The present invention relates to protective relays in general, and more specifically to an improved echo keyed directional comparison blocking system for protecting an electrical power transmission path against internal faults.
Conventional directional comparison blocking systems generally include at each end of the protected transmission path a directional pilot forward-looking relay, a directional pilot reverse-looking relay, an auxiliary logic block and a pilot communication link including a transmitter and receiver for exchanging information representative of the measured power flow conditions at the two ends of the transmission path. The directional pilot forward-looking relays have as their forward zone at least the distance of the transmission path while the reverse pilot relays look in the opposite direction. Generally, the directional pilot forward-looking relays detect a fault condition and the reverse-looking relays determine if that fault condition is within the transmission path. If the fault is determined to be outside of the transmission path, commonly referred to as an external fault, the auxiliary logic block keys the generation of a blocking carrier signal which is exchanged over the communication link between the logic units at the two ends of the transmission path to prevent interruption of current through the transmission path being protected. Otherwise, with an internal fault detection, the current through the transmission path is caused to be interrupted.
An echo key concept to directional comparison blocking relay systems has been proposed in the U.S. Pat. No. 4,484,245 issued to John D. McFall on Nov. 20, 1984 and assigned to the same assignee as the instant application. Among other improvements, this new system eliminates the need for the reverse-looking pilot relays at the two ends of the transmission path being protected and relies solely on the directional pilot forward-looking relays for measurement information. For example, when the directional pilot forward-looking relay at one end of the transmission path detects a fault condition, it causes the logic block to key its transmitter unit to transmit an initial carrier signal for a short time interval over the communication link to the receiver and logic unit at the other end of the transmission path. If the detected fault is external to the transmission path, the carrier signal is echo-keyed by the logic unit at the other end of the transmission path back to the initiating logic unit to act as a blocking signal to prevent interruption of current through the transmission path. The carrier signal communication between the logic units at the two ends of the transmission path is locked in for a predetermined period of time so long as the previously described condition prevails. Of course, should the condition change to reflect an internal fault, then the carrier blocking signal is inhibited to permit current interruption through the transmission path.
In theory, the McFall echo key system appears to be quite adequate in carrying out its functional requirements, but is not without improvement especially under special conditions of practical operation in a power system network. For example, for those protective systems which provide voice service over the same communication link as used for the blocking carrier signal, the carrier receivers may interrupt the voice signalling as a blocking signal and inhibit current interruption during the time the voice channel is operative. Accordingly, the circuit breakers isolating the transmission path may be delayed in operation if the voice channel is being used at the inception of an internal fault. Another example is where the directional pilot forward-looking relays do not operate together upon an internal fault, but rather one is substantially slower in operation than the other. Because of the logic design of the echo key system, the difference in operating speeds of the two relays may cause an undesirable delay in current interruption in response to the detection of an internal fault.
A still further example of an improvement area is where electrical noise may be coupled into the communication channel. Any noise from the channel will echo key the transmitter via the logic block and this keying will be sealed in for a considerable time period. Accordingly, during this seal-in period, the logic unit is blocked from responding to an internal fault condition and thus tripping of the circuit breaker units will be unduly delayed. In addition, McFall's proposed system does not provide full transient blocking to prevent false operation on all conditions of external faults. For example, where there exists a parallel transmission path to the protected path, both being provided electrical energy from sources at their common ends, the McFall system may render a false trip of the protected transmission path upon detection of a power reversal condition brought on by the sequential closure of the circuit breaker units of the parallel path caused by an internal fault thereon.
Applicants' invention as disclosed hereinbelow offers features which include improvements covering the above exemplary cases as well as other improvements. The disclosed embodiment provides for a viable directional comparison blocking system suitable for application in a power system network environment which may give rise to the aforementioned difficulties.