Feeder protection- Distance and line differential

12. Distance Relays



As the name indicates , the distance relay work on the principle of measuring the distance from the relay location to the fault. In other words if we know the votage across the fault and the current flowing through the fault , the ratio of this voltage and the current gives the impedance of the line and hence the relay can take a decision on tripping the associated CB. With obvious reason the relay requires the Voltage input from the VT and the current input from the CT to process this information to arrive at the fault .Unlike over current relays which can be set to operate above the load current , the distance relays can operate for a current less than the load current
Hence in a distance relay the settings are already given based on the line impedance and this value is compared with the measured value using the VT and CT input and if the measured impedance is less than the set impedance , the relay take a decision to trip the associated Circuit breaker
To understand the principle of Distance relays let us Assume a radial feeder , with a infeed from one side with VT and CT connected at the feeding side . Now let us assume there is a line to line fault at some point on the line , now the current seen by the CT is the fault current and the Voltage across the VT is the drop across the line up to the fault and that of the return path ( loop ) . Now if we calculate the ratio of voltage and current , then we get the impedance of the loop of the line and dividing this result with 2 , we get the impedance of the line up to the fault .
Similarly , if there is an phase to earth fault , then the voltage seen by the VT is the voltage drop in the line up to the point of fault and the drop of the earth return path . As the impedance of the earth return path is not equal to that of the line , the above mentioned method of measuring the impedance by direct method will not be applicable here and to get the actual impedance up to the fault , a factor called residual compensation factor to be given to the relay to calculate the actual impedance up to the point of fault . It is to be noted that , irrespective of nature of the fault whether it is line to line or line to earth , the point of fault is measuredby caluculating the impedance of the line up to the point of fault . In short if the impedance of the line up to the point of fault is Z1 and for a line to line fault at this point or line to earth fault at this point should give the same result for the fault location ( That is Z1 in case of line to line fault and the same Z1 in case of line to earth fault ) .

Relay measuremnts and computation during a fault
We know that the input to the Distance relay is the fault current seen by the CT placed at the relay location and the Fault voltage seen by the Voltage transformer placed at the relay location . Now let us try to analyse some faults and and how the relay find out the fault location from the CT and VT inputs
The faults are generally classified as
1. Three phase short circuit with or without ground path
2. Double phase fault
3. Single phase fault

3phase fault and the distance relay measurement
Let us try to analysis this fault by symmetrical component analysis
For a 3 phase fault let us calculate the relay computation of the impedance for the BC element
Let the measured line currents be Ia, Ib,Ic and the measured line to line voltages be Vab,Vbc,Vca
Now for the BC element of the relay
The voltage is Vbc and the current is Ib-Ic ( phasor sum)
We know that from the Symmetrical component Analysis
Ib= Ib1+Ib2+Ib0( +ve, -Ve,Zero sequ components respectively)
IC=Ic1+Ic2+Ic0
IA= Ia1=Ia2+Ia0

Also for 3phase fault Ia0=0 ( = 1/3(IA+IB+IC by defenition)
so also Ib0 and Ic0

Hence IB-IC = Ib1+Ib2 - Ic1-Ic2
Ib2 and Ic2 are Zero for a 3phase fault ( by defenition)
IB-IC= Ib1-Ic1
= a2Ia1-aIa1
= Ia1(a2-a)
Now the loop voltage VBC= (IB-IC)Z1 ( note that IB*Z1 is the drop in line B and ICZ2 is the drop in the line C)
= Ia1(a2-a)Z1
VBC/(a2-a)Ia1 = Z1
Hence from the VT voltage and the CT current we get the impedance at the fault .The term a2-a represent the phase angle

Double phase fault and the distance relay measurement


Let us assume a BC fault
Now Ia=0 (no fault)
IB=(-IC) ( when there is BC fault the loop current IBand IC will be same but in opposit direction )
Ia0=Ib0=Ic0= 1/3(IA+IB+IC)= 0 ( as IA=0 and IB=-IC}
IB-IC= Ib1+Ib2-Ic1-Ic2
= a2Ia1+aIa2-aIa1-a2Ia2
= (a2-a)Ia1- (a2-a)Ia2
Also as IA=0
Ia1+Ia2+Ia0=0
Ia1+Ia2 =0 as Ia0=0
Hence Ia2=-Ia1
Hence IB-IC= 2(a2-a)Ia1
We know that loop voltage = VBC= (IB-IC)Z= Ib1Z1+Ib2Z2-Ic1Z1-Ic2Z2
For a line Z1=Z2
Hence (IB-Ic)Z= Ib1+Ib2-Ic1-Ic2 *Z1
= 2(a2-a)Z1
Hence VBC/Ia1*2(a2-a)= Z1
In this case also the relay measures the impedance Z1 with manipulation

