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Question on Line Differential Application

Question on Line Differential Application

(OP)
HI all,

I would like to tap on protection gurus here. Can a line differential relay work on lines with multiple substation taps on them?

I asked this because in one of our client transmission system, the 69kV overhead line from Main-Tie-Main GIS switchgear has a line differential GE L90 going into a remote substation also having GE L90 and communicates via fiber optic using IEC 61850 protocol. Now between, these lines, a new substation will be tapping off. My concern that they required us to place another line differential which I am doubting if this even going to work.

Your help is very much appreciated for my understanding.

Regards

RE: Question on Line Differential Application

It can if you take the load current into account when setting the line differential (ie, make it less sensitive). However there is a limit as the more current you differential must take into account the less fault protection you have.

RE: Question on Line Differential Application

If you have protection at the tap point, the best solution is to protect it as a three terminal line. Is this what you are planning? No reason it shouldn't work.

With SEL products, you can use tap line protection. This allows you to apply a time overcurrent differential setting that would be set to coordinate with downstream protection off the tap.

Finally, you can make sure the minimum differential pickup is where it won't detect through faults on the new substation transformer. Faults on high side would cause the line to trip in this case.

RE: Question on Line Differential Application

We have several lines with tapped load protected with SEL differential relays, though I don't know anything about GE L90 relays. Line differential with consideration for tapped load is still likely to be better than other protection schemes. Although line differential can be slowed down to fully coordinate with transformer protection, we sometimes allow both the line and transformer protection to trip for a low probability transformer fault when the line has automatic reclosing.

RE: Question on Line Differential Application

We just supervise the differential with a phase and ground distance element. The 87 prevents the overreaching distance elements from operating and under high load we have security against the 87 element operating. It is easier than keep track of the load current. It appears to work and seems more straight forward than adjusting the diff elements to account load current.

I think that you could set negative and ground 87 elements and be good if all the tapped transformers have delta high sides.

RE: Question on Line Differential Application

Yes. But there IS a limit. Depending on the scenario you can probably get away with two tapped stations. The limitation isn't based on actual taps, but the total load of the taps relative to the expected power flow on the line. At lower power flows your differential slope will be lower, so tapped stations could look like an 87 trip.

We also supervise with a distance element, which helps.

RE: Question on Line Differential Application

Took a 2-day refresher on line prots this past Monday & Tuesday where this very topic was discussed extensively; our utility has a number of three-ended ckts where this type of prot is used. We supervise digital line diff prots with instantaneous Zone 2 impedance relays, but do not go to the trouble and expense of integrating the additional tapped stations into this prot scheme [since there aren't that many tapped stations], opting instead to "de-tune" the settings to allow for what we call "leakage"...but there definitely is a limit to just how far you would dare to push this particular envelope.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

RE: Question on Line Differential Application

Leakage makes sense, but seen far more references to spill current in that context than to leakage current.

RE: Question on Line Differential Application

GE L90s or L30s work with 3 terminal lines. The communication protocol for exchanging 87L data is not IEC 61850 but a propietary one.

You should have all the three devices with two communication channels so that each one communicates with the other two simultaneously. In case a link fails, one of the devices remains performing 87L function as a master and sends a transfer trip to the other two in case of a fault.

The multi ended fault location algorithm also works with 3 terminals.

Regards,

Charlie

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