Distance Relay
Distance and Impedance relay
Brief History
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1921 Voltage restrained time over current
1929 balance beam impedance
1950 Induction cup phase comparator
1965 Solid state implementations
1984 Microprocessor based implementation
Distance Relay
Need
• Faults level are higher in high voltage transmission line
• Faults need to be cleared rapidly to avoid instability and
extensive damage
Advantages
• The impedance zone has a fix impedance rich
• Great instantaneous trip coverage with security
• Easier setting calculations and coordination
• Fixed zone of protection that are relatively impedance of the
system change
• Higher independence of load
Basic Principle
• A distance relay has the ability to detect a fault within a pre-set distance
along a transmission line or power cable from its location. Every power
line has a resistance and reactance per kilometer related to its design and
construction so its total impedance will be a function of its length. A
distance relay therefore looks at current and voltage and compares these
two quantities on the basis of Ohm’s law.
Basic Principle Ctd..
Since the impedance of a transmission line is proportional to its
length, for distance measurement it is appropriate to use a
relay capable of measuring the impedance of a line up to a
predetermined point (the reach point). Distance relay is
designed to operate only for faults occurring between the
relay location and the predetermined (reach) point, thus
giving discrimination for faults that may occur in different
line sections. The basic principle of distance protection
involves the division of voltage at the relaying point by the
measured current. The calculated apparent impedance is
compared with the reach point impedance. If the measured
impedance is less than the reach point impedance, it is
assumed that a fault exists on the line between the relay and
the reach point.
Advantage of Distance Relay
• the key advantage of distance protection is that its fault
coverage of the protected circuit is virtually independent of
source impedance variations.
Transmission Line Impedance
• Z ohms/mile = Ra+ j (Xa+ Xd)
• Ra, Xa function of conductor type, length Xd function of
conductor spacing, length Xa+ Xd >> Ra at higher voltages
Line Impedance
Zones of Protection
• Zone 1: this is set to protect between 80% of the line length AB and
operates without any time delay. This “under-reach” setting has been
purposely chosen to avoid “over-reaching” into the next line section to
ensure selectivity since errors and transients can be present in the voltage
and current transformers. Also manufacturing tolerances limit the
measurement accuracy of the relays.
• Zone 2: this is set to protect 100% of the line length AB, plus at least
20% of the shortest adjacent line BC and operates with time delay t2.
(≈0.5s) It not only covers the remaining %20 of the line, but also provides
backup for the next line section.
• Zone 3: this is set to protect 100% of the two lines AB, BC, plus about
25% of the third line CD and operates with time delay t3. (≈1.5s)
P-Q and R-X relationships
If the direction of real and reactive
power is from bus 1 to bus 2, the R-X
coordinates are located in quadrant I. If
the direction of real power is from bus 1
to bus 2 and the reactive power is from
bus 2 to bus 1, then the R-X coordinates
are located in quadrant IV
Tripping Characteristics
The shape of the operation zones has developed throughout the years. An
overview of relay characteristics can be seen below:
Impedance Characteristic:
If the relay’s operating boundary is plotted on an R-X diagram, its
impedance characteristic is a circle with its center at the origin of the
coordinates and its radius will be the setting (the reach point) in ohms.
The relay will operate for all values less than its setting i.e. for all points
within the circle. This type of relay, however, is non-directional. It can
operate for faults behind the relaying point. It takes no account of the
phase angle between voltage and current. It is also sensitive to power
swings and load encroachment due to the large impedance circle.
Mho Characteristic
The limitation of the impedance characteristic can be improved by a
technique known as self polarization. Additional voltages are fed into the
comparator in order to compare the relative phase angles of voltage and
current, so providing a directional feature. This has the effect of moving
the circle such that the circumference of the circle passes through the
origin. Angle 𝜃 is known as the relay’s characteristic angle. It appears as a
straight line on an admittance diagram.
Mho Characteristic Ctd…
By use of a further technique of feeding in voltages from the healthy phases
into the comparator (known as cross polarization) a reverse movement or
offset of the characteristic can be obtained. This is called the offset mho
characteristic.
Mho Circle Component
The mho circle is composed of its impedance maximum reach,
maximum torque angle and relay characteristic angle
• Impedance Maximum Reach:
The mho circle maximum reach is set by the impedance reach Zr of the
protective zone. These impedance reaches vary depending on the zone of
protection such as zone 1, 2, 3 and 4. Each impedance value determines the
diameter of the mho circle.
• Maximum Torque Angle (MTA):
The angle of maximum torque of a distance relay using the mho characteristic is
the angle at which it has the maximum reach. For microprocessor relays, the
MTA is the same as the positive sequence line impedance angle.
