Special high-speed relaying systems with communication channels are installed to protect the line. One such system is the permissive overreaching system, which is illustrated in Figure 53. This system requires either microwave radio, fiber optic, broad-band power line carrier, or wire-line channel as a communication medium. This medium is an audio tone, frequency shift scheme in which the transmitter at each terminal of the protected line sends a continuous guard signal to the remote receiver to block tripping. When a fault occurs within the reach of the zone 2 relay, a trip signal is sent to the local breaker and the local transmitter frequency is shifted to the trip frequency. Tripping occurs if the local receiver receives a trip signal from the remote terminal. Note that the zone 2 timer is bypassed for this operation. Examination of Figure 53 will show that fault 1 and fault 2 will be detected by both zone 2 relays, so that both transmitters will be shifted to the trip frequency and the fault will be cleared with high speed, the only additional delay being that of the communication channel and the audio tone equipment, generally about 4 ms to 16 ms. If the fault is external to the protected line (see Figure 53, fault 3), even though the relays at breaker D may detect the fault and key its transmitter, the relays at breaker C, being directional, will not detect the fault, will not trip breaker C, and will continue to transmit a guard signal to block tripping of breaker D.
A phase comparison relaying system analyzes the location of a fault by comparing the phase relationship of fault currents at each end of a protected line. Power line carrier or audio tone equipment is commonly used as a communication channel. If the fault currents on-the-line are in phase, the fault is considered to be external to the protected line and a tripping will be blocked. If the fault currents on-the-line are out of phase, the fault is internal and tripping normally will take place. In general, the fault is sensed and tripping is initiated in 8 ms to 20 ms. A channel failure, reduced signal level, or noise at the time of fault may cause improper operation, either disallowing a necessary trip or causing an incorrect operation, depending on the fault location. Please note that relay engineers usually speak of the current phase relationship out of the current transformers (CTs) rather than on-the-line. The understanding is that in the way CTs are connected, the currents out of the CTs will be in phase for a fault on the protected line and out of the phase for a fault on-the-line beyond.
Pilot wire relaying is a differential protection scheme designed to protect transmission lines by comparing the polarity of circulating current in a metallic communication circuit to determine if the fault is internal or external to the protected line. An open or short circuit in the communication pair can prevent a correct operation. However, independent monitoring systems are available that can signal the system operator upon the occurrence of a communication cable fault. Some pilot wire systems are now using fiber-optic channels A transferred-tripping scheme is designed to transfer a trip signal from a local protective relaying terminal to a remote terminal for the purpose of tripping the remote breaker. Almost any type of conventional communication channel may be employed. Transferred tripping is used to protect lines and transformers and for breaker failure protection. Direct unsupervised tripping, upon receipt of the trip signal by the receiver, places severe security requirements on the communication channel and related equipment. For security, usually two signals are sent in parallel and received in series. Transmitter-receiver time is 4 ms to 12 ms and channel time is 1 ms to 10 ms, with total tripping initiation times of generally less than 16 ms (see Figure 54).
Figure 54 —Simplified transfer trip system
Backup systems should be provided to protect the power system from the failure of a relay to detect a fault or a breaker to open.
Remote backup can be accomplished by using a zone 3 impedance relay set (see Figure 52) to protect its own line and all of the next line. Proper coordination with the adjacent line is obtained by using a time delay to give the adjacent line breakers a chance to clear before the backup relay operates. With this
system, all faults not cleared in proper time will then be cleared in 1.5 s to 3 s by the backup relay. This system has the disadvantage of being slow, and at times it may be difficult for the zone 3 impedance relay to distinguish between load and far-end faults on the adjacent line. Zone 3 relays are seldom used in modern systems.
Local backup for line faults is accomplished by using redundant relaying systems plus a breaker failure scheme. The OR logic gates of all line relays are combined with an AND logic gate of an overcurrent fault detector for each circuit breaker and connected to a timer to provide coordination. This scheme will detect the failure of a circuit breaker to trip within a preset time—usually 6 to 10 cycles on EHV systems and 10 to 15 cycles on others—and open all other sources of fault current to the faulted element. Certain bus arrangements, such as a ring bus, require a transfer trip system to open a remote breaker if high-speed fault clearing is required. This requirement is particularly true on EHV systems.
Most systems use automatic reclosing after a fault. These additional clearing times should be considered.
Table 9 can be used as a guide. Generally, the higher the system voltage, the faster the fault is cleared and the faster the breakers reclose, although they may reclose fewer times.
Some EHV systems utilize single-phase tripping and reclosing. This arrangement means the remaining phases remain energized and continue to carry load during the normal reclose dead time. The imbalanced current that flows for this brief interval (about 0.3 s to 0.4 s) will generally not cause problems for communication circuits.
Fast, secure, and reliable communications are necessary on permissive overreach, phase comparison, and transfer tripping. A channel failure, increase, or decrease in level, test tone, or noise at the time of fault may cause improper operation, either by causing the communication receiver to squelch and thereby disallowing a necessary trip (for upward of 2 s on some systems) or by causing an unwanted trip, depending on the data transmission logic. Since no fault detector is used with a transfer trip system, an unwanted trip could occur any time any of the previously mentioned channel conditions exist, particularly on a single-channel system.
Table 9 —Typical trip and reclosure sequence Voltage
NOTE 3— Tripping by time-overcurrent ground relay could extend times to 1.5 s on voltages below 230 kV.
NOTE 4— Dead time is followed by closing of the breaker.