Zone 3 of distance relay that can be made immune to voltage collapse

1. Adjusting Third Zone Distance Protection
to Avoid Voltage Collapse
V. C. Nikolaidis, N. Savvopoulos, A. S. Safigianni
Democritus University of Thrace (DUTH)
Xanthi, Greece
{vnikolai,nikosavv,asafig}@ee.duth.gr
C. D. Vournas
National Technical University of Athens (NTUA)
Athens, Greece
vournas@power.ece.ntua.gr
Abstract—Undesirable distance relay operations are possible
to be initiated under heavy stressed system conditions leading to
voltage instability. This can result in cascaded outages in the
transmission system that may cause severe system deterioration.
A number of recent blackouts have been reported to be initiated
from undesired distance protection operations. This paper pro-
poses an adaptive distance relaying scheme that avoids undesira-
ble third zone tripping during the evolution of voltage instability
phenomena. The relaying scheme is based on local measurements
and provides the necessary time for other emergency controls
like undervoltage load-shedding to be applied, if needed, in order
to mitigate system collapse.
Keywords—Adaptive protection; distance relaying; load-
shedding; voltage stability; zone 3
I. INTRODUCTION
The deregulated power market operation and environmental
restrictions impose a serious challenge to the power system
protection engineers especially at ensuring correct and well
coordinated relay settings that can provide adequate system
reliability during heavy stressed system conditions that could
lead to voltage instability. Voltage instability stems from the
load restoration dynamics that attempt to restore power con-
sumption to a load demand which is beyond the capability of
the combined transmission and generation system [1]. This
may occur especially in the case of a contingency, when the
capability of the system is weakened, whereas the load may
still attempt to recover to the pre-contingency level.
Low system voltages and high reactive power flows are
typical features of voltage instability events. It follows that
these events may be identified from the distance relays as a
remote short-circuit causing an incorrect trip. Undesirable dis-
tance relay operations due to voltage instability will mainly be
initiated by the zone with the longest reach, which is usually
the third zone. Zone 3 distance relays are mainly used to pro-
vide remote backup protection for adjacent sections of trans-
mission circuits. However long lines, intermediate infeed and
load encroachment may cause difficulties to obtain that result
in a secure manner [2], [3].
Uncontrolled disconnection of power lines should be
avoided during voltage instability as this will possibly cause
cascading outages when all of the remaining generation capaci-
ty is further used. This behavior is undesirable since it will ag-
gravate the status of the power system in an already severe
situation. Most cascaded blackouts [4], [5], [6] are caused by
unexpected backup relay operations due to low voltage or over-
load state caused by post fault load restoration dynamics.
An adaptive method based on the rate of change of voltage
as an additional criterion to increase relays’ security with re-
spect to voltage instability is used in [6]. In order to prevent
cascaded events and to establish a proper control strategy, a
quasi-steady-state simulation framework is developed in [7] for
identifying possible zone 3 relay operations after an initiating
contingency. In [8] the calculation of the relay margin, mean-
ing the distance of the system trajectory to the zone 3 of a dis-
tance relay, is used to identify critical relays and contingencies
in the transmission system. The sensitivity of this relay margin
to load power is calculated in [9], [10] for determining an op-
timum emergency control strategy based on a multi-agent ap-
proach. A generalized numerical integration approach com-
bined with trajectory sensitivity analysis is presented in [11] for
assessing the possible operation of distance relays after system
disturbances. In [12] a zone 3 blocking method to avoid cas-
caded outages in the transmission system is proposed, based on
calculated overloads from a static load flow program.
In this paper, two different case studies have been simulat-
ed on a small power system to investigate the zone 3 distance
relay performance. An adaptive third zone relaying scheme
based on [13] is implemented in order to avoid undesired cas-
caded events and to secure time to take emergency control ac-
tions (e.g. undervoltage load-shedding), if needed, in order to
mitigate voltage collapse. The organization of this paper is as
follows. Section II includes a brief description of the distance
relay fundamentals. The zone 3 adjusting measures are dis-
cussed in Section III. The studied cases are presented and the
simulation results are reported in Section IV, to establish the
feasibility of the proposed method.
II. DISTANCE RELAY FUNDAMENTALS
A. Distance Relay Operation Principle
Fig. 1 shows a single transmission line connecting buses i
and j. The line is protected by two distance relays (DR) located
at its ends. Each relay receives voltage and current inputs from
voltage and current transformers respectively that are installed
at the appropriate line end. The apparent impedance seen by
the distance relay DRij is given as following:
Paper submitted to Power Systems Computation Conference, August 18-22,
2014, Wroclaw, Poland, organized by Power Systems Computation Confer-
ence and Wroclaw University of Technology.