• Relay Characteristic Angle:
The relay characteristic angle (RCA) of a mho circle is 90°. For purposes of
calculating the maximum relay loadability, the RCA is the angle whose
vertices are made between the load impedance vector and the difference
between the line impedance and load impedance vectors
Mho Circle Component
Relay Impedances - Zr
High Load
•The relay impedances are the values
that a distance relay uses for zones of
protection
•Diameter of each mho circle
is based on the value of each zone of
protection
•During steady-state or normal system
conditions, the load impedance remains
constant and is high enough to
keep it away from the relay zones of
protection
Hint: As load increases, the load
apparent impedance decreases,
moving it closer to the origin
Quadrilateral Characteristic
Modern distance relays offer quadrilateral characteristic, whose
resistive and reactive reach can be set independently. It
therefore provides better resistive coverage than any mhotype characteristic for short lines. This is especially true for
earth fault impedance measurement, where the arc resistances
and fault resistance to earth contribute to the highest values
of fault resistance. Polygonal impedance characteristics are
highly flexible in terms of fault impedance coverage for both
phase and earth faults. For this reason, most digital relays
offer this form of characteristic.
Quadrilateral Characteristic
Zone Protection Calculation
Example
𝑉1 = 138∠0° 𝐾𝑉 = 𝑉𝐿𝑁 = 79.6∠0° 𝐾𝑉
𝑉2 = 138∠30° 𝐾𝑉 = 𝑉𝐿𝑁 = 79.6∠30° 𝐾𝑉
−79.6∠0° 𝐾𝑉 + 𝐼66∠75° + 79.6∠30° 𝐾𝑉 = 0
79.6∠0° 𝐾𝑉-79.6∠30° 𝐾𝑉 10.97 − 𝑗39.8 41.28 𝐾𝑉∠ −74.75°
°
𝐼=
=
=
=
625.45∠
−150
66∠75°
66∠75°
66∠75°
Example Ctd…
S@𝐵𝑢𝑠1 = 79.6𝐾𝑉 ∗ (−537.88 − 𝑗312.5 𝐴𝑚𝑝𝑠)∗
S@𝐵𝑢𝑠1 = 79.6𝐾𝑉 ∗ (−537.88 + 𝑗312.5 𝐴𝑚𝑝𝑠)
S@𝐵𝑢𝑠1 = −42.48𝑀𝑉𝐴 + 𝑗24.87𝑀𝑉𝐴
S@𝐵𝑢𝑠1 = 49.22∠150°
S@𝐵𝑢𝑠1 = −42.48𝑀𝑤𝑎𝑡𝑠 + 𝑗24.87𝑀𝑣𝑎𝑟𝑠
Example Ctd…
𝑍𝑙𝑜𝑎𝑑
79.6∠0°
∗
79.6𝐾𝑉 2 ∠150°
° 𝑂ℎ𝑚𝑠
=
=
=
128.73∠150
∗
49.22𝑀𝑉𝐴
49.22∠150°
𝑍𝑙𝑜𝑎𝑑 = −110.70 + j64.35 Ohms
Example Ctd…
𝑍𝑙𝑜𝑎𝑑 = −110.70 + j64.35 Ohms
The load impedance has a negative R value and a positive jX value, and those
coordinates are located in quadrant II, as indicated in Figure below. This example has
demonstrated that the location of the load impedance depends on the direction of
power. This concept will be emphasized even further when we calculate the maximum
loadability of the relay for different power factor conditions.
Calculating the Maximum Loadability of a Distance
Relay with mho Characteristics
• The relay impedances zones of protection must be selected
carefully in order to avoid load encroachment problems. The
zone of protection with greater risk is zone 3, since it is the
mho circle with the greatest area and closest proximity to the
load impedance. Zone 3 settings are certainly vulnerable to
load encroachment conditions during high load and power
swings conditions, which can cause the load impedance to
travel towards the boundaries of the zone 3 mho circle and
cause an undesired trip.
Example
Maximum relay loadability that zone 3 will calculated by
following steps:
1. Draw the zone 3 impedance vector in the R-X diagram.
2. Draw the load impedance vector at a specified power factor.
For this example, .
3. Draw a right triangle forming the 90 ° relay characteristic
between the load impedance vector and the difference vector
that is made up of Z3 – Zload.
Calculating the Maximum Load ability of a Distance
Relay with mho Characteristics
Example Ctd…
4. Calculate the interior angle that is made between the load and
line impedance vectors. This is done by subtracting the line
impedance angle minus the power factor angle
5. Calculate the load impedance that the relay will experience at
the specified power factor using right triangle properties:
Example Ctd…
6. Calculate the maximum loadability of the relay in MVA by:
Relay will Trip at 269 MVA