The relay that is also called an angle impedance relay is the Mho relay. Let's understand why this relay is referred to as an angle impedance relay.

1. Introduction to the Mho Relay:
The Mho relay is a distance relay used in power systems to protect transmission lines. It operates based on the measurement of impedance, which is the ratio of voltage to current in a circuit. The Mho relay is designed to operate when the impedance seen by the relay falls within a specific range.

2. Understanding Impedance:
Impedance is a complex quantity that represents the resistance and reactance of a circuit. It is denoted by the symbol Z and has both magnitude and phase angle. The magnitude of impedance represents the resistance, while the phase angle represents the reactance.

3. Angle Impedance Relay:
The Mho relay is often referred to as an angle impedance relay because it operates based on the angle of the impedance seen by the relay. The relay measures the impedance between the relay location and the fault location and compares it with a predetermined characteristic known as the Mho characteristic.

4. Mho Characteristic:
The Mho characteristic is a circular characteristic in the impedance plane. It is a circle with its center at the relay location and a radius representing the reach of the relay. The Mho characteristic is defined in terms of angle and impedance.

5. Operation of Mho Relay:
When a fault occurs in the transmission line, the impedance seen by the relay changes. The Mho relay measures the impedance and checks if it falls within the Mho characteristic circle. If the impedance falls within the characteristic circle, the relay operates and initiates the tripping of the circuit breaker.

6. Angle Measurement:
To determine whether the impedance falls within the Mho characteristic circle, the Mho relay measures the angle of the impedance. The angle represents the phase difference between the voltage and current. By comparing this angle with the Mho characteristic angle, the relay determines if the impedance is within the permissible range.

7. Advantages of Mho Relay:
The Mho relay has several advantages, including high speed of operation, reliable performance, and accurate fault detection. It is widely used in transmission line protection due to its ability to operate based on the angle of impedance.

In conclusion, the Mho relay is also called an angle impedance relay because it operates based on the angle of the impedance seen by the relay. It measures the impedance between the relay location and the fault location and compares it with the Mho characteristic, which is defined in terms of angle and impedance. By measuring the angle of the impedance, the relay determines if it falls within the permissible range and initiates the tripping of the circuit breaker if a fault is detected.