Directional OC Protection Concepts

[WiringBoy]

I have been working on basic feeder protection for a while now and recently started focusing on concepts of directional over-current protection and distance protection.

It seems like I am missing some conceptual background to get a handle on these topics. Here is what I would like to start with. I have put enough effort on my own in to undestanding these terms but just could not any here and thought this forum might be able to lend a helping hand.


Q#1 Most common term I run into is Maximum Torque angle:

I went through a lot of literature in books and internet and liked this chapter of GE publising:

http://www.gedigitalenergy.com/multilin/notes/artsci/art02.pdf

Here are my questions:

On Page 16 of 22(pdf)

Why the author says that "However, it is usually desirable that maximum torque occur at some value of theta(angle) other than 90 degrees"To this end, one of the actuating coils may be shunted by a resistor or a capacitor.
Maximum torque will still occur when the coil currents are 90 degree out of phase; but, in terms of the currents supplied from the actuating sources, maximum torque will occur at some angle other than 90 degrees.


I don't understand why author is trying to shunt coil of current with a resistor or capactior, how it helps ? why it is desired that torque occurs other than 90 degree angle.


Q# 2 Fig. 15 shows the vector diagram, why author is considering max. torque of 90 degree between Coil component of I1 rather I1 itsself ?


and How T=K1 I1 I2 Sin(theta-Fi)-K2

turned into T=K1 I1 I2 cos(theta-T)-k2,

I would appreciate your opinion.

Thanks

 

[davidbeach]

Blackburn's Protective Relaying book provides a good explanation.

 

[LiteYear]

Q1: dunno.

Q2: As far as the coil is concerned, it only reacts to current through itself. The shunt resistor only affects the current angle of the supply. So the torque imparted by the coil is related to the coil current, equal to "coil component of I1". I1 itself is coil current plus resistor current.

tau - phi = 90*
=> phi = tau - 90*
=> theta - phi = theta - tau + 90*
=> theta - phi = 90* - (tau - theta)
=> sin(theta - phi) = sin(90* - (tau - theta))
=> sin(theta - phi) = cos(tau - theta)
=> sin(theta - phi) = cos(-(theta - tau))
=> sin(theta - phi) = cos(theta - tau)

So the two equations are equivalent.

 

[WiringBoy]

I also browsed Blackburn book but it jumps to application directly and I guess that was not much help.

I would still appreciate answer to question #1, if somebody might know.

Liteyear, Appreciate your input. I went further and studied the interaction of Current-Voltage relationship within the same document. On page 32, Fig.17 talks about plot of ICos(theta- tao)= Constant , I am just curious how this line can be generated. Could you please shed some light.

 

[LiteYear]

The line is plotted on a polar (aka r(theta)) plot, ie. one where theta is the angle anti-clockwise from the x-axis and I is the distance from the origin. If tau is constant, this gives a straight line. To see this, rearrange to get:

I = k/cos(theta - tau)

If theta = tau, then I = k. This is the point on the line at the tip of Imin. As theta gets larger or smaller, cos(theta - tau) gets small so I gets larger. As theta - tau approaches 90*, I goes to infinity, which gives the extrema of the line.

 

[DiscoP]

When explaining MTA I think of a simple kWh meter. Disc turns fastest (highest torque) when V and I are in phase. Directional elements are similar to kWh meters in this aspect. So a meter has an MTA of 0 deg. There are two poles on. Directional element or kWh meters. They are physically spaced 90 deg apart to achieve an MTA of zero. If I want a different MTA, I need to electrically shift one of the poles which is done using caps and resistors.
Don't know if I have helped or made it worse for you!

 

[electricpete]

On Page 16 of 22(pdf)

Why the author says that "However, it is usually desirable that maximum torque occur at some value of theta(angle) other than 90 degrees"To this end, one of the actuating coils may be shunted by a resistor or a capacitor.

Maximum torque will still occur when the coil currents are 90 degree out of phase; but, in terms of the currents supplied from the actuating sources, maximum torque will occur at some angle other than 90 degrees.

It depends on what your polarizing quantity is.

Let's start with case where we want max torque at 90 degrees.
This can occur if you are comparing a line current compared to a line-to-neutral voltage.
You want the line current in phase with your line to line to neutral voltage.
The polarizing current from line to neutral voltage lags the line to neutral voltage by 90 degrees.
So this in-phase condition is represented by 90 degrees difference between currents and we want max torque there... sufficiently lower torque will indicate approaching a reversal condition.

Now, what if line to neutral voltage is not available but instead you have phase to phase voltage.
This has something like a 30 degree shift from line to neutral voltage.
Now you want max torque for same actual in-phase condition of current to line-to-neutral voltage, but your basing it on comparison of line current with phase to phase voltage. You need to add 30 degree phase shift to the previous approach.

I don't understand why author is trying to shunt coil of current with a resistor or capactior, how it helps ? why it is desired that torque occurs other than 90 degree angle.

It creates the required phase shift (i.e. 30 degrees in example above).

=====================================
(2B)+(2B)' ?

 

[Marmite]

The Areva NPAG Chapter 9 has a good description of the torque angle. Also there is a Schneider Cahier technique which discusses directional protection.
Regards
Marmite

http://files.engineering.com/getfil...-e362c4439f36&file=Directional_Protection.pdf

 

[DTR2011]

I'll take a stab and try to KISS. If you recall ELI the ICE man, in an inductive circuit the current lags the voltage. When a fault occurs, the inductive component dominates, thus ELI is in place. The line will also have a resistive component, which would offset the -90 deg. The line constant you mentioned, I think is what is referred to as the line angle. This is a function of several things including voltage level, conductor spacing, tower type, etc.

If you forget about how clever the original designers of 67 and 21 relays were (the components you have described above) and perhaps think about how a numerical relay operates it may clarify things a bit. The NAGP is a good read BTW, but written from an IEC perspective.

In addition, I would recommend checking the SEL and Basler sites. Both have very good application guides and notes. Additionally, if you have the opportunity to sit down with a relay and test set for a few hours, viewing the phasors via your test set or relay software may unlock some doors