GPS backup system

A robust back up mode which enables the protection system to operate reliably in the absence of the GPS system is very important. Since the alignment of GPS satellites changes during the course of a day, the GPS antenna must be installed carefully with an unobstructed view of the sky, otherwise reception of the GPS time signal may be interrupted. Poor installation practice can result in the signal being unreliable. As well as representing a dependability risk, this has also manifested itself as a security risk where reports of poor front end receiver and algorithm design have resulted in the utilization of erroneous synchronization pulses by protection algorithms causing false tripping of the equipment. We are also aware of military GPS jamming exercises out to sea from various locations in the UK. There are no guarantees given that these will not affect devices on land! A significant satellite anomaly occurred on 1 January 2004 and an interruption to the 1pps signal occurred. It transpired that a failure had occurred in the atomic frequency standard on one of the GPS satellites and caused transmission of misleading information. This anomaly affected GPS receivers over a large geographical area. It is therefore very important that the performance of the protection system is unaffected by loss of GPS and certainly any back up mode should aim to enable the protection system to run as normal with high communication channel delay asymmetry as well as being unaffected by any communication switching events. Although significant anomalies are believed to be a rare occurrence measures have to be taken within the design of the GPS receiver and relay to ensure that even under the most extreme conditions the integrity of the GPS based relay system is maintained. It is worth noting the point that the GPS antenna and GPS receiver must be treated as an integral part of the protection system.

In order to alleviate these problems and other potential causes of interruption of the GPS time signal, the back-up function described below was developed that allows sampling synchronization control to continue during an interruption to the GPS time synchronization signal. Further, the back-up function can maintain sampling synchronization even in the event of the SDH/SONET communication path being switched during the period of interruption to the GPS time synchronization signal. In the event of the GPS signal being lost, the relay is able to control the oscillator's free-running frequency error to within 0.2ppm, by using the correction factor stored immediately prior to the loss of the GPS signal. By this method, the sampling synchronization error between two relays can be limited to within 1.5μs.

MODE 0: operation under normal conditions

During normal reception of the GPS standard time signal, relays A and B continuously record timing information t1 and t2 for the signal received from the other terminal.

Because the sampling time of relays A and B are synchronized by GPS, t1 and t2 are equal to the propagation delay time between relays A and B respectively. Changes in communication path propagation delay time caused by switching of the SDH network have no effect on the sampling timing of the relays.

MODE 1: operation following loss of GPS signal

'Following loss of the GPS signal at terminal B, relay B becomes the 'slave' and controls the frequency of its internal oscillator so that the signal timing information for the signal received from the other end is maintained at a value of t1, where t1 is the propagation delay recorded immediately prior to loss of the GPS signal.

In the event that the GPS time signal is lost at both terminals then the relay which lost its signal first becomes the slave, and synchronization is maintained at both ends by carrying out sampling synchronization control in the back-up mode.

In the event of the SDH communication path being switched during the period of interruption to the GPS time synchronization signal, relay B will detect the sudden change in the timing of the received signal since switching of a path in the SDH network takes a period of several milliseconds. Having detected a sudden change of propagation delay time, the relay records the new value of delay time t1', and continues to carry out sampling synchronization control according to the new delay time.

If relay B is unable to receive the signal sent from relay A due to a failure in the communication link, then it cannot continue to perform sampling synchronization control. Under these conditions the sampling oscillator is allowed to run free. After subsequent recovery of the communication link, the actual transmission time of signals received from relay A differs from the recorded time by an error which is proportional to the length of time for which the oscillator was in a free-running condition. Therefore, Mode 1 operation incorporates a limitation of allowable free-running time, after which the differential protection element is locked, since sampling synchronization can no longer be guaranteed.

MODE 2 : operation without GPS time signal and following a prolonged period of communication failure

If, during MODE 1 operation, the allowable free-running time is exceeded, or if the dc power supply to the relays is removed causing loss of recorded propagation delay data, then back-up operation changes from MODE 1 to MODE 2. MODE2 operation provides sampling synchronization control based on the assumption that the propagation delay times of the send and receive paths are approximately equal. This mode is used after having checked the difference in propagation delay times of the SDH communication link. However, the transition from MODE 1 to MODE 2 may be made automatically by using a check of the phase difference of the current between the terminals at each line end (see below).

Current phase difference monitoring system

In order to continuously monitor the performance of the sampling synchronization control system, the relay is provided with a function for checking the phase difference between the currents at the relay terminals. The function operates when the measured current phase difference exceeds a specified threshold. The threshold can be adjusted since the measured current phase difference may be greatly influenced by the line charging current, particularly in cases of low load current.