LBB & Partial Differentiation

Googling on your question yields: The Ladyshenskaja-Babuska-Brezzi (LBB) condition from

"Numerical treatment of partial differential equations" by Christian Grossmann, Hans-Görg Roos, Martin Stynes Pg. 268

There's much more math there for the truly curious/obfuscated amongst us ...

A better question is why a numerical method in finite element math is in the same question as bus tripping logic, but hey, I'm just a EE/CS sort of guy.

You should get a clear idea first about the question and then answer it.

I wanted to mean LBB= Local Breaker Back up protection and Partial Differentiation protection (is used where bus zone and check zone protections are not in use).

While testing these two logics for down or off feeder, what are the precautions one should follow to avoid bus tripping.

Thanks for the clarification, this is what happens when one's knowledge base covers too many topics. I suppose it would be even better if we all used less acronyms, abbreviations, etc and provided sufficient information when we seek advice from others.

"Partial differential" relaying is a form of overcurrent relaying frequently used to detect main bus overcurrent faults and to back up feeder overcurrent relaying. The basic circuit is shown in the one-line diagram. Note that this is a double-ended substation, with two main breakers and a tie breaker. The partial differential relaying concept cannot be used on a straight radial distribution system.

True bus differential relaying compares all currents entering and leaving a switchgear bus. Within the limits of the accuracy of the CT's and the relays, true bus differential relaying will detect all faults on the protected bus. Since all currents are taken into account, the relays can be very fast. However, bus differential relaying provides no backup to the feeder overcurrent relaying, so additional overcurrent relays are required on main and tie breakers to provide this backup function. Also, high speed bus differential relaying can be quite expensive, and many switchgear users do not feel that it is economically justified. Partial differential relaying sums the currents entering or leaving a switchgear bus through main and tie breakers. If a fault exists on the protected bus, the currents will add in the relays, but if fault current is flowing through the bus to a fault on another bus, the currents will subtract and the relays will not respond. If the fault is on a feeder, the partial differential relays will act as backup to the feeder overcurrent relays.

LBB( local breaker Back up) is used for grid substation to protect the grid stability.<p>

It is mandatory protection requirement as per Indian Grid Code for EHV grid feeders .<p>

I hope you know that grid feeders are protected by distance protection. <p>

Normally distance protection are provided with 3 Zone protection Schemes. In that scheme, when there is a fault in the feeder, distance relay will give trip command to the breaker and the breaker will open to clear the fault.<p>

If the breaker is defective and did not execute the trip command, then the back up relays (located in all the nearby stations to which this station is connected) will trip and clear the fault making this station isolated from the grid. Other wise this station will become dead. This will take 400 to 1000 ms depending upon the scheme of connection.<p>

The delay in clearing the fault upto 1000 ms may be detrimental to the power system stability depending upon the location of fault in the grid.<p>

As anyway the station becomes dead due to operation of the remote end relays in adjacent substations, LBB relay is designed to trip all the other breaker in this station itself, without waiting for other end relays to come for rescue <p>

LBB relay is connected with it's current circuit in series with the Main II protection relay. LBB relay will operate if two conditions are fulfilled. (1) It will operate when current flows above set value ( normally set at 200 Milli-amps ) after 200 ms (2)on receipt of a DC command<p>

This is how it operates.<p>

When fault is detected by the distance relays and trip command send to breaker, another simultaneous DC command is sent to the LBB relay. If the breaker trips the current will cease to zero within 100ms at the maximum and therefor LBB relay will not operate. <p>

On the other hand if breaker did not respond to the trip command,the fault current will continue to flow for period of 400 to 1000 ms. Now the LBB relay will operate in 200ms and send trip command to all the other breakers in this station<p>

This is advantageous not only for grid stability.The operator in this station will identify immediately the failure of operation of the breaker and can normalise the system quickly.

Regarding Partial differential protection. I never heared any such thing.Only differential protection is available. Please explain your query in detail.

Re: If the breaker is defective and did not execute the trip command, then the back up relays (located in all the nearby stations to which this station is connected) will trip and clear the fault making this station isolated from the grid. Other wise this station will become dead. This will take 400 to 1000 ms depending upon the scheme of connection.

Interesting. It sounds like India uses a different transmission / distribution system than I'm used to (in my experience in the mining and steel industry in the US).

What I'm used to for feeding a substation is to tap off the HV line and put a breaker in that tap--there is no opportunity to trip the feed to adjacent substations as there is no breaker in the HV line at the substation that can break the HV line (except after the tap).

(In some situations we have what are called "double ended substations" which are substations with two transformers (fed from separate power sources), each feeding one side of a split bus, with a tie breaker connecting the two halves of the split bus. If the transformer (or something else) feeding one side of the bus is lost, both sides of the bus are fed from the remaining transformer (to keep critical loads powered).)

In India, how is the connection from the HV line made to the substation--do you put breakers "inline" in the HV line on either side of the tap to the substation?

Is the power network always a ring or something like that?

Dear Mr. Kramer,

You are talking about radial station or what is generally termed as distribution substation.<p>

In such stations power will be fed from a particular Grid station or it might be located in the middle of a tie feeder between two grid stations<p>

In these substations, the failure to trip a breaker on the down stream (Say 11Kv feeder breaker in a 110KV/11 KV SS) will be controlled by the breaker on the upstream side like LV Breakers (on the transformer secondary) and HV breakers located on the Transformer HV side.

The scheme of LBB protection is warranted only on Grid-feeders.

Thanks!

So, reading between the lines (maybe), a grid station / feeder is a substation that has one or more generators--it feeds power into the system?

Regarding precautions to be taken, while testing the relay, following to be adopted <p>

If it is electromechanical relay, remove relay from its case, while testing the main protections<p>

If it is not with drawable remove LBB trip fuse from the panel.<p>

In TNEB, it is mentioned in operating instruction for issue of Line clear to the testing personnel that it shall be mentioned in the line clear order that the LBB fuse has been removed.<p>

It is the responsiblity of the operator on duty to remove the LBB trip fuse before issuing Line clear for testing.