2. DR ( , , , ) (1)ij
i i
i j i j
i i j j
ij ij
v
Z v v
v v
R jX

 
 


  

where vi, vj are the voltage magnitudes and θi, θj are the angles
at bus i and j respectively, and Rij, Xij are the transmission line
resistance and reactance respectively.
In general, a distance protection relay DRij operates if a
function of the apparent impedance ZDRij seen by the relay, falls
below a preset threshold ,
k
s ijZ for a predetermined time delay
td,k.
DR , ,( ) (2)ij
k
s ij d kf Z Z AND t t 
In a common time-stepped distance zone scheme there is
more than one threshold settings ,
k
s ijZ assigned to specific ele-
ments of the distance relay, having appropriate time delays td,k
[2]. Typical settings for a three-zone scheme (k = 1,2,3) are
such that the first zone covers the 80%-90% of the transmission
line length with no inherent time delay (td,1 ≈ 0), the second
zone covers the whole line length plus the 50% of the shortest
adjacent line length with a fast time response (td,2 = 0.2÷0.3 s),
and the third zone covers 20% more than the sum of the whole
line length and the length of the longest adjacent line with a
relatively large time delay (td,3 = 0.5÷3 s).
B. Distance Relay Characteristics
It is convenient to plot the impedance trajectory, seen by
the distance relay, together with the relay zone characteristics
on the complex R–X plane. The origin of the plane refers, in
that case, to the substation (correspondingly to the line end)
from which the relay takes the voltage and current measure-
ments. Note that in this paper we are using the plane of primary
ohms for simplicity.
Each distance relay zone setting corresponds to a specific
characteristic on the complex R–X plane. Electromechanical
distance relays support usually a single characteristic type (im-
pedance or mho type), while modern numerical relays provide
a variety of additional characteristics (quadrilateral, lens, etc)
available for every element.
The characteristic of a mho element on the R-X diagram is
a circle whose circumference passes through the origin, as il-
lustrated in Fig. 2. The offset coefficient of mho relays is con-
sidered zero in this paper, so the third zone characteristic pass-
es also through the origin. Angle φ is the characteristic angle of
the transmission line (usually between 70o
-88o
). The Maximum
Torque Angle (MTA) of the relay is considered to be the same
with angle φ in this example. Obviously, the mho element is
inherently directional and such that it will operate only for
faults in the forward direction along the line. Fig. 2 depicts also
the characteristics of a three-zone polygonal relay.
Condition (2) can be further analyzed if the specific zone
characteristic of the distance relay is known. For example,
when mho relays are assumed to be installed at both of the line
ends in Fig. 1, the relay DRij will operate if the following con-
dition lasts for a time period greater than td,k
Fig. 1. Transmission line equipped with distance relays.
Fig. 2. Three zones mho and polygonal characteristics.
DR , (3)ij k k d kZ AND t t   
where ρk is the center and dk is the diameter of the circle of the
k-th zone in the complex plane or:
( ) (4)
2
k
k ij ij
d
R jX  
Hence, for a usual three-zone scheme as in Fig. 2, each of
the three circles corresponds to a different value of ρk and sub-
sequently to a different value of dk (k = 1,2,3).
C. Load Encroachment Function
Modern distance relays incorporate the so called “load en-
croachment” function, which gives the user the ability to define
custom load regions both in the forward and reverse directions
[14]. Fig. 2 depicts the typical shape of the load encroachment
area on the R-X plane. The radius of the two arcs that are part
of the load encroachment characteristic is usually determined
from the maximum load transfer capacity of the transmission
line multiplied by a safety factor. The slope angle of the
straight lines (usually ±30o
÷40o
) is defined from the maximum
and minimum expected load power factor.
As long as the apparent impedance seen by the distance re-
lay lies inside the load encroachment area, the trip command of
the three-phase element is blocked. Only the operation of the
three-phase element is supervised, since load on the transmis-
sion system is primarily a balanced three-phase condition and
voltage instability is a phase symmetrical phenomenon.
When the third zone is large enough as in Fig. 2, undesired
blocking of the tripping command can happen for three-phase
faults that correspond to a point which lies simultaneously
within the third zone characteristic and the load encroachment
area. Otherwise, if load encroachment function is disabled,
large load transfers can cause undesired third zone tripping.

