DIRECTIONAL COMPARISON BLOCKING SCHEME
This is a high-speed protection scheme designed to provide quick tripping for faults anywhere on the protected line, this is the most common pilot relaying scheme which is used to protect extra high voltage power lines. It uses distance relays as directional indicators and blocks initiation for phase faults.
If there is any short circuit in the power system it will affect the entire power system the ability to transfer power in the power system is reduced by this, so in order to prevent this high speed reclosing can be used, if the line is out of service power transferring capability is reduced and automatically restoring the line with minimal delay
DIRECTIONAL COMPARISON PILOT PROTECTION
This pilot protection scheme uses the communication between the terminals to achieve high-speed clearing of faults on the protected line, to implement this protection scheme the pilot protection requires a communication channel for each terminal of the line and thus the remote end will receive the status of each terminal and the terminals can take the decisions according to the data in DCB the relay will send a blocking signal to the remote terminal if any fault occurs in reverse direction and it will indicate that the fault has occurred outside the protected zone and the relay will trip if it sees the fault in forwarding direction
TYPES OF DIRECTIONAL COMPARISON SCHEMES
- Directional comparison blocking scheme
- Directional comparison unblocking the pilot system
- Directional comparison overreaching transfer trip pilot system
- Directional comparison under reaching transfer trip pilot system
OVERREACHING TRANSFER TRIP PILOT SYSTEM
Trip will be initiated when the if the fault is detected from both ends of the line otherwise it will initiate backup protection, this scheme requires an overreaching forward element at both ends of the line zone 2 elements is set to reach 120 to 150 percent of the line length if every relay could detect the fault in forward direction then the faulty section will be internal to the protected line
Relay 4 will trip if there is any fault in forward direction and the permission is received from relay 3
ADVANTAGES OF DIRECTIONAL COMPARISON SCHEME
- Safeguard system stability
- Clear many faults in high speed
- It allows high speed reclose
- Provides coordination
- Long and short line applications
- Reduced equipment damage
- Voltage sag duration is reduced