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Things discussed so far in Industrial switchgear :
• Voice over ppt shared on BBB and 20 discussion meets related to Switchgear.
• Power system Protection: Introduction.
• Differential relay & Protection scheme: Basics
• Biased Differential relay & Protection scheme
EE422 Principles of Switchgear and Protection • Transformer protection relays and schemes.
• (i) Busbar differential and (ii) Induction motor protection
L39-Topic: Reactance relay and Mho relay: Trip Law & • Phase unbalance, single phasing, -ve phase sequence relay for Induction motor.
their performance • Distance relay and protection scheme: Basics
• Simple Impedance relay and its performance
Course Instructor: Gururaj S Punekar
• Today we will discuss: Reactance relay and Mho relay: Trip Law & their
Dept of E & E NITK Surathkal. performance
Type of Distance Relays Relay synthesis & Generic Torque equation
1. Impedance relay • It is possible to synthesis many type of relays based on the operating
and restraining quantities in a relay.
• All the relays can be expressed in terms of Generic Torque equation
with following torque creating quantities:
2. Reactance relay 1. Operating or Restraining torque due to CURRENT
2. Operating or Restraining torque due to VOLTAGE
3. Operating or Restraining torque due to DIRECTIONAL ELEMENT
3. Mho relay 4. Restraining torque due to SPRING.
• Relay would get the sensed VOLTAGE, CURRENT and DIRECTION based
voltage and current signal at the relay location by CT and PT.
• For this a generic Torque equation can be used.
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Generic (Universal) Torque equation Reactance relay realization using Generic (Universal) Torque
equation
T is the Torque on the actuating structure to change the status of
the relay contact (TRIP) from • In the above Universal Torque equation if we choose to have:
either NO contact to CLOSE k1 ……+ve k2=0 and k3…..-ve k4,,,,,neglected
or NC to OPEN
IR is the current sensed at the relay location (fault current)
VR is the voltage sensed at the relay location (fault voltage)
Θ is the fault power factor angle; angle between IR and VR. If T is positive then relay operates. Hence TRIP law is:
is the Maximum Torque Angle (MTA) of the relay.
k1, k2 and k3 are constants……They can be either +ve OR –ve OR
ZERO.
k4 is the Spring Restraining torque.
Trip Law of Reactance relay Characteristics of Reactance Relay
• This trip law can be simplified as:
If MTA=90° and k1/k2 = Xset=Xn
Relay is sensitive only to the Reactance
part of the Impedance up to fault.
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Reactance relay: Behavior under Normal load
Comparing performance of Distance relays (not stable)
1. Behavior under Normal load
2. Behavior with Arcing fault
3. Directional properties of the distance relay.
4. Behavior under Power swings
Reactance relay: Directional properties of the distance relay.
Effect of Arcing fault on the Reactance Distance relay
performance
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Reactance relay: Behavior under Power swings
Mho relay realization using Generic (Universal)
Torque equation
• In the above Universal Torque equation if we choose to have:
K1=0 k2=-ve and k3=+ve k4,,,,,neglected
Then TRIP law is given by:
Characteristic of Mho Relay Mho relay: Behavior under Normal load
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Effect of Arcing fault on the Mho Distance relay performance
Mho distance relay: Directional properties
Mho relay: Behavior under Power swings Comparison of distance relays
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Things to learn and information to gather.
• Distance relay application to
• 3-zone protection &
• 3-phase system protection.
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