Using distance protection vs overcurrent protection 1

[rockman7892]

Is there a typical standard on when to use distance protection as opposed to overcurrent protection?

I'm looking at a protection scheme in a 69kV Switchyard and I notice that all of the outgoing 69kV transmission lines use distance protection but some of the outgoing lower voltage 12.47kV (transformer in yard)lines only use overcurrent protection. These 12.47kV outgoing lines appear to be going to another local Substation.

Is distance protection typically used for larger transmission over longer distances, with long distances being the determining factor for when to use distance protection? Or is it used for networks that aren't necessarily radial and thus have power flowing in different directions?

 

[cranky108]

Protection equipment is like insurance, how much are you risking, and how much can you afford, and afford to lose.

Distance protection costs more and is more difficult to set up, but is in general faster.

If you really need to be cheep, use fuses.

 

[HamburgerHelper]

With distance protection, you don't get hung up on the system changes affecting your protection. The downside is that you need more expensive relays and something to measure the voltage. Just like overcurrent you'll have difficulties setting it for short segments. It also isn't really just an either or. Any relay that has distance protection will allow you to also set overcurrent elements.

I think one thing that might need to be taken into consideration is whether you have sectionalizing. I don't know if distance relaying is compatible with distribution sectionalizing philosophies.

 

[Mbrooke]

Generally speaking if the lines are looped or you must coordinate relays with varying short circuit current its distance relaying. For radial lines, especially those where short circuits do not need to be cleared instantaneously for system stability reasons, the added cost of step distance is not worth it.

Consider the differences via the two:

12.47kv;

1. Lines are radial (with limited or no generation infeed) and thus time over current curves can simply be "stacked" while selectively coordinating with ease.

2. A 12.47kv fault can easily go for 60 to 120 cycles without throwing major generation out of syncronism.

3. Delayed clearing is actually appreciated to coordinate with fuses, series of reclosers and "slow" devices.

4. Line impedances will vary based on switching of the line via switch or loop recloser. Any line can become longer or shorter, pickup up longer segments, segments with different size/conductor spacing or perhaps feed a completely different distribution circuit altogether.

5. Faults, especially L-G faults, can have significant impedances which may fall outside of zone 1 zone 2 and even zone 3 distance elements. On the other hand arc sense technology can pick this up, and on 3 wire lines zero sequence/ground over current elements can pick this up.

6. No phase angles to worry about when re-closing (line is always denergized when the breaker is open) and thus no bus and line VTs are needed. As a result distance protection would actually elevate cost significantly. Ditto for measuring each line's sequence components.

69Kv;


1. Lines are looped/meshed with varying fault current and thus forces the need for discrimination that goes past over current.

2. Critical clearing comes into play, with line clearing times at about 20-25 cycles often being required. Where this is to long POTT can be implemented (standard on modern distance relays) to bring it down to say 6 cycles.

3. Buss and Line VTs are often required based on phase angles / re-closing philosophy alone.

4. Lines usually do not change in distance from switching and thus sequence values remain the same.

 

[jghrist]

12.47 kV lines are often radial with multiple fused taps. Coordinating distance relays with fused taps is not practicable.