Mho relay, an admittance or angle admittance relay, is an important protective device in power systems. It is a type of distance relay that operates based on the measurement of admittance or angle of impedance. The mho relay provides reliable and selective fault detection and protection by comparing the measured impedance or admittance with a preset characteristic impedance or admittance boundary. The mho relay determines the distance to the fault location within the power system by analyzing the phase angle and magnitude of the measured quantities. This relay is widely used in transmission lines and other high-voltage applications to ensure the stability and reliability of the electrical network. This article will delve into the mho relay’s principles, operation, and applications, highlighting its significance in power system protection.
Mho Relay Construction & Operation
A simple form of Mhorelay| Admittance or angle admittance relay is shown in the figure below:
The torque equation is given by T = K1 VI (Φ – α) – K2V2 – K3
The upper and lower poles are energized by a voltage V to produce a polarizing flux. The capacitor connected in series provides memory action. A current energizes the left as the operating quantity. Due to current I, the left pole interacts with the polarized flux due to V producing the active torque K1VI Cos (Φ – α).
The angle α can be varied by adjusting the resistance in the phase-shifting circuit provided on the left pole. The voltage energizes the right-hand side pole, and its flux interacts with the polarizing change for producing the restraining torque, K2V2.
A Mho relay measures a component of admittance |Y| ∠θ. But its characteristic, plotted on the impedance diagram (i.e., R-X diagram), is a circle passing thro’ the origin shown in Fig. It is inherently adirectional relayas it detects the fault only in the forward direction. The relay is called the Mho relay because its characteristic is a straight line when plotted on anadmittance diagram(G-B axes, i.e., conductance – susceptance axes) as in the figure. Read: Directional Over Current RelayRead: Solid State Relay or Static Relay
Mho Relay Characteristic Expression
The operating torque for a Mho relay is by V-I element, and the restraining torque is by voltage element.
Therefore, a Mho relay can be called a voltage-restrained directional relay.
T = K1 VI Cos (Φ – α) –K2V2, neglecting the effect of the spring.
K2V2 < K1VI Cos (Φ – α)
K2V < K1I Cos (Φ – α)
(V/I Cos (Φ – α)) < K1/K2 or (V/I) < (K1/K2) Cos (Φ – α) or Z < (K1/K2)Cos (Φ – α)
At balance conditions, the operating torque is equal to the restraining torque.
i.e., K1VICos (Φ – α) = K2V2
(I/V)Cos (Φ – α) = (K2K1) = K
(1/Z) = (K / Cos (Φ – α)) = Y
Y = K / Cos (Φ – α) = admittance in mho.
There, units of mho relays are used to protect a section of the line. The I unit is high-speed to save 80% to 90% of the line section. The II unit covers the rest of the line section, and its reach extends up to 50% of the adjacent line section. The III unit is meant for backup protection of the adjoining line section. The II and III units operate with a preset time delay, usually 0.2 sec to 0.5 sec and 0.4 sec to 1 sec, respectively. The time distance characteristic is a stepped one, as shown in the figure.
Impedance and Mho Relay in Power Swing
Comparison of Mho characteristic and Impedance Characteristic under power swing conditions AB is the line to be protected. Impedance characteristic relay will trip even for the fault points behind location A, which is nothing but a ‘nuisance tripping.’
Whereas the Mho characteristics relay requiring a comparatively small area of a circle for the line AB does not sense the faults behind A. Hence many points covered by the impedance characteristic are in the negative torque region of the Mho characteristic.
Power Swing Locus and Offset Mho Relay
Locus of the power swing, occurring on the long transmission lines during wrong synchronization, etc., being a temporary phenomenon, is a curve that enters into the operating zone of an impedance relay earlier than that of the Mho relay, a turn in the diagram. This is not a desirable one because before allowing the power swing to die down, the transmission line is tripped under the grab of protection by the impedance relay. However, in the event of a severe and fast occurrence of a power swing, the locus may enter the zone of operation of the Mho relay, which will act and cut off the line, which is also undesirable. Hence, an offset Mho relay is used to avoid this situation, as indicated below.
During the power swing, the locus of the impedance measured by the relay moves along the curve. When it comes within the positive torque region of the offset Mho characteristic (point P), the offset Mho relay acts and blocks the measuring relay for line BC. Therefore the Mho relay does not operate during power swing.
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