The relay used for protecting EHV transformers from magnetising inrush current is:

Explanation:

Relay Used for Protecting EHV Transformers from Magnetising Inrush Current

Definition: A relay is a protective device that detects abnormal conditions in an electrical circuit and initiates appropriate actions to safeguard the system. For Extra High Voltage (EHV) transformers, magnetising inrush current is a transient phenomenon that occurs during transformer energization. This inrush current, caused by the core's magnetic saturation, can be misinterpreted as a fault by conventional protection systems. To address this issue, a specific relay is utilized to differentiate between inrush currents and actual fault currents.

Correct Option: Second Harmonic Restraint Relay

The correct relay for protecting EHV transformers from magnetising inrush current is the second harmonic restraint relay. This relay is specifically designed to distinguish between inrush currents and fault currents based on the harmonic content of the current waveform.

Working Principle:

The second harmonic restraint relay operates on the principle that magnetising inrush currents contain a significant second harmonic component, unlike fault currents. When a transformer is energized, the core's magnetic flux may saturate due to the sudden application of voltage, leading to a high inrush current. This inrush current is rich in harmonics, particularly the second harmonic. The relay measures the harmonic content of the current and restrains its operation if the second harmonic component exceeds a predefined threshold, indicating that the current is an inrush current and not a fault current.

Key Features:

• Harmonic Analysis: The relay continuously monitors the current waveform and performs a harmonic analysis to detect the second harmonic component.
• Restraint Function: If the second harmonic content is above the threshold, the relay restrains its tripping operation, preventing unnecessary interruptions.
• Fault Detection: In the absence of significant second harmonic content, the relay interprets the current as a fault current and initiates tripping to protect the transformer.

Advantages:

• Prevents unnecessary tripping of the transformer during energization, ensuring system stability.
• Provides accurate differentiation between magnetising inrush currents and fault currents.
• Improves the reliability and selectivity of transformer protection systems.

Applications:

• Protection of EHV and high-capacity transformers in power systems.
• Used in scenarios where frequent energization of transformers is required, such as in industrial and grid substations.

Important Information

To further understand the analysis, let’s evaluate the other options:

Option 2: Thermal Relay

A thermal relay operates based on the principle of heat generation due to current flow. It is commonly used for overload protection in motors and other equipment. However, it is not suitable for distinguishing between magnetising inrush currents and fault currents in EHV transformers. Magnetising inrush current is a transient phenomenon, while a thermal relay responds to sustained overcurrents, making it ineffective in this scenario.

Option 3: Balanced Beam Type Relay

A balanced beam type relay operates on the principle of balancing forces generated by current or voltage. It is generally used for differential protection but lacks the capability to analyze harmonic content. Therefore, it cannot differentiate between inrush currents and fault currents, making it unsuitable for protecting EHV transformers from magnetising inrush currents.

Option 4: Attracted Armature Type Relay

An attracted armature type relay uses an electromagnet to attract an armature when a current flows through the coil. This type of relay is simple and primarily used for basic protection applications. It does not have the sophistication to perform harmonic analysis and distinguish between inrush currents and fault currents, rendering it ineffective for this specific application.

Conclusion:

The second harmonic restraint relay is the most appropriate choice for protecting EHV transformers from magnetising inrush currents. Its ability to analyze harmonic content and selectively restrain tripping during inrush conditions ensures reliable and efficient transformer protection. The other relay types, while suitable for specific applications, lack the necessary functionality to address the unique challenges posed by magnetising inrush currents in EHV transformers.