924! My course text in 1964. In 1979, a book, titled, “Applied Protective Relaying”, was published by Westinghouse Electric Corp, Relay Instrument Division. All are available on the Web!

Phil

Does the book cover over current differential?

Basically the industry says to just use a high impedance relay (587Z) to offset saturation, but in all honesty I see no reason why you can't just drop a 351 in have that do the same job.

 

[mbrooke]

Junkhound…
You’re right! Easier for me to do than say… like describing “Ratchet” w/o using your hands! Initially, the simplest procedure to determine Vs was construction of a tangent-line using a 45⁰ isosceles triangle template on a plot of the CT’s characteristic curves (usually plotted on log-log paper). Later, the curve was digitized using first, Fortran, then still later, Excel!

MBrooke…
The first “Silent Sentinels” was published in 1924! My course text in 1964. In 1979, a book, titled, “Applied Protective Relaying”, was published by Westinghouse Electric Corp, Relay Instrument Division. All are available on the Web!

Phil

Slight hang up- do you have a link to the PDF by chance? Scribd wants you to pay.


https://www.scribd.com/doc/316208536/Applied-protective-relaying-Westinghouse

 

[Phil Corso]

... do you have a link to the PDF by chance...

1) No... I have the Doc. 2) $3 seems to be reasonable! I.E.E.E is at least $13, IFF you are a member!

Phil

 

[mbrooke]

1) No... I have the Doc. 2) $3 seems to be reasonable! I.E.E.E is at least $13, IFF you are a member!

Phil

Can you link it via Email?