3. III. ADAPTIVE THIRD ZONE RELAYING
According to our experience from many countries at South
Europe (including Greece), the North Africa and the Middle
East, there exists a large number of mho distance relays at their
transmission lines. The study report [15] includes the results
from a survey conducted between more than 110 utilities with-
in the United States, the Canada, and other national power sys-
tems throughout the world regarding the protective relays mar-
ketplace in the electric utility industry. Based on this survey,
only the 60% of the transmission line relays worldwide are
digital relays with the remaining percentage corresponding to
obsolete static and electromechanical relays. Recently, major
blackout events [5], [6] were accelerated from the third zone
tripping of mho relays. In this paper only mho relays have been
considered. As an alternative, polygonal relays could be as-
sumed since the quadrilateral characteristics together with the
mho ones are the most encountered on existing relays.
This section addresses several options for adjusting the
third zone tripping operation of mho relays [13] in order to
improve system protection against voltage collapse. Any of the
adaptive measures proposed here is assumed to be activated
when the impedance trajectory ZDRij, seen by the distance relay
DRij, enters and stays for an appropriate time delay into the
Third Zone Proximity Area (TZPA). When the impedance tra-
jectory exits the TZPA, the conventional mho operation is in
use even if any measure was applied previously.
The TZPA is an area on the R-X plane, which when en-
croached from the measured impedance trajectory indicates
that the third zone element is close enough to trip. There are
different ways available to implement the TZPA principle. In
this paper we realize the TZPA as a fourth zone, concentric
with the third zone of protection, having a larger reach in the
forward direction than that of the third zone. Equally, this cor-
responds to a fourth zone with a radius ε that is p% larger than
the radius of the third zone:
3
(1 ) (5)
2
d
p  
It is evident that the TZPA covers a section in all four quad-
rants on the R-X plane in the latter case.
Moreover, the TZPA is used to start a timer. If the imped-
ance locus crosses zone 3 before the time delay Te runs out, a
fault is declared. Hence, in case of a fault, none adjusting
measure will be applied because the apparent impedance will
rapidly enter into one of the relay zone characteristics. If the
apparent impedance trajectory crosses zone 3 after the time
delay Te expires, the adusting measure will be applied. The
timer is reset any time the impedance trajectory exits TZPA.
The adaptive protection measures described hereafter are
based on local measurements and there is no need for commu-
nication means. After the application of any of these measures,
the distance relay can still trip if the apparent impedance enters
into the adjusted third zone characteristic or into the first or
second zone characteristic. Note that the third zone tripping
command is always blocked when the apparent impedance lies
inside the load encroachment area.
Inter-tripping is considered at the time when the
non/adjusted third zone element releases a trip command. That
means that if the circuit breaker (CB) at one line end is tripped
by the non/adjusted third zone distance relay element, a pilot
signal will be sent immediately to the relay at the other line end
in order to trip the corresponding CB.
1) Blinders
The blinders are straight lines obtained by using a rotated
reactance characteristic on the R-X plane. Two blinders limit
the operation of the distance relay to a banded area to either
side of the impedance of the protected transmission line. Fig. 2
depicts two blinders in parallel to the transmission line charac-
teristic. Of course, the characteristic angle of the blinders and
that of the line can differ.
The first of the proposed adaptive measures enables a pair
of blinders on the R-X plane, in parallel to the transmission line
characteristic. The blinders’ resistive reach, defined from the
intersection point with the R-axis, is determined considering
maximum load transfer conditions at the protected transmission
line. After blinders have been set, the actual operating area of
the third zone mho element is reduced to that defined from the
section of the third zone mho characteristic and the blinders. A
tripping command will be released from the relay only if the
apparent impedance trajectory enters into this restricted area.
Obviously, if polygonal relays were used instead of mho
ones, the same adjustment could easily be obtained by reducing
the resistive and inductive reach setting of every zone charac-
teristic so as to coincide with the desired blinders position.
2) Characteristic Shape Modification
This measure modifies the standard third zone mho charac-
teristic to a lenticular one in order to reduce the relay zone's
coverage in possible load regions. The lenticular relay charac-
teristic derives from the section of two identical mho relay
characteristics on the R-X diagram. The lenticular characteris-
tic used in this measure (Fig. 3) is a special case where direc-
tional tripping is further desired meaning that this relay will
operate only for faults in the forward direction along the line.
3) Characteristic Shape Rotation
This measure simply rotates the conventional third zone
mho characteristic at an appropriate angle in the opposite direc-
tion of that approached from the measured impedance trajecto-
ry. Practically, the MTA of the third zone element is adjusted
in order to obtain the desired rotation.
For polygonal relays this option cannot be applied as
straightforward as for mho relays since it is dependent on the
particular characteristic that is selected. In general, the resis-
tive/inductive reach of every zone could be reduced and a mod-
ification of the zones inclination angle may also be needed. The
exact implementation is a topic that needs further investigation.
4) Third Zone Reach Adjustment
This measure reduces the reach of the conventional third
zone mho characteristic without alternating the MTA of the
relay. As a special case, the third zone can be practically disa-
bled by reducing its reach so that it coincides with the reach of
the second zone. Then, only the first and the second zone are
active in that case after TZPA is reached.

