Mho Relay or Admittance Relay

Mho Relay or Admittance Relay

A separate unit is needed to make an impedance relay directional. But in mho relay, it made inherently directional by adding a voltage winding called polarizing winding.

The mho relay works on the measurement of admittance Y ∠ θ. This relay is also known as angle impedance relay.

Construction

The mho relay uses induction cup type construction. It has operating coil, polarizing coil, and restraining coil.

The schematic diagram of mho relay is shown in the figure below.

Construction of Mho Relay
Construction of Mho Relay

In this type of relay, the operating toque is developed by voltage and current element. And the restraining torque is developed by the voltage element. Hence, the mho relay or admittance relay is a voltage restrained direction relay.

The operating torque is developed by the interaction of fluxes by the winding surrounded of the poles 1, 2, and 3. And the restraining torque is produced by the interaction of fluxes due to the winding carried by the poles 1, 3, and 4.

Therefore, the restraining torque is proportional to the square of voltage and the operating torque is proportional to the product of voltage and current.

The torque angle is adjusted with the help of series tuning circuit.

Torque Equation

The restraining torque is proportional to the square of voltage and the operating torque is proportional to the product of voltage and current.

Hence, the equation of net torque is;

    \[ T = K_1 V I cos(\phi-\tau) - K_2 V^2 - K_3 \]

Where, K3 is a constant for spring effect.

Generally, in most the practical cases, the spring effect is neglected. Therefore, K_3 = 0.

In balance condition, torque is zero.

    \[ 0 = K_1 V I cos(\phi-\tau) - K_2 V^2 \]

    \[ K_1 V I cos(\phi-\tau) = K_2 V^2 \]

    \[ K_1 cos(\phi-\tau) = K_2 \frac{V}{I} \]

    \[ Z = \frac{K_1}{K_2} cos(\phi-\tau) \]

This is the equation of a circle having diameter \frac{K_1}{K_2} passing through origin. And this constant is ohmic setting of this relay.

Operating Characteristics of Mho Relay

From the equation of torque, the characteristics of this relay is a circle passing through origin and the diameter is \frac{K_1}{K_2}.

The circle is shown in the figure below.

"Operating

The relay operates when the impedance seen by the relay falls within this circle.

Consider two lines AB and AC with mho relay located at the point A. The relay will not operate for the faults occurring in the line AB only. There relay will not operate for the line AC.

Therefore, the mho relay is inherently direction without any additional directional unit.

The angle \tau can be adjusted to be 45˚, 60˚, 75˚, and so on. This angle is maximum torque angle.

The setting of 45˚ is suitable for high voltage distribution system (33-11 kV), 60˚ is used for 66-132 kV lines, and 75˚ is used for 275-400 kV transmission lines.

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