Bus differential protection

[stevenal]

What's your bus protection philosophy; when and where is it used? Is there a minimum voltage level? We're looking to eventually loop some of our now radial sub-transmission system, so I'm trying to plan ahead. I'm looking for comments from those experienced in electric utility protection and design. Google jockeys need not answer. Thanks.

 

[dpc]

For utilities, it's rare to find bus diff on distribution buses, except for new installations of metal-clad switchgear.

Industrial facilities will often use it for their main 15 kV switchgear, especially when fault currents are very high.

Of course, for ring bus systems, bus diff is not really applicable.

For the sub-transmission voltages (34.5 - 69 kV) it seems to be a mixed bag. Above 69 kV, it is pretty common. Retrofitting bus diff protection is tough because of the need for matching CTs.

Other bus arrangements use as main/transfer bus or double-bus/single breaker are more difficult to apply bus diff to because you will have to switch out the bus diff for "off-normal" operating conditions (or switch the CT secondaries which is scary).

Schweitzer has a new digital high-impedance bus diff relay. I'm not about GE/Multlin or ABB. Basler has an older analog solid state bus diff.

 

[stevenal]

DPC,

It is a 69 kV main/transfer bus. When you said above 69, did you mean to exclude that particular voltage, putting it in the mixed bag category? I don't see a difficult application, since all we would do is sum all the currents all the time. In normal mode, the aux breaker has zero current. When switched in, the aux breaker picks up the current of the line breaker it replaces. Sum is still zero.
We would only need to switch out the protection when a mobile transformer is installed, since I wouldn't bother with CT's at the hook up point for this rare event. Thanks.

 

[dpc]

I don't see any problem with what you are proposing, as long as there are no loads normally coming off the transfer bus. And as long as you have the dedicated CTs available that you need.

I'd suggest adding a selector switch to allow the bus diff to be disabled in the trip circuit.

The next consideration would be breaker failure for the 69 kV breakers.

Sounds good.

 

[RRaghunath]

I have come across in a steel plant, 33kV indoor switchgear with bus bar protection.

 

[electengr]

Hi, Happy new year to all.

bus differential is used in 11kv in-door metal-clad Switch gears as well.the philosophy is:sum of all in-coming current should be eqal the sum of all out-going currents
in a substation with multi incoming feeders & many outgoing feeders in different section. sould there be a problem with in the bus section,all the incoming & the all the outgoing feeders will trip isolating the faulty bus section

 

[stevenal]

When I said "philosophy" I didn't mean the operation, but where used. For example, Blackburn suggests a transformer differential use rule of thumb: 10MVA and above. Looking for similar application rules for bus protection. Bus in question is outdoors.

 

[RRaghunath]

Stevanol,

Here in Gulf and in India the outdoor substations at 66kV level and above are provided with Busbar differential protection. Hope this info is useful to you.

 

[peterb]

In addition to everything said above, consider the clearing time required for a bus fault. Can your system ride through a bus fault that is cleared by remote backup protection? Is there any generation connected at this voltage level?

Sounds as though you aren't too concerned about sectionalizing the faulted bus section, but this could be another indicator for deciding where high speed discriminating protection is required.

 

[MarkAthay]

Bus protection circuits are generally installed where there is a desire to minimize the damage from a bus fault. Don't assume that relaibility will be increased from having differential protection, reliability will actually decrease on that particular bus or that section of the system due to the extra complexity introduced and additional possibilities for failure.

If the mechanical results and electrical results of a fault are the same for a particular bus, then I'd seriously suggest you reconsider installing a differential circuit on it.

Things that influence whether or not to install a differential protection system: System stability, very high fault currents and hence severe equipment damage in a fault, repeated fault problems in the past and wanting to protect the rest of the system from it.

There is no "right" answer to this. As Blackburn once told me, the relaying philosophy of a particular company is generally determined by the manager, and not by the technical requirements.

Mark in Utah