Inter-trip Requirement In Question 2

[nightfox1925]

Our design contractor is presently designing an 11kV double-ended substation with normally closed bus tie and supplied to two separate feeders from another 11kV substation (no transformers in between). As a standard protection scheme, our design contractor provided directional phase and ground overcurrent relays on both downstream 11kV incomers + a non-directional phase and ground overcurrent relays. In the upstream 11kV feeeders, non-directional phase and ground overcurrent relays where also provided. In addition, a pilot wire type (line Differential) unit protection is provided. The protection scheme is also provided with inter-tripping schemes (mutual trip of upstream and downtream breakers) on both directional and non-directional schemes.

Now, our client advised as to remove the inter-tripping schemes since directional and pilot wire type unit protection is provided which I disagree. For an incoming line cable fault, inter-trip between upstream and downtsream breakers is a must. Can anyone provide me with more second opinions to back me up.

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[davidbeach]

You client believes that belts are enough and he doesn't also need suspenders. The line differential will clear any line faults by tripping the breakers at both ends. If that works, every time, there is no need for the inter-trip (transfer trip). But what happens on a line fault while the line differential is out of service (relays have to be tested occasionally)? In that case, without transfer trip, you will have sequential tripping. The breaker nearer the fault will see more fault current and trip faster; after that all the fault current will come from the remaining end of the line and then it will trip.

So, all faults will still be cleared, even without the line differential, but if the line differential fails, line faults will take longer to clear than they would if the transfer trip were included.

 

[nightfox1925]

Another point I noticed is that if we assume there is a cable fault on the line (assuming the differential relay is out of service or failed) then the downstream directional relay (67) would set to trip the assosciated downstream incomer while the upstream breaker (where the 51 relay is located) will trip with a set time delay. Eventually, if inter-trip is provided, the fault clearance within the cable zone will be faster (due to the directional relay tripping) and that is what I require for this protection zone. The upstream 51 will serve as back-up for both incoming cable fault and a downstream bus bar fault.

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[nightfox1925]

In addition, I have suggested to provide an electrical interlock between the upstream breaker and the downstream breaker such that everytime the upstream circuit breaker opens,the downstream circuit breaker trips (via send trip relay at the upstream breaker). My point is that either the upstream breaker is opened (manual/fault trip) the downstream breaker trips to avoid current backfeed.

In my opinion, I do not agree with my client's concept that the downstream 67 relay will take care if sombody manually opens upstream. This relay should respond to a true cable fault within its zone of protection and not to an upstream manual breaker opening. If such inter-trip is not provided, then during manual switching of the upstream breaker, the 67 actuates and sends a wrong alarm signal of a fault. Both upstream and downstream breakers are in separate substation with a very significant distance (5kM).

Are both technical justification acceptable. Need your second opinion.

Respectfully...


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[jghrist]

If someone manually opened the upstream breaker under a no-fault condition, why would the downstream 67 actuate? Under a no-fault condition, is it important for both breakers to open if one is manually opened? Why?

Your scheme would require a separate communications channel, assuming that you want it to work when the differential is out of service. I'm not sure I see the need; the client's proposed system seems to handle tripping both breakers under fault conditions. If the differential is out of service, tripping will be slower (with upstream 51), but would the extra speed of tripping be worth the cost and complication of an inter-tripping scheme?

If there is only one incomer in service, or if the bus tie is open, then the downstream 67 relay would not see a fault. In this situation, your inter-trip scheme would not speed up tripping of the upstream breaker anyway.

If you need fast tripping of both breakers under fault conditions with the differential out of service, I would suggest installing a 67 for the upstream breaker and using a POTT scheme. This seems like unnecessary goldplating for an 11 kV system to me, but it wouldn't cost much more than an intertripping scheme.

 

[nightfox1925]

"If someone manually opened the upstream breaker under a no-fault condition, why would the downstream 67 actuate? Under a no-fault condition, is it important for both breakers to open if one is manually opened? Why?"

jighrist, I am under the thinking that opening the upstream circuit breaker (say for some maintainance purposes) would draw a back feed current to flow to the 67 relay due due to a normally closed bus tie at the downstream circuit breaker. This will give a false 67 fault condition if this happens. Furthermore, if an authorized person would conduct some maintainance, it is convenient to send a trip signal to the downstream circuit breaker in order to de-energize the cable circuit and operate the cable grounding switch. What's your opinion.

Respectfully...

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[jghrist]

If there is no fault, what current would flow in the downstream breaker if the upstream breaker is open? Is there load tapped between the upstream and downstream breakers? Maybe I am misunderstanding the system.

Is it like this?
[tt]
Source 1 Source 2
| |
X Upstream Bkr 1 X Upstream Bkr 2
| |
| |
| |
| |
X Downstream Bkr X Downstream Bkr
| |
--+---------X-------------+--
Tie Bkr
[/tt]
If remote operation is desired, why not use SCADA to open the downstream breaker. I would not depend on an intertrip scheme to de-energize a cable circuit that I was working on. I'd want to make sure the breaker was open and tagged out and the ground switch closed.

 

[nightfox1925]

jghrist, your diagram is correct. The source side feeder circuit breakers are connected to a double ended switchgear with NC bus tie (same as downstream switchgear) and some of its associated feeder breakers are also connected to other substations.

I stand corrected. You are right, no reverse current would flow if the upstream breaker opens during no-fault condition. What is present is just a voltage at the load end terminals of the upstream breaker.

In this case, our client is correct about one point: No send trip is required if the upstream breaker is manually opened. However, I still see for a need to have a send/receive trip caused by protective devices on a fault within the incoming cable zone (due to the line differential and directional overcurrent).

How about if a power transformer is inserted in between and the voltage level is say 11kV / 3.3kV? Will a reverse current flow be seen by the downstream directional relay since circulating current may flow through the transformer secondary?

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[jghrist]

As davidbeach pointed out, both breakers would trip for a line differential operation, without intertrip. If the differential relays are out of service, the backup relays will both operate, but slower.

I'm not sure where you would be inserting the transformer such that circulating current would flow. If there are two transformers, one on each incomer, and the primary of one were de-energized by its upstream breaker opening, I don't see where current would circulate.

 

[nightfox1925]

You may be right jghrist, no circulating current will flow (to correct myself). The issue of directional relay operation when the upstream circuit breaker is open will not happen.

With this I would only observe that the automatic tripping of the downstream circuit breaker due to opening of the upstream circuit breaker is an additonal safety measure to ensure that there is no voltage present within the cable zone during maintainance. By the way, the two breakers (upstream and downstream) are in separate substations and both owned and maintained by the plant.

Any other comments and recommendations to add. Thanks for all the support...

respectfully.

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[davidbeach]

With this I would only observe that the automatic tripping of the downstream circuit breaker due to opening of the upstream circuit breaker is an additonal safety measure to ensure that there is no voltage present within the cable zone during maintainance.

Doesn't matter. There is "voltage present" on the interconnecting line until breakers at both ends have been racked out or the disconnecting switches have been opened, and tags and grounds have been applied. No one working in the line will rely on the breaker at the other end having been tripped.