Distance Protection

The impedance relay/distance relay is made to respond to the impedance between the relay
location and the fault point. The impedance is proportional to the distance to the fault and is
therefore independent of the fault current levels.
The V/I ratio at the relay can be termed as the “impedance” seen by the relay. The relay will
operate if the impedance seen by the relay is less than ‘Z’. Since the operating characteristics of
the relay depend upon the ratio of voltage and current and the phase angle between them, their
characteristics can be best represented on an R-X diagram where both V/I ratio and the phase
angle can be plotted in terms of an impedance R+jX. Further, the power system impedance like
fault impedance, power swings & loads. can also be plotted on the same RX diagram. Therefore
response of a particular relay during power swing, faults and other system disturbances can
easily be assessed.
Types of Distance Relays:- Equn:-T=K1|Ir|^2+K2|Vr|^2+K3|Vr||Ir|cos(φ-τ)+K4(spring)
Impedance relay:- Characteristics of an impedance relay on R-X diagram is shown in fig

Put K1=+ve, K2=~ve & K3=0 in above Equation. Preferred for medium transmission line.
The numerical value of the ratio of V to I is shown as the length of the radius vector, such as
Z and the phase angle between V and I determines the position of the vector, as shown.
Operation of the impedance relay is practically independent of the phase angle between V and I.
The operating characteristic is a circle with its center at the origin, and hence the relay is non-
directional. |Zseen|<|Zset| then trip.
Reactance type Distance Relay:- Reactance relay measures V/I Sin Ø (i.e. Z sin Ø). Whenever
the reactance measured by the relay is less than the set value, the relay operates. The operating
characteristic on R-X diagram is indicated below.
Put K1=+ve, K2=0 & K3=~ve in above Equation. Preferred for short line & L-G fault.
The resistance component of impedance has no effect on the operation of reactance relay. The
relay responds solely to reactance component of impedance. This relay is inherently non-
directional, so it can’t used alone but need to supervise by another relay . The relay is most
suitable to detect earth faults where the effect of arc resistance may render other types of relays
to detect faults with difficulty.|Xseen|<|Xn| &|Zseen|Cos(φ-90’)<|Xn| then trip.
Mho relay: - This is a directional impedance relay, also known as admittance relay. Its
characteristic on R-X diagram is a circle whose circumference passes through the origin as
illustrated in Figure below showing that the relay is inherently directional and it only operates for
faults in the forward direction. |Zseen|<|Zn|Cos(φ-τ) then trip.

Put K1=0, K2=~ve & K3=+ve in above Equation. Preferred for Long transmission line.
Modified impedance relay:-Also known as offset Mho relay whose characteristic encloses the
origin on R-X diagram as indicated in the figure above.
This offset mho relay has three main applications: -
i) Bus-bar zone backup.
ii) Carrier starting unit in distance/carrier blocking schemes.
iii) Power Swing blocking.
DISTANCE RELAYS APPLICATION
Relay Setting
Since the distance relays are fed from the secondary of line CTs and bus PTs/line CVTs, the line
parameters are to be converted into secondary values to set the relay as per requirements.
Zsecy = Zpri/Impedance ratio (where Impedance ratio = P.T.Ratio/C.T.Ratio)
Impedance in Ohms per KM is approximately & zone of protection
220 KV- 0.4- 70 to 80 Deg
400 KV- 0.3- 80 to 85 Deg
Zone-1 80% of ZL Instantaneous
Zone-2 100% of ZL + 40-50% of ZSL 0.3 to 0.4 seconds
Zone-3 100% of ZL + 120% of ZSL 0.6 to 0.8 seconds
Zone-4 20% of Reverse
SIR-system impedance ratio. Y=Zs/ZL
Choice of O/C relay for Line protection-
SIR-Large-Short line-DTOC preferred. >2
SIR-Small-Long line-IDMT preferred. <2

Distance Scheme consists of the following major components:-

1. Starters: The starting relay initiates the distance scheme in the event of a fault within the
required reach (more than zone-3). Starting of timer relays for second and third zone &
Switching of the respective faulty phase currents and voltage to the measuring unit in a
switched scheme. The starters are generally of Mho or impedance type.
2. Measuring units: They are generally of a mho or reactance or a combination of mho,
reactance and resistance types.
Phase Fault Units:-These measuring units are fed with line to line voltages (such as Va,
Vb) and difference between line currents (Ia-Ib). They measure the positive sequence
impedance from the relay location to the fault point. They however do not respond
correctly to earth faults.
Earth Fault Units: - These measuring units utilize line to neutral voltage (Van, Vbn, Vcn)
and phase currents (Ia, Ib, Ic). In order to make these units measure the positive sequence
impedance correctly, a zero sequence current compensation is to be provided which is
obtained by: KN = (Z0-Z1)/ 3*Z1. In the current circuit (1+KN) Ia will be fed for the
above measurement.
3. Timers: Timer relays when initiated by starters provide the time lag required for zones.
They also will be used for zone extension purpose whenever required.
4. Auxiliary relays: Distance scheme comprises of several auxiliary relays, which perform
functions such as flag indications, trippings, signaling, alarm etc.

