PILOT WIRE PROTECTION

PILOT WIRE PROTECTION : 

For longer tie lines, the most common protection scheme is the pilot wire differential scheme, as shown in Figure 9-8. A fault (Fext) external to the circuit breakers (CB) will cause current to flow through the restraint coils (R) of ANSI Device No. 87L and little or no current to flow in the operating coils (OP).

 An internal fault (Fint) will cause current to flow through the operating coils, which will result in the opening (tripping) of both circuit breakers. 

PILOT WIRE DIFFERENTIAL RELAYING :

Pilot wire relaying is a form of directional relaying where the phase currents are compared over a pair of metallic wires or fiber optics. 

• Internal Faults: One of the network output voltages (VA) will reverse polarity. The current entering Bus A will be 180o out of phase with the current entering Bus B and probably not equal. VA and VB will also be out of phase and probably not equal. With the voltages opposite most of the current will flow through the operating coils resulting in a trip of both breakers  

• External Faults: The network output voltages have the polarity. The currents entering Bus A and leaving Bus B are equal and in phase; the voltages likewise. The polarity of VA and VB allows them to support a circulating current through the restraint coils and pilot wire and very little current through the operating coils -- the relays will not operate. 

• Pilot Wire Supervision: To detect pilot wire faults (e.g., shorts, opens, grounds, etc.) a continuous dc supervision current is applied to the pilot wire. If the pilot wires are shorted, false tripping can occur. Although it reduces the sensitivity of the protection, a fault detector is often used to prevent false tripping.

The increase of the circulating current initiates an alarm. Depending on the location of the short, one of the relays may not trip for an external fault. If the pilot wires are open no tripping can occur. The interruption or absence of the supervisory current also initiates an alarm.

The relay acts as an overcurrent relay if the pilot wire is open.