For external faults, that lead to CT saturation, the block signal arrives 1st and also the trip command remains inactive. … Compared to a conventional busbar differential relay theme, the newrelay achieves larger stability on external faults and increased sensitivity for internal faults.
In early days solely standard over current relays were used for conductor protection. however it’s desired that fault in any feeder or electrical device connected to the conductor mustn’t disturb conductor system. In viewing of now setting of conductor protection relays square measure created drawn-out. therefore once faults happens on conductor itself, it takes a lot of time to isolate the bus from supply which can came a lot of injury within the bus system.
In recent days, the second zone distance protection relays on incoming feeder, with in operation time of zero.3 to 0.5 seconds are applied for bus protection. however this theme has additionally a main disadvantage. This theme of protection cannot discriminate the faulty section of the bus. Now days, electric power system deals with immense quantity of power. thus any interruption in total bus system causes massive loss to the corporate. therefore it becomes essential to isolate solely faulty section of bus throughout bus fault.
Another disadvantage of second zone distance protection theme is that, someday the clearing time isn’t short enough to confirm the system stability. to beat the on top of mentioned difficulties, differential bus protection theme with associate degree operational time but zero.1 sec., is often applied to several SHT bus systems.
Differential Busbar Protection
Current Differential Protection
The theme of bus protection, involves, Kirchoff’s current law, that states that, total current coming into Associate in Nursing electrical node is precisely adequate to total current exploit the node. Hence, total current moving into a bus section is adequate to total current exploit the bus section.
The principle of differential conductor protection is incredibly easy. Here, secondaries of CTs are connected parallel. That means, S1 terminals of all CTs connected along and forms a bus wire. equally S2 terminals of all CTs connected along to make another bus wire. A tripping relay is connected across these 2 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,
IA+IB+ IC+ ID+ IE and IF. Essentially all the CTs used for differential conductor protection are of same current quantitative relation. Hence, the summation of all secondary currents should even be capable zero.
Now, say current through the relay connected in parallel with all CT secondaries, is iR, and iA, iB, iC, iD, id est and iF area unit secondary currents. Now, allow us to apply KCL at node X.
As per KCL at node X,
So, it’s clear that beneath traditional condition there’s no current flows through the conductor protection tripping relay. This relay is mostly referred as Relay eighty seven. Now, say fault is occurred at any of the feeders, outside the protected zone. therein case, the faulty current can submit to primary of the CT of that feeder. This fault current is contributed by all alternative feeders connected to the bus. So, contributed a part of fault current flows through the corresponding CT of individual feeder. thence at that faulty condition, if we tend to apply KCL at node K, we’ll still get, iR=0
That means, at external faulty condition, there’s no current flows through relay eighty seven. currently think about a scenario once fault is occurred on the bus itself. At this condition, conjointly the faulty current is contributed by all feeders connected to the bus. Hence, at this condition, add of all contributed fault current is adequate to total faulty current. Now, at faulty path there’s no CT. (in external fault, each fault current and contributed current to the fault by completely different feeder get CT in their path of flowing).
The total of all secondary currents is not any longer zero. it’s up to secondary equivalent of faulty current. Now, if we tend to apply KCL at the nodes, we are going to get a non zero price of iR. therefore at this condition current starts flowing through eighty seven relay and it makes trip the fuse love all the feeders connected to the current section of the conductor. As all the incoming and outgoing feeders, connected to the current section of bus ar tripped, the bus becomes dead. This differential conductor protection theme is additionally referred as current differential protection of conductor.
Differential Protection of Sectionalized Bus
During explaining working rule of current differential protection of busbar, we’ve shown an easy non sectionalized busbar. however in moderate high voltage system electrical bus sectionalized in than one sections to extend stability of the system. it’s done as a result of, fault in one section of bus mustn’t disturb alternative section of the system. therefore throughout bus fault, total bus would be interrupted. allow us to draw and discuss regarding protection of conductor with 2 sections.