4. Fig. 3. Lenticular relay characteristic.
IV. SIMULATION RESULTS
A. Test System Description
The eight-bus system shown in Fig. 4 is used for testing the
proposed measures. This system is a slight variation of that
used in [16]. It consists of three restorative loads supplied
through the transmission system from a synchronous generator
and a strong external power system. Resistances are omitted.
Load-Tap-Changer (LTC) mechanisms on the bulk distri-
bution transformers control the secondary voltage by changing
the transformer tap ratio ri (i = 1,2,3). The first operation starts
after a fixed time delay (30 s), while the time delay for the sub-
sequent tap changes has been selected shorter (10 s). Under
stable system conditions, by controlling the transformer sec-
ondary voltage, the power consumption on the distribution side
is restored.
In addition, load restoration mechanisms acting behind the
LTCs are explicitly modeled. Such mechanisms include ther-
mostatically controlled loads, distribution system voltage regu-
lators, etc. To account for this behavior a slow load restoration
is implied for all the loads, modeled as a variable conductance
following the differential equation:
2
, , (6)L m o m m L iT G P G V 
Gm is the conductance and Po,m is the power demand of the
m-th load. VL,i is the voltage magnitude of the m-th load bus i.e.
bus 4, 6, 8 for loads 1, 2 and 3 respectively with regard to Fig.
4. TL is the time constant of the self-restorative load mecha-
nism, which is considered the same for all the loads. Thus the
load is initially constant admittance, but it is trying to restore to
constant power in the long run.
The generator is modeled with an Automatic Voltage Regu-
lator (AVR) and an Overexcitation Limiter (OXL). The OXL is
activated 40 s after the rotor current exceeds its limit and is
enforcing this limit. Thus, after OXL activation the generator
operates on constant excitation. The external power system is
represented by an ideal voltage source Einf connected through a
Thevenin impedance Zth = Rth + jXth at bus 7.
Fig. 4 shows also the distance relays at the transmission
lines. For simplicity, any additional protection relay (e.g. unit
differential relay, backup overcurrent relay etc) that exists in
the power system is not shown in Fig. 4, as this paper focuses
on the transmission line distance protection. The following
settings have been considered for the distance relays, where lij
(resp. jk) is the length of the line connecting buses ij (resp. jk):
 First Zone: 1
, ,10.8 , 0s ij ij dZ l t  
 Second Zone: 2
, ( ) ,20.5 , 0.3s ij ij jk shortest dZ l l t s   
 Third Zone: 3
, ( ) ,31.2 ( ), 1.0s ij ij jk longest dZ l l t s   
 TZPA: p = 30%, Te = 10 s
B. Case Study
In the initial system state, the total power demand Po,tot is
equal to 8.05 pu. The disturbance assumed is a 30% step in-
crease in the total power demand at time t = 1000 s, as shown
with dashed line in Fig. 5, uniformly distributed over all loads.
After that disturbance, the loads try to meet the new power
demand through their restorative mechanisms and the power
consumption increases constantly (solid line). Even when the
generator becomes overexcited, the system remains stable in
the short-term and the total load consumption continues to re-
store, as can be seen in the same figure.
The voltage magnitudes at the transmission level decrease
progressively due to the load restoration mechanisms. Fig. 6
depicts the time evolution of the transmission system voltages.
The sudden decrease in the voltages at t = 4090 s is due to the
overexcitation limit that is enforced on the generator. Note that
the voltage magnitudes remain in an acceptable level during
the system evolution, even after the OXL operates, before line
37 trips at time t = 7570 s. At that time the lowest transmission
voltage V3 is equal to 0.88 pu.
The disconnection of line 37 is initiated from the zone 3 el-
ement of the relay DR73, which trips the CB close to bus 7 after
the apparent impedance trajectory crosses its third zone charac-
teristic (Fig. 7) and remains within it for a time greater than the
corresponding time delay td,3. Simultaneously, a tripping signal
is sent from the relay DR73 to the CB on the opposite line end,
switching off completely the line. When this happens, the im-
pedance trajectory seen by the relay DR23 falls immediately
inside its third zone characteristic (Fig. 8), leading to a cascad-
ed tripping of the line 23 after the expiration of the correspond-
ing third zone time delay td,3.. The same is true for the relay
DR53, whose third zone operates after the disconnection of line
23 causing line 35 to be switched off. Note that these cascaded
outages happen almost simultaneously with the initial discon-
nection of the line 37.
Fig. 4. Eight-bus system.

