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Distance  Protection  

 
Introduction    
 
Ø The problem of combining fast fault clearance with selective tripping of

plant is a key aim for the protection of power systems
...

Ø Distance protection, in its basic form, is a non-unit system of protection

offering considerable economic and technical advantages
...

Ø This is illustrated in figure below, where it can be seen that overcurrent

protection cannot be applied satisfactorily
...

Ø It can also provide both primary and remote back-up functions in a

single scheme
...

Ø 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 apparent
impedance so calculated is compared with the reach point impedance
...

Ø The reach point of a relay is the point along the line impedance locus

that is intersected by the boundary characteristic of the relay
...

Ø The loci of power system impedances as seen by the relay during

faults, power swings and load variations may be plotted on the same
diagram and in this manner the performance of the relay in the presence
of system faults and disturbances may be studied
...
Reach accuracy is a comparison of the actual ohmic
reach of the relay under practical conditions with the relay setting value
in ohms
...
The impedance measuring techniques
employed in particular relay designs also have an impact
...
Depending on the measuring techniques employed in a
particular relay design, measuring signal transient errors, such as those
produced by Capacitor Voltage Transformers or saturating CTs, can also
adversely delay relay operation for faults close to the reach point
...
However, for modern digital
or numerical distance relays, the variation between these is small over a
wide range of system operating conditions and fault positions
...

Ø It was customary to present information on relay performance by

voltage/reach curves, as shown in figure below, and operating time/fault
position curves for various values of system impedance ratios (S
...
R
...
𝑅 =  
 
 

 

𝑍!
 
𝑍!

𝑤ℎ𝑒𝑟𝑒  𝑍! = 𝑠𝑦𝑠𝑡𝑒𝑚  𝑠𝑜𝑢𝑟𝑐𝑒  𝑖𝑚𝑝𝑒𝑑𝑎𝑛𝑐𝑒  𝑏𝑒ℎ𝑖𝑛𝑑  𝑟𝑒𝑙𝑎𝑦  
                                         𝑍! = 𝑙𝑖𝑛𝑒  𝑖𝑚𝑝𝑒𝑑𝑎𝑛𝑐𝑒  𝑒𝑞𝑢𝑖𝑣𝑎𝑙𝑒𝑛𝑡  𝑡𝑜  𝑟𝑒𝑙𝑎𝑦  𝑠𝑒𝑡𝑡𝑖𝑛𝑔  

 

  
 
Zones  of  Protection    
 
Ø Basic distance protection will comprise instantaneous directional Zone 1

protection and one or more time-delayed zones
...

Ø Digital and numerical distance relays may have up to five or six zones,
some set to measure in the reverse direction
...
To determine the
settings for a particular relay design or for a particular distance
teleprotection scheme, involving end-to-end signalling, the relay
manufacturer’s instructions should be referred to
...

Ø For digital/numerical distance relays, settings of up to 85% may be safe
...
Otherwise, there would be a loss of discrimination with
fast operating protection on the following line section
...


Zone 2 Setting
Ø To ensure full coverage of the line with allowance for the sources of

error already listed in the previous section, the reach setting of the Zone
2 protection should be at least 120% of the protected line impedance
...

Ø Where possible, this ensures that the resulting maximum effective Zone

2 reach does not extend beyond the minimum effective Zone 1 reach of
the adjacent line protection
...

Ø In electromechanical and static relays, Zone 2 protection is provided

either by separate elements or by extending the reach of the Zone 1
elements after a time delay that is initiated by a fault detector
...

Ø Zone 2 tripping must be time-delayed to ensure grading with the

primary relaying applied to adjacent circuits that fall within the Zone 2
reach
...


Zone 3 Setting
Ø Remote back-up protection for all faults on adjacent lines can be

provided by a third zone of protection that is time delayed to
discriminate with Zone 2 protection plus circuit breaker trip time for the
adjacent line
...
2 times the
impedance presented to the relay for a fault at the remote end of the
second line section
...
For
example, where the first three zones are set as above, Zone 4 might be
used to provide back-up protection for the local busbar, by applying a
reverse reach setting of the order of 25% of the Zone 1 reach
...
An offset impedance

measurement characteristic is non-directional
...


Distance Relay Characteristics
Plain Impedance Characteristic
Ø This characteristic takes no account of the phase angle between the

current and the voltage applied to it; for this reason its impedance
characteristic when plotted on an R/X diagram is a circle with its centre
at the origin of the co- ordinates and of radius equal to its setting in
ohms
...

Ø The relay characteristic, shown in figure below, is therefore non-

directional, and in this form would operate for all faults along the vector
AL and also for all faults behind the busbars up to an impedance AM
...
Vector AB represents
the impedance in front of the relay between the relaying point A and the
end of line AB
...
AL represents the reach of instantaneous Zone 1

protection, set to cover 80% to 85% of the protected line
...
This can be obtained by the addition of a separate directional
control element
...
At the same time, the
impedance unit is prevented from operating by the inhibiting output of
unit RD
...
Reversal of current through
the relay from IF1 to IF2 when C opens could then result in incorrect
tripping of the healthy line if the directional unit RD operates before the
impedance unit resets
...
In older relay designs, the type of problem to
be addressed was commonly referred to as one of ‘contact race’
...
It cleverly
combines the discriminating qualities of both reach control and
directional control, thereby eliminating the ‘contact race’ problems that
may be encountered with separate reach and directional control
elements
...
Mho impedance

elements were particularly attractive for economic reasons where
electromechanical relay elements were employed
...

Ø For this reason they are still emulated in the algorithms of some modern
numerical relays
...
This demonstrates that the impedance
element is inherently directional and such that it will operate only for
faults in the forward direction along line AB
...
Angle φ is known as the Relay Characteristic
Angle (RCA)
...
 

Example    
 
Ø The system diagram shown in figure below shows a simple 230kV

network
...
All relevant data for this exercise are given
in the diagram
...
Relay parameters used in the example are listed in
Table 11
...


 

 

  

 

The line impedance is:

...
089︎ j0
...
9 + j47
...
42︎∠79
...
42 (magnitude) and 80o (angle) as nearest settable
values
Zone  1  Reach  Setting  
 
Ø The required Zone 1 reach is 80% of the line impedance
...
8 x (48
...
41 o) = 38
...
41 o
Use 38
...

Zone  2  Reach  Setting  
 
Ø Ideally, the requirements for setting Zone 2 reach are:
Ø at least 120% of the protected line
Ø less than the protected line + 50% of the next line
Ø Sometimes, the two requirements are in conflict
...
A setting of the whole of the line between
substations ABC and XYZ, plus 50% of the adjacent line section to
substation PQR is used
...
42∠79
...
5 x 60 x (0
...
41 o))  
= 62
...
41 o
Use  62
...
It is assumed that this constraint is met
...
42∠79
...
2 x 60 x (0
...
41 o))
= 83
...
41 o
Use a setting of 83
...
Independent timers are available for the three zones to ensure this
...
A time delay is used only in cases
where large d
...
offsets occur and old circuit breakers, incapable of breaking
the instantaneous d
...
component, are involved
...

Assuming that this line has distance, unit or instantaneous high-set overcurrent
protection applied, the time delay required is that to cover the total clearance
time of the downstream relays
...
A typical time delay is 350ms, and the normal range is
200-500ms
...
Assuming distance
relays are used, a typical time is 800ms

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