Since electrical power transmission lines are usually quite long in length and pass through an open environment, the potential for electrical power transmission line faults is much greater than that of electrical power transformers and alternators. This is why transmission lines require more protection schemes than transformers and alternators.
Line protection should have certain features, such as-
1.During a fault, the circuit breaker nearest to the fault point should be tripped.
2.If the circuit breaker closest to the fault point fails to trip, the circuit breaker immediately adjacent to that breaker will trip as a backup.
3.The operating time of relays associated with line protection should be minimal to prevent unnecessary tripping of circuit breakers connected to other healthy parts of the power system.
The above requirements cause transmission line protection to be very different from the protection of transformers and other power system equipment. There are three main methods of transmission line protection-
1.Rating time on current protection.
a) Rating time on current protection:
It can also be called as over current protection of electric power transmission line. Let us discuss the different time classification schemes on current conservation.
Protection of Radial Feeder:
In a radial feeder, power flows in only one direction, from the source to the load. This type of feeders can be easily protected by using either fixed time relays or inverse time relays.
Line Protection by Definite Time Relay:
This insurance conspire is exceptionally straightforward. Here the absolute line is separated into various segments and each part is given fixed time transfer. The hand-off nearest to the furthest limit of the line has the base time setting while the time setting of different transfers moves progressively towards the source.
For instance, assume there is a source at point An in the figure beneath.
At point D a circuit breaker CB-3 relay is installed with a fixed time of operation of 0.5 seconds. Subsequently, another circuit breaker CB-2 relay is installed at point C with a specified time of operation of 1 second. Next circuit breaker CB-1 is installed at point B which is closest to point A. At point B, the relay operation time is set to 1.5 seconds.
Now, suppose a fault occurs at point F. Due to this fault, poor current flows through all current transformers or CTs connected in line. But since the operation time of the relay at point D is at least CB-3, the relay connected to it will trip first to isolate the fault zone from the rest of the line. If, for any reason, CB-3 fails to trip, then the next higher time relay will operate to initiate the corresponding CB to trip. In this case, CB-2 will trip. If CB-2 also fails to trip, the next circuit breaker i.e. CB-1 will trip to isolate the larger part of the line.
Advantages of Definite Time Line Protection:
The main advantage of this scheme is simplicity. Another major advantage is that during a fault, only the nearest CB line from the fault point to the source will serve to isolate the specified position.
Disadvantage of Definite Time Line Protection:
If the number of sections in the line is large enough, the time setting of the relay closest to the source will be too long. So any fault close to the source will take longer to isolate. This can have a very destructive effect on the system.
Over Current Line Protection by Inverse Relay:
As we have discussed about transmission line current protection only for a fixed time, this can be easily overcome by using inverse time relay. In an inverse relay, the operation time is inversely proportional to the fault current.
In the above figure, the general time grouping of the hand-off at point D is least and this time succession is expanded for the transfers associated with the focuses towards point A.
In the event of a shortcoming at point F, CB-3 at point D will clearly trip. In the event of inability to open CB-3, CB-2 will be worked on the grounds that the general time setting in this hand-off at point C is high.
Albeit, the time setting of the hand-off nearest to the source is most extreme, it will in any case trip quicker than expected, in the event that a huge shortcoming happens close to the source, on the grounds that the transfer working time is conversely corresponding to the shortcoming current.
Over Current Protection of Parallel Feeders:
To maintain the stability of the system it is necessary to feed a load from the source to two or more feeders in parallel. If a fault occurs in any feeder, only that faulty feeder should be isolated from the system to maintain continuity of supply from source to load. This requirement makes protection of parallel feeders slightly more complicated than simple non-directional over line current protection as in the case of radial feeders. Parallel feeder protection requires using directional relays and grading the time setting of the relays for selective tripping.
Two feeders are connected in parallel from source to load. Both feeders are non-directional at the current relay at the source end. These relays should be inverse time relays. Also both the feeders have directional relays or reverse power relays at the load end. The reverse power relays used here should be of instantaneous type. That is, these relays should be operated as soon as the power flow in the feeder is reversed. The general direction of power is from the source to the load.
Now, suppose a fault occurs at point F, say the fault current is This fault will have two parallel paths from the source, one through circuit breaker A only and the other through CB-B, feeder-2, CB-Q, load bus and CB-P. This is clearly shown in the figure below, where IA and IB are the common fault currents through feeder-1 and feeder-2 respectively.
As per Kirchoff’s current law, IA + IB = If.
Now, IA passes through CB-A, IB flows through CB-P. As soon as the flow direction of CB-P is reversed it will trip immediately. But CB-Q will not trip because the flow of current (power) in this circuit breaker is not reversed. As soon as CB-P trips, the fault current stops flowing through the IB feeder and hence there is no question of further tripping of the inverse time over current relay. IA still continues even CB-P is tripped. Then due to current IA, CB-A will trip. Thus the faulty feeder is isolated from the system.
Differential Pilot Wire Protection:
This is a differential protection scheme applicable to feeders only. A number of different schemes are implemented for line protection, but the Mess-price voltage balance system and the trans lay scheme are the most popular.
Merz Price Balance System:
The working principle of Merz Price Balance System is quite simple. In this line protection scheme, an identical CT is connected to each of the two ends of the line. CTs have the same polarity. The secondary of these current transformers and the operating coil of the two instantaneous relays form a closed loop as shown in the figure below. The pilot wire in the loop is used to connect both the CT secondary and the two relay coils as shown.
Now, it is quite clear from the figure that when the system is in normal condition, no current can flow through the loop because the secondary current of one CT will cancel the secondary current of the other CT.
Now, if a fault occurs in the line portion between these two CTs, the secondary current of one CT will not be equal and opposite to the secondary current of the other CT. So there will be a resultant circulating current in the loop.
Due to this circulating current, the coil of both relays will close the trip circuit of the associate circuit breaker. Therefore, the faulty line will be separated from both ends.