5. Fig. 5. Total load consumption vs. power demand (30% step increase).
Fig. 6. Voltage evolution assuming conventional third zone.
After all the abovementioned transmission line disconnec-
tions, bus 3 is disconnected from the remaining network.
Hence, the system area fed through bus 3 experiences a black-
out and this happens due to the undesired operation of the third
zone element of the distance relays in the transmission system.
The sudden disconnection of load PL,1 causes a sudden de-
crease in the total load consumption (shown with dotted line in
Fig. 5) and considerable overvoltages in the transmission sys-
tem (shown in Fig. 6). Note that if the third zone operation of
the distance relays was avoided, the system would tolerate the
demand increase as shown with the solid line in Fig. 5.
C. Adjusting Relays Third Zone Operation
This section presents how the proposed third zone adjusting
measures can prevent the cascaded line disconnections that
cause the blackout in the system area fed from bus 3.
Remember that the proposed protective measures can be
applied only if the following conditions are met simultaneously
on a distance relay:
 A phase symmetrical phenomenon is observed.
 The apparent impedance trajectory lies outside the load
encroachment area.
 The apparent impedance trajectory passes slowly through
TZPA (dashed line in Fig. 9), so that time delay Te expires.
The first measure examined applies blinders on the R-X
plane in order to reduce the third zone mho characteristic area.
Since resistances are omitted in the test system, the
transmission line impedances are pure reactances
corresponding to a line characteristic angle of 90o
. Thus,
vertical blinders are set on the R-X plane, having a resistive
reach equal to ±Rb.
Fig. 7. Third zone tripping of line 73.
Fig. 8. Cascaded tripping of line 23.
The magnitude of the resistive reach Rb is determined from
the maximum permissible power that can be delivered by the
line under the worst inductive/capacitive power factor
according to:
min
max
(7)
3
b
V
R
I


where Vmin is the minimum expected normal operating voltage
and Imax the maximum expected load current. The sign of Rb
indicates the power flow direction with respect to the reference
bus; the positive sign corresponds to active power flow out of
the bus while the negative sign to active power flow into the
bus.
The blinders for the power system under investigation are
shown with dotted vertical lines on the R-X plane of Fig. 9. In
the same figure the conventional mho characteristics are also
illustrated with solid lines.
The second adaptive measure rotates the conventional third
zone mho characteristic to the direction that moves it away
from the apparent impedance trajectory. The angle α by which
the MTA is rotated is set to a value that brings the adjusted
characteristic (depicted with dotted lines in Fig. 9) in a tangent
position relative to the blinder which, if existed, would be the
first reached from the apparent impedance trajectory. In this
example, angle α equals to 10o
.
The next protection measure transforms the zone 3 mho
characteristic to a lenticular one, shown with solid line in Fig.
9. The reactance reach of the lens is selected to be equal to that
of the original third zone. This is equivalent with only reducing
the resistive reach of the original third zone characteristic by an
appropriate percentage.