Here, bus section A or zone A is finite by CT1, CT2 and CT3 wherever CT1 and CT2 ar feeder CTs and CT3 is bus CT. equally bus section B or zone B is finite by CT4, CT5 and CT6 wherever CT4 is bus CT, CT5 and CT6 ar feeder CT. Therefore, zone A and B ar overlapped to make sure that, there’s no zone left behind this conductor protection theme. ASI terminals of CT1, a pair of and three ar connected along to make secondary bus ASI; BSI terminals of CT4, five and six ar connected along to make secondary bus BSI. S2 terminals of all CTs ar connected along to make a typical bus S2. Now, conductor 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 theme operates in some manner easy current differential protection of conductor. That is, any fault in zone A, with trip solely CB1, CB2 and bus CB. Any fault in zone B, can trip solely CB5, CB6 and bus CB. Hence, fault in any section of bus can isolate solely that portion from live system. In current differential protection of conductor, if CT secondary circuits, or bus wires is open the relay could also be operated to isolate the bus from live system. however this is often not fascinating.
DC Circuit of Differential Busbar Protection
A typical DC circuit for busbar differential protection scheme is given below.
Here, CSSA and CSSB ar 2 selector that are wont to place into service, the bus protection system for zone A and zone B severally. If CSSA is in “IN” position, protection theme for zone A is in commission. If CSSB is in “IN” position, protection for zone B is in commission. typically each of the switches ar in “IN’ position in traditional operational condition. Here, relay coil of 96A and 96B ar nonparallel with differential bus protection relay contact 87A-1 and 87B-1 severally. 96A relay is multi contacts relay. every fuse in zone A is connected with individual contact of 96A. Similarly, 96B is multi contacts relay and every fuse in zone-B is connected with individual contacts of 96B. though here we have a tendency to use only 1 tripping relay per protected zone, however this can be higher to use one individual tripping relay per feeder. during this theme one protecting relay is provided per feeder fuse, whereas 2 tripping relays one for zone A and alternative for zone B ar provided to bus section or b
us coupling fuse. On an interval fault in zone A or bus section A, the various bus protection relay 87A, be energized whereas throughout internal fault in zone B, the various relay 87B are energized. As presently as relay coil of 87A or 87B is energized various no. contact 87A-1 or 87B-1 is closed.Hence, the tripping relay ninety six can trip the breakers connected to the faulty zone. to point whether or not zone A or B bus protection operated, relay thirty is employed. for instance, if relay 87A is operated, corresponding “No” contact 87A-2 is closed that energized relay 30A. Then the No contact 30A-1 of relay 30A is closed to energized alarm relay seventy four. direction relay ninety five of various zone is additionally energized throughout internal fault, however it’s a time delay of three second. So, it reset as presently because the fault is cleared and thus doesn’t develop zone bus wire shorting relay 95x that successively shorts out the bus wires. Associate in Nursing alarm contact is additionally given to the current auxiliary 95x relay to point that CT is open circuited. No potential unit relay eighty is provided in each trip and non-trip section of the DC circuit of differential bus protection system to point any separation of D. C. supply.
Voltage Differential Protection of Busbar
The current differential theme is sensitive only if the CTs don’t get saturated and maintain same current magnitude relation, point in time error below most faulty condition. this can be typically not eighty, significantly, within the case of an external fault on one in all the feeders. The CT on the faulty feeder is also saturated by total current and consequently it’ll have terribly massive errors. because of this massive error, the summation of secondary current of all CTs during a specific zone might not be zero. therefore there is also a high probability of tripping of all circuit breakers related to this protection zone even within the case of associate external massive fault. to stop this maloperation of current differential bus protection, the eighty seven relays square measure supplied with high acquire current and enough time delay. the best hard explanation for current electrical device saturation is that the transient dc element of the contact current. This difficulties may be overcome by using air core CTs. This current electrical device is additionally referred to as linear mechanical device. because the core of the CT doesn’t use iron the secondary characteristic of those CTs, is line. In voltage differential bus protection the CTs of all incoming and outgoing feeders square measure connected serial rather than connecting them in parallel.
The secondaries of all CTs and differential relay type a closed-loop system. If polarity of all CTs ar properly matched, the total of voltage across all CT secondaries is zero. thence there would be no resultant voltage seems across the differential relay. once a buss fault happens, total of the all CT secondary voltage is not any longer zero. Hence, there would be current flow into within the loop because of the resultant voltage. As this loop current conjointly flows through the differential relay, the relay is operated to trip all the circuit beaker related to protected bus zone. Except once ground fault current is severally restricted by neutral electrical phenomenon there’s sometimes no property downside once such a tangle exists, it’s resolved by use of a further additional sensitive relaying instrumentality as well as a oversight protecting relay.