Inter-trip Requirement In Question
Inter-trip Requirement In Question
(OP)
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.
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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RE: Inter-trip Requirement In Question
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.
RE: Inter-trip Requirement In Question
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RE: Inter-trip Requirement In Question
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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RE: Inter-trip Requirement In Question
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.
RE: Inter-trip Requirement In Question
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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RE: Inter-trip Requirement In Question
Is it like this?
Source 1 Source 2
| |
X Upstream Bkr 1 X Upstream Bkr 2
| |
| |
| |
| |
X Downstream Bkr X Downstream Bkr
| |
--+---------X-------------+--
Tie Bkr
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.
RE: Inter-trip Requirement In Question
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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RE: Inter-trip Requirement In Question
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.
RE: Inter-trip Requirement In Question
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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RE: Inter-trip Requirement In Question