What is KD in relay protection
In relay protection, "KD" refers to a type of compensator distance relay used for phase fault detection and tripping in power systems.
Overview of KD Relays
The KD relay is a polyphase compensator distance relay designed to provide a single zone of phase protection for all three phases. It is capable of detecting phase-to-phase faults, two-phase-to-ground faults, and three-phase faults, and provides essentially instantaneous tripping within its reach and sensitivity settings (ABB KD-4, KD-10) . KD relays are widely used in directional and impedance relaying schemes.
Function of the Compensator
The "KD" designation is associated with the compensator mechanism within the relay. The compensator is typically a current-to-voltage air gap transformer (designated as T, TAB, or TBC) that produces a voltage proportional to the phase current. One section of the compensator winding is connected subtractively in series with the relay terminal voltage, effectively vectorially subtracting a voltage proportional to the current from the terminal voltage. This allows the relay to measure impedance accurately and respond to faults within a specific zone .
Role in Relay Operation
The KD relay uses the compensator output to:
- Adjust the phase angle relationship between the primary current and induced secondary voltage.
- Ensure accurate fault detection for different combinations of phase faults.
- Provide instantaneous tripping when the measured impedance falls within the relay's set zone. The relay may also include tapped auto-transformers (S and M taps) to fine-tune the reach and sensitivity, allowing precise control over the relay's response to system conditions .
Summary
In essence, "KD" represents a compensator-type distance relay that combines current and voltage measurements to detect faults accurately and initiate tripping. Its design allows for phase compensation, adjustable sensitivity, and directional operation, making it a critical component in medium-voltage and high-voltage protection schemes .
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