Line Protection

Since the length of the electricity transmission line is generally quite long and passes through an open atmosphere, the probability of failure of the electricity transmission line is much greater than that of transformers and alternators of electricity. That’s why a transmission line requires many more protection schemes than a transformer and alternator.

Line protection must have some special features, such as:

In the event of a fault, the single switch closest to the point of failure must trip.

If the switch closest to the defective point does not trip, the switch right next to this switch trips as a backup.

The operating time of the relay associated with the line protection must be as short as possible to avoid the unnecessary intervention of circuit breakers associated with other sound parts of the power supply system.

These aforementioned requirements cause protection of the transmission line very different from protection of the transformer and other equipment of the power supply system. The three main methods of protecting the transmission line are:

Time graduated with respect to the current protection.

Differential protection.

Remote protection.

Time graduated with respect to the current protection

This can also be defined simply as overcurrent protection of the electricity transmission line. Let’s take a look at several time frames classified on current protection.

Radial feeder protection

In the radial feeder, the power flows only in one direction, which goes from the source to the load. This type of power supply can be easily protected by defined timed relays or reverse timed relays.

Line protection by defined timed relay

This protection scheme is very simple. Here the total line is divided into several sections and each section has a defined time relay. The relay closest to the end of the line has a minimum time setting, while the time setting of other relays subsequently increases towards the source.

protection of radial feeder

In point D, the CB-3 switch is installed with a defined relay run time of 0.5 seconds. Subsequently, at point C another CB-2 switch is installed with a defined relay operating time of 1 second. The following CB-1 switch is installed in point B, which is closest to point A. In point B, the relay is set at the time of operation for 1.5 seconds.

Now suppose that an error occurs at point F. Because of this error, the defective current flows through all the current transformers or CTs connected on the line. But since the operating time of the relay in point D is minimal, the CB-3, associated with this relay, will trip first to isolate the defective area from the rest of the line. In the event that, for any reason, the CB-3 does not trip, the next relay with the highest time will work to trip the associated CB. In this case, CB-2 will fire. If not even CB-2 trips, then the next switch, or CB-1, trips to isolate most of the line.

Advantages of protecting the defined timeline

The main advantage of this scheme is simplicity. The second main advantage is that, during the fault, only the isolator closest to the source from the fault point will function to isolate the specific position of the line.

Disadvantage of protecting the defined timeline

If the number of sections on the line is large enough, the time setting of the relay closest to the source would be very long. Therefore, during any fault closer to the source, it will take a long time to isolate itself. This can cause a serious destructive effect on the system.

Overcurrent protection through reverse relay

The drawback that we discussed just in defined time about the current transmission line protection can be easily overcome by using the reverse time relays. In the reverse relay, the operating time is inversely proportional to the fault current.

In the figure above, the general relay time setting at point D is minimal and subsequently this time setting increases for the relays associated with the points at point A.

In case of failure at point F, of course CB-3 will start at point D. In case of failure to open CB-3, CB-2 will be activated since the general time setting is greater on that relay at point C.

However, the time setting of the relay closest to the source is maximum, but it trips in a shorter period if a serious fault occurs near the source, since the operating time of the relay is inversely proportional to the current. defective.
Differential protection of the pilot cable

This is simply a differential protection scheme applied to power supplies. Various differential schemes are applied for the protection of the line, but the voltage balance system of the price of the disorder and the transmission scheme are the most used.

Merz price balancing system

The principle of operation of the Merz price balance system is quite simple. In this line protection scheme, identical CTs are connected to each of the two ends of the line. The polarity of the CTs is the same. The secondary of these current transformers and the operating coil of two instantaneous relays are formed in a closed circuit as shown in the following figure. In the loop, the pilot wire is used to connect the CT secondary coil and the relay coil, as shown. 

However, the time setting of the relay closest to the source is maximum, but it trips in a shorter period if a serious fault occurs near the source, since the operating time of the relay is inversely proportional to the current. defective.
Differential protection of the pilot cable

Now, from the figure, it is quite clear that when the system is in normal conditions, there would be no current flowing through the circuit since the secondary current from one CT will cancel out the secondary current from another CT.

Now, if an error occurs in the part of the line between these two CTs, the secondary current from one CT will no longer be the same and opposite to the secondary current from another CT. Therefore, there would be a resulting circulating current in the circuit.

Related Posts

Leave a comment

You must be logged in to post a comment.