Barra Protection | Barra Diffarential Protection Scheme
A tafin lokaci na gida an yi amfani da mafi girman relays ta karamin hanyar tsakiyar karamin kasa daga busbar protection. Amma yana neman cewa abu mai lalacewa ko mutumai da ake kara da busbar ba zan iya haifar da systemin busbar. Saboda haka, wakar tsakitsakin relays masu busbar protection suna yi ne a lokacin da ke da zaman. Don haka, idan abu mai lalacewa ya faru a kan busbar, zai iya yi rike da zaman don in bude bus daga mafi girma wanda ya zama da lafiya da sauransu.
A tafin lokacin, an yi amfani da relays masu hanyar tsakitsa da biyu a kan feeders masu inganta, da tsawonsa na 0.3 zuwa 0.5 detari don busbar protection.
Amma wannan yanayi na da mafi kyau. Wannan yanayi ba zan iya sanya babban abubuwan da suka haifar da busbar.
A tafin lokacin, systemin karamin kasa na yi aiki da karamin kasa mai yawa. Saboda haka, idan akwai hadin karamin kasa a duk systemin bus, zai kasance halaye mai yawa ga kamfanin. Saboda haka, ita ce da ya danganta in bude kawai babban abubuwan da suka haifar da busbar a lokacin da abu mai lalacewa ya faru.
Wannan yanayi na da mafi kyau wajen tsakitsa da hanyar tsakitsa da biyu shine, idan lokacin da ake bude abu mai lalacewa, tsawon bude ba zai zama da tsawon da ya danganta don in tabbatar da ciyaron system.
Don in taimaka da kuma haɗa da abubuwan da aka bayar, an yi amfani da yanayi masu tsakitsa da hanyar busbar differential protection da tsawonsa na 0.1 detari, a kan duk SHT bus systems.
Differential Busbar Protection
Current Differential Protection
Yanayin busbar protection, tana ƙunshi, Kirchoff’s current law, wanda ya ce, jami'ar karamin kasa da ke samu cikin node elektriki ce da ke fito da jami'ar karamin kasa da ke rage node.
Saboda haka, jami'ar karamin kasa da ke samu cikin bus section ce da ke fito da jami'ar karamin kasa da ke rage bus section.
Prinsipin busbar differential protection ce da ya fiye. A wannan, secondaries of CTs suna kara da parallel. Yana nufin, S1 terminals of all CTs connected together and forms a bus wire. Similarly S2 terminals of all CTs connected together to form another bus wire.
A tripping relay is connected across these two bus wires.
A nan, a cikin figure above, ana ƙarin cewa, idan lokacin da ya da shiga, A, B, C, D, E and F carries current IA, IB, IC, ID, IE and IF.
Idan kana ƙarin cewa, saboda Kirchoff’s current law,
Essentially all the CTs used for differential busbar protection are of same current ratio. Hence, the summation of all secondary currents must also be equal to zero.
A nan, cewa karamin kasa ta relay connected in parallel with all CT secondaries, is iR, and iA, iB, iC, iD, iE and iF are secondary currents.
A nan, za a ƙara KCL at node X. As per KCL at node X,
Saboda haka, ita ce da ya danganta cewa lokacin da ya da shiga, ba ake samu karamin kasa a kan busbar protection tripping relay. This relay is generally referred as Relay 87. A nan, cewa abu mai lalacewa ya faru a kan duk feeders, waje protected zone. A wannan lokacin, abu mai lalacewa zai taka primary of the CT of that feeder. This fault current is contributed by all other feeders connected to the bus. So, contributed part of fault current flows through the corresponding CT of respective feeder. Hence at that faulty condition, if we apply KCL at node K, we will still get, iR = 0.
Yana nufin, lokacin da ya da abu mai lalacewa waje, ba ake samu karamin kasa a kan relay 87. A nan, za a ƙara abin da abu mai lalacewa ya faru a kan bus itself.
A wannan lokacin, abu mai lalacewa tana ƙunshi, abubuwan da suka haifar da bus. Saboda haka, a wannan lokacin, jami'ar abubuwan da suka haifar da abu mai lalacewa ce da ke fito da jami'ar abu mai lalacewa.
A lokacin da ya da abu mai lalacewa, ba ake samu CT. (a lokacin da ya da abu mai lalacewa waje, both fault current and contributed current to the fault by different feeder get CT in their path of flowing).
Jami'ar abubuwan da suka haifar da abu mai lalacewa ba zai zama da zero. Ita ce da ke fito da secondary equivalent of faulty current.
A nan, idan a ƙara KCL at the nodes, zai iya samu ma'anar iR.
Don haka, a wannan lokacin, karamin kasa tana bude a kan 87 relay and it makes trip the circuit breaker corresponding to all the feeders connected to this section of the busbar.
As all the incoming and outgoing feeders, connected to this section of bus are tripped, the bus becomes dead.
This differential busbar protection scheme is also referred as current differential protection of busbar.
Differential Protection of Sectionalized Bus
During explaining working principle of current differential protection of busbar, we have shown a simple non sectionalized busbar. But in moderate high voltage system electrical bus sectionalized in than one sections to increase stability of the system. It is done because, fault in one section of bus should not disturb other section of the system. Hence during bus fault, total bus would be interrupted.
Let us draw and discuss about protection of busbar with two sections.
Here, bus section A or zone A is bounded by CT1, CT2 and CT3 where CT1 and CT2 are feeder CTs and CT3 is bus CT.
Similarly bus section B or zone B is bounded by CT4, CT5 and CT6 where CT4 is bus CT, CT5 and CT6 are feeder CT.
Therefore, zone A and B are overlapped to ensure that, there is no zone left behind this busbar protection scheme.
ASI terminals of CT1, 2 and 3 are connected together to form secondary bus ASI;
BSI terminals of CT4, 5 and 6 are connected together to form secondary bus BSI.
S2 terminals of all CTs are connected together to form a common bus S2.
Now, busbar protection relay 87A for zone A is connected across bus ASI and S2.
Relay 87B for zone B is connected across bus BSI and S2.
This section busbar differential protection scheme operates in some manner simple current differential protection of busbar.
That is, any fault in zone A, with trip only CB1, CB2 and bus CB.
Any fault in zone B, will trip only CB5, CB6 and bus CB.
Hence, fault in any section of bus will isolate only that portion from live system.
In current differential protection of busbar, if CT secondary circuits, or bus wires is open the relay may be operated to isolate the bus from live system. But this is not desirable.