Measurement for Earth faults and Residual Compensation

As explained in the forgoing paragraph , for a line to line fault , the impedance up to the point of fault can be calculated more directly where as for a line to earth fault , the impedance up to the point of fault can not be found out directly due to the different impedance of the line and that of the earth return path . Let us analysis the above by a single line to earth fault with line impedance of Z1 and earth return path impedance of ZE
Now for a single phase to earth fault we know that the loop impedance is Z1+ZE and the voltage drop across the faulted loop is
Vph.n = IaZ1 + InZE
= Z1( Ia + In ZE/Z1)
Where Ia is the line current and the In is neutral current or Earth fault current ( of course in this case Ia=In and In is used in the equation for more generalization of the result )

Now from the above equation ,
Vphn/ (Ia+ In ZE/Z1) = Z1 which is the impedance of the line up to the point of fault
We know that Vphn is the line to neutral Voltage of the VT and the Ia is the current measured by the CT . It is clear from the above equation that , in thedenominator of the equation there is second term apart from the CT current Ia , that is a component of neutral current or residual current . The factor by which this neutral current is multiplied (ZE/Z1) is called residual compensation factor
Now for a transmission line or cable we know that , the zero sequence impedance Z0= Z1 +3ZE ( by definition )
Hence ZE = Z0-Z1/3
Substituting for ZE ,then we have another equation for residual compensation factor as Z0-Z1/3Z1

Zero sequence compensation factor

 From the foregoing discussion , the poertion of the neutral current added to the fault current Ia to obtain Z1
= In ZE/Z1
= In Z0-Z1/3Z1
= In/3 Z0-Z1/Z1
= I0 Z0-Z1/Z1 where I0 is the Zero sequence current which is equal to In/3
= I0 K0 ( where K0 = Z0-Z1/Z1 is called Zero sequence compenstation factor )

Effect of source impedance on the distance relay measurements


Zones of protection

The protection Zones in case of a distance relay applied to a transmission line is generally devided in to 3 zones
Zone 1 - The measured impedance of the relay depend upon the measured value of voltage and the current . Naturally if there is any error in the measurements these error will also be introduced in the measured impedance . Hence a relay which is supposed to measurre the fault impedance may see an impedance more than that of the actual impedance or it may see an impedance lees than the actual impedance . In otherwise the relay may under reach or it may over reach . Hence to take care of the situation and to avoid the relay overreach ( to avoid the relay to see the fault in the next line from the substation ) an allowance is usually made in the setting . Hence if it is intend to protect a line in between two stations , the setting is usually done to cover 70-80 percent of the line .This is called Zone 1 seting .The timing is usually instatanious for this zone .
Zone 2
In the previous paragraph we understood that , the Zone 1 protection is set to cover 70 to 80 % of the line to be protected . The zone 1 reach is set to this level to avoid any overreach of the relay due to the error introducedby the CT or in general, measurement . Hence the remaining 20 to 30 % of the line is still left unprotected . This portion is covered by another Zone called Zone 2 . To avoid any error in reach measurement , the zone 2 is set to cover the remaining 20 t0 30 % plus say 50% of the next shortest line from the remote substation . As the Zone 2 is set to cover ideally some portion of the next line also . Hence if we give an instatanious operation for the Zone 2 , It may operate for a actual fault at the next line which should have cleared by the Distance relay and asociated CB at thet remote station, To avoid such an indiscriminative tripping the Zone 2 is always with time delay typically say 400 msec . Hence Zone 2 is not a back up protection by intention evethough it acts so in certain cases .
From the above discussion we understand that the 20 to 30% of the line is tripped with time delayed Zone 2 operation which is undesirable . Hence to avoid the delay and to make the operation faster , we go for communication aided tripping schemes which is discussed later in this discussion

.There is another backup zone set in the relay known as Zone 3 which is set to cover the 100% of the protected line and 100% of the next largest line from the remote station . This timing is usually set to 800msec . Care also should be taken to avoid the zone 3 setting to cover the fault impedance . Hence the maximum loading condition shall also be taken in to consideration before setting the Zone 3 reach ( otherwise load will be treated as fault and results in unwanted relay operation )

Zone 4 settings

There is an another setting above the Zone 3 but below the load impedance which is used to detect the power swings in the system which characterized by the slowly varying impedance which passes through different Zones . During a fault the fault , the change from the load impedance to the fault impedance is very fast whereas in case of power swings the variation of impedance during the power swing is in comparatively slow . Hence the power swing is detected by the time taken for the impedance curve to cross from the Zone 4 to Zone 3 and decision is taken by the relay accordingly.

