Saturday, 16 April 2016

Distance Protection Philosophy

Distance protection is a non-unit system of protection offering considerable economic and technical advantages. Unlike phase and neutral over current protection, the key advantage of distance protection is that its fault coverage of the protected circuit is independent of source impedance variations. Let us take an example of this to understand how distance protection is independent of source impedance. Consider the figure below.

In the figure above, R1 is an over current relay which is used for the protection of Transmission Line. If there is a fault at F1,

Equivalent source impedance Zs = 10×10/20 = 5 Ω

Impedance up to the point of Fault = 5+4 = 9 Ω

Fault current IF1= 220×103/1.732*9 = 220×103/15.588 = 14113.5 A

Therefore the setting of over current Relay should be more than 14113.5 A.

Now consider the case,

Here fault is not on the Transmission Line but it is assumed to be inside Switchyard and only one source is feeding the power to the network. Proceeding in the similar manner,
Fault Current IF2 = 220×103/1.732*10 = 12702A

Therefore for the protection of Transmission Line, the setting of Relay shall be kept less than 12702 A. But for earlier case we saw that setting of Relay R1 shall be more than 14113.5 A thus overall the setting shall be > 14113.5 but <12702 A which is impractical. Therefore over current Relay is not suitable here and it depends on the source impedance.

Distance protection is therefore used for the protection of Transmission Line. It is simple to apply and fast in isolating the faulty section from the healthy network. Distance Protection provides primary as well as back-up protection to the protected line. I will show this back-up protection function latter in this post.


Since the impedance of a transmission line is proportional to its length, for distance measurement it is justified to use a relay capable of measuring the impedance of a line up to a predetermined point. This predetermined point is called Reach of the Relay.

Such a relay is described as a distance relay and is designed to operate only for faults occurring between the relay location and the selected reach point, thus giving discrimination for faults that may occur in different line sections. The basic principle of distance protection involves the division of the voltage at the relaying point by the measured current. The apparent impedance so calculated is compared with the reach point impedance which is settable in the Relay. If the measured impedance is less than the reach point impedance, it is assumed that a fault exists on the line between the relay and the reach point and issues trip command to the concerned Breaker Trip Coil either through Master Trip Relay or directly (in case of single pole tripping of breaker, assuming single pole Auto Reclosure is allowed).

If measured value of impedance V/I is less than setting z then Relay assumes a fault as clear from the above diagram.


Consider the figure below and carefully observe.

Here there are three sub-stations namely A, B and C. For sub-station A, the distance protection is divided into three zones Z1a, Z2a and Z3a which are called Zone-1, Zone-2 and Zone-3 protection. Similarly for sub-station D the three zones will be Z1d, Z2d and Z3d.
Zone-1 is normally set to 80% of total length of Line (here line length is AB between two consecutive substation). Zone-2 is set to 150 % of total line length and Zone-3 set at 120% of (100% line length + 100% of Longest Line from Remote substation i.e. B). It should be noted that all Zones are setting is done in terms of impedance.

Assume the distance between A and B = 200 KM

Total Impedance of Line AB = 61 Ohm

Current Transformer ratio = 1000/1A

Potential Transformer ratio = 400 kV / 110 V

So for Zone-1 Impedance setting = 80 % of Total Line Impedance = 80% of 61

                                                      = 0.8×61 = 48.8 Ohm ????? (Will it be????)

It won’t be…..because you need to consider CT & PT ratio for calculating the impedance as the Relay is sensing current and voltage through CT and PT only.

PT/CT Ratio = 1000/(400×103/110) = 1000×110/400,000 = 0.28

So the required setting for Zone-1 = 48.8×0.28 = 13.66 ohm. Which means if the distance Relay senses Impedance less than 13.66 Ohm then it will pick-up for Zone-1.

In the same manner, Setting for Zone-2 = (150% of 61) × CT/PT ratio

                                                                = 1.5×61×0.28 = 25.62 ohm

Which means if the distance Relay senses Impedance less than 25.62 Ohm then it will pick-up for Zone-2.

Setting for Zone-3 = 120% of (Impedance of Line AB+ Impedance of Longest Line from                        substation B)

Assume the Longest Line from substation B is having an impedance of 61 Ohm.

Therefore Setting for Zone-3 = (120% of (61+61)) × CT/PT ratio

                                              = 1.2×122×0.28 = 41 Ohm

Which means if the distance Relay senses Impedance less than 41 Ohm then it will pick-up for Zone-3.

