Transformer in Zone-2 of cable protection 2

[nkn5]

Hello all,

I have a case where a long underground cable to be protected by step distance protection. This cable feeds a unit substation with a power transformer which effectively falls in zone-2 of the cable to be protected by distance protection. I would like to understand if zone-2 and zone-3 are required and if so, how zone-2 and zone-3 impedance are calculated in this case.

I will be thankful if anybody can share their experiences or any literature addressing this issue.

 

[davidbeach]

Does the transformer have its own protection and a high-side interrupting device? I wouldn't count fuses. Is the transformer the only load on the cable?

How fast do you trip underground cable? Are you trying to get the cable fault off the system as quickly as possible do you delay tripping a bit to ensure enough cable damage to have a better chance of finding the fault?

For an overhead line radially feeding only one transformer, I'd set the zone 1 reach at 125% of the line, but less than the line plus the transformer; if the transformer also trips it's a transformer fault but if only the line trips it is a line fault. There's nothing to be gained by having part of the line trip slower. Add other lines or more transformers at the receiving end and there are more decisions to make. Cable complicates this somewhat since you may want slower tripping and undoubtedly won't be reclosing.

If the transformer is a delta-wye with the delta on the cable side you can set the ground distance well "into" or even "through" the transformer since it won't actually reach through.

How many zones to set depends in part on what your back-up protection philosophy and goals are. If the transformer is fuse protected I'd definitely try to stay ahead of the fuses to the extent possible, especially for through faults. Resetting targets and closing a breaker is much less expensive and much quicker than replacing transformer fuses.

You can just add the impedances, once they're on the same base. You can also simply run faults in the model and find out what the relay sees.

 

[nkn5]

thanks David. transformer has its own differential protection and over current as backup at the unit substation. As you rightly mentioned, there are two more similar cables from the Primary station feeding to receiving unit station with transformers. Attached a oneline diagram. Each of the cables have separate differential and distance as a backup protection.

We intend the differential protection, zone-1 of distance protection to act with no intentional delay. I'm not sure about the fault locating methods when it is operational, I will try to find them out.

So I would set the zone-1 to 80% of the line and zone-2 (with a 200ms delay) to 100%of line plus 50% of transformer impedance to reach be into it. Further zones not required as there is enough protection (current differential) to transformer and further downstream.

Please advice if miss out anything here.

 

[davidbeach]

Don’t forget that batteries do fail. I’d want fully redundant DC systems - chargers, batteries, distribution, trip coils, etc. before I’d begin to consider omitting remote backup protection. Others may be willing to live more adventurously.

 

[RRaghunath]

nkn5, As Davidbeach originally suggested, it is appropriate to include part of transformer under Distance protection Zone-1.
You will appreciate that the cable itself will have very small impedance and it may not be easy to set the distance relay to 80% of cable itself.

Rompicherla Raghunath