Installation, Maintenance, Testing &
Commissioning of Substations
A Study of Distance Protection
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Distance Protection:
1.
◼ Transmission lines
◼ Busbar
◼ Transformer/ Generator
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.
4.
85%
12%
3%
Measuring loop for single- phase faults
The distance protection relays are always set based on the phase
impedance to the fault. The measured Impedance is a function of
positive and zero sequence impedance
◼ Transient faults
◼ are common on transmission lines, approximately 80-85%
◼ lightnings are the most common reason
◼ can also be caused by birds, falling trees, swinging lines etc.
◼ will disappear after a short dead interval
So, the input quantities to the distance relay are voltage and current and the
output it gives is V/I which is impedance (Z). Since it uses two input
quantities, its reliability is more than that of overcurrent relay, which uses
only one input quantity (i.e., current)
Power lines have impedances of size 0,3- 0,4 ohm/km and normal angles of
80-85 degrees.
Fault Types:
◼ Persistent faults
◼ can be caused by a broken conductor fallen down
◼ can be a tree falling on a line
◼ must be located and repaired before normal service
2.
5.
Distance Protection Zones
There could be usually 03 to 05 impedance zone depending upon the
application.
Measuring loop for three- phase faults
The distance protection relays are always set based on the phase
impedance to the fault. The measured impedance is equal to the
positive sequence impedance up to the fault location
The principle of distance protection
ZL =R+jX
The line impedance must be converted to secondary values with the formula:
𝒁𝒔𝒆𝒄 = 𝒁𝒑𝒓𝒊𝒎 𝒙
𝑽𝑻𝒔𝒆𝒄 𝑪𝑻𝒑𝒓𝒊𝒎
𝒙
𝑽𝑻𝒑𝒓𝒊𝒎 𝑪𝑻𝒔𝒆𝒄
3.
Measuring loop for two- phase faults
The distance protection relays are always set based on the phase
impedance to the fault. The measured impedance is equal to the
positive sequence impedance up to the fault location
____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Prepared By: Babar Shahzad
Date: 18-03-2023 Rev:00
Page 1 of 2
Installation, Maintenance, Testing &
Commissioning of Substations
A Study of Distance Protection
___________________________________________________________________________________________________________________________________________________________________________________________
therefore cleared with no intentional time delay.
Zone 1:
9.
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/VT 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/VT errors, it may
appear to the relay that the fault is on the protected line, thereby
tripping the protected line, which is a wrong operation.
When energizing a power line onto a forgotten earthing no measuring
voltage will be available and the directional measuring can thus not
operate correctly.
A special SOTF function is thus provided. Different principles can be
used, from one phase current to unidirectional impedance measuring.
7.
Zone 2:
Permissive Over Reach Transfer Trip Scheme (POTT)
The working principle of the POTT (used for the short lines) pattern is
that if there is a disturbance felt by the two-distance relay zone, the
relay will send the signal to the other end terminals and
the other ends of the terminals will only work if the
interference is also felt by the two relays.
1. Mainly intended to cover the remaining 10 % to 20 % of the
protected line and provide backup for the adjoining lines (50% of the
adjoining 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 length or 100% of the protected
line length plus the 50% of the shortest adjoining line length,
whichever is the less.
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.
6.
SOTF condition can either be taken from the manual closing signal
activating the CB input or it can be detected internally by a logic.
10. Broken Conductor
Zone 3:
1. it is intended to give full back up to the adjoining 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
safety margin of 20 %.
4. The operating time of zone-III should be slightly more than the zone-ii
operating time. for a 220 kV line, it is about 700ms.
Switch onto Fault Logic (SOTF)
8.
Stub protection function
It is not possible for the distance protection relay to measure impedance
when the line disconnector is open. Not to risk incorrect operation the
distance protection must be blocked and a Stub protection is released.
The Stub protection is a simple current relay
Phase-phase or phase-ground faults lead the current to increase
appreciably and therefore they can easily be detected in most
applications. Open circuit faults are a different type of faults that can
happen in electrical networks. These faults can be caused by broken
conductors, blown fuses or mis-operation of a pole of a circuit-breaker.
Series faults will not lead to an increase in phase current and therefore
they cannot be easily detected by common overcurrent relays. However,
this type of faults produce an unbalance that creates negative phase
sequence current, which can be detected. Broken conductor function
measures the ratio between the negative and the positive phase
sequence current and issue a tripe as the delay time elapsed.
Permissive Under Reach Transfer Trip Scheme (PUTT)
To provide fast fault clearance for all faults, both transient and
permanent, along the length of the protected circuit, it is necessary to
use a signal aided tripping scheme. The simplest of these is the
Permissive Under Reach (PUR) protection scheme. The channel for a
PUTT scheme is by operation of the underreaching zone 1 elements of
the relay. If the remote relay has detected a forward fault upon receipt
of this signal, the relay will operate with no additional delay. Faults in
the last 20% (If Zone-1 set to cover 80%) of the protected line are
End of Training;
Hope after going through all above, each participant has a greater level of
understanding regarding distance protection.
____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Prepared By: Babar Shahzad
Date: 18-03-2023 Rev:00
Page 2 of 2