So we now know how to calculate the setting for different Zones of Distance Protection.
Now suppose our substation is A and we are providing distance protection so Relay is located at A. For fault in Zone-1, obviously we need to isolate the fault without any time delay. Now say our breaker at A opened but as we are connected to the substation B so their breaker at B shall also trip so as to isolate the fault completely otherwise fault will be feed from substation B side even though our breaker at A opened. Thus if fault in Zone-1 occurs then Distance Relay shall trip Breaker at A and send a signal to Remote Substation B by receiving which Remote substation B shall trip their breaker at B. This signal is called Carrier Signal which is sent through Power Line Carrier Communication (PLCC) Line. This is the purpose of PLCC. I will post on PLCC latter so be there.

Thus for Zone-1, time delay = 0. Got it? (If no then write in comment box I will be happy to clear your doubt)

Next, suppose there is a fault in Zone-2 then our breaker at A shall not trip rather Remote Substation breaker at C shall trip (If fault is in section CD in figure above) as it will be in their Zone-1. So we need to introduce some time delay in our Distance Relay to operate for Zone-2 fault. This time delay is usually kept around 350 ms. If within 350 ms Remote substation breaker at B trips then our Breaker at A won’t trip but if suppose because of any Reason Remote Substation breaker at C fail to trip then our breaker at A will definitely trip.

See how Zone-2 is working as Back-up protection for line CD. Got it friend?

Now if there is a fault in the remaining 20% of line which is protected by Zone-1 at our substation A then it will be sensed by our Relay at A in Zone-2 but for Remote substation B it will be Zone-1 so their breaker at B will instantaneously trip but our breaker at A also need to trip otherwise our substation will continue to feed the fault by receiving carrier signal.

Now coming to Zone-3, if there is a fault in Zone-3 then our breaker at A is not supposed to trip rather Remote substation breaker at C &D is supposed to trip. Therefore we introduce some time delay for the operation of Zone-3 which is typically of the order of 1s. If because of any reason breaker at C & D fail to trip within 1s then our Distance relay will operate to open our Breaker at A.

There is one more Zone in modern Distance Relay which is called Reverse Zone or Zone-4. As the name Reverse Zone implies it is back-up protection of the Substation where Distance Relay is installed, in our case to the substation A. The setting for zone is normally 10% of the impedance of protected line.

Distance Relay Zone Characteristics on R-X Plane:

The reach point of a relay is the point along the line impedance locus that is intersected by the boundary characteristic of the relay.

Thank you! Waiting for your comments…….


Balaji Govindarajan said...

Hi your explanations are really good. But i need some more clarity in Zone 3 operation, could you explain me in brief? Also please do let me know how busbar differential works, if you have any explanation like this please share. My email

Aditya Kumar said...

Thank you! Let me take a real situation which we are adopting for new Switchyard. There are four zone of protection Zone-1, Zone-2, Zone-3 and Zone-4. Zone-3 setting is done in such a manner that it should operate when there is a fault on Remote end. Like if the distance between station A and station B = 200 Km and the longest line from station B is of 201 km (say station C) then as per the setting when a fault occurs in a line at station C then Zone-3 shall operate but first we shall wait for the operation of Zone-1 of station C and Zone-2 of station B, therefore a delay of 1 s is introduced in Zone-3. This is basically time grading. Hope it might help you.

For bus bar differential protection, I will say that it is much easier and will post on it soon and send you a doc. Thank you very much gain.

Pawan Pareek said...

Such a good explanation......& examples....

Unknown said...

Great description!!

Gaurang Nandha nandha said...

Great description!!

Amaan said...

Thank you Aditya for providing valuable information

I have one query.

as per the explanation, distance Zone reach settings are
Z1=13.66 ohms, Z2=25.62 ohms, Z3=41 ohms.

Consider If the fault occurs on Zone1, so let us assume the impedance seen by the relay will be around 10 Ohms.

So obviously this value (10 Ohms) lies under all the zone-1,2&3 settings.

So I understand that Z1 will operate at 0 seconds and zones (Z2 & Z3) will start and it will trip the breaker as per its time settings only when its remote breaker fail to open or fault still exists in the line.

Is my understanding is correct?

Aditya Kumar said...

Yes Amaan. You are correct!

Anonymous said...

Hi Aditya,

Great explanation

Some concerns with regards to the consideration of the CT/PT ratio in calculating the setting for the Line Impedance. Is there a standard where it is based? Since as i always do in practice particularly in testing distance relays, i always consider the percentage of the line not considering the CT/PT ratio, please advise




Great this what I was searching for .... I think I will visit your blog regularly.

Thank you very much for this information sharing.


Aditya Kumar said...