Barra de Distribuição Proteção | Schema Protectionis Differentialis Barra

In early days only conventional over current relays were used for protectionem busbar. But it is desired that fault in any feeder or transformer connected to the busbar should not disturb busbar system. In viewing of this time setting of busbar protection relays are made lengthy. So when faults occurs on busbar itself, it takes much time to isolate the bus from source which may came much damage in the bus system.
In recent days, the second zone distance protection relays on incoming feeder, with operating time of 0.3 to 0.5 seconds have been applied for protectionem busbar.
But this scheme has also a main disadvantage. This scheme of protection can not discriminate the faulty section of the busbar.
Now days, electrical power system deals with huge amount of power. Hence any interruption in total bus system causes big loss to the company. So it becomes essential to isolate only faulty section of busbar during bus fault.

Another drawback of second zone distance protection scheme is that, sometime the clearing time is not short enough to ensure the system stability.
To overcome the above mentioned difficulties, differential busbar protection scheme with an operating time less than 0.1 sec., is commonly applied to many SHT bus systems.

Differential Protectionem Busbar

Protectionem Differentialis Currentis

The scheme of protectionem busbar, involves, Kirchoff’s current law, which states that, total current entering an electrical node is exactly equal to total current leaving the node.
Hence, total current entering into a bus section is equal to total current leaving the bus section.

The principle of differential busbar protection is very simple. Here, secondaries of CTs are connected parallel. That means, 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.

Here, in the figure above we assume that at normal condition feed, A, B, C, D, E and F carries current IA, IB, IC, ID, IE and IF.
Now, according to 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.

Now, say current through the relay connected in parallel with all CT secondaries, is iR, and iA, iB, iC, iD, iE and iF are secondary currents.
Now, let us apply KCL at node X. As per KCL at node X,

So, it is clear that under normal condition there is no current flows through the protectionem busbar tripping relay. This relay is generally referred as Relay 87. Now, say fault is occurred at any of the feeders, outside the protected zone. In that case, the faulty current will pass through 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.

That means, at external faulty condition, there is no current flows through relay 87. Now consider a situation when fault is occurred on the bus itself.
At this condition, also the faulty current is contributed by all feeders connected to the bus. Hence, at this condition, sum of all contributed fault current is equal to total faulty current.
Now, at faulty path there is no CT. (in external fault, both fault current and contributed current to the fault by different feeder get CT in their path of flowing).

The sum of all secondary currents is no longer zero. It is equal to secondary equivalent of faulty current.
Now, if we apply KCL at the nodes, we will get a non zero value of iR.
So at this condition current starts flowing through 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 Protectionem Sectionalis Busbar

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 protectionem busbar 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 protectionem differentialis busbar 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.

DC Circuit of Differential Protectionem Busbar

A typical DC circuit for protectionem differentialis busbar is given below.