Additional Features in distance schemes: -

1. Power Swing blocking:- Distance relay which respond to balanced 3-phase changes in
the impedance will be affected by power swings. In case of fault, the transition from
period of impedance locations to fault impedance is sudden whereas during power
swings, the transition to swing impedance is slow; the PSB relays use this difference to
block the tripping during swings.
2. VT fuse failure relay:- The distance relays being voltage restraint O/C relays, loss of
voltage due to main PT fuse failure or inadvertent removal of fuse in one or more phases
will cause the relay operation. The fuse failure relay will sense such condition by the
presence of residual voltage without residual current and blocks the relay.
3. Fault Locator:-
4. Auto Reclosing Scheme:-Three types of fault:-1) Transient Fault, 2) Semi-permanent
Fault, 3) Permanent Fault. About 80-90% of the faults occurring are transient in nature.
Hence automatic reclosure of breaker will result in the line being successfully re-
 energized, thereby decreasing outage time, Improving reliability, & Improving system
stability.
Dead Time: The time between auto-reclose scheme being energized and the operation of the
contacts which energize the circuit breaker closing circuit. Lower limit is decided by de-
ionising time of circuit breaker. Upper limit is decided by transient stability and synchronism
Long transmission lines require longer dead time for single phase faults. The dead time for
high speed auto-reclosing scheme with EHV system is 0.3-0.8 Sec.
Reclaim Time: The time between first and second auto-reclosure. This should not be set to
such a low value that the operating cycle of breaker is exceeded when two fault incidents
occur close together. The reclaim time will be in the range of 10-30 Sec, depending on the
breaker opening and closing mechanisms.
Types of Auto-reclosing schemes (based on phase):
a) Three Phase Auto-reclosing
b) Single Phase Auto-reclosing
Types of Auto-reclosing schemes (based on attempts of reclosure)
a) Single shot Auto-reclosing:-
b) Multi-shot Auto-reclosing:-
i) Circuit Breaker Limitations:
ii) System conditions
Types of Auto-reclosing (depending on speed)
a) High speed Auto-reclosing:-Characteristics of protection schemes and circuit breaker.
b) Low speed or Delayed Auto-relcosing:-This is suitable for highly interconnected
systems where the loss of a single line is unlikely to cause two sections of the system to
drift apart and loose synchronism.

Factors affecting distance relay operation:-

1) Fault resistance:- Fault resistance has two components:
a) Arc resistance. b) Ground resistance.
In a fault between phases, only arc resistance is involved.
For a fault at F, the actual line impedance = R + JX = ZL
Due to the presence of fault resistance, the impedance measured
by the relay = R + JX + RF = ZR (where ZR > ZL)
Fault arc resistance is given by Warrington’s formula:
Rarc = 8750 xL /I 1.4 where L = length of arc in ft, I = fault current in Amps
The arc resistance has little effect on accuracy of zone-1 unit as it
operates instantaneously before the arc can stretch appreciably except in case of short
lines. Reactance relays are therefore used for short lines where the fault resistance may be
comparable with that of the protected lines and also for ground faults where the ground
resistance is high.
The arc resistance will have greater impact on accuracy of backup zones (time delayed)
as the arc stretches appreciably.
2) Infeed effect:-A fault at F on the line BC is at a distance of Z1+Z2 for the relay at station
A. But when current I2 flows from bus D, the impedance to the fault as seen by the relay
at A is Z1 + Z2 + Z2 x (I2/I1).Thus the fault is seen by the relay as farther than what it
 really is, i.e. distance relay under reaches due to the infeed effect.

3) Branching-off effect:- A fault at F is at the distance of Z1+Z2 for the relay at station A.
But when current I1 gets distributed as I2 & I3 at station B, the impedance to fault seen
by the relay at station A will be (Z1 + I3/I1 * Z2) which is less than (Z1+Z2). Then the
fault is seen by the relay as nearer than what it really is i.e. distance relay overreaches due
to branching-off effect.

4) Load encroachment:- The two conditions i.e. operation at heavy load and short circuit
differ by virtue of phase angle between voltage and current. For the load impedance, the
Phase angle will be within +30 to - 30 Deg. While during short circuits, the fault
impedance has a phase angle of 60 to 80 deg.

The following points should be observed while testing.

1) Reach check for all zones including reverse zone if provided.
2) Switch on to fault, power swing blocking, Stub Protection, Fault locater initiation, Fuse
failure, LBB relay operation and Auto reclose functioning.
3) Trip and annunciation check.