Distance relays are used to protect transmission and sub-transmission power lines. They work by measuring the impedance, or distance, between the relay location and the point of fault on the line. The main types of distance relays are impedance, reactance, and mho relays. Impedance relays specifically measure the impedance of the line, which includes resistance and reactance. For the relay to operate, the measured impedance must be less than a preset constant, indicating a fault within the protected zone. The relay uses current and voltage measurements to calculate impedance and determine if a fault has occurred.

1. Advanced Power System Protection
Relay and Its Types
By
Anas Ahmad
22FMELE072

2. Distance Relay
Relay:Electrically operated switches that open and close the circuits
by receiving electrical signal from out side source.
• Distance Protection relay is used for the protection of
Transmission and sub-transmission line, usually 33kV,66kV,132kV
and 220kV lines.
• Distance relay measures the distance between the relay location
and the point of fault in terms of impedance, reactance etc. The
relay operates if the fault lies within the protected zone of line.
• Types:There are different types of Distance relays. The important
types are impedance, reactance and mho relays.

3. Impedance Relays
• Impedance relay measures the impedance of the line between the
relay location and the point of fault.
• The term impedance includes both resistance as well as reactance.

4. Operating Principle
• Impedance relay needs two actuating quantities i-e Current and
Voltage. Current is compared with the voltage at relay location.
Z=
𝑉
𝐼
• Current produces a positive Torque (Operating Torque) and Voltage
produces negative torque (Restraining Torque).
• For operation of Relay the following condition must satisfied.
• Z<K

5. Cont’d
• The Relay operates if the measured value of impedance Z is less than
the given constant.
• The impedance of a transmission line is directly proportional to its
length, it can easily be concluded that a distance relay can only
operate if fault is occurred within a predetermined distance or length
of line.

6. Operating Characteristics
The operating characteristic of the impedance
relay is shown in the figure.
• The positive torque region of the
impedance relay is above the operating
characteristic line. In positive torque region,
the impedance of the line is more than the
impedance of the faulty section.
• In negative region, the impedance of the
faulty section is more than the line
impedance.

7. Contd
• In normal operating condition, restoring torque is more than
deflecting torque. Hence relay will not operate. But in faulty
condition, the current becomes quite large whereas voltage becomes
less. Consequently, deflecting torque becomes more than restoring
torque and dynamic parts of the relay starts moving which ultimately
close the contacts of relay.
• Hence, it can be said that the relay will only operate when the
impedance of the line becomes less than predetermined impedance
(voltage/current).
• When relay opearte it gives signal Ckt Breaker which disconnect the
healty section from faulty section