Distance Relay Charecteristics

The Distance relay measures the impedance up to the point of fault and compares this value with the set value in the relay .However for any type of faults we know the there are certian amount of additional resistance added to the impedance measuremnt due to the arc

Charecteristics on RX diagram

As the impedance of the line consists of the line reactance and the resistance , it can be plotted in an RX plane . Now we can define this characteristics in four quodrants . Let us assume that , the line impedance looking forward from the relay location as positive and the line impedance looking reverse from the relay location as negative . Then First quadrant represnts the +X and +R meaning that ,it is forward inductive reactance and the resistance of the line and -ve X and +R which comes in the fourth quadrant represent the Capacitive reactance and the resistance of the line in forward direction ( see the sign change of X when the reactance is capacitive ) . Similarly if we use -Ve sign for the above concepts ie -ve X and -ve R which means the inductive reactance of the line looking in reverse direction which is in the third quadrant and the +ve X and -Ve R meaning the capacitive reactance in the reverse direction.

Now if we can measure the reactance and the resistance based on its direction from the relaying point , then we can say whether the fault is in forward direction or reverse direction

Let us see how this can be achieved in the relay by using the Current input and the Voltage input we have . Now let us take the current as reference and try to draw the voltage phasor up to the point of fault ,

we will see that IR drop in same phase as that of the fault current and IXL drop leading the Voltage by 90deg for the forward inductive faults - The point lies in the first quadrant

Similary IR drop in same direction of fault current and the IXc drop lagging the Current by 90deg and this point lies in the Fourth quodrant

Similarly if the current I is refersed with respect to the Relaying point , meaning a fault in the reverse direction , the current phsor changes its direction and hence the -IR drop which is in phase with the fault current -IXL drop which is leading the current by 90deg which lies in the fouth quadarnt

and finally a reverse capacitive fault , -IR drop in phase with I and IXC drop lagging the I by 90deg which comes in the third quadrant

Now we have got coordinates in all four qudarant in line with the RX quadrant concept but with multiplication by I . this can be scaled to calculate the impedance .

Now if we know the line impedance ( to be set in relay )then we have

Z(sq) = R(sq)+X(sq) which is the equation of a circle with Z as radious and if the measured impedance is less than this set impedance , the relay will operates- This is called impedance charecteristics

From the forgoing discussion we understand that if we just take the magnitude ofthe impedance for the relay measuremnt with out considering the angle asociated with it , then the relay will operate for the faults in both forwrd and reverse direction which is undeisrable and indescriminative tripping will result . Hence inorder for the relay to become directional and trips for only positive faults an additional charectristics is added to the impedance charectristics to make it directional which will respond only for forward direction

Mho charecteristics to be continued




Quadrilateral characteristics Distance relay schemes


The Zone 1 of the distance relays are generally set 70 to 80 % of the line to be protected. The balance 20 to 30% is given as tolerance in order to avoid the relay overreaching due to various errors introduced in measurements if the relay were set at 100% . The balance portion of the line is protected by the backup Z2 of the distance protection and hence it is time delayed .
In order to avoid the time delay for a fault at the far end of the line which would come under the Z2 of the protection, different schemes are used which makes use of communication aided inter tripping so that the tripping takes place at a faster time .
There are different schemes used to achieve the aided tripping that are discussed below

Permissive under reach Scheme .

In this scheme , the Zone 2 tripping at this end is made faster if he Zone 1 starter at the remote end picks up and sends a signal to this end thru communication media . The whole idea is based on the fact that , for a fault seen in Zone 2 of this end relay would normally be seen in Zone 1 by the other end of the line . Hence for such a fault , the other end trips first and simultaneously sends a signal to this end , as this end relay starter has already picked up due to the fault seen in Zone 2 , it trips faster as confirmation of the fault is received from the other end . The point to be noted here is that eventhough , the fault is seen by the relay at this end by its forward looking Zone 2 , this fault has to be identified as forward by the relay at the other end to make sure that , the fault is within the protected line . ( Zone 2 of this end will pick up for faults which is beyond the next station for which the relay at this end need not be tripped in Zone 1 time)

Permissive Over reach scheme .

