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Examoogle • User • info@examoogle.com • EE-2024-8821
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MHO relay can be used for protection of
EHV
UHV
Both A and B
None of above
Both A and B
Quick Summary: The MHO relay is a directional distance relay whose characteristic is a circle passing through the origin in the R-X plane. Due to its inherent directional property and ability to tolerate power swings, it is the standard choice for the protection of long EHV (Extra High Voltage) and UHV (Ultra High Voltage) transmission lines.
The MHO relay is a directional distance relay whose characteristic is a circle passing through the origin in the R-X plane. Due to its inherent directional property and ability to tolerate power swings, it is the standard choice for the protection of long EHV (Extra High Voltage) and UHV (Ultra High Voltage) transmission lines.
Z=Zncos(θ−ϕ) — MHO relay characteristic equation where θ is the impedance angle and ϕ is the relay phase angle
T=K1VIcos(θ−ϕ)−K2I2 — Torque equation for the relay
The relay operates based on the ratio of voltage and current (Z=IV). The MHO relay characteristic is defined by the equation Z=Kcos(θ−ϕ), where ϕ is the relay's maximum torque angle. Because the circle passes through the origin, it inherently provides directionality without needing a separate directional unit, making it robust against line oscillations in high-voltage networks.
The MHO relay characteristic is inherently directional.
It is less sensitive to power swings compared to impedance relays.
The diameter of the circle is determined by the relay setting.
It is specifically designed for long transmission lines where load impedance is low.
Inherent directional characteristic
Less susceptible to power swings
Small size and weight compared to other distance relays
Reduced sensitivity for faults near the relay location
Complex internal design
Protection of long transmission lines
Back-up protection for generators
EHV and UHV power system networks
EHV is typically defined as voltages above 345 kV; UHV covers systems above 765 kV.
The MHO relay is also known as a 'voltage-restrained directional relay'.
C is correct — MHO relays are the standard choice for protecting both EHV and UHV transmission lines due to their directional properties and power swing immunity.
Always remember: Impedance relays are for medium lines, MHO relays are for long lines (due to power swings), and Reactance relays are for short lines (to avoid resistance arc interference).