6. Fig. 9. Third zone adjusting.
Finally, the last examined measure reduces the
conventional third zone reach in a way that it coincides with
the second zone reach without adjusting the MTA.
By simulating the case under investigation and by
individually applying all of the abovementioned measures, any
line tripping was avoided in all of the simulated scenarios and
the system was finally stabilized serving the increased total
load demand. Fig. 10 illustrates the transmission system
voltages, that show the same time response in any of the
simulated scenarios since none relay tripped its CB.
D. Emergency Control
Obviously, if a larger increase in the total load demand was
supposed, third zone tripping would probably not be avoided
even after any adaptive measure was applied, as far as load
self-restoration dynamics were considered. However, this
would happen with a substantial time delay, which provides
necessary time for other emergency measures to be applied.
Let us assume a 50% step increase in the total power de-
mand, as shown with dashed line in Fig. 11, uniformly distrib-
uted over all loads. The increased total power demand cannot
be restored in the long-term. If no distance relay existed, the
entire system would collapse after Δto = 10910 s. At this time
instant (marked with an asterisk in Fig. 11) the system equa-
tions cannot be solved. However, since distance relays exist in
a real power system the sudden decrease shown with solid line
in Fig. 11 would be observed after Δt1 = 1950 s from the dis-
turbance. This sudden decrease is caused from the cascaded
outages of lines 37, 23, and 35 that disconnect the bus 3 from
the remaining system. The system does not entirely collapse,
only because the total area load PL1 is unintentionally lost.
Fig. 11 depicts also the total power consumption when
blinders (dashed line), rotated zone 3 mho shapes (dotted line),
or lenses (dashed-dotted line) are applied. Reducing the reach
of zone 3 is not prefered because it causes a lack of backup
protection for faults in the nearby lines. It is evident that valua-
ble time is gained if any of the adaptive measures is applied.
Indeed, the loss of PL1 is delayed by Δt2 = 2680 s, Δt3 = 3230 s,
or Δt4 = 10310 s if blinders, zone 3 mho rotation or lenticular
zone 3 characteristic are considered respectively. The time de-
lay is sufficient enough to perform an emergency control action
for the case of lenses, but it is relatively small when blinders or
zone 3 mho shape rotation measure is applied. On the other
hand, a lens is prone to fail if high resistance faults occur.
Fig. 10. Voltage evolution after third zone adjustment (30% step increase).
Fig. 11. Total load consumption vs. power demand (50% step increase).
Fig. 12. Power and voltage at bus 6 for lenticular zone 3 (50% step increase).
Actually, the maximum available time to take an emergen-
cy control action is restricted from the instability detection on
LTC feeding load PL2. The instability is locally detected as a
continuous decrease both in the secondary voltage V6 and the
transformer tap ratio r2 [17], starting at time t = 4190 s. The
subsequent transformer tap changes cause a drastic decrease in
voltage V6 and in the power consumption of load PL2 (Fig. 12).
Note that the instability is not reflected in the total power
consumption Ptot. When the lower tap limit r2,min is reached, the
load consumption PL2 slightly increases due to the combined
action of the restoration mechanisms at the remaining system.
However, due to the cascaded line disconnections, load PL2 is
totally lost, deteriorating further the system. A considerable
decrease in the transmission voltage magnitudes is also ob-
served after instability appears, as shown in Fig. 13.

7. Fig. 13. Transmission voltages for lenticular zone 3 (50% step increase).
Fig. 14. Transmission voltages after load shedding
Hence, the total time given to the power system operators
in order to take an emergency measure is about one hour (3190
s). Assume now that a closed-loop undervoltage load-shedding
scheme [18], [19] was available with the following simple op-
eration principle:
 If a transformer primary (transmission) voltage drops
below 0.85 pu, shed 5% of the secondary (distribution) bus
load without any time delay.
 No more than four successive load-shedding steps are
allowed.
In addition, assume that this load-shedding scheme is com-
bined with one of the adaptive protection measures described
in this paper. We choose to adjust the conventional mho char-
acteristics to lenticular ones, since this measure provides the
larger time delay. By applying the combined protection
scheme, the cascaded line outages are avoided and the system
voltages are stabilized as shown in Fig. 14. Besides, the total
system load is restored and none area is blacked out. The total
amount of shed load is 1.125 pu or 9.32% of the total system
load demand.
V. CONCLUSIONS
Undesirable third zone distance relay operations can result
in cascaded outages in the transmission system that may cause
severe system deterioration. This paper proposes an adaptive
relaying scheme that can avoid undesirable outages by
adjusting the zone 3 characteristic in various ways. Zone 3
tripping cannot be avoided entirely even after its adjustment if
the apparent impedance trajectory continues to move towards
its characteristic, meaning that the final outcome depends on
the disturbance itself and the system dynamics. However, this
paper shows that valuable time is saved if the proposed adap-
tive scheme is applied and that voltage collapse can be avoided
if undervoltage load-shedding is further applied.
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