If the fault is high impedance type or if the protected line length is very less, there is a chance for the remote end relay to see the fault in Zone 2 and if the zone1 start at the remote end is used to send signal to this end , then the relay at both ends may not trip at all for such faults . Hence Z2 start of the remote end is used to send a signal to this end so that, the tripping at this end takes place at a faster time . As the fault is seen by the remote end relay also in the Zone 2 , it will not trip instantaneously but trips in aided trip once it receives signal from the remote end .
Please note that this is a case where in the relay at both end see the fault in Zone 2 forward due to the error introduced in the measurements due to various reason like , Ct errors , High fault impedance etc due to which the relay at both the ends of the protected line see the fault in Zone 2 measurement even though the actual fault is within the protected Zone .
The disadvantage of the relays operating in the permissive scheme is that , the inter tripping takes place only if the signal is reached from the remote end . In any case if the signal received is attenuated , then there is a possibility for the non operation of the relay for actual faults and tripping of the scheme for spurious signals if proper filtration is not provided

Blocking scheme


In the permissive scheme , discussed above the tripping due to the aided tripping takes place only after getting the signal from the remote end . Hence availability of the communication channel and signal strength is a prerequisite for the tripping to take place . However the so called blocking schemes works in the opposite way . It always block the tripping if the fault is outside the zone , otherwise allows the tripping . Hence the signal is required for blocking only . This is preferred if the tripping is to be ensured in all the cases .
To understand the concept more clearly let us assume a fault near to the bus of the next substation which is seen by this end relay in zone 2 , Now the other end relay see this fault in forward direction hence it sends no signal . This end relay wait for some signal from the remote end for a preset time say 100msec . As there is no signal from the remote end in this case it trips the CB at this end after the time delay , in this case 100 msec
Now suppose the fault is beyond the bus of the remote station . In this case also , this end relay see the fault in say Zone 2 distance . However the remote end relay see the fault in reverse direction and it immediately send a signal to this end for blocking the relay operation which otherwise would have tripped after the preset time of 100msec
Obviously , there is no signal required for the tripping to take place , it is required only in case of blocking . Hence such schemes are particularly suitable where the communication signal is attenuated due to long distance ( of course with possibility of unwanted tripping ) .
It shall be noted that , the reverse zone setting of this end shall be such that it is covering more than the zone 2 setting from the remote end otherwise unwanted operation will occur at the remote end . To be more clear , assume a reverse zone fault at this end which is seen by Z2 of the emote end( The Zone 2 pick up of the remote end is used to send a signal to the local end for aided tripping) . Now the remote end relay wait for some time before it issues a trip signal . As the fault is in the reverse direction at this end and a blocking signal shall be sent from this end relay to remote end to block the tripping of the remote end . This end relay can only give blocking if the fault is covered under revrese zone setting of this relay . If the revrese zone is not covering the zone2 reach covered by the remote end relay , then this end relay will not see the fault at all. Hence it will not send the blocking signal and an unwanted tripping occurs at the remote end . Hence it shall be ensured that , the reverese zone setting is set more than the reach as seen from the Z2 of the remote end for the proper operation

Echo scheme

In case of a condition wherein the CB of the remote end is open , then the relay at the remote end will not see any fault at all . under such condition of the CB open at the remote end , the relay at this end sends a signal to the remote end which will be echoed back to this end as the CB is open and hence the relay at this end take a decision to trip the CB faster .

UNBLOCKING SCHEME
In case of permissive schemes , with power line carrier , there is a possibility for the signal to be lost during the fault as the signal is sent through the power line itself . Hence the relay may not trip for such faults . To over come this difficulty , an unblocking schme can be used . Here a continues guarding signal is sent thru the power line carrier under normal condition and this signal will be lsot during the fault . This loss of signal is considered as an indication of fault if the relay also identifies the fault . Hence tripping is ensured . the disadvantage of this scheme is the requirement of the continues signal during the normal condition



DEF Protection

The main disadvantage of the distance protection is that its inability to sense the earth fault with very low fault current . There are some miminimum current requirement for the distance relay to start the zone measurement and if the fault current is lees than this setting . In order to overcome such a situation , a distance relay is always complimented with directional Earth fault protection ( DEF ) which can be set a very sensitive currents compared to the distance relays . It also uses communication aided tripping to make the relay responds to a fault only in the protected Zone

to be continued ..




 
To be continued....