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# A question about differential and distance protection of transmission lines5

## A question about differential and distance protection of transmission lines

(OP)
I'm going to a job interview next week and I have 3 questions.

I know that differential protection of transmission lines (Mostly done as aerial lines in my country and still being expanded on) has (probably) been a common practice for the past few years due to it's fast sensing and reliability.
In regards to that I have the following questions and how I think it works, and what I don't understand so please correct me if I'm wrong.

1) Can differential protection detect a short circuits? As far as my understanding goes, differential protection reacts only to internal faults where current measurements deviates in such a way that the sum of the currents are not zero.

I've derived the equations for voltage and current during short circuits, or well, the more common ones (LG, LL, LLG and LLL). And as far as I can tell; If we have a LG short circuit, the current in the other phases are zero except the faulted one, so the sum of currents is non-zero and the relay should operate. However for a 3-phase short circuit the current in all phases are the same and I would almost argue that since the currents are balanced the sum is zero, and the relay will not detect the short circuit? If that's the case, would the same happen for 2-phase short circuits not involving ground? Because something tells me that even for 2-phase not involving ground the sum of currents "could" equate to zero since the currents in the two faulted phases are equal while the unfaulted one has zero current going through it. This is of course looking at it from a theoretical perspective.

2) Why are we even remotely interested in differential protection on transmission systems?

It makes sense why we would want differential protection on generators and motors, because asymmetrical currents are quite damaging. However I find it to be a real mystery why we want it for transmisson lines, because as far as I know we "technically" don't care about how much current is being drawn by the aerial line as long as it doesn't exceed the maximum permissible temperature of the line itself (I know this sounds a bit arrogant but you get the idea). I can however understand if the differential currents cause the current in one phase to be too high, and then obviously we want the relay to operate. However when that's said it doesn't feel like that's the one single reason why we want differential protection on the line. Is it related to what's on the other side of the transmission line, such as the load, generator etc?

3) Is there one single distance relay characteristic that's more desired over the others or does it come down to voltage level, cables or aerial lines and other parameters?

I'm familiar with the relay characteristics (I think they're called that at least), such as impedance, mho, polygon etc. So I know that for a polygon characteristic we don't have to worry about the impedance resistive increase due to a potential arc, which is a benefit compared to if we had a strict mho or impedance characteristic. However is there a characteristic that's favourable and more "cost effective" than another? Because to be quite honest I have no clue if relays are pre-programmed to have a specific characteristic or are they manufactured such that "you" the engineer choose both the characteristic the relay supports AND the parameters?

I know this is a bit long, but I just wanted to convey what I know and explain why I don't know to hopefully get a more clear answer.

### RE: A question about differential and distance protection of transmission lines

With no fault, the current entering one end is the same as the current leaving the other end. With properly connected CTs one is 180 degrees different from the other. When summed vectorially they sum to zero. Any internal fault breaks that relationship. Instead of summing to zero you get some larger value.

The line diff I’m most familiar with divides the current at one end by the current at the other end. A result of 1 at 180 degrees is perfect through current and restrains. The restraint region is an arc that accounts for various errors. Internal faults show up close to zero degrees. The relay looks at each phase individually as well as negative sequence and zero sequence quantities.

### RE: A question about differential and distance protection of transmission lines

1)
Differential protection does detect short circuit. Internal fault are a type of short circuit that occurs within the zone of differential protection. Differential works by summing the incoming and outgoing currents, NOT the three phases current from any one side. You need to google basic differential protection principle.

2)
line differential is super important in transmission system because to clear a transmission line fault, you usually need to clear the fault from both sides. Although you can do it through overlapping distance protection, line differential is faster and more reliable.

3)
In terms of which one is better, mho or polygon? i'd say mho circle as it is more commonly used. the polygon offers more customization of the impedance zone but not it is not a must have.

### RE: A question about differential and distance protection of transmission lines

The answer to 3) that protoslash provided is evidence of a ANSI/IEEE influenced take on things (same as mine). The IEC take on things is rather different and there quads and other polygons are much preferred. In a mho environment more mhos are better with quads reserved for special needs. In a quad/polygon environment, more quads are better and mhos a bit of an oddity. No right answer per se, but best to stick with the surrounding practice.

Interesting

### RE: A question about differential and distance protection of transmission lines

I look at it like this:

Step distance is a really nice backup to differential.

Differential is a nice backup if loose my voltage inputs.

Of course this is more after the job then to land the job.

1) Differential will catch short circuits if its within the differential zone of protection.

2) Differential offers high speed clearing that zone 2 step distance does not. Differential is immune to power swings. Not effected by mutual coupling as far as I know.

3) Binders can help on load ability.

These might help:

http://www.pes-psrc.org/kb/published/reports/D4_Ap...

### RE: A question about differential and distance protection of transmission lines

#### Quote (WHiPCPL)

I've derived the equations for voltage and current during short circuits, or well, the more common ones (LG, LL, LLG and LLL). And as far as I can tell; If we have a LG short circuit, the current in the other phases are zero except the faulted one, so the sum of currents is non-zero and the relay should operate. However for a 3-phase short circuit the current in all phases are the same and I would almost argue that since the currents are balanced the sum is zero, and the relay will not detect the short circuit? If that's the case, would the same happen for 2-phase short circuits not involving ground? Because something tells me that even for 2-phase not involving ground the sum of currents "could" equate to zero since the currents in the two faulted phases are equal while the unfaulted one has zero current going through it. This is of course looking at it from a theoretical perspective.
You are confusing differences in phase currents with differences in currents on the ends of the line. The line differential relay compares the current going into one end with the current going out of the other end. If they are the same, then there is no fault. If they are different, then there is a fault.

### RE: A question about differential and distance protection of transmission lines

He practice in the Great Britaina used to be to a protect each 400 kV line with two systems each with different principles eg differential and distance. I assume that is still the case.

(OP)

### RE: A question about differential and distance protection of transmission lines

However, line capacitance phase to ground and phase to phase will throw off a differential system on long transmission lines. Distance relays work well on long distance lines but not very well on short lines. Differential works well on short lines but not well for long distances where capacitance is a problem for it.

There are also sophistocated relays that use the arrival times of the pulses from arcing faults to estimate distance but this only works if both ends of the line have a source.

There are also differential protection relays that encode the secondary currents of the CTs for sending and comparison over microwave or fiber optic circuits. This eliminates the distance limitation of hardwired CTs like in a double ended switchboard.

### RE: A question about differential and distance protection of transmission lines

Modern line diff relays can account for charging current. Line length is not a limitation.

Traveling wave relays can work single ended.

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