Hello friends ! In this post you will get to know everything about the Distance Protection for the Transmission Lines. For 220 KV Lines Distance Protection is the Main Protection and Overcurrent Protectio is the Back up Protection.

What is the Principle of Distance Protection?

Distance Protection is a Non-unit System of Protection, which measures the Impedance between the Relay Location and the point where the fault is incident and compares it with the Set Value. If the measured Impedance is less than the Set Value , the Relay operates and Isolates the Faulty Section.Since, the Line Impedance is directly Proportional to Line Length, we get the exact Location of the Fault in Kms.Since it protects a certain Length of Transmission Line, it is called a Distance Relay.If, the Measured Impedance < Setting Impedance, the Relay Operates.

So, the Input Quantities to the Distance Relay is Voltage and Current and the Output it gives is V/I which is Impedance (Z). Since it uses two Input Quantities, its Reliablity is more than that of Overcurrent Relay, which uses only one Input Quantity (i.e. current).

What are the Faults covered by Distance Protection?

1. Line Faults – They are the Primary Side Phase to Phase & Phase to Earth Faults.

2. Secondary Side Faults: These Include, Broken Conductor and VT Fuse Fails.

3. Dead Line Charging : They are also called as SOTF (Switch On to Fault).

4. Abnormal System Condition : It covers the Power Swing.

What are the Types of Distance Relays?

Depending upon the Distance Relay Characteristics, there are following types of Distance Relays:

1. Impedance Relay:
2. Mho Relay:

What are zones of Distance Protection?

In order to provide Reliability, Distance Protection is divided into number of zones, which are given in the Tab Module below.

Zone 1:

1. Mainly intended to cover the entire length of the Protected Line Length and set to operate instantaneously i.e with no intentional Time Delay.
2. To avoid the Loss of Discrimination with the Zone 1 Protection of the following Line Section, Zone I Distance is set at 80 to 90 % of the Line and not 100%. Hence, it is called as an Under-reaching  Element.
3. This Safety Margin of 10 to 20 % is kept for Relay/CT/PT Errors, Infeed/Outfeed Effects and inaccuracies in line Impedance parameters. Suppose the Line is set at 100% of the Protected Line and the Fault Occurs on the Adjacent Line, but due to CT/PT Errors, it may appear to the Relay that the Fault is on the Protected Line, therby Tripping the Protected Line, which is a Wrong Operation.

Zone 2:

1. Mainly intended to cover the remaining 10 %  to 20 % of the Protected Line and provide Backup for the Adjoining Lines (50% of the Adjoning Length).
2. It set to cover Remote End Busbar and Hence it is called as an Overreaching Element.
3. It is set at 150% of the Protected Line lenghth or 100% of the Protected Line Length plus the 50% of the shortest Adjoning Line Length, whichever is the Less. It is set in such a way, so that the Zone 2 of he Adjacent lines do not overlap, so as avoid loss of Discrimination.
4. The Operating Time of Zone 2 is Delayed by 15-45 cycle time, so as to be selective with Zone 1 of the Adjacent Line i.e the Zone 1 Relays that are supposed to Trip get a chance to do their job first. For a 220KV Feeder, the operating Time of the Zone 2 it is about 400ms.
5. Zone II should not overlap
6. In case of Long Line, followed by Short Line, the above mentioned Formula may not give us margin against possible underreaching. In such cases, Zone II can be set  to Cover 120% of the Protected Line Length.

Zone 3:

1. It is intended to give Full Backup to the Adjoning Line Section.
2. Zone III is primarily intended to provide Backup against External Uncleared Faults and Hence set to cover the Longest Adjoining Line.
3. It covers Full Protected Line Length and Full Adjacent line plus the Safty Margin of 20 %.
4. Zone III covers, 100% of the Line Length Plus the 100% of the Longest Adjacent Line Length Plus a 10 Km of additional Length is considered, to clear the Bus Fault at the Remote End.
5. The Operating Time of Zone III should be slightly more than the Zone II Operating Time. For a 220 KV Line, it is about 700 ms.
6. In case of Long Heavily Loaded Line, the Zone III Settings should be checked for possible Load Encroachment

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